Transition-Metal-Promoted Hydroboration of Alkenes:  A Unique Reversal of Regioselectivity

Abstract

When hydroboration of 1-octene is carried out in the presence of catalytic amounts of rhodium trichloride followed by the usual oxidation (hydrogen peroxide in aqueous alkali), only minor proportions of 1-octanol (2.4%) are formed accompanied by very significant amounts of 2- (17.4%), 3- (36.9%), and 4-octanol (43.3%). These product compositions are obtained in good overall yield when the borane-THF complex is slowly added to a stirred solution of 1-octene in THF solvent containing the rhodium trichloride. Isomerization of 1-octene to 2-, 3-, and 4-octene in the presence of rhodium trichloride alone is far too slow to account for the foregoing results. The mechanism likely involves multiple and reversible addition/elimination of a Rh-activated B-H species across the double bonds.

Recent Advances in Organic Synthesis Using Transition Metal-Catalyzed Hydroborations

“…As a result, a considerable amount of research has focussed on catalyzed hydroborations using alternate borane sources [51]. Catalytic amounts of trialkyltin chlorides have been used in the hydroboration of diethyl(1propynyl)borane with tetraethyldiborane(6) [55] and rhodium trichloride has been used in the addition of borane to alkenes (as a route to isomerize the alkene) [56,57] and alkenyl nitriles [58]. Catalytic amounts of trialkyltin chlorides have been used in the hydroboration of diethyl(1propynyl)borane with tetraethyldiborane(6) [55] and rhodium trichloride has been used in the addition of borane to alkenes (as a route to isomerize the alkene) [56,57] and alkenyl nitriles [58].…”

Exploring the mechanism of the hydroboration of alkenes by amine–boranes catalysed by [Rh(xantphos)]+

“…This pathway is similar to that reported for using the [Rh(P i Bu 2 i Pr) 2 ] + catalyst system, 14 as well as late transition metal hydroboration systems that use, for example, HBCat. 37 and no H/D exchange at the α-position of the linear product is measured, showing that insertion from C to form D (vi) is neither kinetically competent nor reversible. Insertion from C to give the linear intermediate E is reversible (iv), as there is significant (40%) H/D exchange at the β-position in the final product, as well as into the free alkene when D 3 B•NMe 3 is used, that suggests that β-H-elimination from E occurs.…”

Rhodium‐Catalyzed Hydroboration Reactions with Sulfur and Nitrogen Analogues of Catecholborane

“…[16] A unique reversal in the regioselectivity has also been reported in the hydroboration of 1-octene catalyzed by rhodium trichloride. [17] Since the discovery of the transition-metal-catalyzed hydroboration reaction, a number of research groups have used catecholborane (1) [1,3,2-benzodioxaborolane abbreviated as HBCat where Cat = 1,2-O 2 C 6 H 4 ] and recently pinacolborane (2) [4,4,6,6-tetramethyl-1,3,2-dioxaborolane abbreviated as HBPin where Pin = 1,2-O 2 C 6 H 12 ] in rhodiumcatalyzed transformations with much attention focused on the mechanistic, regio-and stereoselectivity implications of the reaction. [1][2][3][4][10][11][12][13] More recently, attention has been focused on the enhancement of Rh-type catalysts with an emphasis on different phosphane ligands.…”