SYNTHESIS OF 2-STANNOLENES AND 3-STANNOLENES VIA ADDITION OF TRIMETHYLTIN ALKOXIDES TO 3-DIETHYLBORYL-4-ETHYL-1,1-DIMETHYLSTANNOLE

Trimethyltin alkoxides (2) react stereoselectively with 3-diethylboryl -4-ethyl-1,1-dimethylstannole (1) via addition of the Me(3)Sn group to C(2) to the C(2)=C(3) bond and a 1,2 shift of an ethyl group from bor on to C(3) to give the 2-stannolenes 3. The molecular structure of 3f' [R = (S)-2-Bu] was determined by single-crystal X-ray analysis, confi rming the cis positions of the Et(RO)B and the Me(3)Sn group. These 2- stannolenes 3 undergo, upon heating to ca. 80 degrees C, facile rearra ngement by irreversible allylic migration of the Et(RO)B group to the 3-stannolenes 4 in which the cis positions of the boryl and the stanny l group are retained. All 2-stannolenes (in contrast to the 3-stannole nes) are readily deprotoborylated to give the 3-stannolene 5. The stru ctures of 3, 4, and 5 follow conclusively from H-1-, B-11-, C-13-, and Sn-119-NMR spectra. The negative sign of the geminal coupling constan ts (2)J(SnSn) was determined in the case of 3, 4, and 5 by 2D Sn-119/H -1 heteronuclear shift correlations.