Rhodium(I) catalysed diboration of (E)-styrylboronate esters: molecular structures of (E)-p-MeO–C6H4–CHCH–B(1,2-O2C6H4) and p-MeO–C6H4–CH2C{B(1,2-O2C6H4)}3

Diboration of the styrylboronate esters (E)-p-R–C6H4–CHCH–Bcat (1) (R=H, (1a) MeO (1b); cat=1,2-O2C6H4), with B2cat2 in the presence of a variety of rhodium phosphine catalysts gives predominantly either p-R–C6H4–CH2C(Bcat)3 (2), which contains three boronate ester groups on one carbon atom, or its isomer p-R–C6H4–CH(Bcat)CH(Bcat)2 (3). The formation of 2 apparently involves regiospecific insertion of the vinylboronates into a Rh–B bond followed by β-hydride elimination, another regiospecific insertion of the 2,2-vinyl bis(boronate) into the remaining Rh–B bond followed by C–H reductive elimination leading to 2,2-diboration and a 2,1-hydrogen shift. Wilkinsonʹs catalyst (7) gives the highest yields of 2 with 75 and 71% yields from 1a and 1b, respectively, while [Rh(COE)2(μ-Cl)]2 (10) with two equivalents of P(o-tol)3 gave 3 in highest yield with 50 and 49% from 1a and 1b, respectively. The crystal structure of (E)-p-MeO–C6H4–CHCH–Bcat shows the molecule to be essentially planar, with the phenyl ring twisted slightly out of the plane. The tris(boronate) p-MeO–C6H4–CH2C(Bcat)3 crystallises with two independent molecules in the unit cell, which differ in the rotational orientation of the phenyl ring in relation to the catecholate groups; the latter are arranged in a propeller-like fashion.