Introduction 1

1 Introduction David Morales - Morales

1.1 Introduction

Since their discovery in the mid -1960s palladacycle compounds have represented a very interesting topic of research [1] – fi rst identifi ed as important intermediates in palladium mediated organic synthesis [2] and more recently due to their unique physical properties, these compounds have experienced a renaissance that has been fundamental in the recent development of homogeneous catalysis. This is particularly true in the case of C− C cross - coupling reactions [3] . In general, these compounds can be synthesized in a very facile manner, making it possible to modulate both their steric and electronic properties or even include chiral motifs in their structures to enable them for potential applications in enantioselective transformations as chiral auxiliaries [4] . Other important areas where palladacycles have found recent applications include their use as mesogenic [5] and photoluminescent agents [5h, 6] as well as biological applications for cancer treatment (bio- organometallic chemistry) [7] . Consequently, the present chapter covers some general concepts regarding palladacycle compounds such as a general defi nition, a brief historical overview, a proposal of a general classifi cation based on some excellent recent reviews and, fi nally, a brief description of the future outlook for these very interesting species.

1.2 Deﬁ nition

In general, a palladacycle (Figure 1.1 ) can be deﬁ ned as any palladium compound containing one palladium– carbon bond intramolecularly stabilized by one or two neutral donor atoms (Y), where the organic moiety acts as a C - anionic four - electron donor ligand or as a C- anionic six - electron donor ligand.

Palladacycles: Synthesis, Characterization and Applications. Edited by Jairton Dupont and Michel Pfeffer Copyright © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 978-3-527-31781-3 2 1 Introduction

R1 2 R Y R2 C X C Pd X Pd X R1 Y Y R1

Y = NR2, =NR, PR2, AsR2, SR, SeR, etc. R1, R2 = alkyl, aryl, etc. X = Cl, Br, I, OTf, OAc, solvent, etc. Figure 1.1 Structural deﬁ nition of a palladacycle.

Cp Ni Ni Cp N 2 N N -CpH N

(1) Scheme 1.1

1.3 Historical Overview

Historically, there are probably three different events that have deﬁ ned the development of the chemistry of palladacycles, one being the discovery of the cyclometallation reaction in 1963 by Kleinman and Dubeck [8] when they reacted

azobenzene with NiCp2 to obtain a ﬁ ve - membered metallacycle (1) (Scheme 1.1 ). The structure originally proposed by Kleinman and Dubeck considered the nickel center to be coordinated η 2 to the N = N π - bond (2) [8] . This chemistry was soon extended to other group- 10 transition metals. Thus, between 1965 [9] and 1968 [10] Cope, Siekman and Friedrich carried out analogous reactions of azobenzene

and N,N- dimethylbenzylamines, this time using PdCl2 or Li2 PdCl4 , to afford the ﬁ rst isolated, well- characterized palladacycles (Scheme 1.2 ).

Ni Cp N N

(2) 1.4 Classiﬁ cation of Palladacycles (Types) 3

Cl Pd N Li2PdCl4 N N MeOH, RT N 2

(3) Scheme 1.2

The physical properties these compounds exhibited, in particular the high thermal stability in the solid state, led to the third and probably most important fact, which was the introduction by Herrmann et al. in 1995 of the cyclopalladated tri - o - tolyl - phosphine complex (4) as catalyst precursor for palladium- catalyzed Heck and other cross- coupling reactions [11] . This raised high expectations for this class of compounds, as these species could activate more economic substrates than those applied thus far (aryl iodides or aryl triﬂ ates), such as aryl chlorides, hence potentially enabling the industrial application of these cross- coupling reactions mediated by palladacycle catalysts [12] . Since then, palladacycles have been ubiquitous in catalytic transformations, playing important roles as catalyst precur- sors or active intermediates in cascade transformations leading to complex molec- ular architectures and so forth [2, 3] .

CH3 R R O O P Pd Pd P O O R R CH3 R = o-Tol (4)

1.4 Classiﬁ cation of Palladacycles (Types)

According to the established deﬁ nition, palladacycles can be divided into two different classes based on the organic fragment: anionic four- electron (CY) or six- electron donor (YCY) complexes [1t, 1w, 1x].

Y C X Pd C Pd X Y X Y CY YCY 4 1 Introduction

Hence, palladacycles of the type CY usually exist as halogen (5) or acetate (6) bridged dimers (Scheme 1.3 ) [1w, 13a] , as two geometric isomers, cisoid and transoid conformations.

