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Small Molecule Activation and Capture by Preorganized Frustrated Lewis Pairs Bertini, F.
2013
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Download date: 02. Oct. 2021 Chapter 1
Small Molecule Activation by Main Group Compounds
Federica Bertini, J. Chris Slootweg, Koop Lammertsma
Abstract: This introductory chapter describes recent spectacular discoveries with respect to the new and fascinating field of small molecule activation by main group compounds. Chapter 1
1.11.11.1.1.1 ... IIIntroductionIntroduction
The activation of small molecules like H 2, CO 2, NH 3, P 4 and N 2O among others, and their subsequent utilization for synthetic purposes, is of fundamental importance in chemistry. Creation of new synthetic methodologies based on small molecule activation would create new opportunities for material development from cheap, readily available and renewable feedstock. For example, there is great interest in understanding how to use carbon dioxide as a feedstock in green processes and as a carbon source for the production of more complex molecules,1 while the development of novel strategies for the transformation of white phosphorus (P 4) is of great importance owing to the high demand of organophosphorus compounds and to environmental concerns. 2 Small molecules are generally quite stable thermodynamically and key to their successful utilization is to provide low barrier reaction pathways, which can be achieved through binding and activation processes typically mediated by transition metal ions. A great amount of fundamental chemistry research has therefore been aimed to understand how metal complexes coordinate to such small and often rather inert molecules, how they modulate their reactivity and how to use the gained knowledge for the development of new catalytic processes. 3 This work has led to a deep understanding that has significantly impacted the fields of organometallic chemistry and catalysis. The terrific success of transition metal based catalysts for the activation of small molecules perhaps has overshadowed for many years the possibility of the utilization of main group elements for the same purpose. In recent years, there has been an intense drive towards developing “green” chemical processes using more environmentally benign chemicals, reagents, solvents and catalysts. 4 A part of this drive is to avoid or minimize the use of transition metals in chemical reactions, as these are often toxic and difficult to dispose properly in large quantities. Moreover, the difficulty of their separation leaves a chance of their contamination of the product. The presence of a metal, even at the lowest level, in pharmaceutical products is closely regulated. Thus, a transition metal free process is desired as a part of the requirements for the chemical industry as well as clean environment. 4 Hence, the development of metal free systems that can replace the use of transition metals is highly desired. The past decade witnessed extraordinary discoveries in the field of small molecule activation by main group species. The first major breakthrough occurred in 2005, when
Power and co workers discovered that the germanium species ArGeΞGeAr (Ar = C 6H3
2,6(2,6 diisopropylphenyl) 2) reacts with molecular H 2 under mild conditions to give the
5 hydrogenated products Ar(H)Ge=Ge(H)Ar, Ar(H) 2GeGe(H) 2Ar and Ge(H) 3Ar. This finding
2 Small Molecule Activation by Main Group Compounds
gained enormous attention in the scientific community since H 2 activation had long been known to occur at transition metal centers, but the reaction of H 2 with a main group compound under mild conditions was unprecedented. In 2006, the group of D. W. Stephan reported on the reversible activation of the dihydrogen molecule under ambient conditions, using a unimolecular phosphine borane
6 Lewis pair. The ability of such phosphine borane pair to activate H 2 was attributed to the contemporary presence of unquenched Lewis basic and Lewis acidic centers, which could synergically interact with the dihydrogen molecule, leading to the heterolytic splitting of the H―H bond. Mutual quenching of the reactive phosphorus and boron atoms was prevented by bulky substituents, which inhibited Lewis adduct formation due to steric hindrance. This finding opened a window of new opportunities in the field of metal free small molecule activation and catalysis and was the starting point from which the chemistry of the so called “frustrated Lewis pairs” (FLPs) rapidly developed. 7 In the same years, it was increasingly recognized that also low valent main group species such as carbenes 8 and their heavier congeners 9 posses the ability to activate small molecules. As in the case of frustrated Lewis pairs, the ability of carbene like species to split strong bonds heterolytically is due to the to the contemporary presence of an electrophilic vacant orbital and a nucleophilic lone pair of electrons. Consequently to these discoveries, the ability of such main group low valent species to activate small molecules has been extensively studied. This chapter illustrates recent developments with respect to an emergent and fascinating topic in main group chemistry: transition metal free small molecule activation. This chapter is not intended to furnish a comprehensive treatment of the literature on such a broad topic, but rather to illustrate the most remarkable discoveries, with emphasis on those main group species which gain their ability to activate small molecules owing to the contemporary presence of acidic empty orbitals and basic lone pairs of electrons. We will first describe a few selected examples of small molecule activation by low valent main group species. In particular, we will describe how carbenes, as well as their heavier congeners, can mimic the behavior of transition metals owing to their ambiphilic nature, and we will briefly comment on the chemistry of the related nitrenes and phosphinidenes, which developed differently. Additionally, a few remarkable examples of small molecule activation by phosphenium ions will be also described. Finally, we will illustrate the chemistry of the newly discovered frustrated Lewis Pairs (FLPs), which is the main focus of this PhD thesis.
3 Chapter 1
1.21.21.2.1.2 ... Singlet CCCarbenesCarbenes
Carbenes are a family of organic molecules composed of a neutral divalent carbon atom with a sextet of electrons and two substituents. 10 They can be regarded as singlet or triplet species depending upon their electronic structure, which is dictated by the nature of the substituents on the carbene carbon atom.11 Triplet carbenes react as diradicals and are therefore less interesting from a synthetic viewpoint since their reactions are non stereospecific.12 In contrast, singlet carbenes are closed shell species which possess a lone pair of electrons and an empty orbital and react stereospecifically participating in cheletropic reactions which occur in a single step, as exemplified by the widely documented 1,2 cycloaddition to double bonds to yield cyclopropanes. 10 13 Carbenes have long been known as very reactive and short lived species that could not be isolated and were usually studied by observing the reactions they undergo. However, strategies for the isolation of persistent carbenes have been developed and nowadays stable carbenes, such as the Arduengo type 14,15 or the Bertrand type,16,17 are very well known. Key to their stabilization is the presence of one or two heteroatoms (such as nitrogen or phosphorus) adjacent to the carbene carbon atom, which decrease the electron deficiency of the empty orbital by donating electron density through resonance, while stabilizing the lone pair on the carbon by inductively withdrawing electron density. Bulky substituents on the heteroatom(s) provide additional stabilization by steric protection of the reactive carbon atom. Despite the provided stabilization, such persistent carbenes remain fairly reactive species. In 2006, the group of Bertrand discovered that acyclic and cyclic (alkyl)(amino)carbenes (aAACs and cAACs) react with CO to afford the corresponding ketenes 18 (as exemplified by aAAC 1 that reacts with CO to form 2; Scheme 1). This led to the question of whether singlet carbenes could mimic the chemical behavior of transition metals, which appeared reasonable since singlet carbenes possess both a lone pair of electrons and an accessible vacant orbital and therefore resemble, at least to some extent, transition metal centers. Consequently to this discovery, the ability of stable singlet carbenes to activate other small molecules was explored. In 2007, Bertrand reported that cAACs and aAACs are also capable of heterolytic H–H splitting under mild conditions (as illustrated by the reaction of 1 with H 2, which results in the formation of 3; Scheme 1), a reaction that has long been known to occur with transition metals only. 19 In contrast to transition metals that act as electrophiles toward dihydrogen, these carbenes primarily behave as nucleophiles. A computational analysis on the mode of H 2 activation by cAACs and aAACs showed that the H–H bond activation arises
4 Small Molecule Activation by Main Group Compounds
from the primary interaction of the carbene`s lone pair with the antibonding orbital of H 2, creating a hydride like hydrogen atom which then attacks the carbon centre. This nucleophilic behavior allows these carbenes to activate the N–H bond of NH 3 (resulting in 1,2 addition products, like 4; Scheme 1), which is a difficult task even for transition metals because their typically electrophilic character favors the formation of Werner type
19 LnM–NH 3 complexes rather than the coordination to the metal via the N–H bond. In fact, examples of N–H bond activation through N–H oxidative addition by transition metals are
20 rare to such extent that the first example of NH 3 activation to afford a product containing
21 both M–H and M–NH 2 groups was reported only in 2005. More recently, Bertrand and co workers have also demonstrated that AACs can activate enthalpically strong bonds such as Si−H, B−H and P−H bonds. 22 The more popular N heterocyclic carbenes (NHCs), which are
23 well established organocatalysts, are currently known to be inert towards CO, H 2 and
NH 3. The higher reactivity of cAACs and aAACs with respect to NHCs arises from the fact that for (alkyl)(amino)carbenes the singlet triplet gap is much smaller, and the HOMO much higher in energy than for the NHCs and consequently, they are more nucleophilic but also more electrophilic, which confers them an increased reactivity. 19 Interestingly, Bielawsky reported that also cyclic di(amido)carbenes (cDACs), despite their higher
24 electrophilicity with respects to cAACs and aAACs, can activate NH 3 and can reversibly react with CO. 25
tBu CO tBu C O 2 (iPr) N (iPr)2N 2 1 tBu H2 H H 3 (iPr)2N
tBu NH3 H 4 NH (iPr) N 2 2
Scheme 1. Reaction of aAAC 1 with CO, H 2 and NH 3.
