Establishing the Hydride Donor Abilities of Main Group Hydrides Zachariah M

Article

pubs.acs.org/Organometallics

Establishing the Hydride Donor Abilities of Main Group Hydrides Zachariah M. Heiden* and A. Paige Lathem Department of Chemistry, Washington State University, Pullman, Washington 99164, United States

*S Supporting Information

ABSTRACT: Interest in reductions with main group hydrides has been reinvigorated with the discovery of frustrated Lewis pairs. Computational analysis showed that the borohydride of the commonly used Lewis acid B(C6F5)3 was determined to be 15 kcal/mol less reducing than borohydride ([BH ]−), 22 4 − kcal/mol less reducing than aluminum hydride ([AlH4] ), and 41 kcal/mol less reducing than superhydride ([HBEt ]−). In − 3 addition to [HB(C6F5)3] , a hydride donor ability scale with an estimated error of ∼3 kcal/mol includes 132 main group hydrides with gradually changing reducing capabilities spanning 160 kcal/mol. The scale includes representatives from organosilanes, organogermanes, organostannanes, borohydrides, boranes, aluminum hydrides, NADH analogues, and CH hydride donors. The large variety of reducing agents and the wide span of the scale (ranging from 0.5 to 160 kcal/mol in acetonitrile) make the scale a useful tool for the future design of metal-based or main group reducing agents.

■ INTRODUCTION noncoordinating base.10 DuBois and co-workers have worked fi Sodium borohydride has become the most important hydride- extensively in this eld, employing the hydride donor ability of metal hydride complexes to better understand the activation of based reductant used on the industrial scale, with a market 11 1 dihydrogen with metals. Using the pK for the deprotonation share exceeding 50%. Some of the key drivers governing its a of a metal hydride (eq 1), the two-electron oxidation of a metal interest are that it is the least expensive commercially available complex (eq 2), and the reduction of a proton to a hydride (eq metal hydride (on a hydride equivalent basis), it is safe with 3), the hydride donor ability of a metal hydride can be regard to storage and use, the industrial implementation estimated (eq 4).11 requires no or limited equipment investment, the ease of workup (boron salts), solvents such as water and methanol can ++ LnnMH→+ L M HGK °= 1.37(p a ) (1) be employed, and both chemo- and diastereoselectivity can be achieved.1 Although [BH ]− is the reducing agent of choice 4 L M→+ L M2+− 2eGE °=° 46.1[ (II/0)] from an industrial standpoint, several other main group nn (2) hydrides have been widely used in the reduction of organic +− − +− 2 − H+→ 2e HGE °=−° 46.1[ (H /H )] (3) substrates, as [BH4] is not the ideal reducing agent for every chemical reaction. Main group hydrides are primarily used as sum: stoichiometric reagents in both industrial and academic applications, but with the advent of frustrated Lewis pairs L MH++−→+ L M2 H (FLPs)3 the possibility of catalytic main group reducing agents nn has been realized. The emphasis on chemical reactions avoiding GKE°=1.37(pa ) + 46.1[ ° (II/0)] + 79.6 (4) precious metals has become a concern of the pharmaceutical − industry due to increased restrictions on trace-metal Utilizing eqs 1 4, DuBois and co-workers have been able to impurities.4 The discovery of catalytic main group reducing show metal hydrides with hydride donor abilities ranging from agents opens up the possibility of their use in reductions on the 26 to 89 kcal/mol (see Supporting Information, Table S4). industrial scale. Chase and co-workers have recently been able Although, these values are helpful in the determination of H2 to show that the use of a chemical scavenger promotes the activation, they provide little insight into the capability of these catalytic hydrogenation of imines using a sterically bulky borane metalhydridecomplexesasreducingagentsinorganic at catalyst loadings as low as 0.5 mol %.5 Although the use of reactions. This discrepancy becomes problematic, as evaluating main group Lewis acids to promote the catalytic reduction of the reducing capacity of metal hydrides versus main group organic substrates seems promising, their reducing capacity for hydrides, which are widely used in organic and industrial organic substrates has not been quantified beyond the reductions, would greatly aid in the design of metal-based and − exploration of substrate scope.6 9 metal-free reduction catalysts. Interest in the quantification of hydride transfer from elemental hydrides has originated through the investigation of Received: November 14, 2014 the heterolytic activation of H2 with a metal complex and a Published: May 6, 2015

The use of the thermodynamic cycle described in eqs 1−4 Geometry optimizations were undertaken using the M06-2X/6- can be very helpful in describing the hydride donor abilty of 31G(d,p) level of theory in the gas phase.27 The M06-2X transition metal complexes, but becomes more problematic in functional has been shown to be one of the more accurate DFT the analysis of main group hydrides. A problem that arises in functionals for main group and FLP chemistry.17,20,28,29 More attempting to use eqs 1−4 for main group hydrides is that most accurate energies were obtained using single-point energy main group hydrides cannot be deprotonated in the presence of calculations at the M06-2X/6-311G++(d,p) level of theory in a base. Also, electrochemical analysis of the parent Lewis acids MeCN, employing the optimized molecular geometries at the − often results only in a one-electron reduction wave,12 14 which M06-2X/6-31G(d,p) level of theory. Acetonitrile was chosen as does not make eq 2 valid for main group hydrides. To remedy the solvent for computational analysis to evaluate the calculated the inability to use a thermodynamic cycle for the hydride donor abilities versus values previously reported for determination of the hydride donor ability of main group metal complexes (see Supporting Information). Although the hydrides, one could examine the equilibrium constant between coordination of acetonitrile to the resulting boranes was a metal complex with a known hydride donor ability and a main considered, CD3CN solutions of HSiEt3 and [lutidinium][HB- group Lewis acid. This technique has been employed in the (C6F5)3] showed no hydride loss over the course of a week and analysis of the hydride donor ability of superhydride were assumed to have no effect on the hydride donor ability of − ([HBEt3] ), where a solution of BEt3 was titrated with the examined main group hydride. A similar assumption is used HRh(dmpe)2, resulting in an estimated hydride donor ability in the determination of the hydride donor ability of metal − 15,16 30−32 of 26 kcal/mol for [HBEt3] . This technique can be hydride complexes. The PCM-UFF solvation model problematic with main group hydrides, as the hydride donor (default for Gaussian 09) was used.27 The IEFPCM-UA0 ability of metal complexes tends to be measured in polar solvation model has been previously employed in the solvents, which are not compatible with highly Lewis acidic determination of hydride donor abilities of transition metal − 30,32 main group complexes. To date, [HBEt3] is the only main complexes, but the PCM-UFF solvation model was found group hydride species where the hydride donor ability has been to give slightly better results in the calculated hydride donor − Δ − 16 experimentally determined. Equation 4 shows the hydride ability of [HBEt3] ( GH = 26 kcal/mol, experimentally); donor ability of metal hydrides; in a similar fashion, eq 5 can be see below. applied to determine the hydride donor ability of main group Common Main Group Hydrides in Organic Reduc- Δ hydrides ( GH−). tions. The most common reducing agents employed in −− industrial scale organic synthesis are NaBH4,NaBH3CN, [Lewis Acid H]→+Δ Lewis Acid H GH− (5) 2 LiAlH4, and NaHBEt3 (superhydride). From a reactivity To investigate the hydride donor ability of main group metal standpoint, superhydride is considered a stronger reducing hydrides and obtain reasonable values to be utilized in agent than lithium aluminum hydride, and lithium aluminum experimental design, computational methods were employed hydride is a stronger reducing agent than borohydride,2,33,34 but with the advantage of avoiding experimental dilemmas as to date, measurement of the reducing power of these main described above. Papaí and co-workers have previously group hydrides has been qualitative as opposed to quantitative. computed the ability of boranes to accept a hydride to aid in In this study, we aimed to quantify the reducing power of 17,18 the understanding of H2 activation with FLPs, but this main group hydrides through computational analysis. To study was limited to the analysis of only 12 boron-based Lewis determine the validity of our computational values, we initially − acids. Krossing and co-workers have very recently investigated determined the hydride donor ability for [HBEt3] , which was the effect of alkoxy substitution on a boron-based Lewis acid computed to be 24 kcal/mol. The computed value of 24 kcal/ both experimentally and computationally, in addition to mol agrees fairly well with the experimentally determined value computing the gas phase hydride, fluoride, chloride, and of 26 kcal/mol,16 which is estimated to have an experimental methyl ion affinity of 34 other main group compounds using error of ±2 kcal/mol. high-level calculations.19 Gilbert (computationally)20 and Piers To further validate the computational model, we compared a (experimentally)21 have probed the effect of fluorine computational versus experimental relative Lewis acidity of a fl substitution on the Lewis acidity of uorinated boranes, but borane of interest versus B(C6F5)3. The ratio of the computed generated contradicting conclusions. To experimentally quanti- hydride donor abilities of the borohydride of interest to − fy the Lewis acidity of newly synthesized Lewis acids, the [HB(C6F5)3] was determined to yield a computational relative Gutmann−Beckett or Childs’ Lewis acidity tests are − Lewis acidity to B(C6F5)3 (see Supporting Information, Table utilized.22 25 Although these tests can provide insight into S2). Comparison of the experimentally determined Lewis the Lewis acidity of a Lewis acid of interest with respect to acidities, determined from the Gutmann−Beckett meth- fl 26 23−25,35 B(C6F5)3, often con icting results can be obtained. od, generated comparable Lewis acidities (within Herein we expand upon the current reports and describe the about 4%) to the computational values (see Supporting hydride donor abilities of main group hydrides with reducing Information, Table S3). The resulting differences between the capabilities spanning 160 kcal/mol, including representatives experimental and computational values yield an estimated error from silanes, boranes, borohydrides, aluminum hydrides, of about 3 kcal/mol for the calculated hydride donor abilities. NADH analogues, and CH hydride donors. In addition to Upon achieving satisfactory results with the chosen level of the described main group hydrides, we also investigate chiral theory, we expanded this analysis to the common reducing main group hydrides to gain insight into the observed agents in synthetic laboratories and borohydrides relevant to enantioselectivities in reduction reactions. the chemistry of frustrated Lewis pairs. When comparing the reducing ability of borohydride, computations showed that − ■ RESULTS AND DISCUSSION Δ − [BH4] ( GH = 50 kcal/mol) was about 26 kcal/mol less − The reducing capacity of 132 main group hydrides was likely to transfer a hydride than [HBEt3] . The ability to investigated using ab initio calculations and Gaussian 09. transfer a hydride became less favorable by 18 kcal/mol than

