Coordination Behavior of Sterically Protected Phosphaalkenes on the AuCl Moiety Leading to Catalytic 1,6-Enyne Cycloisomerization

Mes*-substituted 2,3-dimethyl-1,4-diphosphabuta-1,3-diene, 1,2-diphenyl-3,4-diphosphinidenecyclobutene, 2,2-bis(methylsulfanyl)-1-phosphaethene, and 3,3-diphenyl-1,3-diphosphapropenes (Mes* = 2,4,6-tri-tert-butylphenyl) were employed as P ligands of gold(I) complexes. The (E,E)-2,3-dimethyl-1,4-diphosphabuta-1,3-diene functioned as a P2 ligand for digold(I) complex formation with or without intramolecular Au-Au contact, which depends on the conformation of the 1,3-diphosphabuta-1,3-diene. The 1,2-diphenyl-3,4-diphosphinidenecyclobutene, which has a rigid s-cis P=C-C=P skeleton, afforded the corresponding digold(I) complexes with a slight distortion of the planar diphosphinidenecyclobutene framework and intramolecular Au-Au contact. In the case of the 2,2-bis(methylsulfanyl)-1-phosphaethene, only the phosphorus atom coordinated to gold, and the sulfur atom showed almost no intra- or intermolecular coordination to gold. On the other hand, the 1,3-diphosphapropenes behaved as nonequivalent P2 ligands to afford the corresponding mono- and digold(I) complexes. Some phosphaalkene-gold(I) complexes showed catalytic activity for 1,6-enyne cycloisomerization without cocatalysts such as silver hexafluoroantimonate.

Synthesis, Structure, and Reactivity of Novel 3,4-Diphosphacyclobutenes Bearing Silyl Groups

[reaction: see text] Copper-mediated homocoupling of sterically hindered 2-(2,4,6-tri-tert-butylphenyl)-1-trialkylsilyl-2-phosphaethenyllithiums afforded 1,2-bis(trialkylsilyl)-3,4-diphosphacyclobutenes (1,2-dihydrodiphosphetenes) through a formal electrocyclic [2+2] cyclization in the P=C-C=P skeleton as well as 2-trimethylsilyl-1,4-diphosphabuta-1,3-diene. Reduction of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes followed by quenching with electrophiles afforded ring-opened products, (E)-1,2-bis(phosphino)-1,2-bis(trimethylsilyl)ethene and (Z)-2,3-bis(trimethylsilyl)-1,4-diphosphabut-1-ene. The structures of the ring-opened products indicated E/Z isomerization around the C=C bond after P-P bond cleavage of 5, and the isomerization of the P-C=C skeleton. Ring opening of 1,2-bis(trimethylsilyl)-3,4-diphosphacyclobutenes affording (E,E)- and (Z,Z)-1,4-diphosphabuta-1,3-dienes was observed upon desilylation.

Experimental and theoretical studies on the conjugation of the phosphorus-carbon double bond with a cyclopropyl group

X-ray structural analysis for (Z)-2-cyclopropyl-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaethene (2) was performed to confirm that the cyclopropyl group largely interacts with the P[double bond]C group compared with its carbon analogue, vinylcyclopropane (1). Absorption spectrum and redox properties of 2 were also studied to prove the conjugation. Theoretical investigation for nonsubstituted derivatives (4) indicated conjugative interaction between the P[double bond]C and cyclopropyl groups and revealed the physicochemical similarities between the P=C and C[double bond]C.