A rearrangement of 1,1-linked four-membered rings to bicyclic bridgehead five-membered rings: A phosphorus turn

Novel rearrangements of 2,3,2′,3′-tetra-tert-butyl-4,4′-bis-trimethylsilanyloxy-1,2,1′,2′-tetrahydro-[1,1′]biphosphetyl (7) to 1,2,5,6-tetra-tert-butyl-3,7-bis-trimethylsilanyloxy-1H,5H-[1,2]diphospholo-[1,2-a][1,2]diphosphole (9) and of 3,4,3′,4′-tetra-tert-butyl-[1,1′]biphosphetanyl-2,2′-dione (10) to 2,3,6,7-tetra-tert-butyl-tetrahydro-[1,2]diphospholo[1,2-a][1,2]diphosphole-1,5-dione (12) were observed during sublimation at 150 °C in 0.1 mm vacuum. The C–P–C internal angles of the four-membered rings, which reflect steric strain in (7), were 71.9(2)° and 77.9(1)°; the C–P–P internal angles of the five-membered rings of 9 were 90.88(6)° and 93.02(6)° after rearrangement. Reaction of 7 with metallic caesium provided the caesium salt of 3,4-di-tert-butyl-3,4-dihydro-phosphet-2-ol (15), which is the first example of a 2λ-3-phospha-1-enolate being incorporated into four-membered ring. Due to its large size, the caesium cation has a rare coordination mode with both the phosphorus and oxygen atoms of the 2λ-3-phospha-1-enolate, forming infinite polymeric chains in the solid state of 15. Two tert-butyl groups in the vicinal positions of compounds 7, 9, 10, 12 and 15 dictate the “gauche” conformations in compounds 7, 9, 10, 12 and 15, enforcing this stereoselectivity around the C–C–C portion of the rings in the enlargement reaction.