UNSATURATED GERMANIUM AND PHOSPHORUS-COMPOUNDS - REACTIONS OF GERMAPHOSPHENES WITH ALPHA-ETHYLENE ALDEHYDES AND KETONES

Germaphosphenes MeS2Ge=PR (R: 2,4,6-triisopropylphenyl (Is), 1; 2,4,6- tri-tert-butylphenyl (Ar), 2) react with a-ethylene aldehydes and keto nes to give [2 + 2] and [2 + 4] cycloadditions (for aldehydes) and [2 + 4] cycloadditions and 1,2-additions (for ketones). Four- and six-mem bered ring derivatives can be easily differentiated by P-31 NMR chemic al shifts which are, respectively, +36 to +89 ppm and -33 to -77 ppm. 5a', obtained from 2 and crotonaldehyde, has been structurally charact erized by X-ray diffraction: the six-membered ring (germaoxaphosphorin ene) conformation is a sofa form, with large folding along the Ge-C ax is. A NMR study at various temperatures for 3a, 3a', and 5a' displays dynamic phenomenon including phosphorus and ring inversion. The low DE LTAG values (respectively 18.2,13.7, and 13.2 kcal/mol) seem mainly d ue to substitution of phosphorus by the electropositive germanium and particularly to large steric effects; the 2,4,6-tri-tert-butylphenyl g roup lowers the inversion barrier by 5 kcal/mol when compared with the 2,4,6-triisopropylphenyl group.