1819 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

− [BH4] when a hydride was replaced with a cyanide ligand in in borohydrides 26 and 43 kcal/mol more likely to transfer a − − [BH3CN] . As expected, the addition of a more electron- hydride than [HB(C6F5)3] . As expected, the increased withdrawing group, such as [CN]−, resulted in a reduced electron-donating ability and increased steric bulk of the hydride donor ability, while the addition of an electron- mesityl ligand38 favor hydride loss and generation of a three- donating ethyl group showed an increase in the calculated coordinate borane when compared to the respective phenyl hydride donor ability. Aluminum hydride, which is widely derivatives. An alternative approach to increasing the steric bulk − 2 considered to be more reducing than [BH4] , was found to be surrounding the boron center was recently described by Neu − only 7 kcal/mol more reducing than [BH4] and 19 kcal/mol and Stephan, through the insertion of sterically bulky − − 42 less reducing than [HBEt3] . To experimentally verify the diazomethane groups into a B C6F5 bond. Insertion of − − computationally determined hydride donor ability of [AlH4] ,a (trimethylsilyl)diazomethane or phenyldiazomethane into a B Δ − tetrahydrofuran solution of LiAlH4 ( GH = 43 kcal/mol) and C6F5 bond of B(C6F5)3 resulted in complexes with the Δ − BPh3 ( GH = 36 kcal/mol) resulted in no hydride transfer to respective borohydrides being 6 or 5 kcal/mol more favorable ° − the boron center, even up to a temperature of 50 C, indicating then [HB(C6F5)3] , respectively, to transfer a hydride. As that the hydride prefers to reside on the aluminum center. In anticipated, the increase in steric bulk upon insertion of an 38 − the case of reductive aminations, some organic chemists have electron-rich carbene fragment into a B C6F5 bond resulted preferred the use of NaHB(OAc)3, as opposed to the potential in a more reducing borohydride species by reducing the steric 36 cyanide source, NaBH3CN. Computational analysis showed bulk around the boron center upon loss of a hydride ligand. − − ff that [HB(OAc)3] has a similar reducing power to [BH4] and In an attempt to further probe the e ects of borane is about 19 kcal/mol more favorable to transfer a hydride than substituents in FLP chemistry, several groups have generated a − [BH3CN] . family of highly Lewis acidic boranes with varying degrees of − Hydride Donor Ability of [HB(C6F5)3] and halogenated ligands. One of the simplest approaches to − [HBR(C6F5)2] . The recent discovery of frustrated Lewis pairs manipulating the chemistry of a Lewis acidic borane is the has led to the interest in the reducing capacity of the sterically generation of boranes via hydroboration.43 Hydroboration of − 6 44,45 bulky borohydride, [HB(C F ) ] . Computational analysis of norbornene, cyclohexene, and styrene with [HB(C6F5)2]2 6 5 3 − the hydride donor ability of [HB(C6F5)3] yielded a hydride resulted in boranes with their respective borohydrides being 12, donor ability of 65 kcal/mol, which is 15 kcal/mol less reducing 12, and 10 kcal/mol more likely to give up a hydride than − − − than [BH4] and 3 kcal/mol more reducing than [BH3CN] . [HB(C6F5)3] . These results correspond with an increase in − The diminished ability of [HB(C6F5)3] to transfer a hydride electron density on the boron center by the introduction of a − when compared to [BH4] is attributed to the electron- greater electron-donating substituent. Stephan and co-workers withdrawing nature of the pentafluorophenyl groups, which recently reported that the replacement of the para-fluorine − increases the Lewis acidity at the boron center. Several atom of [HB(C6F5)3] with a hydrogen atom leads to a FLP fi modi cations of B(C6F5)3 have been recently undertaken to exhibiting reversible H2 activation and a borane 97% the Lewis 46 alter the ability to heterolytically activate H2 with a sterically acidity of B(C6F5)3. Computational analysis showed that the bulky base and to promote the reduction of organic substrates. introduction of a hydrogen atom in the para-position of a fl − Erker and co-workers recently showed that the 1,1-carbobora- triaryl uoroborane, [HB(C6F4-p-H)3] , promotes the loss of a − tion of diphenylacetylene or 3-hexyne with B(C6F5)3 results in hydride by only 2 kcal/mol when compared to [HB(C6F5)3] , alkenylboranes.37 Computational analysis of the respective which is on par with the experimental Lewis acidity borohydrides shows that the 1,1-carboboration of B(C6F5)3 determination (see Supporting Information). Piers recently 21 with diphenylacetylene or 3-hexyne leads to borohydrides that reported the related B(C6F4-o-H)3, where computational are about 14 or 9 kcal/mol more reducing, respectively, than analysis of the hydride donor ability of the respective − [HB(C6F5)3] . The generation of less electron-withdrawing borohydride resulted in a borohydride 2 kcal/mol more likely 38 − ligands and increased steric bulk through carboboration to transfer a hydride than [HB(C6F4-p-H)3] . The increased − suggests the production of a more reducing borohydride ability to transfer a hydride from [HB(C6F4-o-H)3] than from − fi species. [HB(C6F4-p-H)3] veri es the experimental observations Other groups have approached reducing the Lewis acidity by where substitution in the para-position as opposed to the substitution at one of the pentafluorophenyl groups of ortho-position resulted in a slightly more Lewis acidic borane.21 − − 26 [HB(C6F5)3] with a phenyl group ([HB(C6F5)2(Ph)] ). In an analogous conversion, replacing the para-hydrogen of − fl − This substitution resulted in a borohydride complex about 10 [HBPh3] with a uorine in [HB(C6H4-p-F)3] results in a kcal/mol more favorable to transfer a hydride than borohydride 1.5 kcal/mol less likely to lose a hydride than − fl − [HB(C6F5)3] . Subsequent phenyl for penta uorophenyl [HBPh3] , which is of similar magnitude to the substitution of fl − group exchanges continued to exhibit complexes more reactive the para- uorine atom of [HB(C6F5)3] with hydrogen. toward hydride loss by 10 and 9 kcal/mol for [HB(C6F5)- Oestreich and Paradies recently reported the synthesis of − − fl fl (Ph)2] and [HBPh3] , respectively. Yoon and co-workers have tris(2,4,6-tri uorophenyl)borane and tris(2,6-di uorophenyl)- 39,40 47,48 recently employed K[HBPh3] in the reduction of ketones. borane, respectively, where the respective boranes were The increase in reactivity of the respective borohydrides toward experimentally determined to exhibit 85% and 82% the Lewis hydride loss is expected by changing from the strongly electron- acidity of B(C6F5)3. Computational determination of the fl − withdrawing penta uorophenyl group to the less electron- transfer of a hydride from [HB(2,4,6-F3-C6F2)3] and − withdrawing phenyl group. [HB(2,4,6-F3-C6F2)3] resulted in borohydrides more favorable Soos and co-workers have recently demonstrated the use of to transfer a hydride by 13 and 14 kcal/mol, respectively, in − increased steric bulk to promote the selective activation of H2 comparison to [HB(C6F5)3] , which suggests similar Lewis with small Lewis bases.41 Computational analysis showed that acidities to the experimental values (see Supporting Informa- − fl [HB(C6F5)2(Mes)] was 14 kcal/mol more reducing than tion, Table S2). Substitution of a uorine group of B(C6F5)3 in − fl [HB(C6F5)3] . Addition of subsequent mesityl groups resulted the ortho-position with a penta uorophenyl group has been