Ph Ph P Ph P Ph

AgOAc, Me2CO Pd Pd Cl AcO 2 2 (5) (6) Scheme 1.3

C X C Y X C Pd Pd Pd Pd Y X Y C X Y

cisoid-palladacycle transoid-palladacycle Additionally, CY species can be divided into neutral, cationic (7) [14] or anionic (8) [15] ; the neutral species can be found as monomers (9) [16] , dimers (10) [10] or bis - cyclopalladated (11) [17] complexes, depending on the nature of the other ligands X. - + Me o-Tol o-Tol Ph2 N Me P P - NR + Pd PF6 4 P Pd Ph2 Cl Cl (7) (8)

But OAr Me But O Me P OAr N P But Pd But Pd Pd But t Cl PCy3 Cl P Bu t 2 Ar= C6H3-2,4-Bu 2 (9) (10) (11) The position of the C −H bond to be activated with respect to the donor atom Y, as well as the hybridization of the carbon atom in the C− H bond being metallated, undoubtedly inﬂ uences the ease of cyclometallation, and although formal energetic considerations regarding the strength of aromatic and aliphatic C− H bonds have been performed [13b, 18], these data are of little utility due to the complex combination of various factors determining the metallation process. However, from analyses of the available experimental results, it can be concluded that for the vast majority of known complexes the metallated carbon is usually an aromatic sp2 carbon [10, 15– 17] (species 8 – 11 ) and less commonly an sp3 aliphatic (12) [19] , benzylic (13) [11, 20] ) or sp2 vinylic (14) [21] carbon. 1.4 Classiﬁ cation of Palladacycles (Types) 5

H O N Ph H 2 NO Cl N 2 Cl Cl Pd Pd Pd P N o-Tol 2 S 2 O2N Tol-o NO2 H H Me

(12) (13) (14)

On the other hand, the position of the C− H bond with respect to the Y donor atom determines the size of the palladacycle. Thus, although CY- type metallated rings can vary from 3 to 11 members, the most common palladacycles are usually ﬁ ve- or six- membered rings. Palladacycles of three and four members are usually unstable, as are those larger than six members, which generally undergo facile reductive elimination [1u, 2b, 22] ; consequently, examples of well - characterized compounds of this kind are rare. The structures of some isolated, well - characterized palladacycles are shown here of three (15) [23] , four (16) [24] , ﬁ ve (17) [25] , six (18) [26] , seven (19) [27], eight (20) [28] , nine (21) [29] and ten (22) [29] members.

O t t N PPh Bu Bu Cl Pd 3 Pd S Cl Me P Cl N Cl Pd Me Me PPh3 N

(15) (16) (17)

Ph Ph F3C S Ph CF3 Py Ph Pd P Cl Cl S Cl Pd Pd S N 2 Me S 2 Me Me

(18) (19) (20)

Et Et Et Cl H Me Pd H CN Et Cl CN Ph Pd Ph Fe Fe Ph Ph

(21) (22) 6 1 Introduction

The above discussion is also valid for YCY palladacycles or pincer- type complexes [1o, 1r, 1s, 30]. The most common arrangement found for these species is that having two equivalent ﬁ ve - membered rings (23) [31] . In addition, recently, unsymmetrical mixed ﬁ ve- and symmetric six- membered (24) [32] and six - membered complexes (25) [33] have been isolated and characterized.

i O PPr 2 O PPri O 2 i PPr 2 Pd Cl Pd Cl Pd Cl i i i PPr 2 PPr 2 O PPr 2 O O

(23) (24) (25)

On the other hand, the donor atoms (Y), the other important part of palladacycles, can theoretically infl uence the palladation process by the basicity and the coordination ability of the donor atom. However, studies carried out with phosphines differing in the nature of their substituents at the phosphorus atoms revealed that these factors are relatively insignifi cant [34] . Thus, complexes derived from numerous phosphines can be synthesized by similar synthetic methods – even YCY symmetric fi ve- membered palladium compounds containing the

P(C 6 F 5 )2 fragment (26, 27) [35] , were synthesized in a very facile manner via a C − H activation process (Scheme 1.4 ). Conversely, the analogous YCY compound − derived from the ﬂ uorinated thioether SC6 F 5 (28) has not yet been synthesized (Scheme 1.5 ) [36] ; this is probably being due to the low availability of the electron pair in the sulfur. These results clearly call for more detailed studies to shed more light on the potential effect of the Y donor atom in the cyclometallation process.

