Hydroboration catalysed

Other convenient routes to carboranes, selected from the growing number of recently reported syntheses, are as follows. Monocarbon carboranes can be prepared in good yield by the transition-metal catalysed hydroboration of alkenes followed by thermal rearrangement of the intermediate product, c.gP" ... 

The metal catalysed hydroboration and diboration of alkenes and alkynes (addition of H-B and B-B bonds, respectively) gives rise to alkyl- or alkenyl-boronate or diboronate esters, which are important intermediates for further catalytic transformations, or can be converted to useful organic compounds by established stoichiometric methodologies. The iyn-diboration of alkynes catalysed by Pt phosphine complexes is well-established [58]. However, in alkene diborations, challenging problems of chemo- and stereo-selectivity control stiU need to be solved, with the most successful current systems being based on Pt, Rh and An complexes [59-61]. There have been some recent advances in the area by using NHC complexes of Ir, Pd, Pt, Cu, Ag and Au as catalysts under mild conditions, which present important advantages in terms of activity and selectivity over the established catalysts. 

Tab. 2.2 Recent rhodium-catalysed hydroborations with P,N-ligands products of H2O2 oxidazition...

Hydroboration of alkenes with BH3 and R2BH proceeds at room temperature without a catalyst. As an exception, the reaction of the less reactive catecholborane (CBH, 529) proceeds at 70-100 °C. It was found that the slow reaction of CBH is accelerated by a Rh catalyst, and the catalyzed and uncatalyzed hydroborations give different products [200]. By the uncatalyzed reaction, the ketone in the enone 530 is attacked to form 531. On the other hand, the alkene is hydroborated to give 532 by the catalysis of RhCl(Ph3P)3 [201]. Different regio- and stereoselectivities are observed in the Rh-catalysed hydroboration of cyclohexenol (533) with catecholborane and uncatalysed reactions with 9-BBN-H [202],... 

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... 

Rhodium-catalysed hydroboration is a powerful tool for introducing chirality into a styrene-derivative. (Figure 1.1)[2] This was in competition to the established route based on chiral resolution using separation of diastereomers formed from reaction of the racemic amines with homo-chiral acids (natural pool). However, although the process appeared favourable from the chemical synthetic route, the process was practically impossible owing to there being no supplier of catecholborane on large scale at the time. 

Figure 1.1 Rhodium-catalysed hydroboration of a styrene derivative.

Proposed mechanisms of transition metal catalysed hydroboration of alkenes and diboration of alkynes... 

The regioselectivity in the metal-catalysed hydroboration of alkyl-substituted alkenes is the same as that with the non-catalysed system, in which the boron atom becomes attached to the less-hindered carbon atom of the alkene. However,... 

Use of bis(catecholato)diboron (2.178) in the rhodium-catalysed hydroboration results in enantioselective diboration and a number of simple trans-alkenes such as trans- 3-methylstyrene (2.179) are converted into the 1,2-diol in high ee by diboration/oxidation using (nbd)Rh(acac) in combination with (S)-QUINAP (2.169) as catalyst. 

While these reactions allowed ready access to oxygen heterocycles having the adjacent hydroxy and vinyl groups in a cis-disposition, access to the corresponding trans-derivatives was more difficult due to a lack of generally applicable routes (16, 17) to E-y-alkoxy allylboronates. We therefore eventually developed a new route to this entity based on a rhodium or zirconium catalysed hydroboration of ynol ethers. (18)... 

Carroll, A.-M. O SuUivan, T. R Guiry, F. J. The Development of Enantioselective Rhodium-Catalysed Hydroboration of Olefins. Adv. Synth. Catal. 2005, 347,609-631. 

Scheme 15.36 Copper-catalysed hydroboration of internal alkenes.

DFT has been used to study both the associative and disociative mechanisms of Rh(I) catalysed hydroboration of alkenes. " Data obtained from low-temperature X-ray diffraction studies on [Rh(acac)(ri -H2C = CH2)] and its perfluoro analogue have been compared with data obtained from DFT calculations ... 

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-l-butene with the NHC-derived borane and catalytic HNTf2 (Tf=trifluoromethanesulfonyl, CF3SO2) affects hydroboration at room temperature. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations." Different NHC-CuCl complexes catalyse hydroborations of propargylic alcohols and ethers with opposite regioselectivity a 6-NHC-CuCl (23) complex provides a-selectivity, whereas / -selectivity is achieved using a 5-NHC-CuCl (24) complex." ... 

Regioselectivity of the copper(I)-catalysed hydroboration of unsymmetrical internal alkynes r1C=CR2 (R1 = Ar, COjR, amide, CHjOR, CH2NR2, CH2CH2OR R = alkyl, SiMe3) has been shown to be controlled by the choice of the catalytic species (copper hydride or boryl copper). Thus, the reaction with PinBH (Pin = pinacol), catalysed by CuCl chelated to an electron-rich diphosphine, affords R C(BPin)=CHR in the presence of t-BuONa switching to pinjBj and electron-poor diphosphine results in the formation of the opposite isomer, in both cases with >99 1 regioselectivity. ... 

Regioselectivity of the CuCl-catalysed hydroboration of propargylic alcohols and ethers R C=CCH(OR )R with Pin2B2 can be controlled (from 98 2 to 2 98) by the steric bulk of the OR group and the NHC ligand to copper. A rationale has been provided by assuming different approach of the reactive species to the individual substrates (87)-(89).S9... 

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