1820 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article recently shown by O’Hare and co-workers to result in a borane in a borohydride 2 kcal/mol more reducing. In the case of 49 fl 113% the Lewis acidity of B(C6F5)3. Computational analysis placing an electron-withdrawing penta uorophenyl group on an − of [HB(C6F4-o-C6F5)3] yielded a hydride donor ability 6 kcal/ alkyl borane, such as 9-BBN (where BBN = borabicyclo[3.3.1]- − mol less likely to transfer than [HB(C6F5)3] , verifying the nonane), hydride transfer from the resulting borohydride experimentally determined Lewis acidity and demonstrating complex decreased by 14 kcal/mol when compared to fl − that a penta uorophenyl group is more electron withdrawing [HBEt3] , exhibiting a hydride donor ability similar to fl − − than a uoride. Oestreich and co-workers recently reported the [HB(C6H4-p-F)3] and [HB(Mes)2(C6F5)] (see Supporting synthesis of tris(5,6,7,8-tetrafluoronaphthalen-2-yl)borane and Information). The decrease in ability to transfer a hydride from 50 − experimentally determined a Lewis acidity 98% of B(C6F5)3. [(B-C6F5)(9-H-BBN)] is of similar magnitude to the removal Computational analysis of the hydride donor ability of the of a pentafluorophenyl group through a 1,1-carboboration of respective hydride suggested a less Lewis acidic borane than B(C6F5)3. observed experimentally, 16 kcal/mol more likely to transfer a Alkoxy Borohydrides. Although alkoxy and aryloxy − hydride than [HB(C6F5)3] . We propose that tris(5,6,7,8- boranes have not met much success in the activation of tetrafluoronaphthalen-2-yl)borohydride is a better hydride dihydrogen with FLPs, primarily due to ligand redistribution at donor than experimentally determined due to a similar the boron center,26 we probed their ability to transfer a hydride − structure and hydride donor ability to [HBPh2(C6F5)] . from their respective borohydrides computationally. As Ashley and co-workers have recently described the use of expected, the replacement of a pentafluorophenoxy group for 51,52 fl − perchlorinated derivatives of B(C6F5)3 in FLP chemistry. a penta uorophenyl group on [HB(C6F5)3] resulted in a − Analysis of the respective hydrides showed that [HB(C6Cl5)3] , borohydride complex more favorable to transfer a hydride by − − − [HB(C6F5)(C6Cl5)2] , and [HB(C6F5)2(C6Cl5)] were 5, 4, 12, 18, and 21 kcal/mol than [HB(C6F5)3] for [HB(OC6F5)- − − − and 4 kcal/mol, respectively, more favorable to transfer a (C6F5)2] , [HB(OC6F5)2(C6F5)] , and [HB(OC6F5)3] , re- − hydride than [HB(C6F5)3] . Intriguingly, the ability to transfer spectively. The increase in hydride donor ability is attributed to a hydride from [HB(C Cl ) ]−, [HB(C F )(C Cl ) ]−, and the partial donation of the lone pair of the aryloxy groups, − 6 5 3 6 5 6 5 2 [HB(C6F5)2(C6Cl5)] was nearly identical even though their which reduces the Lewis acidity of the parent borane. A similar steric bulk varies by almost 30° (cone angles of 213°, 202°, and phenomenon is observed in the CBS catalysts,56 where addition 185°, respectively), indicating more of an electronic effect. of a Lewis acid is needed to turn on the Lewis acidity of the Marder and co-workers have recently reported the synthesis of oxazaborolidine (see below). Substitution of the first air-stable boranes containing fluoromesityl groups.53 Computa- pentafluorophenoxy group resulted in the largest increase in tional analysis showed that the respective borohydride, the ability to transfer a hydride, where each subsequent t fl − fl ff [HB(C6H4-p- Bu)(2,4,6-tris(tri uoromethyl)phenyl)2] ,re- penta uorophenoxy group substitution a ected the ability to sulted in a hydride donor ability about 19 kcal/mol more transfer a hydride half as much as the previous substitution. If a − − reducing than [HB(C6F5)3] and on the order of [BH4] . phenoxy group is present on the borohydride complex as Ashley and co-workers recently reported that tris(3,5-bis- opposed to a pentafluorophenoxy group in the case of fl − (tri uoromethyl)phenyl)borane is more Lewis acidic than [HB(OPh)(C6F5)2] , hydride loss becomes more favorable 54 − B(C6F5)3, but computational analysis of the respective by about 6 kcal/mol than [HB(OC6F5)(C6F5)2] . If all of the fl − fl − borohydride [HB(3,5-bis(tri uoromethyl)phenyl)3] showed penta uorophenoxy groups in [HB(OC6F5)3] are replaced that it was 12 kcal/mol more favorable to transfer a hydride with phenoxy or thiophenoxy groups, hydride loss becomes − than [HB(C6F5)3] . The increase in the ability to transfer a more favorable by about 13 and 2 kcal/mol, respectively. hydride is attributed to the diminished electron-withdrawing DuBois and co-workers have previously shown that borate fl t nature of a 3,5-bis(tri uoromethyl)phenyl group when esters such as B(O Bu)3 and B(OSiMe3)3 do not accept a fl − compared to a penta uorophenyl group, as the respective hydride from [HBEt3] even in the presence of 10 equivalents 38 − 16 borohydrides exhibit similar steric bulk. Investigation of the of [HBEt3] . Computational analysis showed that [HB- fl t − − − electron-withdrawing nature of halogens instead of penta uor- (O Bu)3] , [HB(OSiMe3)3] , and [HB(OH)3] are 23, 19, and − ophenyl groups in [HB(C6F5)3] resulted in the transfer of a 16 kcal/mol more favorable to transfer a hydride than − − − hydride from [HBCl3] or [HBF3] to be 1 kcal/mol less [HBEt3] , thus verifying the experimental observations. − favorable or 16 more favorable than [HB(C6F5)3] , respec- Since catecholate boranes have been recently explored in tively. FLP chemistry,57 we exploited this motif to probe the effect of Reducing Ability of Alkyl Borohydrides. Reduction fluorine substituents on an arene ring of the catecholate ligand. chemistry employing alkyl boranes is primarily governed by Computational analysis showed that hydride transfer from − − reactions with selectride ([HB(sec-butyl)3] ) and superhydride [(C6F5)-BH(catecholate)] (which is 31 kcal/mol more − 34,55 − ([HBEt3] ). Computational analysis of the hydride donor favorable than for [HB(C6F5)3] ) becomes less favorable by ability of selectride and superhydride showed that selectride 3 kcal/mol by substitution of fluorine in the 3- or 4-positions of was about 4 kcal/mol more favorable to transfer a hydride than the catecholate ligand (see Supporting Information), with superhydride. This observed reactivity is attributed to the substitution in the 3-position yielding a slightly more reducing increased steric bulk surrounding the borohydride center in borohydride. The substitution of two fluorine atoms of 38 ff − ff selectride. To examine the e ect of changing from an alkyl [(C6F5)-BH(catecholate)] exhibited a di erence of about 1.5 group to an arene group, we probed the hydride donor abilities kcal/mol between substitution at the 3,6-positions versus the of the respective borohydrides upon replacing ethyl groups with 3,4- or 3,5-positions, with substitution at the 3,6-positions phenyl groups. Replacement of an ethyl group with a phenyl yielding the less reducing borohydride. Substitution of three − − fl group of [HBEt3] yielded borohydrides about 2 6 kcal/mol uorine atoms in the 3,4,5-positions yielded a borohydride as less reducing with each subsequent replacement for reducing as the substitution of two fluorine atoms in the 3,6- − − fl [HBEt2Ph] and [HBEtPh2] , respectively. Replacement of positions. Substitution of three uorine atoms in the 3,5,6- − − the three ethyl groups of [HBEt3] with methyl groups resulted positions of [(C6F5)-BH(catecholate)] resulted in a borohy-

