Unusual [3+1] Cycloaddition of a Stable Silylene with a 2,3-Diazabuta-1,3-diene versus [4+1] Cycloaddition toward a Buta-1,3-diene

Cycloaddition reactions of the thermally stable N-heterocyclic silylene LSi: 1 {L = CH[(C=CH2)-CMe][N(Ar)](2)], Ar = 2,6-iPr(2)C(6)H(3)} with acetone azine (1, 1,4,4-tetramethyl-2,3-diazabuta-1,3-diene) and buta-1,3-diene derivatives have been probed. Unexpectedly, acetone azine undergoes a unique [3+1] cycloaddition to give the 1-sila-2,3-diazacyclobutane 2 and its 1-sila-2,3-diazacyclobutane isomer 3. The latter rearranges further to decrease ring strain, affording the corresponding 1-sila-4, 5-diazacyclohex-3-ene 4. In contrast, reaction of I with isoclectronic 2,3-dimethylbuta-1,3-diene furnishes the expected [4+1] cycloaddition product silacyclopentane 5. The new compounds 2-5 were spectroscopically characterized, including single-crystal X-ray analyses of 2, 4, and 5.