Organometallics, 28, 6777-6788 (2009).

© American Chemical Society

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

Lawrence A. Huck and William J. Leigh*

Contribution from the Department of Chemistry and Chemical Biology, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

Fast kinetic studies of the reactions of isoprene and 2,3-dimethyl-1,3-butadiene (DMB) with diphenylgermylene (GePh2, 2a) and of isoprene with a series of diarylgermylenes bearing polar ring substituents (GeAr2, 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 109 M-1s-1, and appear as discrete reaction intermediates exhibiting λmax = 285 nm and lifetimes of 2 - 670 μs depending on the identity of the germylene and the diene. The variations in the rate constants 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 × 106 – 3 × 108 M-1s-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 GePh2 with isoprene, and of the rate constant for decay of the corresponding vinylgermirane, allow most aspects of the potential energy surface to be defined quantitatively.

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