1821 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

Figure 1. List of calculated hydride donor abilities for chiral boranes relevant to the chemistry of frustrated Lewis pairs. In the case of two possible diastereomers, only the dominant diastereomer is shown. The numbers shown are in units of kcal/mol. The hydride ligand that undergoes hydride transfer is bold and colored in red. dride about 1.5 kcal/mol less reducing than substitution in the pyrrolidine nitrogen center, the reducing capacity of the 3,4,5-positions. The substitution of four fluorine atoms on the borohydride decreases by about 19 kcal/mol (Figure 1), catecholate ligand results in a borohydride that is 11 kcal/mol resulting in a borohydride with a similar reducing capacity to less likely to transfer a hydride than the parent catecholate [HB(Mes) (C F )]−. In the catalytic reductions of ketones − 2 6 5 complex, [(C6F5)-BH(catecholate)] . with the CBS catalysts, hydride transfer occurs from the bound Chiral Boranes. In asymmetric reductions, chiral boranes BH3 center to the substrate as opposed to a hydride that is are used to promote the interaction of the borohydride with bound to the boron center in the oxazaborolidine ring.56 one face of the substrate over another. To achieve Coordination of BH3 to the pyrrolidine ring (3) resulted in the enantioselectivities greater than 90% ee, transition-state hydride donor ability of the BH3 group of the three-coordinate energies between the approaching hydride and the two faces oxazaborolidine (Figure 1) to have a similar reducing ability to of the prochiral substrate need to be 1.8, 2.0, and 2.2 kcal/mol NH3BH3 (see below). for reactions at 25, 60, and 100 °C, respectively. To promote Recent developments of chiral boranes have focused on the reductions with high enantioselectivities, a combination of metal-free reduction of imines using FLPs. The first chiral steric bulk and a regulated hydride donor is needed in the borane (4) used in asymmetric hydrogenations was developed design of a desired reductant to achieve the required energy by Klankermayer and co-workers through the hydroboration of ff α 59 di erence between the prochiral faces of a substrate. We have (+)- -pinene with Piers borane ([HB(C6F5)2]2), resulting in recently reported that an elemental hydride exhibiting a cone enantioselectivities of 13%.60 Computational analysis of these angle greater than 165° promotes enantioselective reductions of borohydrides showed that the most thermodynamically >90% ee.38 Although the steric bulk of an elemental hydride favorable borohydride was 10 kcal/mol more reducing than − can be used to promote asymmetric reductions, the [HB(C6F5)3] , which is of similar magnitude to the related thermodynamics of hydride transfer also play a part in the hydroboration reactions (see above). The minor diastereomer resulting enantioselectivity. To investigate the magnitude of the (3 kcal/mol less stable than the major diastereomer) exhibited ability of chiral borohydrides to transfer hydrides, we examined a hydride donor ability 1 kcal/mol more reducing than the the hydride donor ability of nine chiral borohydrides related to borohydride of the major diastereomer, suggesting that a the chemistry of FLPs. greater energy difference is needed to result in reductions with Chiral oxazaborolidine (CBS) catalysts developed by Corey, greater ee. Klankermayer and co-workers then expanded on the Bakshi, and Shibata have been widely used in the asymmetric previous result with the use of a camphor-derived borane (5), reduction of ketones to chiral alcohols.56 Given that the B- resulting in enantioselectivities of up to 83%.61 Computational pentafluorophenyl derivative has been reported and is capable analysis showed that the major diastereomer (exo-product) of reducing ketones to alcohols in 88% ee,58 we investigated generated in the FLP H reactions had a reducing ability about 2 − computationally its reducing capacity versus other main group 10 kcal/mol more favorable than [HB(C6F5)3] and is less hydrides. As expected, the parent CBS catalyst (1) is a poor susceptible to retrohydroboration. The minor diastereomer Lewis acid and a strong hydride donor (ΔG − = 19 kcal/mol), (endo-product) was about 6 kcal/mol less stable, and the H − exhibiting a reducing capacity similar to [HB(sec-butyl)3] , with respective borohydride was 3 kcal/mol less reducing than the the opposite diastereomer being 15 kcal/mol more reducing. major diastereomer’s respective borohydride, suggesting that Although we initially probed the addition of a hydride ligand to the higher enantioselectivity is attributed to the energy the boron center, the reduction of ketones with CBS catalysts is differences between the two diastereomers as opposed to steric 56 ° ° 38 proposed to undergo a mechanism involving BH3. To activate bulk (cone angles of 197 and 200 ). the CBS catalysts, BH3 binds to the pyrrolidinyl nitrogen center Several other groups have generated chiral boranes, but have to increase the Lewis acidity at the metal center. Computational not yet employed them in FLP chemistry. Jaklë and co-workers analysis shows that upon coordination of BH3 (2) to the have synthesized a planar chiral borane generated from

1822 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

Figure 2. List of calculated hydride donor abilities for amine-boranes of borenium cations relevant to the chemistry of frustrated Lewis pairs.

