Complexes in hydroboration

Abstract A variety of structural types of transition metal complexes containing a-borane ligands are reviewed. Their structure and bonding are discussed. Compared with other types of a-complexes, a-borane complexes display quite different structural and bonding properties because of the electron unsaturated boron center in the a-borane ligands in the precoordination state. The availability of an sp3-hybridized orbital at the boron center allows stronger back-donation interaction without breaking the coordinated B-H bond. The role played by a-borane complexes in hydroboration and borylation reactions has also been reviewed. 

Hydroboration. The advantage of this borane (as dimethyl sulfide complex) in hydroboration is that the adducts can be oxidized directly to carboxylic acids in excellent yields. Thus a synthesis of y-valerolactone from 4-(p-nitrobenzoyloxy)-l-pentene can be easily achieved. This particular ester is used in this case because it is reduced very slowly (in comparison with the acetate). 

The proposed model [B] implies that syn selection in catalyzed hydroboration should decrease as the o-accepting character of the anti substituent decreases. This explains the higher syn selectivity in catalyzed hydroboration of allylic trifluoroacetates as compared with reactions of allylic acetates and carbamates [25]. The less diastereoselectivity of cationic complexes in hydroboration of chiral allylic systems than in the neutral catalyst systems [29, 30] is rationalized, as the latter have more electron density to shed via back-bonding. The anti selectivity in catalyzed hydroboration of the allylic acetate and trifluoroacetate is rationalized in terms of competition between the phenyl and acetate groups for the role of a acceptor. The further evidence is obtained in the case of pentafluo-... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

The mechanistic complexity of hydroboration-oxidation stands in contrast to the simplicity with which these reactions aie caiiied out experimentally. Both the hydrobo-ration and oxidation steps are extremely rapid reactions and are performed at room temperature with conventional laboratory equipment. Ease of operation, along with the fact that hydroboration-oxidation leads to syn hydration of alkenes and occurs with a regioselectivity opposite to Maikovnikov s rule, makes this procedure one of great value to the synthetic chemist. 

A convenient route to both saturated and unsaturated acylsilanes lies in the hydroboration-oxidation of alkynylsilanes (Chapter 7). Recent improvements (6) to this method involve the use of the borane-dimethyl sulphide complex for hydroboration, and of anhydrous trimethylamine A-oxide for the oxidation of the intermediate vinyl boranes. 

A solution of BH3THF complex in THF is generally used as the hydroborating agent. Primary hydroboration products 19 and 20 are transformed into the corresponding 1-boraadamantane THF complexes 21 under reflux in THF (1-4 h), or in 4-5 days at room temperature (Scheme 2). 

Attempts to utilize rhodium complexes for hydroboration of vinylarenes (Equation (2)) were somewhat complicated by discrepancies in the results from different research groups. Thus, it was found that [Rh(PPh3)3Cl] catalyzed the addition of HBcat to styrene to afford a quantitative yield of the branched product 15... 

Nonetheless, among bidentate diphosphines and with the notable exception of BINAP 23, there have been only sporadic examples of ligands whose rhodium complexes give enantioselectivities above 85% in hydroboration Knochel s dicyclohexylphosphine 34,80 Togni s Josiphos 41,85 and TADDOL derivatives 48, 50-52.90 Even... 

Ligand 73 was prepared directly from a single enantiomer of the corresponding naphthol of QUINAP 60, an early intermediate in the original synthesis, and both enantiomers of BINOL. Application in hydroboration found that, in practice, only one of the cationic rhodium complexes of the diastereomeric pair proved effective, (aA, A)-73. While (aA, A)-73 gave 68% ee for the hydroboration of styrene (70% yield), the diastereomer (aA, R)-73 afforded the product alcohol after oxidation with an attenuated 2% ee (55% yield) and the same trend was apparent in the hydroboration of electron-poor vinylarenes. Indeed, even with (aA, A)-73, the asymmetries induced were very modest (31-51% ee). The hydroboration pre-catalyst was examined in the presence of catecholborane 1 at low temperatures and binuclear reactive intermediates were identified. However, when similar experiments were conducted with QUINAP 60, no intermediates of the same structural type were found.100... 

The two-substituted-Quinazolinap-derived rhodium complexes proved extremely efficient catalysts for the hydro-boration-oxidation of vinylarenes (Table 6). For styrene derivatives, in most cases quantitative conversions were obtained after just 2 h at the relevant temperature (entries 1-6). Higher enantioselectivities were afforded with a 4-methoxy substituent (up to 95% ee, entry 3) compared to the 4-chloro or unsubstituted styrene analogs (entries 5 and 1), a trend also observed in hydroboration with rhodium complexes of QUINAP 60. This highlights that both the electronic nature of the substrate combined with the inherent steric properties of the catalyst are important for high asymmetric induction. It is noteworthy that in most cases, optimum enantioselectivities were afforded by the... 

In an attempt to rationalize the factors that control selectivity in the Rh- and Ir-catalyzed hydroboration reactions, Fernandez and Bo [35] carried out experimental and theoretical studies on the H—B addition of catecholborane to vinylarenes with [M(C0D)(R-QUINAP)]BF4, (QUINAP = l-(2-diphenylphosphino-l-naphthyl) isoquinoHne). A considerable difference was found in the stability of the isomers when the substrate was coordinated to the iridium(I) or rhodium(I) complexes. In particular, the difference between pro-R B1 and pro-S B2 isomers was not so great when the metal center was iridium and not rhodium (Figure 7.1), which explains the low ee-values observed experimentally when asymmetric iridium-catalyzed hydroboration was performed. Structurally, the energy analysis of the n2 and Tti interactions [36] seems to be responsible for the extra stabilization of the B2 isomer in the iridium intermediates (Figure 7.1). The coordination and insertion of alkenes, then, could be considered key steps in the enantiodifferentiation pathway. 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide.145 (see entry 6, in Scheme 4.8). Use of the dibromoborane-dimethyl sulfide complex for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation, gives carboxylic acids.146... 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

The significant interest that the prospects of transition metal complexes of boron received over the past few years is not only due to a hitherto unknown type of metal-boron linkage. Boryl complexes in particular became a highly rewarding target due to their potential application for the functionalisation of hydrocarbons. They are well known to be key intermediates in the metal catalysed hydroboration and related reactions,58 65... 

Probably the most important developments in this field over the past 10 years, however, have been in the area of enantioselective see Enantioselectivity) hydroborations using cationic rhodium complexes of the type [Rh(diene)L ]+ (L = chiral ligand). An excellent review on this topic has recently been published. New chiral see Chiral) catalyst systems are typically tested in hydroborations of vinyl arenes. Although catalyzed hydroboration of vinyl arenes can be used as a mild and efficient route to preparing 1-arylethanol... 

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