Regioselectivity of nucleophilic additions to substituted (η4-diene)Co(CO)3BF4 complexes

Abstract

The addition of several nucleophiles to a series of (η4-diene)Co(CO)3BF4 complexes has been examined in order to probe the effect of diene substituents and the nature of the nucleophile on the regioselectivity of attack on the coordinated diene. The dienes include 1-substituted dienes: trans-1,3-pentadiene (piperylene), 1-trimethylsilyl-2,4-pentadiene; 2-substituted dienes: 2-methyl- (isoprene), 2-ethyl-, 2-phenyl-, 2-para-fluorophenyl-, and 2-trimethylsilylmethyl-1,3-butadiene; and the disubstituted diene 2-methyl-1,3-pentadiene. The set of nucleophiles includes NaBH3CN, C6H5MgBr, P(CH3)3, C5H5N (pyridine), and N(CH2CH3)3. The reactions occur generally with a high degree of regioselectivity according to the following trends: (1) addition of nucleophiles to most complexes occurs preferentially or exclusively at C(4) (terminal diene carbon remote from the substituent); (2) noteworthy exceptions include the isoprene complex, in which modest C(1) addition selectivity is observed with Nu = NaBH3CN, C6H5MgBr, P(CH3)3 and the dimethyl derivative (for Nu = pyridine); and (3) the selectivity for C(4) attack is nucleophile dependent, decreasing in the order N(CH2CH3)3 > pyridine > P(CH3)3 > C6H5MgBr > NaBH3CN. A combination of steric and frontier molecular orbital effects is proposed to explain the observed regioselectivity.