62 29 naphthylferrocene (6). Computationally, we determined that of 2,6-lutidine adducts of BHCl2 (15). Computational the generated borohydride is about 17 kcal/mol more reducing analysis revealed the hydride donor ability to be 8 kcal/mol − − than [HB(C6F5)3] . Computational analysis also showed that less favorable than [HB(C6F5)3] and 7 kcal/mol less favorable − upon oxidation of the iron center, the ability to transfer a than [HBCl3] . hydride decreases by about 5 kcal/mol. Zhong and co-workers In an attempt to examine the effect of Lewis basicity on the recently described the synthesis of a phosphine-borane derived hydride donor ability of catecholborane, we determined the 63 from [2.2]paracyclophane, and Paradies recently described a hydride donor ability of the NH3 (16), DABCO (17), NEt3 ff 64 t chiral borane containing the [2.2]paracyclophane sca old. (18), P Bu3 (19), 2,6-lutidine (20), 1,3-bis(2,6- Given that this is a chiral scaffold, we probed the hydride donor diisopropylphenyl)imidazol-2-ylidene (21 (R = Dipp)), and ability of a paracyclophane-derived borohydride (7) in the 4- 1,3-bis(isopropyl)imidazol-2-ylidene (22 (R = iPr)) Lewis base position (Figure 1),64 which resulted in a borohydride complex adducts. The hydride donor abilities ranged 15 kcal/mol − about 10 kcal/mol more reducing than [HB(C6F5)3] and (Figure 2), with the weakest hydride donor being the DABCO − similar to [HB(C6F5)2(Ph)] . Piers and co-workers have adduct and the strongest hydride donor being the 1,3- ′ previously synthesized 2-B(C6F5)2-1,1 -binaphthyl (8) and 2- bis(isopropyl)imidazol-2-ylidene adduct. No clear correlation ′ ′ 65 ′ 71,72 B(C6F5)2-2 -methyl-1,1 -binaphthyl (9). [2-BH(C6F5)2-1,1 - between Lewis basicity (pKa in MeCN) and hydride donor binaphthyl]− was determined to be about 12 kcal/mol more ability of the Lewis base-catecholborane adduct could be − reducing than [HB(C6F5)3] and 1 kcal/mol less reducing than determined (Figure 2). We have previously shown that about ′ ′ − [2-BH(C6F5)2-2 -methyl-1,1 -binaphthyl] . To further exploit half of the cone angle of the Lewis base contributes to the cone the chemistry of binaphthyl groups, Oestreich and co-workers angle of the Lewis base-borane adduct,38 but no clear have recently shown that the axially chiral borane (10) correlation between steric bulk and hydride donor ability containing one pentafluorophenyl group can be used for the could be determined. The absence of a correlation indicates asymmetric hydrosilylation of imines, achieving 33−63% that a combination of steric and electronic effects needs to be ee.66,67 Computational analysis showed that the thermodynami- taken into account when tuning the hydride donor ability of a cally favored borohydride was about 19 kcal/mol more Lewis base-borane adduct. − − reducing than [HB(C6F5)3] and similar to [HB(C6F5)(Ph)2] . With recent interest in the use of ammonia-borane as a Boranes Derived from Borenium Cations. In an effort to possible hydrogen storage material,73 we included the generate Lewis acidic boranes employing commercially examination of hydride transfer chemistry of NH3BH3 in this available reagents, focus has moved to the generation of air- study. Hydride transfer from the neutral NH3BH3 was 8 kcal/ 68 − and moisture-stable borenium cations. Crudden and co- mol less favorable to transfer a hydride than [HB(C6F5)3] .If workers recently discovered that a DABCO-HB(pinacolate) deprotonation of the amine center occurs prior to hydride − 69 − (11) is more reducing than [HB(C6F5)3] . In agreement with transfer, the ability to transfer a hydride from [NH2BH3] the experimental observations, computational analysis showed becomes more favorable than NH3BH3 by about 60 kcal/mol that DABCO-HB(pinacolate) is about 23 kcal/mol more likely and is as reducing as [HB(sec-butyl) ]−. − 3 to lose a hydride than [HB(C6F5)3] . The related 2,6-lutidine- Aluminum Hydrides. Although most of the synthesis of HB(pinacolate) (12) complex exhibited a hydride donor ability new Lewis acids for FLP chemistry has focused around the about 6 kcal/mol greater than the DABCO adduct. In addition implementation of boron, several examples of Lewis acids to Crudden, Stephan and Ingleson have employed borenium employing aluminum have been reported. Stephan and co- cations in FLP chemistry. Stephan and co-workers have workers have recently described FLP chemistry with the 74,75 employed borenium cations derived from N-heterocyclic aluminum analogue of B(C6F5)3. Computational analysis − carbenes and 9-BBN for the catalytic hydrogenation of showed that the hydride donor ability of [HAl(C6F5)3] was 6 70 − imines. Computational analysis showed that 1,3-bis- kcal/mol more reducing than [HB(C6F5)3] . A similar (isopropyl)imidazol-2-ylidene-H-BBN (13) and the related observation was observed by Timoshkin and Frenking, where 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-H-BBN (14) they found that B(C6F5)3 was a stronger Lewis acid than 76 complexes resulted in boranes that were 18 and 17 kcal/mol, Al(C6F5)3 for the addition of a hydride. The related phenyl − respectively, more likely to give up a hydride than complex [HAlPh3] was determined to have a hydride donor − − [HB(C6F5)3] . Ingleson has recently investigated the chemistry ability 20 kcal/mol less than [HAl(C6F5)3] and about 3 kcal/