Remarkably, the carbene mediated activation of white phosphorus (P 4) recently became an active research topic. 26 White phosphorus is of industrial interest as it is the typical starting material for the large scale preparation of organophosphorus compounds. 27
Typically, P4 is treated with Cl 2 gas to make PCl 3 or PCl 5, from which the chlorine atoms are subsequently substituted with organic substrates. Consequently to the increasing demand in phosphorus derivatives and the increasingly stringent environmental regulations, new
5 Chapter 1 processes using white phosphorus, but avoiding chlorine, are highly desirable. While many
2 examples of P 4 activation by transition metals have been reported, the catalytic conversion of white phosphorus to useful products remains elusive, although important steps forward towards the achievement of this goal have been made. 2
The first example of P 4 activation by a stable singlet carbene dates to 2007, when
Bertrand reported that cAAC 5 can open the P4 cluster and stabilizes the resulting acyclic
28 P4 species affording product 7 (Scheme 2; Dipp = 2,6 diisopropylphenyl). Since 7 features a diphosphene and two phosphalkene fragments, which are highly reactive functional groups, it can be used further for the construction of more complex molecules, as exemplified by the diastereoselective [4+2] cycloaddition between the diphosphene moiety and 2,3 dimethylbutadiene, to give organophosphorus compound 9.28 The intermediacy of triphosphirene 6 in the formation of 7, predicted by calculations, 28 was demonstrated by trapping the transient species with 2,3 dimethylbutadiene, which lead to the desired [4+2] cycloaddition product 8 (Scheme 2). 28 Shortly after, also NHCs proved to be reactive towards P 4. For example, NHC 10 reacts with P 4 similarly to cAAC 5, forming
29 the P 4 degradation product 12 (Scheme 3). Additionally, the NHC stabilized P 12 cluster 13 , obtained upon heating 12 , was isolated in high yield (Scheme 3). 29 The lower thermal stability of 12 with respect to 7, which allowed the isolation of cluster 13 , was attributed to the fact that NHCs are less basic than cAACs and therefore better leaving groups, favoring the formation of 13 . On the other hand, the higher electrophilicity of cAACs provides higher stabilization to the acyclic P 4 fragment in 7, by strengthening the P−C
30 bonds. The intermediacy of the [triangle+1] form of P 4 (analogous to 6) in the formation of 12 was again demonstrated by trapping this transient species with 2,3
30 dimethylbutadiene (forming 11 ; Scheme 3). Interestingly, the [triangle+1] form of P 4 could be isolated with aAAC 14 30 (15 ; Scheme 4), which is strongly basic, but also one of the most electrophilic carbenes known.31 In fact, aAAC 14 is sufficiently nucleophilic to open the P 4 tetrahedron, as well as so electrophilic that it undergoes a cyclopropanation reaction with the [triangle+1] form of P 4 (to give 15 ), rather than inducing its ring
30 opening as in the case of 5 and 10 . With carbenes 16 and 19 , P2 and P 1 bis(carbene) adducts 17 (Scheme 5) and 20 (Scheme 6) were isolated, which are of great interest since most synthetically useful organophosphorus derivatives contain only one or two phosphorus atoms, and therefore it is of primary importance to induce the fragmentation
30 of P 4 into smaller P n (n = 1,2) units. The few examples described above nicely illustrate the diversity of the modes of P 4 activation that could be achieved by simply varying the electronic properties of the carbene used, offering new transition metal free strategies for the synthesis of organophosphorus compounds. 32
6 Small Molecule Activation by Main Group Compounds
Scheme 2. Activation of P 4 by cAAC 5.
Dipp N Dipp Dipp Dipp Dipp P 70 °C N N N P4 N P P N 16h P P P P Dipp P P N N N N P P 12 P Dipp Dipp Dipp Dipp P P P P 10 13 Dipp P4 N P P P P N Dipp 11
Scheme 3. Activation of P 4 by NHC 10 .
7 Chapter 1
Scheme 4. Isolation of a carbene stabilized [triangle+1] form of P 4 (15 ).
Scheme 5. Isolation of a P 2 bis(carbene) adduct ( 17 ).
Scheme 6. Isolation of a P1 bis(carbene) adduct ( 20 ).
Singlet carbenes are well known to be extremely versatile ligands for transitions metals 33 and even well established organocatalysts 23 and consequently are of great importance in catalysis. In addition, the recent developments described herein proved their ability to activate small molecules and robust chemical bonds, further increasing the potential use of such low valent carbon species in catalysis. We note that very recently another remarkable achievement has been reported, namely, the N heterocyclic carbene
34 complexation of the greenhouse gas N 2O.
8 Small Molecule Activation by Main Group Compounds
1.31.31.3.1.3 ... Heavier GGGroupGroup XIV CCCarbeneCarbene AAAnaloguesAnalogues
The reactivity of the heavier group XIV carbene analogues, in particular silylenes, germylenes and stannylenes, has also been extensively explored. As heavier analogues of carbenes, they generally posses a singlet ground state, consequently to a larger singlet triplet energy gap, 35 which makes them valuable candidates for the activation of chemical bonds.
1.3.1. Silylenes
While until two decades ago silylenes were considered to be extremely elusive species, 35 since the isolation in 1994 of the first stable N heterocyclic silylene (NHSi),36 new types of NHSis have been developed and their reactivity has been subject to several reviews. 37 In the past few years, important developments in silylene chemistry have been made in connection with the emerging field of metal free small molecule activation, which demonstrated that the intriguing electronic properties of certain silylenes offer new opportunities for the metal free activation of C–H, C–X, Si–X, N–H , P–H, As–H, O–H, S–H, P–P, C–O, N–O, O–O, S–S, Se–Se, Te–Te bonds. 37 Outstanding results, in this context, were obtained with silylene 21 38 (Dipp = 2,6 diisopropylphenyl) and its derivatives. Due to its peculiar ylide like zwitterionic structure (Scheme 7), its chemistry is remarkably different from that of other known NHSis. In fact, 21 exhibits an electron rich butadiene moiety in the backbone, with the exocyclic methylene group that can behave as an additional nucleophilic group and cooperates with the lone pair and empty orbital of the silicon centre making it more reactive towards both nucleophiles and electrophiles. Thus, NHSi 21 features three reactive sites instead of two: the basic lone pair on the Si(II) centre, a formally empty acidic orbital at silicon, and a basic butadiene moiety. For this reason, shortly after its isolation, 38 its ability to activate small molecules gained significant attention. 37a
Scheme 7. Silylene 21 and its ylide like resonance structure 21`.
9 Chapter 1
Thanks to the unsaturated Si atom, 21 manifests a carbene like behavior towards unsaturated organic molecules undergoing a variety of cycloaddition reactions, 39 offering new synthetic methodologies for the synthesis of organosilanes. For example, with terminal alkynes both C–H activation ( 22 and 23 ) and 1,2 cycloaddition to the CΞC triple bond (24 ) were observed at different reaction temperatures (Scheme 8).40
Dipp Dipp Dipp N 21 RT H N H N Si Si Si N RT N H N R = H Dipp R Dipp Dipp Dipp N H R Si 22 23 N R = H or Ph Dipp Dipp H -78 °C N 21 Si N Dipp R
24 Scheme 8. Reactivity of 21 towards terminal alkynes.
Like cAACs and aAACs, 21 can activate the N–H bond of NH 3 (25 ; Scheme 9) and also of hydrazine and methylhydrazine, forming the 1,1 N–H insertion products.41 In a similar
42 fashion, 21 reacts with PH 3 (forming 26 ; Scheme 9), even though longer reaction times and excess of PH 3 are required to achieve complete conversion. By contrast, the reaction with AsH 3 is again very fast, due to the greater Brønsted acidity of AsH 3. However, the 1,1 As–H activation product 27 could only be detected in solution, since it tautomerizes to 28, due to the presence of the additional nucleophilic centre (Scheme 10).42
Scheme 9. Activation of NH 3 and PH 3 by 21 .
10 Small Molecule Activation by Main Group Compounds
Scheme 10. Activation of AsH 3 by 21 .
The reaction of 21 with H 2S, which resulted in the isolation of 29 (Scheme 11), nicely illustrates the unusual ambivalent reactivity of 21 by combining two different types of reactivity involving S–H bond activation: 1,4 and 1,1 addition. 43 In this case, all three reactive sites of 21 have been involved in a single reaction. Other examples that illustrate the reactivity of 21 , involve the activation of C–H bonds (pentafluorobenzene and trifluorobenzene), 44 C–F bonds (hexafluorobenzene, pentafluoropyridine and octafluorotoluene), 44 C–X bonds (X = Cl, Br, I) 45 and Si–X bonds
45 (HSiCl 3 and MeSiCl 3).
Scheme 11. Activation of H 2S by 21 .
In 2007, Driess and co workers reported that 21 reacts with P 4 forming two different activation products, namely 30 (Scheme 12), which results from the Si(II) insertion into a
P–P bond of the P 4 tetrahedron, and 31 (Scheme 12), which results from the insertion of a second equivalent of 21 .46 Interestingly, such mode of activation, which is well known for
2 transition metal/P 4 chemistry, was calculated to be the most stable for both parent
47 48 silylene :SiH 2 and methylene :CH 2 , but has not yet been observed with carbenes.
49 Notably, unlike 21 , the germylene analogue of 21 is resistant towards P 4 even in boiling toluene, owing to the lower reduction potential of Ge(II) versus Si(II). By contrast, an analogous mode of P 4 activation has been reported for related carbene like aluminum and gallium derivatives 32 and 33 (Scheme 13). In fact, the aluminum derivative 34 was
4 reported by Roesky already in 2004 as the first main group complex containing the [P 4]
11 Chapter 1 fragment (Scheme 13),50 while the related gallium derivative 35 was reported several years later. 51
Scheme 12. Activation of P 4 by 21 .
Scheme 13. Activation of P 4 by carbene like Al and Ga species 33 and 35 .
More recent work on P4 activation by silylenes lead to the isolation of a P 4 chain ( 37;
52 Scheme 14) and a P4 cage ( 39; Scheme 15). Reacting the three coordinated Si(II) bis(trimethylsilyl)amide 36 with P 4 in toluene at −10 °C, resulted in the isolation of 37, which possesses a Z diphosphene unit, as revealed by X ray diffraction. The reaction of P4 with disilene 38, which is known to exist in equilibrium with the corresponding silylene 38` in solution,53,54 in a 1:1 molar ratio in toluene at ambient temperature, yielded 39, with a triply opened P 4 tetrahedron (Scheme 15).
Scheme 14. Activation of P 4 by three coordinated Si(II) species 36 .
12 Small Molecule Activation by Main Group Compounds
Scheme 15. Activation of P 4 by bis(silylene) 38 .
Interestingly, the nucleophilicity of the Si(II) centre of silylenes can be increased by stabilization of the Lewis acidic orbital of the silylene with a strong donor (D). This strategy to enhance the nucleophilicity of the Si(II) atom broadened significantly the applicability of silylenes for small molecule activation. For example, although silylene 21 is inert towards
55 55 56 the greenhouse gas N2O, its Lewis base stabilized analogues 21a , 21b and 21c react
55 57 with N 2O affording the corresponding silanones (40a, 40b and 40c ; Scheme 16). Additionally, when DMAP is used as the stabilizing group, the resulting silanone (40c ) can
56 activate NH 3 under mild conditions, after replacement of the DMAP ligand (forming 41 and 42; Scheme 16), while the reaction with H 2S gave 43.
Dipp Dipp NH3 N N NH2 NH2 Si Si OH O N N Dipp Dipp Dipp Dipp N D N2O N D Si Si 41 42 N (-N2) N O (D = DMAP) Dipp Dipp Dipp H S N 21a-c 40a-c 2 S Si OH DMAP N Dipp a: D = 1,3,4,5-tetramethylimidazol-2-ylidine b: D = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidine 43
c: D = 4-dimethylaminopyridine (DMAP)
Scheme 16. Reduction of N 2O by 21a c and activation of NH 3 and H 2S by 40c.
