ORGANOTRANSITION-METAL METALLACARBORANES .24. (C5ME5)CO(ET2C2B3H4-R) METALLACARBORANE SANDWICH COMPLEXES AS VERSATILE SYNTHONS - INTRODUCTION OF ELECTRON-WITHDRAWING SUBSTITUENTS

As part of our development of small nido-metallacarborane complexes as organometallic building-blocks, the controlled syntheses of (C5Me5)Co(Et2C2B3H4-R) and (C5Me5)Co(Et2C2B3H3-R2) derivatives in which R is an acyl, trifluoroacyl, ester, nitrile, nitro, chloro, bromo, or iodo group were investigated. The attack of electrophilic reagents (R+) on the B-B edge of the (C5Me5)Co(Et2C2B3H4)- anion (1-), which was demonstrated earlier (preceding article and earlier papers) to give regiospecific B(5)-R substitution, in the present work produced B(5)-R and/or B(4/6)-R derivatives. Efficient syntheses of the B(5)-chloro, -bromo, and -iodo derivatives were achieved via treatment of 1- with MeSO2Cl, BrCH2CN or BrCH2NO2, and CF3I, respectively; further reaction of the monochloro compound gave the dichloro and trichloro species. In contrast to this B(5)-substitution pattern, reactions of 1- with C(O)CF3Cl gave exclusively the B(4/6)-C(O)CF3 derivative. Treatment of neutral 1 with N-halosuccinimides gave exclusively the monohalo [B(4/6)-X] derivatives (X = Cl, Br, I) in nearly quantitative yield; further halogenation of these species using the same reagents gave the B(4,6)-X2 dihalo products quantitatively. The reaction of 1- with acetyl chloride in THF at -78-degrees-C did not form the expected B(5)-acetyl complex, giving instead the B(5)-(2-vinyl acetate) derivative 2 in good yield; the structure was proved via X-ray crystallography. Base-catalyzed cleavage of 2 readily afforded the B(5)-acetyl compound 3. The formation of 2 does not involve the acetyl species 3 as an intermediate and is proposed to occur via formation of a B-C-B acetyl-bridged intermediate which undergoes O-acetylation and rearrangement to give 2. The remarkable formation of 2 differs from the usual enol acetate formation in organic chemistry in that, in the present case, the only primary source of -(C=O)- is the acylating agent itself. New compounds were isolated in most cases as air-stable crystals and characterized via H-1, C-13, and B-11 NMR, IR, UV-visible, and mass spectra. Crystal data for 2: space group P2(1)/n; Z = 4; a = 8.368 (2), b = 18.757 (5), c = 14.234 (4) angstrom; beta = 97.25 (2)-degrees; V = 2216 (2) angstrom3; R = 0.037 for 3856 reflections having F(o)2 > 3.0-sigma(F(o)2).