1823 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

− mol less likely to transfer a hydride than [HBPh3] . Although Et3SiH, suggesting that the increase in Lewis acidity favors − [HAl(C6F5)3] appears to be more reducing than adduct formation as opposed to hydride transfer. − [HB(C6F5)3] , the aluminum hydride experimentally exists Hantzsch esters have been recently utilized in asymmetric with two Lewis acids coordinated to the hydride.75 As expected, transfer hydrogenation reactions using chiral Brønsted the coordination of the second aluminum complex decreased acids88,89 and have been shown to transfer a hydride to 90 the ability to transfer a hydride by 27 kcal/mol, indicating a B(C6F5)3. Calculation of the hydride donor ability of the neutral Hantzsch ester showed that it is more likely to give up a stabilization by the addition of a second Lewis acid. A similar − observation was perceived in the determination of the hydride hydride than [HB(C6F5)3] by 6 kcal/mol and has a similar − − − − donor abilities of [HAlCl3] , [HAlBr3] , and [HAlI3] , where hydride donor ability to [HAl(C6F5)3] . Deprotonation of the the ability to transfer a hydride became less likely by 18, 13, and nitrogen center leads to activation of the hydride, with the 17 kcal/mol, respectively, upon addition of the second Lewis hydride complex becoming 33 kcal/mol more reducing. acid. Krossing and co-workers recently described the synthesis Whereas a Hantzsch ester exhibited a hydride donor ability similar to [HAl(C F ) ]−, another organic hydrogen source, of a strong Lewis acid, Al(OC(CF3)3)3, for the protonation of 6 5 3 − 77,78 mesitylene in the presence of an alcohol. Computational formate, displayed a similar hydride donor ability to [HAlPh3] . analysis of the respective hydride resulted in an aluminum Some of the interest in the reduction chemistry of Hantzsch hydride species that was 3 kcal/mol less favorable to transfer a esters is related to their structural similarity to the biological − hydride transfer agent NADH. A hydride source related to hydride than [HB(C6F5)3] ; similar results were obtained by Krossing and co-workers.19 NADH is 5,10-methylenetetrahydromethanopterin, which in combination with a monometallic iron cofactor activates Phosphorus Cations. Interest in the development of Lewis 91 acids that exhibit greater air and moisture stability have led hydrogen in the Hmd hydrogenase. The hydride donor ability of 5,10-methylenetetrahydromethanopterin was found to Stephan and co-workers to recently demonstrate the − fl fl have almost an identical hydride donor ability to [BH4] . [9-H- dehydro uorination of uoroalkanes and the hydrosilylation + of olefins with phosphorus(V) cations.79,80 The proposed 10-Me-acridinium] has been recently employed as a carbon- based Lewis acid for the promotion of the hydrosilylation of catalytic cycles consist of the formation of HFP(C6F5)3. 83 imines and heterolytic cleavage of H2. Computational analysis Computational analysis of HFP(C6F5)3 resulted in a hydride − showed 10-methylacridane to be about 3 kcal/mol less reducing species that was 14 kcal/mol less reducing than [HB(C F ) ] − 6 5 3 than [HB(C F ) ] . for the phosphorus(V) species with HF in the cis-configuration; 6 5 3 fi To examine the trend of hydride donor abilities upon moving the trans-con guration was 1 kcal/mol less stable and more down the periodic table, we investigated the H(E)Ph motif, reducing. The reduced ability to transfer a hydride is on par 3 + where E = C, Si, Ge, and Sn. Trityl cation was found to be the with the experimental observations that [FP(C6F5)3] is a 80 most Lewis acidic tetrel cation, with triphenylmethane being stronger Lewis acid than B(C6F5)3. The related cis-HFPPh3 about 6 kcal/mol more stable than triphenylsilane toward the derivative was 35 kcal/mol more reducing than the fl loss of a hydride. Determination of the hydride donor ability of penta uorophenyl derivative. triphenylgermane and triphenylstannane resulted in a hydride Tetrel Hydride Donors. To explore the scope of the complex more favorable to transfer a hydride than the previous hydride donor ability of main group hydrides beyond triel and tetrel by about 5 kcal/mol. The observed trend suggests that pnictogen hydrides, we looked to examine tetrel hydrides. 81 the Lewis acidity of the tetrels decreases as one moves down Silanes have often been considered analogues of H2, and the periodic table. triaryl silylium ions have been recently shown to heterolytically 82 cleave H2 in the presence of trimesitylphosphine. The most ■ CONCLUSIONS reducing silane analyzed in this study was HSi(C Me ) , which 6 5 3 The quantification of the reducing ability of main group was computed to be about 9 kcal/mol less reducing than − hydrides is critical to catalyst design. To give insight into the [HB(C6F5)3] . The increased ability to transfer a hydride of reducing capacity of main group compounds that are widely HSi(C6Me5)3 over other silanes is attributed to an increased used in reduction chemistry, a computational study was steric bulk around the silicon center.38 The more commonly i conducted. We described the hydride donor abilities of 132 used silanes for hydrosilylation reactions, HSi( Pr)3,Me2SiHPh, 83−85 main group hydrides consisting of borohydrides, boranes, and Et3SiH, resulted in hydride donor abilities about 20 − organosilanes, organostannanes, and carbon-based hydrides kcal/mol less reducing than [HB(C6F5)3] . Intriguingly, the spanning a range of about 160 kcal/mol (Figure 3, Supporting organosilanes commonly used in hydrosilylation reactions were Information). The commonly used Lewis acid, B(C6F5)3,in calculated to be about 10 kcal/mol less reactive than H2, FLP chemistry yielded a borohydride 15 kcal/mol less reducing suggesting their propensity to model sigma complexes of − − than [BH4] , 22 kcal/mol less reducing than [AlH4] , and 41 H .81,86 Substitution of a phenyl group in HSiPh with a − 2 3 kcal/mol less reducing than superhydride ([HBEt3] ). Analysis hydride decreased the ability of the organosilane to transfer a of periodic trends showed that as one goes down the tetrels, the hydride by 7, 4, and 13 kcal/mol for Ph2SiH2, PhSiH3, and Lewis acidity decreases and the ability to transfer a hydride SiH4, respectively. Replacement of a phenyl group in increases by 5 kcal/mol. Further analysis of the hydride donor Me2SiHPh with a chloride yielded a silane 13 kcal/mol less abilities of organosilanes resulted in hydrides that are less reducing, which is expected with the introduction of a greater reducing than dihydrogen by about 10 kcal/mol, thus verifying electron-withdrawing substituent. Piers and co-workers recently their use as hydrogen analogues and the propensity to form 87 reported the formation of a Et3SiH-borole adduct. Computa- sigma complexes with metals. Investigation of chiral borohy- tional analysis of hydride transfer from 4,5,6,7-tetrafluoro-1,2,3- drides, related to the chemistry of FLPs, showed that most of trispentafluorophenyl-1H-benzo[b]borol-1-uide resulted in a the chiral borohydrides exhibited a reducing capacity similar to hydride that was 4 kcal/mol less likely to transfer than [BH ]−. Further scrutiny of the hydride donor abilities of chiral − 4 [HB(C6F5)3] and 17 kcal/mol less likely to transfer than borohydrides suggested that the less stable diastereomer results