13 Chapter 1
While activation of CO 2 by the Lewis base stabilized derivatives of 21 described above (21a-c) was unsuccessful, this could be achieved with siloxy silylene 44, which is obtained
58 from 21 upon reaction with H 2O. In fact, 44 is remarkably nucleophilic and proved to be able to activate both greenhouse gasses CO 2 and N 2O forming the same silanoic silyl ester
59 45 upon liberation of N 2 or CO (Scheme 17).
Scheme 17. Activation of CO 2 and N 2O by 44.
Very recently, also two examples of CO 2 and N 2O activation by bis(silylenes) were reported: the group of Baceiredo reported that bis(silylene) 46 reacts with 4 equivalents
60 of CO 2 to form the carboxylato bis silicate 47 (Scheme 18), while Roesky reported that
61 bis(silylene) 48 reacts with 6 equivalents of N 2O forming the siloxy compound 49 upon
62 liberation of N 2 (Scheme 19).
Scheme 18. Activation of CO 2 by bis(silylene) 46.
14 Small Molecule Activation by Main Group Compounds
Scheme 19. Activation of N 2O by bis(silylene) 48.
Other examples of silylene mediated bond activation include the activation of calcogens 57,63 (S, Se, Te) and a large variety of reactions with selected unsaturated organic functional groups,64 which highlight the highly versatile chemistry of such low valent silicon species.
15 Chapter 1
1.3.2. Germylenes and Stannylenes
Like silylenes, also germylenes and stannylenes, as well as related Ge(II) and Sn(II) species, gained significant attention for their ability to activate chemical bonds. In the past decade, Holl and co workers reported a considerable amount of work on the indirect activation of different types of C–H bonds by germylenes and stannylenes.65 66 Initially, germylene 50 (Scheme 20) was observed to insert into α C–H bonds of organic nitriles,
65 when employing THF solutions of salts such as LiCl, MgCl 2 or LiBr. Subsequently, Holl reported examples of selective C–H activations of ethers and alkanes mediated by germylenes 50 and 51 (Scheme 20) in the presence of in the presence of aryl halides (Ph–X, X = I, Br, Cl).66a Benzene, derived from the aryl halide, is produced along with the
C–H activation products (R 2GeXR’). The authors proposed a radical mechanism for the C–H activation, involving free phenyl radical, while oxidative addition of aryl halide to the germylene was observed as a concentration dependent side reaction. High yields in the C– H activation products were obtained through the use of high dilution techniques. 66a In 2006, this chemistry was extended to the acyclic N,N disubstituted stannylene 52 (Scheme 20), which was employed for the C−H activation of alkanes and ethers 66b and in 2008, to the cyclic (alkyl)stannylene 53 (Scheme 20), for the C–H bond activation of alkynes 66c and alkenes. 66d Additionally, the products obtained from the reactions of 52 with alkanes and ethers were used in subsequent Stille type C(sp 3)–C(sp 2) cross coupling reactions to form new C–C bonds. 66e In 2009, Holl also reported on the reactivity of NHSi 54 (Scheme 20) to activate the C– H bond of alkanes and ethers with Ph−X and further extended this chemistry for NHSi 54 and germylene 50 to alkyl amines. 66f Similarly, the reaction of stannylene 53 and Ar−I with alkanes, alkenes, alkynes and ethers resulted in the corresponding C–H activation products.66g
Scheme 20. Germylenes, stannylenes and silylene used for the activation of C –H bonds in the presence of aryl halides.
16 Small Molecule Activation by Main Group Compounds
Noteworthy, is also the reactivity of related Ge(II) and Sn(II) hydrides, which were studied for their ability to activate small molecules. 67 In fact, it was observed that, unlike the tetravalent group XIV hydrides, the Ge(II) and Sn(II) hydrides can activate a large number of small molecules in the absence of any added catalyst. For example, Ge(II) and
Sn(II) hydrides ( 55a ,b; Scheme 21) react with CO 2 to give the Ge(II) and Sn(II) esters of formic acid ( 56a ,b; Scheme 21). 68,69 Similarly, Ge(II) and Sn(II) hydrides react with carbonyl groups, alkynes, C=N bonds, and also with azo and diazo compounds and azides. 68 70,71,72 All these reactions proceed via the insertion of the small molecule into the Ge–H or Sn–H. The activation of small molecules by such Ge(II) and Sn(II) hydrides, and by related Si(II) hydrides, has been the subject of a recent review. 67
Scheme 21. Activation of CO 2 by Ge(II) and Sn(II) hydrides.
Remarkably, certain low valent silicon and germanium species, in analogy to carbenes, are also able to split the H–H bond of H 2. As we have mentioned in the introductory paragraph (1.1.), the first example of H 2 activation by a main group compound under mild conditions was reported in 2005, when Power discovered that the germanium species
ArGeΞGeAr ( 57; Ar = C6H3 2,6 (2,6 diisopropylphenyl)2) reacts with molecular H 2 forming a mixture of hydrogenated species (58−60 ; Scheme 22).5 By contrast, the Sn analogue
(61; Ar = C 6H3 2,6 (2,6 diisopropylphenyl) 2) reacts with dihydrogen to give only ArSn( –
73 H) 2SnAr (62; Scheme 23).
Scheme 22. The first example of H2 activation by a main group compound.
More recently, also germylenes and stannylenes were found to be able to react with H2 under relatively mild conditions. 74 75 In 2008, Power reported on the ability of stannylene
17 Chapter 1
SnAr 2 (63: Ar = C6H3 2,6 (2,6 diisopropylphenyl)2) to activate H2 and even NH 3, affording dimeric products bearing two –H (62) or –NH 2 (64) bridging ligands (Scheme 23), with concurrent elimination of ArH. The corresponding lack of reactivity towards H 2 and NH 3
# # observed for SnAr 2 (Ar = C6H3 2,6(Mes)2) and Sn[N(TMS) 2]2 (53), suggested that the higher reactivity of SnAr 2 (63) may be attributed to the increased triplet character of 63 in the ground state, accordingly to the wide C–Sn–C angle (117.6 °). 75 Interestingly, related examples of N–H and H−H bond activation have been reported for a low valent aryl gallium(I) species (65/65`), which react with both NH 3 and H2 forming gallium species 66 and 67 under ambient conditions (Scheme 24). 76
Scheme 23. Activation of H 2 and NH 3 by stannylenes.
Scheme 24. Activation of H 2 and NH 3 by Ga(I) species.
18 Small Molecule Activation by Main Group Compounds
In contrast, extension of this chemistry to germylenes showed the formation of the tetravalent monomeric products. Germylenes 68 and 69 react with H 2 to give germanes
70 and 71 , and with NH 3 forming germanes 72 and 73 (Scheme 25). The different reactivity of 68 towards H 2 most probably results from the increased steric bulk on the germanium atom, which favors arene elimination and addition of a second equivalent of H 2
77 to form 70 . The germanium analogue of silylene 21 (74 ), also reacts with NH 3, but in contrast to the Si analogue 21 , which undergoes formal oxidative addition to the N–H bond forming the 1,1 addition product ( 25 ), with 74 the 1,4 addition product 75 is observed (Scheme 26). Primary amines react with 74 in a similar fashion. 78
Ar Ar H Ar H NH3 2 H Ge Ge Ge H Ar NH2 Ar (-ArH) H 72 68 70
Ar = C6H3-2,6-(2,6-diidopropylphenyl)2
# # Ar# Ar H Ar H NH3 2 H Ge Ge Ge # # H Ar# NH2 Ar Ar 73 69 71 # Ar = C6H3-2,6-Mes2
Scheme 25. Activation of H 2 and NH 3 by germylenes.
Scheme 26. Activation of NH 3 by germylene 74 .
In 2009, Power reported the first example of room temperature reactions of CO with heavier carbene analogues. Interestingly, while carbenes are known to react with CO to give ketenes 18 (par. 1.2.) germylenes 76 and 77 reacted with CO, to give germanium oxides 78 and 79 (Scheme 27).79
19 Chapter 1
Scheme 27. Activation of CO by germylenes.
In addition, the first example of gentle activation of P 4 by a low valent Sn compound
80 has been reported recently. The reaction of P 4 with the novel, unsymmetrically coordinated, bis(stannylene) 80 afforded the butterfly like bicyclo[1.1.0]tetraphosphabutane derivative 81, by insertion of one P 4 tetrahedron into the Sn−Sn bond of 80 (Scheme 28). Unlike in the case of 80 , in 81 both Sn atoms are four coordinated, as revealed by single crystal X ray diffraction studies. We note that while
28 30 46,52 a few examples of P 4 activation by carbenes and silylenes have recently appeared in the literature, this is the first example of P 4 activation by a low valent Sn compound, while no gentle activation of P 4 by low valent Ge species has yet been reported. Furthermore, as we mentioned in the previous paragraph, Driess reported that the germylene analogue of
49 silylene 21 , in contrast to the silicon species ( 21 ), is unreactive towards P 4.
Consequently, the search for the P 4 activation by low valent Ge species appears to be an attractive target, to fill in the gap in this series, in the hope that these interesting compounds will pave the way for the development of a transition metal free and environmentally friendly P 4 activation for industrial processes.
Scheme 28. First example of P 4 activation by a low valent Sn(I) species.
20 Small Molecule Activation by Main Group Compounds
1.1.1.41. 444.... Group XV CCCarbene Carbene AAAnalogues: Analogues: PPPhosphinidenes Phosphinidenes and PPPhospheniumPhosphenium IIIonsIons
Having highlighted the ability of stable singlet carbenes and their heavier group XIV analogues to react with small molecules, the chemistry of their group XV analogues
+ 81 deserves also attention. Phosphenium ions (R 2P ), in particular, are of interest for the metal free activation of small molecules because, due to the positive charge on the two coordinate phosphorus atom, they are highly electrophilic 82 and are known as singlet species. 83 In contrast, free phosphinidenes (RP) generally possess a triplet ground state 84 and consequently they reacts as diradicals, which makes them inadequate for synthetic applications. So far, singlet phosphinidenes are accessible only upon metal complexation, which allowed a substantial body of fundamental research on the reactivity of singlet P(I) species.85,86 In case of nitrenes, 87 which also posses a triplet ground state, the singlet triplet gap is generally even larger than for phosphinidenes, as for the parent systems imidogen H−N 88 (36 kcal mol 1) and phosphinidene H−P 89 (22 kcal mol 1). Additionally, for nitrenes, the lowest energy singlet state is typically predicted to be open shell. In this paragraph, we will limit our discussion to singlet phosphinidenes and phosphenium ions.
1.4.1. Singlet Phosphinidenes
While strategies for the isolation of stable singlet carbenes, silylenes, germylenes and stannylenes have been developed, metal free phosphinidenes are known as transient species, whose existence has been inferred mainly by trapping reactions and spectroscopically, which explains why their chemistry developed rather differently than the one of their group XIV relatives. Substituents that have δ type lone pair electrons ( i.e.
