Irradiation of 1-aryl-4-(pentamethyldisilanyl)-1,3-butadiynes (1a-1c) with acetone yields site specific and regioselective 1:1 adducts (2-6) via silacyclopropene intermediates. The silacyclopropene intermediates are formed from singlet excited states in 1-phenyl-4-(pentamethyldisilanyl)-1,3 -butadiyne(1a) and 1-(p-methoxyphenyl)-4-(pentamethyldisilanyl)-1,3 -butadiyne (1b), but from the triplet excited state in 1-(p-nitrophenyl)-4 -(pentamethyldisilanyl)-1,3-butadiyne (1c). The triplet excited state of the silacyclopropene reacts with acetone to give acetone addition photoproducts. Photoreaction of 1-aryl-4-(pentamethyldisilanyl)-1,3 -butadiynes with dimethyl fumarate gives two-atom insertion products (7 and 11) via silacyclopropene and/or [2+2] photocycloaddition products (8, 9, 10, and 12). In this reaction, substituents on the arylring affect the reaction pathway which can be explained by the stability of silacyclopropene intermediates and the nucleophilic character of the radical-like triplet state of 1-aryl-4-(pentamethyldisilanyl)-1,3 -butadiynes.
$PdCl_2(PPh_3)_2$-catalyzed photolysis of 1-phenyl-4- (pentamethyldisilanyl)-1,3-butadiyne (1a) in dry benzene gives 1,4 -disilacyclohexa-2,5-diene type dimerization products(14-17) via silacyclopropene. The silacyclopropene formed from the singlet excited state of 1a reacts with $(PPh_3)_2Pd^0$ to form palladasilacyclobutene. In this reaction, the silylene-palladium complex is generated and reacts with 1a to give another silacyclopropene. $PdCl_2(PPh_3)_2$-catalyzed photolysis of 1a with other alkynes supports the involvement of this silylene complex.
Irradiation of 1,4-bis(pentamethyldisilanyl)butadiyne(1d) in methanol gives the 1:1 photoaddition products (20 and 21) and 1:2 photoadduct (22) through silacyclopropene intermediates formed from the singlet excited states of 1d. The 1:2 photoadduct is a secondary photoproduct of the 1:1 photoadduct. Irradiation of 1d with acetaldehyde and acetone in deaerated methylene chloride yields only 1:1 photoadduct (23-25). It is probably due to the steric hindrance of the 1-oxa-2-silacyclopent-3-ene ring which prevents the formation of silacyclopropene from 1:1 photoadducts (23-25).