1824 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

description of main group compounds and the way it accounts well for the dispersion interactions of weakly bound complexes.27,28,92 In all calculations, the ultrafine integration grid was employed to ensure the stability of the optimization procedure for the molecules of interest. Each stationary point was confirmed by a frequency calculation at the same level of theory to be a real local minimum on the potential energy surface without an imaginary frequency to be a minimum structure or through a potential energy surface scan in the case of fl − + [HB(3,5-bis(tri uoromethyl)phenyl)3] , [DABCO-B(catecholate)] , + and [n-Bu3Sn] , where the optimized structures consistently exhibited one imaginary frequency.28 The 6-311++G(d,p) basis set was used to computemoreaccurateelectronic energies for the optimized geometries.97,98 All reported free energies are for acetonitrile solutions at the standard state (T = 298 K, P = 1 atm, 1 mol/L concentration of all species in MeCN) as modeled by a polarized continuum model.99 The energy values given here correspond to solvent-corrected Gibbs free energies that are based on the M06-2X/6-311++G(d,p) electronic energies and all corrections calculated at the M06-2X/6-31G(d) level. For structures with greater than one possible conformer, each possible conformer (ranging from two to six structures) was optimized independently, and only the lowest energy conformer is reported within. An estimated error for the computed hydride donor abilities is 3 kcal/mol. ■ ASSOCIATED CONTENT *S Supporting Information 3D coordinates and energies of all calculated structures, table of computed and experimentally determined Lewis acidities, tables of free energies and enthalpies for the hydride donor ability of all computed main group hydrides. Complete ladder diagram showing the hydride donor abilities of the main group hydrides described within compared to known metal complexes. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/om5011512. ■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This research was supported with startup funds from Washington State University. ■ REFERENCES Figure 3. Calculated hydride donor abilities for selected main group (1) Sodium Borohydride; Dow Chemical Company; accessed hydrides in acetonitrile at the M06-2X/6-31G(d)//M06-2X/6-311G+ September 11, 2014. http://www.dow.com/sbh/products/sodium- +(d,p) level of theory. Please see the Supporting Information for a borohydride.htm. more complete ladder diagram. (2) Reductions. In Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; John Wiley & Sons, Inc., 2010. in a more reducing borohydride, and the larger energy (3) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49,46−76. difference between the two borane diastereomers resulted in (4) USP Elemental Impurities - Limits [online early access]. Published online: 2014. http://www.usp.org/sites/default/files/usp_ the highest enantiomeric excess in catalytic hydrogenation fi reactions. The investigation and implementation of main group pdf/EN/USPNF/key-issues/c232_ nal.pdf. (5) Thomson, J. W.; Hatnean, J. A.; Hastie, J. J.; Pasternak, A.; elements in reduction chemistry is a topic of interest in our Stephan, D. W.; Chase, P. A. Org. Process Res. Dev. 2013, 17, 1287− laboratory. 1292. (6) Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.; Hastie, ■ EXPERIMENTAL SECTION J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.; Welch, G. Computational Methods. All structures were fully optimized C.; Ullrich, M. Inorg. Chem. 2011, 50, 12338−12348. without symmetry constraints using the M06-2X92 functional as (7) Stephan, D. W.; Erker, G. Frustrated Lewis Pair Mediated implemented in Gaussian 09,93 using the 6-31G** basis set.94,95 The Hydrogenations. In Frustrated Lewis Pairs I; Erker, G., Stephan, D. W., Stuttgart basis set with effective core potentials was employed for all Eds.; Springer: Berlin, 2013; Vol. 332,pp85−110. tin, germanium, and iodine atoms.96 Exchange−correlation functional (8) Mahdi, T.; Stephan, D. W. J. Am. Chem. Soc. 2014, 136, 15809− M06-2X was chosen for the good overall performance observed in the 15812.

1825 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

(9) Scott, D. J.; Fuchter, M. J.; Ashley, A. E. J. Am. Chem. Soc. 2014, (45) Peuser, I.; Neu, R. C.; Zhao, X.; Ulrich, M.; Schirmer, B.; 136, 15813−15816. Tannert, J. A.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker, G.; Stephan, (10) Curtis, C. J.; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J. Am. D. W. Chem.Eur. J. 2011, 17, 9640−9650. Chem. Soc. 2002, 124, 1918−1925. (46) Ullrich, M.; Lough, A. J.; Stephan, D. W. Organometallics 2010, (11) DuBois, M. R.; DuBois, D. L. Chem. Soc. Rev. 2009, 38, 62. 29, 3647−3654. (12) Cummings, S. A.; Iimura, M.; Harlan, C. J.; Kwaan, R. J.; Trieu, (47) Keess, S.; Simonneau, A.; Oestreich, M. Organometallics 2015, I. V.; Norton, J. R.; Bridgewater, B. M.; Jakle,̈ F.; Sundararaman, A.; 34, 790−799. Tilset, M. Organometallics 2006, 25, 1565−1568. (48) Greb, L.; Daniliuc, C.-G.; Bergander, K.; Paradies, J. Angew. (13) Kwaan, R. J.; Harlan, C. J.; Norton, J. R. Organometallics 2001, Chem., Int. Ed. 2013, 52, 5876−5879. 20, 3818−3820. (49) Binding, S. C.; Zaher, H.; Mark Chadwick, F.; O’Hare, D. (14) Lawrence, E. J.; Oganesyan, V. S.; Wildgoose, G. G.; Ashley, A. Dalton Trans. 2012, 41, 9061−9066. E. Dalton Trans. 2013, 42, 782−789. (50) Mohr, J.; Durmaz, M.; Irran, E.; Oestreich, M. Organometallics (15) DuBois, D. L.; Blake, D. M.; Miedaner, A.; Curtis, C. J.; DuBois, 2014, 33, 1108−1111. M. R.; Franz, J. A.; Linehan, J. C. Organometallics 2006, 25, 4414− (51) Ashley, A. E.; Herrington, T. J.; Wildgoose, G. G.; Zaher, H.; ̈ ’ 4419. Thompson, A. L.; Rees, N. H.; Kramer, T.; O Hare, D. J. Am. Chem. − (16) Mock, M. T.; Potter, R. G.; Camaioni, D. M.; Li, J.; Dougherty, Soc. 2011, 133, 14727 14740. W. G.; Kassel, W. S.; Twamley, B.; DuBois, D. L. J. Am. Chem. Soc. (52) Zaher, H.; Ashley, A. E.; Irwin, M.; Thompson, A. L.; Gutmann, ’ − 2009, 131, 14454−14465. M. J.; Kraemer, T.; O Hare, D. Chem. Commun. 2013, 49, 9755 9757. (17) Rokob, T. A.; Hamza, A.; Papai, I. J. Am. Chem. Soc. 2009, 131, (53) Zhang, Z.; Edkins, R. M.; Nitsch, J.; Fucke, K.; Steffen, A.; − Longobardi, L. E.; Stephan, D. W.; Lambert, C.; Marder, T. B. Chem. 10701 10710. − (18) Rokob, T. A.; Papai, I. Top. Curr. Chem. 2013, 332, 157−212. Sci. 2015, 6, 308 321. (54) Herrington, T. J.; Thom, A. J. W.; White, A. J. P.; Ashley, A. E. (19) Bohrer, H.; Trapp, N.; Himmel, D.; Schleep, M.; Krossing, I. − Dalton Trans. 2015, 44, 7489−7499. Dalton Trans. 2012, 41, 9019 9022. (20) Durfey, B. L.; Gilbert, T. M. Inorg. Chem. 2011, 50, 7871−7879. (55) Hubbard, J. L.; Dake, G. Lithium Tri-sec-butylborohydride. In e- (21) Morgan, M. M.; Marwitz, A. J. V.; Piers, W. E.; Parvez, M. EROS Encyclopedia of Reagents for Organic Synthesis; John Wiley & − Sons, Ltd, 2001. Organometallics 2013, 32, 317 322. − (22) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982, (56) Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37, 1986 60, 809−812. 2012. − (57) Dureen, M. A.; Lough, A.; Gilbert, T. M.; Stephan, D. W. Chem. (23) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225 255. − (24) Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106, Commun. 2008, 4303 4305. (58) Korenaga, T.; Kobayashi, F.; Nomura, K.; Nagao, S.; Sakai, T. J. 1235−1257. Fluorine Chem. 2007, 128, 1153−1157. (25) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse, M. (59) Parks, D. J.; von H. Spence, R. E.; Piers, W. E. Angew. Chem., Int. B. Inorg. Chem. Commun. 2000, 3, 530−533. Ed. Engl. 1995, 34, 809−811. (26) Neu, R. C.; Ouyang, E. Y.; Geier, S. J.; Zhao, X.; Ramos, A.; (60) Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130−2131. Stephan, D. W. Dalton Trans. 2010, 39, 4285−4294. (61) Chen, D.; Wang, Y.; Klankermayer, J. Angew. Chem., Int. Ed. (27) Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc. 2008, 120, 215−241. 2010, 49, 9475−9478. (28) Goerigk, L.; Grimme, S. Phys. Chem. Chem. Phys. 2011, 13, (62) Chen, J.; Venkatasubbaiah, K.; Pakkirisamy, T.; Doshi, A.; 6670−6688.   Yusupov, A.; Patel, Y.; Lalancette, R. A.; Jakle, F. Chem. Eur. J. 2010, (29) Clark, E. R.; Del Grosso, A.; Ingleson, M. J. Chem. Eur. J. − − 16, 8861 8867. 2013, 19, 2462 2466. (63) Wang, G.; Chen, C.; Du, T.; Zhong, W. Adv. Synth. Catal. 2014, (30) Nimlos, M. R.; Chang, C. H.; Curtis, C. J.; Miedaner, A.; Pilath, − − 356, 1747 1752. H. M.; DuBois, D. L. Organometallics 2008, 27, 2715 2722. (64) Greb, L.; Paradies, J. Top. Curr. Chem. 2013, 334,81−100. (31) Raugei, S.; DuBois, D. L.; Rousseau, R.; Chen, S.; Ho, M.-H.; (65) Morrison, D. J.; Piers, W. E.; Parvez, M. Synlett 2004, 2429− Bullock, R. M.; Dupuis, M. Acc. Chem. Res. 2015, 48, 248−255. − 2433. (32) Kovacs, G.; Papai, I. Organometallics 2006, 25, 820 825. (66) Hermeke, J.; Mewald, M.; Oestreich, M. J. Am. Chem. Soc. 2013, (33) Paquette, L. A.; Ollevier, T.; Desyroy, V. Lithium Aluminum 135, 17537−17546. Hydride. In e-EROS Encyclopedia of Reagents for Organic Synthesis; John (67) Mewald, M.; Fröhlich, R.; Oestreich, M. Chem.Eur. J. 2011, Wiley & Sons, Ltd, 2001. 17, 9406−9414. (34) Zaidlewicz, M.; Brown, H. C. Lithium Triethylborohydride. In e- (68) Piers, W. E.; Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. EROS Encyclopedia of Reagents for Organic Synthesis; John Wiley & 2005, 44, 5016−5036. Sons, Ltd, 2001. (69) Eisenberger, P.; Bailey, A. M.; Crudden, C. M. J. Am. Chem. Soc. (35) Adamczyk-Wozniak,́ A.; Jakubczyk, M.; Jankowski, P.; 2012, 134, 17384−17387. Sporzynski,́ A.; Urbanski,́ P. M. J. Phys. Org. Chem. 2013, 26, 415−419. (70) Farrell, J. M.; Hatnean, J. A.; Stephan, D. W. J. Am. Chem. Soc. (36) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. 2012, 134, 15728−15731. A.; Shah, R. D. J. Org. Chem. 1996, 61, 3849−3862. (71) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic (37) Kehr, G.; Erker, G. Chem. Commun. 2012, 48, 1839−1850. Solvents; Blackwell Scientiﬁc Publications: Oxford, 1990; Vol. 35. (38) Lathem, A. P.; Treich, N. R.; Heiden, Z. M. Isr. J. Chem. 2015, (72) Kaljurand, I.; Kuett, A.; Soovaeli, L.; Rodima, T.; Maeemets, V.; 55, 226−234. Leito, I.; Koppel, I. A. J. Org. Chem. 2005, 70, 1019−1028. (39) Yoon, N. M.; Kim, K. E.; Kang, J. J. Org. Chem. 1986, 51, 226− (73) Stephens, F. H.; Pons, V.; Tom Baker, R. Dalton Trans. 2007, 229. 2613−2626. (40) Yoon, N. M.; Kim, K. E. J. Org. Chem. 1987, 52, 5564−5570. (74) Menard, G.; Tran, L.; Stephan, D. W. Dalton Trans. 2013, 42, (41) Eros, G.; Mehdi, H.; Papai, I.; Rokob, T. A.; Kiraly, P.; Tarkanyi, 13685−13691. G.; Soos, T. Angew. Chem., Int. Ed. 2010, 49, 6559−6563. (75) Menard, G.; Stephan, D. W. Angew. Chem., Int. Ed. 2012, 51, (42) Neu, R. C.; Stephan, D. W. Organometallics 2012, 31,46−49. 8272−8275. (43) Brown, H. C. Hydroboration; W. A. Benjamin, Inc.: New York, (76) Timoshkin, A. Y.; Frenking, G. Organometallics 2008, 27, 371− NY, 1962. 380. (44) Jiang, C.; Blacque, O.; Fox, T.; Berke, H. Dalton Trans. 2011, 40, (77) Kraft, A.; Beck, J.; Steinfeld, G.; Scherer, H.; Himmel, D.; 1091−1097. Krossing, I. Organometallics 2012, 31, 7485−7491.