−NX 2, −PX 2, −OX, −SX) are predicted to lower the singlet triplet energy gap or even favor the singlet state. 84 While the most stabilized singlet ground states were predicted for P−SF and P−SCl, from a practical point of view, amino (P–NR 2) and phosphino (P–PR 2) derivatives bearing large alkyl groups (R) are the most plausible and feasible targets for preparing phosphinidenes possessing a closed shell singlet ground state. However, appropriate precursors for such species have not yet been developed. 90 So far, close shell singlet phosphinidenes, which are suited for synthetic applications, are accessible only upon complexation with a transition metal stabilizing group which confers them either a nucleophilic or an electrophilic character. 91
21 Chapter 1
Nucleophilic phosphinidene complexes 86 are significantly stabilized to such extent that many could be isolated, however, they typically react via the P=M double bond, as exemplified by their most characteristic reactions, i.e. the phospha Wittig reaction with carbonyl compounds, the addition to protic reagents and the [2+2] cycloadditions with alkynes. 86 Since all these reactions involve the participation of the metal centre, they will not be discussed further herein. Electrophilic phosphinidene complexes 85 are strikingly more reactive and are typically generated as transient species from appropriate precursors. The best known and most typical reaction of electrophilic phosphinidene complexes is the 1,2 cycloaddition to carbon carbon double or triple bonds, which occurs with retention of configuration, to give 3 membered phosphiranes and phosphirenes, respectively.85 In a similar fashion, they react also with hetero olefinic C=X double bonds, (X = N, O, Si, S, P), giving access to CXP 3 membered rings. 85 Nevertheless, their reaction with C=X double bonds may also result into a variety of products. For example, the reaction with carbonyl containing compounds leads to different products depending on the carbonyl compound used. 92 Numerous phosphinidene C–H and C–C insertion products have been reported, but in most cases these products result from the initial phosphinidene addition to an unsaturated bond of the substrate, followed by intramolecular rearrangement into the insertion product.85 A remarkable example of a direct phosphinidene insertion reaction is the insertion of phosphinidene complexes R−P–M(CO) 5 (R = Ph, Me or CH 2CH2Cl; M = Mo, W) into a C–H bond of one of the Cp ligands of ferrocene (82; Scheme 29), which highlighted its strongly electrophilic character.93
Scheme 29. Insertion of electrophilic phosphinidene complexes into a C−H bond.
Although a substantial body of elegant work has accumulated on electrophilic phosphinidenes complexes, utilization of their chemistry for synthetic purposes is significantly hampered by the limited amount of suitable and easily accessible precursors and also by the difficult demetallation step required to create metal free species from the resulting reaction products.
22 Small Molecule Activation by Main Group Compounds
1.4.2. Phosphenium Ions
+ Phosphenium cations (R 2P ) are isoelectronic to carbenes and phosphinidenes and represent another class of low valent, 6 electron, organophosphorus compounds. 81 They differ from phosphinidenes for the fact that they bear two substituents on the phosphorus atom and consequently they are positively charged, while free phosphinidenes bear only one substituent and are therefore neutral. More importantly, in contrast to free phosphinidenes, they possess a singlet ground state. 83 Like phosphinidenes, phosphenium cations are typically generated in situ , although certain phosphenium cations have been isolated as stable species, owing to the presence of two amino substituents on phosphorus, which stabilize the low coordinate P(III) centre through a push pull effect, similar to the N heterocyclic carbenes. 84,95 Additionally, also a few stable N,C bound phosphenium cations are known. 94 Phosphenium cations 81 have been known for more than 40 years and interestingly, the isolation of free N heterocyclic phosphenium cations (NHPs) in 1972 95 preceded the one of their carbon analogues (NHCs),14 although they have garnered much less attention. Due to the increasing number of publications relative to the activation of small molecules by low valent main group species, the potential use of phosphenium cations for this purpose has recently been recognized. Due to the presence of a strongly electrophilic vacant orbital, phosphenium cations react with Lewis bases 96 such as phosphines, 96a c to give the Lewis base adducts of the phosphonium ion. Additionally, they are known to insert into C–H bonds. The first example was reported in 1982 by Cowley and co workers, who observed the insertion of
+ − [( iPr 2N) 2P] (83; [AlCl 4] as counterion) into the C–H bond of stannocene and plumbocene (84a,b; Scheme 30).97 Subsequently, other examples of C–H activation by phosphenium
+ ions have been reported, such as the intramolecular rearrangement of [(Me 5C5)( tBu)P] (85) into phosphonium salt 86 (Scheme 30).98
Scheme 30. Examples of insertions of phosphenium cations into C–H bonds.
23 Chapter 1
Also cycloaddition reactions to dienes are well documented. 99 Typically, the reaction of phosphenium ions to 1,3 dienes leads to the formation of the 1,4 cycloaddition products, namely phospholenium salts (87; Scheme 31). It is supposed, based on experimental observations, that the reaction undergoes via a concerted [1+4] cycloaddition. 99c Also the reaction with 1,4 dienes has been investigated. For example, [ iPr 2NPCl][AlCl 4] (88) reacts readily with 1,4 pentadiene or 1,4 hexadiene to afford phosphorus bi and tri cyclic compounds 89 and 90 (Scheme 31).99c These cycloaddition reactions, as well as the
+ insertion reactions mentioned above, firmly established the carbenoid nature of [R2P] .
Scheme 31. Examples of cycloaddition products for the reaction of phosphenium ions with 1,3 and 1,4 dienes.
In 1994, Burford reported the reaction of the neutral, zwitterionic aluminum derivative
91 and the ionic N,N stabilized phosphenium salt [( iPr 2N) 2P][AlCl 4] ( 83) with elemental
100 sulfur , which gave the new bis(spiro)tricyclo dialumina tetra aza dithia diphosphetane 92 and the previously reported 101 thiophosphoryl chloride aluminum trichloride complex
(iPr 2N) 2P(Cl)S AlCl 3 (93), respectively (Scheme 32). The aluminum derivative 91 is also known to react with P=N bonds,102 as illustrated by the reaction of 91 with
(TMS) 2N−P=N(TMS) which gave by cyclic zwitterion 94 , formed upon insertion of the imminophosphine (P=N) unit of (TMS) 2N−P=N(TMS) into the P−N bond of 91, followed by an unusual ring opening and cycloaddition involving the formation of three bonds (Scheme 33).
24 Small Molecule Activation by Main Group Compounds
Scheme 32. Activation of S 8 by phosphenium cations.
Scheme 33. Reaction of phosphenium cation 91 with P=N bonds.
Yet, the most remarkable achievements with regard to the activation of small molecules by phosphenium cations were obtained with the activation of white phosphorus. In 2001, Krossing and co workers reported the insertion of the in situ prepared, highly
+ electrophilic [X2P] cations ( 95; X = Br, I) into one of the P−P bonds of the P 4 tetrahedron,
+ yielding phosphorus rich binary cage cations [X2P5] (96; Scheme 34). Although highly reactive, 96 could be successfully isolated using non oxidizing, weakly coordinating
– 103 counteranions of type [Al(OR) 4] (R = C(CF 3)3). This mode of P 4 activation is analogous
46 to the one observed with silylenes (see silylene P4 adduct 30; par. 1.3.1.) and contrasts
28 30 to the mode of activation by carbenes, which attack the P 4 skeleton nucleophilically. In
+ a similar fashion, such [X2P] cations, which are generated by silver salt metathesis of
Ag[Al(OR) 4] and PX 3, also insert into X–X and P–X bonds of X 2 and PX 3 (X = Br, I), forming
+ + – 103b [X 4P] (97) and [X 5P2] (98) salts of the weakly basic anion [Al(OR) 4] . In 2009,
+ Wiegand extended this chemistry to [Ph 2P] (99) and reported the preparation of mono ,
25 Chapter 1
+ 2+ 3+ di and tri cationic clusters [Ph 2P5] (100 ), [Ph 4P6] (101 ) and [Ph 6P7] (102) (Scheme
104 + 34). In this work, the source of [Ph 2P] is a molten medium, readily obtained upon mixing Ph 2PCl and GaCl 3 with varied stoichiometries. In a previous study, the molten
105 medium obtained from a 1:1 mixture of Ph 2PCl/GaCl 3 had proved to be able to react with cyclic polyphosphine Ph 5P5 to give P–P insertion product 2,3,4,5 cyclo tetraphosphanyl 1,4 diphosphonium dication 103,105 which suggested the use of the melt for the activation of the P 4 cluster.
Scheme 34. Insertion of phosphenium ions into P–P bonds.
Similarly, cyclo 1,3 diphospha 2,4 diazane 104, which acts as phosphenium ion source in the presence of a Lewis acid, reacts with P4 in a stepwise manner to form the novel
+ 2+ 106 clusters [P 4((Dipp)NP) 2Cl] (105) and [(P 4)2((Dipp)NP) 2] (106) (Scheme 35). In a similar fashion, also Si based and Al based relatives 107 and 91 react with 1 equivalent of
+ P4 yielding compounds 108 and 109, which feature the [P5] unit (Scheme 35). These results highlight the potential use of phosphenium cations for the transformation of white phosphorus. The next and most important target is the development of P transfer reactions, ideally with the use of only catalytic amounts of activating agent, allowing the direct synthesis of organophosphorus compound from P 4. In view of the growing attention towards low valent main group species for the metal free activation of small molecules, further discoveries in this respect are to be expected in the coming years.
26 Small Molecule Activation by Main Group Compounds
Scheme 35. P4 activation by heterocyclic phosphenium ions.
27 Chapter 1
1.1.1.51. 555.... Frustrated Lewis Pairs
1.5.1. Discovery of Frustrated Lewis Pairs (FLPs)
A novel and promising approach to the activation and utilization of small molecules has emerged very recently after the seminal discovery, by the group of D. W. Stephan, that certain sterically encumbered Lewis acid and Lewis base combinations, which do not undergo the ubiquitous neutralization reaction to form “classical” Lewis adducts (later termed as “frustrated Lewis pairs”), could cleave the robust H–H bond of the dihydrogen molecule. 7
In 2006, D. W. Stephan reported on the reversible metal free H 2 activation by phosphinoborane 110 , which effects heterolytic H–H cleavage under mild conditions to form phosphonium hydroborate 111 (Scheme 36). 6 Similarly to the case of singlet carbenes and their heavier group XIV analogues, the ability of phosphinoborane 110 to effect heterolytic H–H cleavage arises from the contemporary presence of unquenched Lewis acidic and Lewis basic centers.
Scheme 36. Reversible H 2 activation by phosphinoborane 110.
