Kinetic and Mechanistic Studies of the Formal (1+2)- and (1+4)-Cycloadditions of Germylenes to Conjugated Dienes

Fast kinetic studies of the reactions of isoprene and 2,3-dimethyl-1,3-butadiene (DMB) with diphenylgermylene (GePh 2, 2a ) and of isoprene with a series of diarylgermylenes bearing polar ring substituents (GeAr 2, 2b − g ) have been carried out in hexanes solution. Though the major stable products of the reactions with isoprene are the corresponding 1,1-diarylgermacyclopent-3-ene derivatives, the results indicate that the major initial products are the corresponding transient 1,1-diaryl-2-vinylgermiranes ( 6a − g ) resulting from formal (1+2)-cycloaddition to the less-substituted C═C bond of the diene. These compounds are formed reversibly and with rate constants in excess of 10 9 M −1 s −1, and appear as discrete reaction intermediates exhibiting λ max = 285 nm and lifetimes of 2−670 μs depending on the identity of the germylene and the diene. The variations in the lifetimes with aryl substituents are shown to be most consistent with a stepwise mechanism for vinylgermirane → germacyclopent-3-ene isomerization, involving (reversible) dissociation to the free germylene and diene followed by (irreversible) (1+4)-cycloaddition. The bimolecular rate constants for (1+4)-cycloaddition to isoprene, calculated from the data on the basis of this model, vary over the range of 5 × 10 6 to 3 × 10 8 M −1 s −1 depending on the aryl ring substituent(s). The variation in the rate and equilibrium constants for reaction of 2a − g with isoprene indicates that the germylene plays the role of an electrophile in both the (1+2)- and (1+4)-cycloaddition processes and demands the involvement of a polarized steady-state intermediate in the (1+2)-addition reaction. The temperature dependence of the experimental rate and equilibrium constants for reaction of GePh 2 with isoprene, and of the rate coefficient for decay of the corresponding vinylgermirane, allows most aspects of the potential energy surface to be defined quantitatively.