Chemistry of Heavy Carbene Analogues R2M (M = Si, Ge, Sn). 8. Germylenes: Singlets or Triplets? Cheletropic Cycloadditions of Me2Ge and GeI2 to Conjugated Dienes

Article abstract of DOI:10.1021/ja00342a043

Thermally generated germylenes Me2Ge undergo in solution under mild conditions (70-150 deg C) concerted 1,4-additions of the linear <2+4> cheletropic type to certain 1,3-dienes.Thus, the highly reactive meso diallene 10a gives, in a about 1:1 ratio, only the two isomeric 1-germacyclopentenes 11a and 11b expected from front- and back-side attack of Me2Ge, whereas the D,L-diallene 10b produces only the third isomer, 11c, in accordance with this mechanism.The yields are about 70percent; the isomer purity is >/= 98percent.Likewise, two other E,E-1,4-disubstituted 1,3-butadienes give only the corresponding cis-2,5-disubstituted 1-germacyclopentenes 7a or 9a in 80percent or 30percent yields with an isomer purity of >/= 98percent (limit of the NMR analysis).Similarly, with GeI2 and 10a, only 14a and 14b are found, and 14c with 10b, as expected for a cheletropic reaction.Methylation of 14a-c yields the corresponding compounds 11a-c.Other mechanisms for these 1,4-additions are discussed but are highly unlikely: thermal germylenes Me2Ge and Ge2I behave as singlets.