This discovery gained enormous attention in the scientific community, since it was the first example of reversible transition metal free H2 activation under mild conditions. In fact, at that time, the only other main group compound known to react with molecular hydrogen under mild conditions was Power’s digermyne ( 57 ). 5 Consequently to this remarkable discovery, also the reactivity of simple stoichiometric phosphine/borane mixtures (112 ―119 ) towards H 2 was explored, using toluene as
107 solvent. Phosphine/borane frustrated Lewis pairs tBu 3P/B(C 6F5)3 (112) and
Mes 3P/B(C 6F5)3 (113) reacted smoothly with H 2 forming the corresponding phosphonium hydroborate salts [ tBu 3PH][HB(C 6F5)3] ( 120 ) and [Mes 3PH][HB(C 6F5)3] ( 121 ) in quantitative yield, while frustrated Lewis pair tBu 3P/BPh 3 (114) effected analogous H–H cleavage, but required longer reaction times and the product ( 122 : [ tBu 3PH][HBPh 3]) was
28 Small Molecule Activation by Main Group Compounds isolated in low yield (33%), most likely as a consequence of the reduced Lewis acidity of the borane. Frustrated Lewis pairs Mes 3P/BPh 3 (115), (C 6F5)3P/B(C 6F5)3 (116) and tBu 3P/BMes 3 (117) gave no reaction upon exposure to an H 2 atmosphere at ambient conditions, while phosphine/borane pairs PPh 3/B(C 6F5)3 (118) and PMe 3/B(C 6F5)3 (119) formed stable Lewis adducts and proved to be unreactive towards H 2.
These observations brought across the take home message that H 2 activation by such phosphine/borane pairs occurs only under favorable steric and electronic conditions. In particular, steric constrains must be sufficient to preclude the mutual quenching of the Lewis basic and acidic centers, and the Lewis acidity and basicity of the phosphorus and boron atoms must be properly matched. Especially, the use of strongly electron withdrawing pentafluorophenyl substituents on the boron atom seemed to be required to enhance the Lewis acidity of the borane and consequently, the reactivity of the FLP. Due to this observation, most FLPs are based on B(C 6F5)3 or a –B(C 6F5)2 fragment as the Lewis acidic component. By contrast, a large variety of Lewis bases have been used in FLP chemistry. 108 On the other hand, it was soon envisioned that also weakly bonded Lewis adducts could manifest FLP reactivity. In 2007, the group of Erker reported that phosphinoborane
Mes 2PCH 2CH 2B(C 6F5)2 (123 ), which forms a classical (intramolecular) Lewis adduct, as indicated by multinuclear NMR spectroscopy and by calculations, reacts smoothly with H 2 at ambient conditions to give phosphonium borate Mes 2P(H)CH 2CH 2(H)B(C 6F5)2 (124 ; Scheme 37).109 The FLP like behavior of 123 was attributed to the weakness of the intramolecular P−B bond which allows its dissociation to give a reactive Lewis pair 123’ . Subsequent investigations revealed that 123 possesses an extraordinary FLP type reactivity towards a large variety of molecules 110,146 and is an active metal free catalyst for the hydrogenation of enamines and imines. 121a
Scheme 37 . Lewis pair 123 /123’ and its reactivity towards H2.
Two years later, the group of D. W. Stephan reported on the peculiar case of the
Lutidine/B(C 6F5)3 pair (125 ), which is “at the boundary of classical and frustrated Lewis pair reactivity” , since it forms a classical Lewis adduct and also exhibits FLP behavior in the
29 Chapter 1
activation of H 2, due to partial dissociation of the Lewis adduct which occurs in solution, probing that classical and frustrated Lewis pairs are not mutually exclusive. 111 More astonishing, was the discovery that the strongly bonded, classical Lewis pair
Ph 3P–B(C 6F5)3 (118), for which no evidence of dissociation was observed by NMR spectroscopy, consistent with the calculated P–B bond dissociation energy of 39 kcalmol 1,112 undergoes P/B addition to the triple bond of phenylacetylene in an E fashion, to give phosphonium borate Ph 3PC(Ph)=(H)B(C 6F5)3 (128), as observed for FLPs ( o tol) 3P/B(C 6F5)3
113 (126 ) and ( o tol) 3P/Al(C 6F5)3 (127 ). This observation revealed that even classical, strongly bonded Lewis adducts, previously thought to be unreactive, might manifest FLP type reactivity, broadly extending the potential of FLP chemistry for small molecule activation.113
1.5.2. Proposed Mechanisms for the FLP-mediated H–H Bond Splitting and Association in Frustrated Complexes
Since the discovery that FLPs could split the H 2 molecule heterolytically, computational effort has been devoted to unravel the mechanism for the H 2 uptake by such main group phosphine borane species. 114,115 120 Guo and Li were the first ones to propose a concerted mechanism for the heterolytic splitting of H2, resulting from the synergic interaction of the boron and phosphorus atoms with the dihydrogen molecule. 115 The enlightening work of Papai, 116 provided insight into the nature of the bimolecular
FLP tBu 3P/B(C 6F5)3 (112). With the aim to unravel the mechanism of the H 2 uptake by
112, initially, they examined the possibility that the H 2 activation could be initiated by the side on interaction of the H 2 molecule with the borane, resulting in the donation of the H−H σ bond electrons into the Lewis acidic empty orbital of the boron centre. This assumption was based on the analogy with the mode of H 2 activation by transition metals
117 and considered previous studies on the existence of weakly bound BH 3−H 2 adduct. Subsequent H + migration to the phosphorus atom would lead to the final phosphonium borate species. However, the calculations indicated that this interaction is actually repulsive, in agreement with the observation that for B(C 6F5)3 there is a significant delocalization of the aryl π electrons into the p(B) vacant orbital of B(C 6F5)3, which prevents the σ donation from H 2 into the boron vacant p orbital and consequent formation of a H 2−B(C 6F5)3 complex. Nonetheless, we note that an example of FLP mediated H 2 activation which occurs through the initial side on interaction of the H 2 molecule with the
30 Small Molecule Activation by Main Group Compounds boron centre, followed by the deprotonation by the Lewis base, has been recently
F 118 proposed for Lewis pair Ar 2BH/NEt 3. As an alternative scenario, the end on approach of tBu 3P to H 2 was also considered, which is supported by previous low temperature matrix isolation work which demonstrated that phosphines can weakly interact with H 2, presumably via an end on interaction between the lone pair on the phosphorus atom and
116 the empty σ* orbital of the H 2 molecule. However, also such end on interaction resulted unfavorable, owing to Pauli repulsion. Based on these observations and considering the ease of the activation of H 2 by FLPs, an alternative picture had to be sorted out. Importantly, it was envisioned that secondary interactions may lead to weak association between the molecules of the FLP . Indeed, a weakly bound tBu 3PB(C 6F5)3 complex was identified as a minimum on the PES (Figure 1). The bonding in this adduct was characterized as a combination of multiple C−HF hydrogen bonds and dispersion interactions. The association energy was predicted to be of −11.5 kcal mol −1, which was surprisingly large, suggesting a certain degree of association even at room temperature.
Another characteristic feature of the described tBu 3PB(C 6F5)3 complex is its structural flexibility, which originates from the dominance of weak, non directional long range forces.
Figure 1. Structure of the tBu 3PB(C 6F5)3 complex reported by Pápai . C–HF type hydrogen bonds (with d(H –F) < 2.4 Å) are indicated with dotted lines.
According to this new model proposed by Pápai, which involves the pre organization of the FLP into a loosely bound frustrated complex, a small H 2 molecule can then insert into the reactive pocket of this flexible FLP and interact simultaneously with both reactive centers. The H −H cleavage was described to occur through simultaneous interaction of the phosphorus` lone pair with the σ* orbital of the H 2 molecule and of the σ(H−H ) bonding
31 Chapter 1
electrons with the vacant orbital of B(C 6F5)3 and implies progressive weakening of the H— H bond along the reaction pathway. The flexibility of such frustrated complex was further probed by additional computational work by the group of Rhee, 119 aimed to unravel the origin of the stability of FLP 112. In fact, their calculations further indicated that dispersive interactions between the phosphorus` lone pair and the delocalized π system in the borane render the system highly flexible and provide considerable amount of entropic stabilization in the pair formation. Additionally, they observed that a substantial stabilization of the frustrated complex also arises from the non covalent interaction between the boron and phosphorus atoms. Once the importance of the secondary non covalent interactions, which are responsible for the formation of a “ frustrated complex ”, was recognized, subsequent computational studies by Grimme`s group 120 shed some doubt on the linear P−HH−B arrangement in
116 the transition state previously proposed by Pápai for the reaction of 112 with H2. In fact, it was noted that a linear P−HH−B arrangement is not possible for phosphinoborane 123 , although this Lewis pair also had proved to react efficiently with H 2 at ambient conditions. The authors suggested that the almost linear P−HH−B arrangement observed in the transition state computed by Pápai is likely to be an artifact arising from the insufficient theoretical treatment of intramolecular London dispersion forces between the large substituents. Furthermore, based on their calculations which include dispersion forces, they presented a simpler mechanistic picture of the basic activation step that emphasizes on the polarization of H 2 induced by the electric field of the FLP inside its cavity. In fact, computing a relaxed two dimensional potential energy surface (PES) with a fixed linear P−H−H−B unit and with the most important H−H and
P−B distances as variables, revealed that once the H2 molecule is within the FLP cavity, the H−H dissociation is actually barrierless and that the observed barrier should rather be attributed to some kind of entrance process of H 2 into the FLP`s reactive pocket. A detailed analysis of the transition state for the reaction of 112 with H2 confirmed that the entrance of the H 2 along with the initial opening of the FLP, is the key step of the reaction responsible for the observed barrier and that once the H 2 inside the FLP cage, the reaction proceeds without a barrier. This conclusion describes the reaction of 112 with H 2 as an effectively bimolecular process between a prepared Lewis pair and H 2. Analogous conclusions were reported also for the unimolecular Lewis pair 123 . Furthermore, the authors proposed a simpler mechanistic picture where the FLP itself is neglected entirely and replaced by an electric field. Almost exact (FCI/aug cc pVQZ) potential energy curves for dihydrogen dissociation in an electric field of varying strength applied along the H−H
32 Small Molecule Activation by Main Group Compounds bond axis were computed. Above a critical field strength of about 0.05–0.06 a.u. the potential energy curves start to exhibit a maximum, which indicates heterolytic dissociation to the H +/H− ion pair. The corresponding barrier and its position are strongly field dependent: with increasing field strength the transition state moves to smaller H−H distances and the activation barriers is reduced. A rational conclusion is that the magnitude of the field along the bond axis in the region of the H 2 molecule should be as large as possible for small molecule activation by FLPs.