+ P(C6F5)2 P(C6F5)2 [Pd(NCMe) ][BF ] 4 4 2 Pd NCMe BF - MeCN 4

P(C6F5)2 P(C6F5)2

(26)

LiCl MeCN

P(C6F5)2

Pd Cl

P(C6F5)2

(27) Scheme 1.4 1.4 Classiﬁ cation of Palladacycles (Types) 7

SC6F5 SC6F5 [Pd(NCMe) Cl ] 2 2 Pd Cl MeCN

SC6F5 SC6F5

(28) Scheme 1.5

Nevertheless, a multitude of Y donor atoms have been able to provide an equal number of palladacycles. Hence, palladacycle compounds of the type CY and YCY can be found containing a wide number of functional groups, such as azoben- zenes, imines, amines, oximes, phosphines, arsines, thioethers, oxazolines, different heterocycles, including NHC- heterocyclic carbenes, ethers, selenoethers, and so forth. However, despite this rich structural variety, the most common palladacycles are derived from tertiary amines, usually exhibiting fi ve - or six- membered rings. Palladacycles derived from primary and secondary amines are rather rare, since ortho- palladation of primary amines is diffi cult. In addition, the possibility of further reactions of the acidic protons of the amine with the palladium center or with additional substrates increases the possibility of undesired or side products. Nonetheless, in recent years effi cient synthetic methods to attain such compounds have been reported [37], including the effi cient cyclometallation of amino - acid derivatives (29) [38] .

CO2Me H NH2 Pd Br N

Me (29)

Additionally, due to their easy synthesis, and modular properties, these compounds have been functionalized to include chiral motifs on their structures. These species have been used in enantioselective transformations and as chiral resolving agents [1r, 1w]. As their achiral counterparts these complexes can be classiﬁ ed according to where the stereogenic center is located in the palladacycle. Thus, there are cyclopalladated compounds that have a stereogenic carbon atom directly σ- bonded to the metal (30) [39] , those where the stereogenic center is the donor atom (Y), asymmetrically substituted and bound directly to the palladium center; this generally occurs for amine, phosphine, arsine and thioether donor groups (31) [13b] . The most common type of chiral functionalized palladacycles, though, are those where the stereogenic center is not directly bonded to the palladium but located elsewhere in the palladated ligand (32) [40] . Finally, some compounds exhibit planar chirality, which is generally conferred by the 8 1 Introduction

presence of a ferrocene- like moiety forming part of the palladated ligand (33) [41] .

But t t o-Tol t Bu Bu But Bu P P Cl P Cl Pd Cl Pd NH H Pd PPh 3 Ph PPh3 Me H Me Me H (30) (31) (32)

2 Cy Cl Pd Cy P

(33)

1.5 Final Remarks

Many palladacycles were fi rst discovered as C− H activation products of a given substrate, and although some specifi c methods have been designed for the synthesis of other palladacycles not easily available by this method (Chapter 2 ), the C− H activation process remains the most straightforward method for attaining of these species (Chapters 3 and 4 ). This is relevant not just because a fairly general and facile method is now available for the synthesis of these compounds but also because in the process of understanding this synthetic method researchers have advanced their knowledge and understanding of the activation of C − H bonds [42] . This is of considerable importance since C− H activation is one of the fundamental steps in alkane dehydrogenation, which has long been considered as one of the holy grails in chemistry [43]. Thus, in recent years researchers have focused on this most interesting fact, attaining recently not dehydrogenative processes with palladium but, as a consequence of the good understanding of the C −H activation process with this metal, C −C couplings without the use of preactivated aromatic carbon fragments [44] . The relevance that palladacycles have acquired in the last decade is refl ected in the continuous research and application of these compounds in many different fi elds, such as medical applications, sensors, optical and electronic devices, catalysis and so forth. This has been mani- fested in the growing number of publications that include palladacycles (Figure 1.2 ). Clearly, the development of the chemistry of palladacycle compounds is both a viable option in the development of new areas of chemistry and a very important References 9

Published items in each year Citations in each year 140 4500 120 4000 3500 100 3000 80 2500 60 2000 40 1500 1000 20 500 0 0 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Years Years Figure 1.2 Evolution of the number of publications including palladacycles and the steadily growing number of references to these papers in the last 15 years. tool in the consolidation of present ones. The study of palladacycles, these easy to synthesize, robust and versatile species, represents a very promising and proﬁ table ﬁ eld of research for the future.

Acknowledgments

I gratefully acknowledge the support and enthusiasm of former and current group members and colleagues. The research from our group described in this chapter is supported by CONACYT (J41206- Q; F58692) and DGAPA- UNAM (IN114605; IN227008).

References

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