Incomplete Electrocyclization of a Sterically Hindered 1,4-Diphosphabutadiene

Butadiene is the simplest neutral acyclic closed-shell π-conjugated system and is typically sufficiently stable enough to avoid electrocyclization to cyclobutene. In contrast, most congeners of butadiene containing heavier elements are easily converted into the corresponding 4-membered cyclobutene system. Herein, we demonstrate that the gauche 1,4-diphosphabutadiene (P=C-C=P) skeleton in a sterically encumbered 2,3-bis(phosphanylidene)-1,4-disilinane can be remarkably perturbed due to "incomplete electrocyclization" where P=C-C=P partially form the corresponding 1,2-dihydrodiphosphete (3,4-diphosphacyclobutene) by [2+2] electrocyclization. ³¹ P NMR data obtained in solution indicated that the coexistence of a closed ring substantially reduces the open-ring characteristics of the P=C-C=P moiety. However, the ³¹ P CP-MAS spectrum of 2,3-bis(phosphanylidene)-1,4-disilinane showed that the P=C-C=P structure is predominant in the solid-state. Single-crystal X-ray analysis revealed that decreasing the temperature promoted the generation of small amounts of incomplete 1,2-dihydrodiphosphete system in the crystalline state. Furthermore, the 1,2-dihydrodiphosphete units disappeared upon warming the single crystal, and this unique solid-state electrocyclization reaction was reversible.

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.

Formation of Phosphaethyne Dimers: A Mechanistic Study

High-level ab initio (CCSD(T), CBS-QB3 and CASSCF, CASPT2, MR-ACPF, MR-ACPF-2) and density functional theory (B3LYP) calculations were carried out to study the dimerization of phosphaacetylene or phosphaethyne (HCP). Seventeen low energy closed-shell and five open-shell phosphaacetylene dimers were found on the potential energy surface. Two head-to-head, one head-to-tail and three other dimerization reaction pathways were determined, all with high activation barriers, suggesting that closed-shell minima are usually kinetically stable. An open-shell head-to-head reaction pathway has also been found with moderate initial barrier (95.0 kJ mol(-1)) leading to 1,2- and 1,3-diphosphacyclobutadiene, suggesting that polymerization of HCP and oligomerization of its derivatives have open-shell mechanism. Formation of 1,2-diphosphacyclobutadiene is both thermodynamically and kinetically favored over 1,3-diphosphacyclobutadiene. A head-to-head reaction involving LiBr as a catalyst was also studied. It has been pointed out that LiBr catalyze the closed-shell mechanism. All the four possible reaction channels of this reaction yield 1,4-diphosphatriafulvene with a fairly low activation Gibbs-free energy (44.8 kJ mol(-1)), suggesting that this compound could be synthesized. This finding fully supports the experimental results.

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.