In conclusion, it is nowadays commonly accepted that FLPs activate H 2 (and chemical bonds in general) by polarization of the chemical bond, which occurs owing to the electric field created by their donor/acceptor atoms and that the observed reaction barriers are mainly due to the formation of a prepared FLP.
1.5.3. Reactivity of Frustrated Lewis Pairs
Hydrogenation reactions
7 Since their discovery, the ability of FLPs to activate H 2 has been extensively studied. The most interesting aspect, is the fact that the phosphonium hydridoborate salts that result from the facile H–H splitting by the phosphine/borane pairs, have (in some cases) the ability to transfer the H +/H – couple to organic substrates, with concurrent regeneration of the FLP, acting as unique metal free catalysts for the hydrogenation of organic molecules.121 This was first demonstrated with FLP 110 , which was used for the hydrogenation of imines and aziridines.121c Such FLP catalyzed imine reductions proceed via the initial imine protonation, followed by hydride transfer from the hydridoborate. Also phosphinoborane 123 , among others, soon proved to be a very active metal free catalyst for the hydrogenation of imines and enamines. 121a Similarly, also amine/borane based FLPs were used for metal free catalytic hydrogenations of imines and enamines.121d Having demonstrated the metal free catalytic hydrogenation of imines by FLPs, the groups of Stephan 121e and Klankermayer 121f probed that the imine itself could be used as the Lewis base of the FLP, be it that sufficient steric bulk on the imine must be provided to avoid quenching of the borane catalyst B(C 6F5)3 upon Lewis adduct formation with the
121e imine itself or with the resulting amine. In addition, B(C 6F5)3 and the additional base
PMes 3 were found to catalyze the reduction of electron poor imines and protected nitriles.
The rate acceleration observed with PMes 3 as additional base is presumed to be due to the rapid reaction of PMes 3/B(C 6F5)3 with H 2, giving [Mes 3PH][HB(C 6F5)3], which reduces the imine.121e
33 Chapter 1
Analogous borane catalyzed reductions of imines were subsequently reported by Chen and Klankermayer, which also reported the first example asymmetric reduction of an imine, which gave an enantiomeric excess of 13%, using a chiral borane. 121f Despite the poor enantioselectivity, this work served as a proof of principle demonstrating the possibility to extend FLP chemistry to asymmetric synthesis. Consequently to these initial findings, more effective chiral boranes were targeted for application in asymmetric hydrogenations and the first examples of highly enantioselective hydrogenations of
121g prochiral imines with chiral FLPs were reported (see FLP H2 adduct 129 ; Scheme 38).
Scheme 38. Asymmetric hydrogenations of prochiral imines using chiral FLP based hydrogenation catalysts.
In the few years that followed the discovery of frustrated Lewis pairs, their use for hydrogenation purposes has been rapidly developing and a substantial amount of work has been reported since then. 7 A comprehensive review 7e illustrating the variety of FLP catalysts that have been studied and the range of substrates where FLP reductions have been shown to be effective in catalyzing the hydrogenation reaction has been published very recently, thus we will not discuss FLP hydrogenations more in detail, in this chapter. Instead, we will conclude this paragraph by pointing out that the group of D. W. Stephan has recently reported another remarkable achievement, the FLP mediated reduction of aromatic rings. 122 Although the scope studied to date is still limited, it is clear that this finding should provide synthetic chemists with an unconventional strategy to cyclic amine derivatives.
Dehydrogenation reactions Interestingly, FLPs have also been observed to promote de hydrogenation reactions.
For example, Miller reported that FLP 112 reacts with aminoboranes NH 3−BH 3 and
Me 2NH−BH 3 to afford dehydrocoupling products (NH 2−BH 2)n and (Me 2N−BH 2)2 and
123 phosphonium borohydride salt [tBu 3PH][HB(C 6F5)3] ( 120 ).
34 Small Molecule Activation by Main Group Compounds
Additionally, it was observed that the combination of amines iPr 2NH and iPr 2NEt with
B(C 6F5)3 resulted in 1:1 mixtures of the corresponding ammonium salts
[iPr 2NH 2][HB(C 6F5)3] ( 130 ) and [iPr 2NEt(H)][HB(C 6F5)3] (131 ) with the zwitterionic products of amine dehydrogenation (iPr)(H)N=C(CH 3)CH 2B(C 6F5)3 (132 ) and
124 iPr 2N=C(H)CH 2B(C 6F5)3 (133 ; Scheme 39), while the sterically demanding carbene 1,3 di tert butylimidazolidin 2 ylidene and B(C 6F5)3 form a frustrated Lewis pair (134 ), which in the absence of reactants, exhibits self dehydrogenation reactivity to give a mixture of an imidazolidinium borate (135 ) and an abnormal carbene borane adduct (136 )(Scheme 40).125
Interestingly, Roesky and co workers recently employed a NHC/B(C 6F5)3 1:1 mixture for the synthesis of germylenes,126 obtained by the FLP mediated dehydrogenation of the appropriate germanium precursors.
Scheme 39. Examples of self dehydrogenation reactivity of aminoborane Lewis pairs.
Scheme 40. Self dehydrogenation reactivity of a carbene/borane Lewis pair.
35 Chapter 1
1,2 additions and C−H activations FLPs have also been shown to undergo a large variety of reactions with organic molecules, which we do not intend to discuss in this chapter. We will limit our discussion to saying that typically, FLPs undergo 1,2 addition to double bonds, as first demonstrated by
D. Stephan by exposing a solution of the bimolecular FLP tBu 3P/B(C 6F5)3 (112) to ethylene, which resulted in the straightforward formation of the zwitterionic species
[tBu 3P(C 2H4)B(C 6F5)3] (137 ; Scheme 41). Similarly, the reactions with propylene and 1 hexene gave products 138 and 139, respectively (Scheme 41). Mechanistically, activation of the alkene by the Lewis acid, which is bound to the less hindered carbon atom in the final products, is thought to initiate these reactions. 127 Upon reaction with conjugated dienes, FLPs typically undergo 1,4 additions. 128
Scheme 41. Reactivity of intermolecular FLP 112 with olefins.
With terminal alkynes both C–H and C ≡C bond activation may occur. This is exemplified by the phosphine/borane pairs tBu 3P/B(C 6F5)3 (112) and (o tol) 3P/B(C 6F5)3 (126 ) and phosphine/alane pairs tBu 3P/Al(C 6F5)3 (140 ) and (o tol) 3P/Al(C 6F5)3 (127), which gave phosphonium borate salts [ tBu 3PH][PhCCB(C 6F5)3] (141 ) and [ tBu 3PH][PhCCAl(C 6F5)3]
(142 ) and C ≡C addition products (o tol) 3P(Ph)CC(H)B(C 6F5)3 (143 ) and (o
113 tol) 3P(Ph)CC(H)Al(C 6F5)3 (144 ) (Scheme 42). In addition, the reactivity of FLPs towards olefins and alkynes has also been exploited to effect intramolecular cyclizations of sterically encumbered amines with intramolecular olefin or acetylene fragments in the presence of a Lewis acid, affording five and six membered heterocyclic ammonium borate species.129
Scheme 42. Reactivity of intermolecular FLPs with phenylacetylene.
36 Small Molecule Activation by Main Group Compounds
THF ring opening The FLP mediated ring opening of the THF molecule is also well documented. 111,130 Interestingly, the first example of THF ring opening by a Lewis pair was reported back in
1950 when Wittig described the reaction of Ph 3CNa with THF(BPh 3) which afforded, quite surprisingly, the anion [Ph 3C(CH 2)4OBPh 3] (145 ), resulting from the ring opening of the THF molecule (Scheme 43).131 Since this early study, the ability of Lewis acidic centers to promote THF ring opening reactions in the presence of Lewis bases has been observed for a consistent number of systems. 130
Scheme 43. First example of THF ring opening by a FLP.
Activation of NH 3 and other amines In an effort to further probe the activation of chemical bonds by FLPs, the activation of the N–H bond of NH 3 and of other amines was also investigated. Carbene based FLP
NHC/B(C 6F5)3 146, which is also capable of cleaving heterolytically the H–H bond of the
132,133 dihydrogen molecule forming ionic complex [NHC(H)][HB(C 6F5)3] (147), proved to be
133 able to split the N–H bond of NH 3 to give 148 (Scheme 44) and other amines. Such reactions were performed by pre coordinating the amine to the Lewis acidic borane
B(C 6F5)3 and subsequently reacting the resulting amine borane Lewis adducts with the carbene. The high nucleophilicity (basicity) of the carbene is then capable of abstracting a H+ from the ammonium moiety. It has to be noted that Bertrand observed no reaction upon reacting NHC type carbenes alone with either H2 or NH 3, whereas both H 2 and NH 3 reacted with the more reactive AACs. 19
Scheme 44. Activation of H 2 and NH 3 by a carbene based FLP 146.
37 Chapter 1
Activation of P 4
Carbene based FLP 146 has also been used for the controlled activation of P 4, to afford an adduct (149) in which an abnormal carbene and B(C 6F5)3 are bound in a trans ,trans fashion to a butterfly bicyclo[1.1.0]tetraphosphabutane moiety (Scheme 45).134 Interestingly, the selective cleavage of a single P–P bond has rarely been observed since
2,32 degradation of P 4 typically proceeds further.
Scheme 45. Reaction of NHC/B(C 6F5)3 frustrated Lewis pair 146 with P 4.
Considering the great interest in the development of new methodologies for the activation of white phosphorus using main group elements and compounds,32 it is quite surprisingly that no other examples of P 4 activation by FLPs have yet been reported.
Reactions with N 2O Another remarkable finding, was the discovery that FLPs have the ability to form stable adducts with N 2O. N 2O is only a minor component of the atmosphere but it was estimated
135 to be circa 300 times more potent as greenhouse gas than CO 2. However, N 2O is a potentially strong and environmental friendly oxidant (the side product is N 2), but its high kinetic stability has hampered its use. Main reason for this is that N 2O is a very poor ligand, due to its inability to act as either good σ donor or π acceptor and consequently very few N2O complexes are known.136 In 2009, Stephan reported on the complexation of N 2O by FLPs tBu 3P/B(C 6F5)3 (112) and tBu 3P/B(C 6F5)2Ph ( 150 ), forming FLP–N2O complexes 151 and 152 (Scheme 46) which were notably the first N 2O complexes ever characterized by X ray diffraction. 137
Scheme 46. First examples of FLP N2O complexes.