1826 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827 Organometallics Article

(78) Kraft, A.; Trapp, N.; Himmel, D.; Böhrer, H.; Schlüter, P.; Scherer, H.; Krossing, I. Chem.Eur. J. 2012, 18, 9371−9380. (79) Perez,́ M.; Hounjet, L. J.; Caputo, C. B.; Dobrovetsky, R.; Stephan, D. W. J. Am. Chem. Soc. 2013, 135, 18308−18310. (80) Caputo, C. B.; Hounjet, L. J.; Dobrovetsky, R.; Stephan, D. W. Science 2013, 341, 1374−1377. (81) Kubas, G. J. Adv. Inorg. Chem. 2004, 56, 127−177. (82) Schafer,̈ A.; Reißmann, M.; Schafer,̈ A.; Saak, W.; Haase, D.; Müller, T. Angew. Chem., Int. Ed. 2011, 50, 12636−12638. (83) Clark, E. R.; Ingleson, M. J. Angew. Chem., Int. Ed. 2014, 53, 11306−11309. (84) Parks, D. J.; Piers, W. E. J. Am. Chem. Soc. 1996, 118, 9440− 9441. (85) Gutsulyak, D. V.; Vyboishchikov, S. F.; Nikonov, G. I. J. Am. Chem. Soc. 2010, 132, 5950−5951. (86) Kubas, G. Metal Dihydrogen and σ-Bond Complexes; Kluwer Academic/Plenum: New York, 2001. (87) Houghton, A. Y.; Hurmalainen, J.; Mansikkamaki,̈ A.; Piers, W. E.; Tuononen, H. M. Nat. Chem. 2014, 6, 983−988. (88) Ouellet, S. G.; Walji, A. M.; Macmillan, D. W. C. Acc. Chem. Res. 2007, 40, 1327−1339. (89) You, S.-L. Chem. Asian J. 2007, 2, 820−827. (90) Webb, J. D.; Laberge, V. S.; Geier, S. J.; Stephan, D. W.; Crudden, C. M. Chem.Eur. J. 2010, 16, 4895−4902. (91) Shima, S.; Pilak, O.; Vogt, S.; Schick, M.; Stagni, M. S.; Meyer- Klaucke, W.; Warkentin, E.; Thauer, R. K.; Ermler, U. Science 2008, 321, 572−575. (92) Zhao, Y.; Truhlar, D. G. J. Chem. Theory Comput. 2008, 4, 1849−1868. (93) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc.: Wallingford, CT, 2013. (94) Dill, J. D.; Pople, J. A. J. Chem. Phys. 1975, 62, 2921−2923. (95) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257−2261. (96) Andrae, D.; Haußermann,̈ U.; Dolg, M.; Stoll, H.; Preuß, H. Theoret. Chim. Acta 1990, 77, 123−141. (97) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650−654. (98) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654−3665. (99) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem. 2003, 24, 669−681.

1827 DOI: 10.1021/om5011512 Organometallics 2015, 34, 1818−1827