The 1-silyl-1H-phosphirene/1,2-dihydrophosphasilete rearrangement and its preparative significance

Photolysis of the silyl-substituted 1H-phosphirene 3a proceeds selectively with cleavage of a silicon-silicon bond and ring expansion to furnish the 1 ,2-dihydro-1,2-phosphasilete 4a, The corresponding heterocyclic germanium p roduct, 4b, is prepared analogously from 3b, The preparative significance o f 4a is reflected not only in its numerous addition reactions to multiple-b ond systems, such as alkynes 8 and 12 and ketene 10, but also in its substi tution reactions with the chloro compounds 15, 18, and 21, The latter react ions proceed through the formation of chlorotrimethylsilane and the novel 1 ,2-dihydro-1,2-phosphasiletes (16, 19, and 22) which, depending on the subs titution pattern at the phosphorus atom, undergo various isomerization reac tions (16-->17, 19-->20), The hydrolysis of 4a to 23 by mere traces of wate r in carefully purified tetrahydrofuran demonstrates the extreme sensitivit y of this compound towards moisture.