38 Small Molecule Activation by Main Group Compounds
Subsequently to this discovery, the ability of FLP N2O complexes to undergo Lewis acid exchange reactions was explored. 138 The reaction of 151 with the toluene adduct of
Zn(C 6F5)2 resulted in the partial formation of a new compound ( 156), which showed no
11 signals in the B NMR suggesting that exchange of the borane B(C 6F5)3 moiety of N 2O complex 151 with the Zn(C 6F5)2 had occurred. To facilitate the Lewis acid exchange, N 2O complex tBu 3P—N2O—B(C 6H4F) 3 (153) was prepared following an analogous protocol, and its reactivity towards (tol)Zn(C 6F5)2 was exploited. Indeed, the Lewis acid exchange resulted facilitated due to the reduced Lewis acidity of the borane (B(C 6H4F) 3 instead of
B(C 6F5)3) bound to the tBu 3P−N2O fragment. Reacting tBu 3P−N2O−B(C 6H4F) 3 (153) with 1,
1.5 and 2 equivalents of (tol)Zn(C 6F5)2 resulted in the facile formation of compounds 154, 155 and 156, respectively (Scheme 47). While 154 proved to be the dimer of the expected Lewis acid exchange product, X ray analysis of 155 showed a single pseudo tetrahedral Zn centre that bridges two [tBu 3P−N2O−Zn(C 6F5)2] units where the Zn atoms are coordinated to the O and N atoms of the N 2O fragment yielding two chelating four membered [ZnN 2O] rings. Compound 156, whose formation was previously observed in the preliminary experiments with tBu 3P−N2O−B(C 6F5)3 (151), was obtained in high yield upon reacting tBu 3P−N2O−B(C 6H4F) 3 (153) and (tol)Zn(C 6F5)2 in 1:2 ratio. Importantly, the characterization of 154, 155 and 156 illustrates multiple binding modes for the interaction of an N 2O fragment with a metal.
C6F5 C6F5 (tBu) PN C6F5 C6F5 Zn 3 (tBu)3P N (a) N O (c) Zn C F N O O N B(C6H4F)3 6 5 153 (tBu) P N O Zn Zn N P(tBu)3 3 N C F C F C F 6 5 6 5 6 5 (b) 156 154 C6F5 C6F5
C6F5 Zn C6F5
C6F5 Zn O O Zn C6F5 NN NN (tBu)3P P(tBu)3 155
Scheme 47. Lewis acid exchange reactions. (a) 1 eq. of (tol)Zn(C 6F5)2; (b) 1.5 eq. of
(tol)Zn(C 6F5)2; (a) 2 eq. of (tol)Zn(C 6F5)2
39 Chapter 1
Recently, this chemistry has been extended to other tBu 3P/borane combinations, using boranes B(C 6F5)2Mes, B(C 6F5)2(OC 6F5), B(C 6F4 p H) 3, B(C 6H4 p F) 3 and diborane 1,4
(C 6F5)2BC 6F4B(C 6F5)2 (with 2 equivalents of tBu 3P) as the Lewis acidic component. Room temperature reactions yielded mono and bis zwitterionic species tBu 3P(N 2O)B(C 6F5)2Mes
(157), tBu 3P(N 2O)B(C 6F5)2OC 6F5 (158), tBu 3P(N 2O)B(C6F4 p H)3 (159), tBu 3P(N 2O)B(C6H4
139 p F)3 (160 ) and tBu 3P(N 2O)B(C 6F5)2C6F4(C 6F5)2B(ON 2)P tBu 3 (161) (Scheme 48).
N2O capture was similarly achieved using FLP Cy 3P/B(C 6F4 p H) 3 (162) yielding the zwitterionic species Cy 3P(N 2O)B(C 6F4 p H) 3 (163) (Scheme 48). Attempts to extend the chemistry to other phosphorus or nitrogen bases failed, indicating that N 2O capture is strongly dependent on the nature of the base and suggesting that both σ donation and π acceptance stabilize the tBu 3P–N2O fragment. In this respect, it is noteworthy to point out that N 2O complexation by NHCs, which are highly Lewis basic compounds, has been
34 recently reported and the resulting NHC—N2O adducts proved to be remarkably stable.
Reaction of 160 with [Ph 3C][B(C 6F5)4] resulted in facile transfer of the robust tBu 3P(N 2O)
+ fragment to the stronger Lewis acid Ph 3C generating [ tBu 3P(N 2O)CPh 3][B(C 6F5)4] ( 164) (Scheme 48). In a similar manner, compounds 152, 157, 158, 159 and 161, can be obtained from 160 upon the exchange of the weaker Lewis acid B(C 6H4 p F) 3 with the more Lewis acidic boranes. 139 Similar Lewis acid exchange reactions of 160 with titanocene and zirconocene cations generate transition metal and phosphine stabilized
140 nitrous oxide salts, of the form [ tBu 3P(N 2O)MCp 2Me][MeB(C 6F5)3], (M = Zr or Ti).
(tBu)3PN (tBu)3P N F F N O N O B(R1)2(R2) (C6F5)2B B(C6F5)2 O N 157: R = C F ;R = Mes 1 6 5 2 F F N P(tBu) 158: R1 = C6F5;R2 = OC6F5 3 159: R = R = C F -p-H 1 2 6 4 161 160: R1 = R2 = C6H4-p-F
Cy3PN (tBu)3PN B(C6F5)4 N O N O B(C6F5-p-H)3 CPh3 163 164
Scheme 48. FLP N2O complexes.
40 Small Molecule Activation by Main Group Compounds
Reactions with NO Recently, the group of Erker further extended FLP chemistry to another oxide of nitrogen: nitric oxide (NO). Remarkably, this lead to the discovery of a new strategy that allows easy access to a novel family of aminoxyl radicals.
Phosphinoborane Mes 2P(CH 2)2B(C 6F5)2 (123 ) reacts with NO to form a persistent heterocyclic N oxyl radical 165 (Scheme 49), related to the well known TEMPO. 141 X ray analysis of 165 revealed that both the P and B atoms had bound to the N atom of the incorporated NO. More importantly, while NO itself is a poor H atom abstractor, owing to the weakness of the resulting H−ON bond (approximately 47 kcal mol 1),142 FLP NO complex 165 proved to be able to easily undergo H atom abstraction reactions. This is illustrated by the reaction with 1,4 cyclohexadiene (C−H bond dissociation energy of circa 76 kcal mol −1 )142 which gave 166 and benzene as by product, and the reaction of 165 with cyclohexene or ethylbenzene, which gave 1:1 mixtures of 166 and 167 or 168 , respectively (Scheme 49).141
Interestingly, the reaction of the bimolecular FLP tBu 3P/B(C 6F5)3 112 with NO did not give the analogous N oxyl radical: instead, the already known 112 N2O complex 151,
141 along with the formation of oxide tBu 3P O B(C 6F5)3, was observed. This reaction follows a course related to the well known disproportionation of NO to give N 2O and R 3P=O upon
143 reaction of NO with phosphines PR 3, which highlights the fact that the ability of the intramolecular FLP 123 to form a P/B chelate with NO plays a critical role in the isolation of the NO adduct 165. FLP NO species 165 represented a novel addition to the important family of persistent N oxyl free radicals and expands FLP chemistry to radical species. In fact, we note that recently this chemistry has been further extended to structurally related P/B frustrated Lewis pairs (in particular, P and B on adjacent carbon atoms, which allow N,N addition of the FLP in a chelate fashion), which showed analogous reactivity towards NO, and resulted in a unique family of easily accessible aminoxyl radicals. 144
41 Chapter 1
1/2 Mes2P B(C6F5)2 Mes2P B(C6F5)2 N N + 1/2 O OH 1/2 166 1/2 165
Mes2P B(C6F5)2 + 166 Mes2P B(C6F5)2 + 166 N N O O 168 167
Scheme 49. Novel N oxyl radical 165 and its reactions with C−H bonds.
Reactions with C=O bonds and CO 2 capture Frustrated Lewis pairs also undergo 1,2 addition reactions to carbonyl compounds.
110d Typical examples are the reaction of FLP Mes 2P(CH 2)2B(C 6F5)2 (123 ) with benzaldehyde or the FLP addition to the C=O bond of isocyanates.145,147 Remarkably, FLPs were also discovered to react with CO 2 which, owing to its role as a greenhouse gas and potential use as a C1 chemical feedstock, is gaining increasing attention. 1
The first example of CO 2 capture by frustrated Lewis pairs dates up to 2009, when
Stephan and Erker reported, in a joint publication, on the reversible complexation of CO 2 by tBu 3P/B(C 6F5)3 (112) and Mes 2PCH 2CH 2B(C 6F5)2 (123 ), to give CO 2 adducts 169 and
146 170 , respectively. Liberation of CO 2 from 169 occurs upon heating at 70 °C, while CO 2 adduct 170 , formed under pressurized conditions (2 atmosphere CO 2) rapidly loses CO 2 in solution at temperatures above −20 °C, to reform the starting material 123 (Scheme
146 50). In addition, we reported the CO 2 capture by P/B based geminal FLP 171 (see
Chapter 2) which, in contrast to the previously reported FLP CO 2 adducts 169 and 170 ,
147 resulted remarkably stable towards loss of CO 2. Following these initial reports, we further extended this chemistry to P/Al based FLPs and reported on the reversible CO 2 capture by the geminal P/Al pair 173 (Scheme 51; see
148 Chapter 3). While CO 2 adduct 174 is stable at ambient conditions, liberation of CO 2 occurred smoothly upon heating at 135 °C under vacuum for 2 minutes, which resulted in the complete reformation of FLP 173 . More recently, our group (see Chapter 4) and the group of Fontaine reported, independently, on the related CO 2 capture by methylene
42 Small Molecule Activation by Main Group Compounds bridged, P/Al based dimeric Lewis adducts ( 175a-c; Scheme 51), probing the concept that FLP chemistry can be accessible also from classical, strongly bonded Lewis adducts. 149
Scheme 50. CO 2 capture by P/B based Lewis pairs.
Ph Ph
+CO2 , RT Mes2P AltBu2 Mes2P AltBu2 -CO2 , 135 °C O 173 O 174
R Me R P Al Me +CO2 , RT R2P AlMe2 Al P Me R O Me R a: R = tBu b: R = Me O c: R = Ph 175a,b,c 176a,b,c
Scheme 51. CO 2 capture by P/Al based Lewis pairs.
