Rhodium-Catalyzed Hydroborations and Related Reactions

In the interim period, results have accumulated steadily, in endeavors to address and extend the chemistry beyond the initial perceived limitations. These limitations include the following (a) the effective catalytic syntheses are confined to the reactions utilizing catecholborane (b) the scope of alkenes for which efficient rate, regio- and enantio-selectivity can be achieved is limited, and (c) the standard transformation mandates the oxidation of the initially formed (secondary) boronate ester to a secondary alcohol, albeit with complete retention of configuration [8]. Nonetheless, for noncatalytic hydroboration reactions that lead to the formation of a trialkylborane, a wide range of stereo-specific transformations may be carried out directly from the initial product, and thereby facilitate direct C-N and C-C bond formation [9]. 

Modem Rhodium-Catalyzed Organic Reactions. Edited by P. Andrew Evans Copyright 2005 WILEY-VCEI Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30683-8 

It is a striking feature of metal-catalyzed hydroboration that alkoxyboranes are more effective than borane or its alkyl derivatives. Analysis of the electronic features of the two classical reagents, catecholborane and 9-BBN, reveal only modest differences in the B-H region (Fig. 2.3). This by itself does not explain the narrow convergence of 

For adoption by the synthetic-organic community, a new method must pass certain tests. If it is catalytic, then the turnover must be efficient so that the quantity of (presumably expensive) catalyst employed is small. The reaction needs to be selective in all the desirable ways, where appropriate including chemo-, regio-, and stereospecificity. Much of the work on hydroboration has been aimed at progress toward those goals. The rhodium-catalyzed reaction of catecholborane with a simple styrene frequently forms part of the standard catalytic screening procedures for a novel ligand. 

As would be expected, catalytic hydroboration is effective for alkynes as well as al-kenes, and prior examples have been reviewed [6]. An interesting development has been the diversion of the normal syn- to the anti-addition pathway for a terminal alkyne, with 99% (catechoborane) and 91% (pinacolborane) respectively (Fig. 2.5) [20]. The new pathway arises when basic alkylphosphines are employed in combination with [Rh(COD)Cl]2 as the catalyst in the presence of Et3N. Current thinking implies that this is driven by the initial addition of the rhodium catalyst into the alkynyl C-H bond, followed by [1,3]-migration of hydride and formal 1,1-addition of B-H to the resulting alkylidene complex. The reaction is general for terminal alkynes. 

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Hydroborations. Addition of Catecholborane to alkenes is accelerated by Wilkinson s catalyst, and other sources of rhodium-(I) complexes. Unfortunately, the reaction of Wilkinson s catalyst with catecholborane is complex hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37) however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases. Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson s catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use. The 5yn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives. ... 

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