The products of catalysed diboration of bis(p-tolyl)ethyne and of 4-cyanophenyl-ethyne by bis(catecholato-O,O')diboron

Both (Z)-1,2-bis(1,3,2-benzodioxaborolan-2-yl)-1,2-bis(4-methylphenyl)ethene [(I), C28H22B2O4] and (Z)-4-[1,2-bis(1,3,2-benzodioxaborolan-2-yl)vinyl]benzonitrile [(II), C21H13B2NO4] have a cis arrangement of two boronate ester substituents on a C=C double bond, which shows a slight twist to relieve steric crowding of the substituents. The interplay of steric and electronic factors results in approximate coplanarity of only one substituent with the alkene unit in (I), while this is achieved for two mutually trans substituents in the less crowded (II). A consistent pattern of bonding and conformational geometry is found for a series of related alkenes with boronate ester substituents. The catecholboronate group shows almost invariant geometry across a range of compounds, with a slight but probably real lengthening of the B-O bonds on coordination to transition metals.