Of particular interest is the fact that the CO 2 capture may offer the opportunity to convert this cheap and renewable C1 source into useful chemicals. Efforts in developing
43 Chapter 1
homogeneous as well as heterogeneous processes that utilize CO 2 to produce CO, formic acid or methanol have been undertaken, 1 however further improvements are required. Methanol is considered a very valuable product because it serves as precursor to many useful organic chemicals, as a substitute for fossil fuels and for the generation of electricity in fuel cells. 150 Additionally, it can be stored and transported safely. In 2009, Ashley and co workers demonstrated that the frustrated Lewis pair
151 TMP/B(C 6F5)3 (177) can be used to convert CO 2 into MeOH. Addition of CO 2 to the already known FLP 177 (4 equivalents) under an H 2 atmosphere showed quantitative conversion (after 6 days, at 160 °C) into (CH 3O)B(C 6F5)2 via formato complex
[TMPH][H(CO 2)B(C 6F5)3] (179), formed upon CO 2 insertion into the H–B bond of phosphonium borate salt [TMPH][HB(C 6F5)3] ( 178), which results from the H–H heterolytic splitting by the TMP/B(C 6F5)3 pair. Vacuum distillation at 100 °C lead to the isolation of
151 MeOH (17–25% yield) as the sole C1 product, alongside C 6F5H and TMP by products.
The same TMP/B(C 6F5)3 FLP 177 was employed later also by the group of Piers to
152 effect the transformation of CO 2 into CH 4 using triethylsilane as the reducing agent. It has to be noted that a similar transformation of CO2 using silanes as sacrifical reducing agents was reported in 2009 by Zhang and Ying: in this work an NHC was used as organocatalyst for the hydrosilylation of CO 2 to give R 3SiOCH 3 which can be easily
153 transformed into R 3SiOH and methanol upon quenching with H2O.
In 2010, Stephan`s group reported on the room temperature reduction of CO 2 to
MeOH by Al based FLPs using ammonia borane as the reducing agent. CO 2 is first reacted with a 2:1 mixture of AlX 3 (X = Cl or Br) and PMes 3; the resulting species
Mes 3P(CO 2)(AlX 3)2 (180a,b: (a) X = Cl or (b) X = Br; Scheme 52), which are rare examples of double activation of CO 2, are stable towards loss of CO 2 even upon heating at 80 °C under vacuum and react rapidly with ammonia borane at room temperature to give MeOH as final product after quenching the reaction mixture with water. Unfortunately, this reaction requires the consumption of a stoichiometric amount of FLP. 154
More recently, the same authors reported that these Mes 3P(CO 2)(AlX3)2 compounds
155 180b (X = Br) and 180c (X = I) also perform the reduction of CO 2 to CO. Allowing a mixture of Mes 3P, AlX 3 (X = Br or I) stir under a CO 2 atmosphere for several hours, afforded new products which were identified as [Mes3PX][AlX 4] (X = Br or I) and the novel compounds Mes 3PC(O 2AlX 2)O(AlX 3) ( 181b,c: (b) X = Br; ( c) X = I; Scheme 52). The additional oxygen atom in 181b,c inferred the reduction of CO 2 to CO, which was subsequently confirmed by IR spectroscopy of the headspace gas and upon trapping of CO with ruthenium complex Cp*RuCl(PCy 3) to give the corresponding carbonylated complex
Cp*RuCl(CO)(PCy 3).
44 Small Molecule Activation by Main Group Compounds
Scheme 52. CO 2 capture by P/Al based Lewis pairs and reduction of CO 2 to CO.
45 Chapter 1
1.1.1.61. 666.. Concluding RRRemarksRemarks
In this introductory chapter, we described recent spectacular discoveries with respect to the new and fascinating field of small molecule activation by main group compounds. The activation of small molecules has traditionally been the domain of transition metals which, owing to the availability of both vacant and filled d orbitals, are able to activate chemical bonds, resulting in powerful and extremely versatile catalysts. In transition metal based catalysis, the most important reaction types are oxidative additions, reductive eliminations and insertions. Most steps in homogenous catalytic cycles involve one or more of these reactions, usually with small molecules, such as H 2 or olefins among others, under mild conditions. Until recently, no comparable reactions with main group molecules were known. However, it has been increasingly realized that also low valent main group compounds such as singlet carbenes and their heavier congeners hold the potential to activate chemical bonds. In fact, the ability of such low valent species to undergo reactions such as insertions into C−H bonds and additions to unsaturated substrates has long been known and their high reactivity has been the major obstacle to their isolation, until appropriate strategies for their stabilization have been discovered. The scenario changed quite significantly after the seminal discovery that H 2 activation could be achieved with non transition metal species, which consisted in a major breakthrough in main group chemistry and was followed by an increasing number of studies which reported examples of transition metal free heterolytic H 2 cleavage under mild conditions, along with other remarkable discoveries. On one hand, Guy Bertrand importantly pointed out that carbenes, as well as related low valent main group species, bear donor and acceptor frontier orbitals separated by modest energies and therefore it seemed very reasonable to expect that they could manifest a reactivity comparable to the one of open shell transitions metal species and probed this concept by reporting on remarkably facile reactions of carbenes with CO, H 2 and NH 3. The groups of Driess, Roesky and Power among others, significantly extended this chemistry to the heavier group XIV species and to a large variety of important substrates, of which P 4, NH 3, H 2, N, CO and CO 2 are only a few (though very remarkable) examples, demonstrating that these compounds offer the opportunity to activate numerous and diverse chemical bonds. On the other hand, the discovery of frustrated Lewis pairs by the group of Douglas Stephan revealed a significantly important strategy for the metal free activation of small molecules, which nicely offers an ulterior and alternative approach to the metal free activation of small molecules under mild conditions, and can complement the use of low
46 Small Molecule Activation by Main Group Compounds valent species. Perhaps, the FLP approach is even more versatile, due to the possibility of varying independently the donor and acceptor sites and to the wide availability of both Lewis acids and Lewis bases. In the past five years, numerous examples of heterolytic H−H splitting have been reported using the newly discovered FLPs, which have already been shown to be efficient catalysts for the hydrogenation of organic substrates and even found complementary applications in which they are instead used for de hydrogenation purposes. Remarkably, even their potential in asymmetric hydrogenations was recently demonstrated. Moreover, their chemistry was extended to a large variety of substrates, such as alkenes, alkynes, the greenhouse gasses and potentially useful reagents CO 2 and
N2O, white phosphorus, NO and many organic molecules containing unsaturated functional groups. In conclusion, the discovery that also main group compounds can perform a variety of reactions, which were previously known only for transition metals, opened a new window of opportunities for bond activation and catalysis providing unique routes to new synthons and processes. Despite the fact the emerging field of small molecule activation and catalysis by main group species is only in its infancy, it is clear from the data collected up to date that important developments are to be expected in the coming years, which will significantly broaden the scope of the discovered reactivity.
47 Chapter 1
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[108] Phosphines and nitrogen bases are the most common Lewis bases in FLP chemistry, and allow variation of the substituents on the P or N atom. However, also carbenes and thioeters have been employed. For carbene based FLPs, see references: 125, 126, 132, 133, 134. For thioethers, see: a) M. A. Dureen, C. C. Brown, D. W. Stephan, Organometallics 2010 , 29, 6594−6607; b) C. A. Tanur, D. W. Stephan, Organometallics 2011 , 30 , 3652–3657.
[109] Erker`s FLP: P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme, D. W. Stephan, Chem. Commun. 2007 , 5072–5074.
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57 Chapter 1
2036; b) T. A. Rokob, A. Hamza, I. Pápai, J. Am. Chem. Soc. 2009 , 131 , 10701–10710; c) A. Hamza, A. Stirling, T. A. Rokob, I. Pápai, J. Quantum Chem . 2009 , 109 , 2416–2425; d) R. Rajeev, R. B. Sunoj, Chem. Eur. J. 2009 , 15 , 12846–12855; e) G. Lu, H. Li, L. Zhao, F. Huang, .Z. X. Wang, Inorg. Chem . 2010 , 49 , 295–301.
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[121] Catalytic hydrogenations by FLPs: a) with Erker’s Lewis pair 123 : P. Spies, S. Schwendemann, S. Lange, G. Kehr, R. Fröhlich, G. Erker, Angew. Chem. Int. Ed . 2008 , 47 , 7543–7546; b) with the bisphosphinonaphtalene based FLP: H. Wang, R. Fröhlich, G. Kehr, G. Erker, Chem. Commun. 2008 , 5966–5968; c) with Stephan`s first FLP 110 : P. A. Chase, G. C. Welch, T. Jurca, D. W. Stephan, Angew. Chem. Int. Ed. 2007 , 46 , 8050– 8053; d) with amine borane FLPs (molecular tweezers): V. Sumerin, F. Schulz, M. Atsumi, C. Wang, M. Nieger, M. Leskelä, T. Repo, P. Pyykkö, B. Rieger, J. Am. Chem. Soc. 2008 , 130 , 14117–14119; e) P. A. Chase, T. Jurca, D. W. Stephan, Chem. Commun . 2008 , 1701–1703; f) first asymmetric immine reduction: D. Chen, J. Klankermayer, Chem. Commun . 2008 , 2130–2131; g) enantioselective hydrogenations using chiral boranes: D. Chen, Y. Wang, J. Klankermayer, Angew. Chem. Int. Ed. 2010 , 49 , 9475–9478; h) selective hydrogenations of CΞC triple bonds with 123 : B. H. Xu, G. Kehr, R. Fröhlich, B. Wibbeling, B. Schirmer, S. Grimme, G. Erker, Angew. Chem. Int. Ed. 2011 , 50, 7183– 7186; i) G. Erόs, H. Mehdi, I. Pápai, T. A. Rokob, P. Király, G. Tárkányi, T. Soόs, Angew. Chem. Int. Ed. 2010 , 49 , 6559–6563; j) hydrogenation of imines employing the Lewis acid 1,8 bis(dipentafluorophenylboryl)naftalene: C. F. Jiang, O. Blacque, H. Berke, Chem.
58 Small Molecule Activation by Main Group Compounds
Commun. 2009 , 5518–5520; k) transfer hydrogenation catalysis using iPr 2PH as H source and B(C 6F5)3 as catalyst: J. M. Farrell, Z. M. Heiden, D. W. Stephan, Organometallics 2011 , 30 , 4497–4500; l) D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie, S. J. Geier, J. M. Farrel, C. C. Brown, Z. M. Heiden, G. C. Welch, M. Ullrich, Inorg. Chem. 2011 , 50 , 12388–12348; m) diastereoselective hydrogenations of chiral imines with B(C 6F5)3: Z. M. Heiden, D. W. Stephan, Chem. Comm. 2011 , 47, 5729–5731; n) hydrogenations of N based heterocycles: G. Eris, K. Nagy, H. Mehdi, I. Pápai, P. Nagy, P. Kiraly, G. Tarkanyi, T. Soos, Chem.–Eur. J. 2012 , 18 , 574–585.
[122] For metal free aromatic hydrogenations, see: T. Mahdi, Z.M. Heiden, S. Grimme, D.W. Stephan, J. Am. Chem. Soc. 2012 , 134 , 4088–4091.
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59 Chapter 1
[131] First example of THF ring opening: G. Wittig, A. Rückert, Justus Liebigs Ann. Chem. 1950 , 566 , 101–113.
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