A series of new agostic (CH3···Rh)
(π-allyl)-closo-rhodacarboranes (π-allyl
= 1,1-dimethylallyl,
1,2-dimethylallyl, 1,1,2-trimethylallyl, 1,2,3-trimethylallyl), stable
in the solid state, have been synthesized via one-pot reactions between
the K+ salts of the [7-R-8-R′-7,8-nido-C2B9H10]− monoanions
(1a, R = R′ = Me; 1b, R,R′
= μ-(o-xylylene); 1c, R,R′ = μ-(CH2)3) and the di-μ-chloro
cyclooctene rhodium dimer [(η2-C8H14)4Rh2(μ-Cl)2] (2) in the presence of a 3-fold excess of the conjugated 1,3-dienes
2-methylbuta-1,3-diene (isoprene, 3), 2,3-dimethylbuta-1,3-diene
(4), and 3-methylpenta-1,3-diene (5). The
agostic structures of [3-{(1–3-η3)-1,1-dimethylallyl}-1,2-(CH3)2-3,1,2-closo-RhC2B9H9] (7a) and [3-{(1–3-η3)-1,1,2-trimethylallyl}-1,2-(CH3)2-3,1,2-closo-RhC2B9H9] (8a) have been unambiguously confirmed by single-crystal X-ray
diffraction studies. Two of these π-allyl complexes prepared
were evaluated for their efficacy in hydroformylation of the model
alkenes under syngas (CO/H2) using supercritical carbon
dioxide (scCO2) as the solvent, and both display excellent
conversion and high regioselectivity in the formation of aldehyde
products.