173166-52-4

Aromatic hydrogenation is a challenging transformation typically requiring alkali or transition metal reagents and/or harsh conditions to facilitate the process. In sharp contrast, the aromatic heterocycle 2,4,6-tri-tert -butyl-1,3,5-triphosphabenzene is shown to be reduced under 4 atm of H2 to give [3.1.0]bicylo reduction products, with the structure of the major isomer being confirmed by X-ray crystallography. NMR studies show this reaction proceeds via a reversible 1,4-H2 addition to generate an intermediate species, which undergoes an irreversible suprafacial hydride shift concurrent with P - P bond formation to give the isolated products. Further, para-hydrogen experiments confirmed the addition of H2 to triphosphabenzene is a bimolecular process. Density functional theory (DFT) calculations show that facile distortion of the planar triphosphabenzene toward a boat-conformation provides a suprafacial combination of vacant acceptor and donor orbitals that permits this direct and uncatalyzed reduction of the aromatic molecule.

Cycloaddition reactions of the kinetically stabilized phosphaalkynes 1 with the imidovanadium(v) trihalides 9 furnish the 1,2,4-azaphosphavanada(v)cyclobutenes 10. The stability of these novel metallacyclic compounds depends solely on the substitutents of the imido unit. Thus, the imidovanadium(v) species 9 with tertiary alkyl groups on the N atom form stable addition products with 1 while in the cases of compounds 9 with a lower degree of substitution at N (primary and secondary alkyl groups) the primarily formed adducts 10 undergo irreversible decomposition to afford the 1H-1,2,4-azadiphospholes 13. Reactions of an excess of the phosphaaalkyne 1 with the vanadium complexes 9 furnish the corresponding triphosphabenzenes 8 in good yields (36-68%). A catalytic reaction course has been demonstrated for the all-tert-butyl system 1a/9a in which the metallacyclic species 10a serves as the catalytically active species. Poisoning of the catalyst leads to a second reaction pathway, which results in formation of the azatetraphosphaquadricyclanes 16. By means of the stepwise use of different phosphaalkynes 1a,b this methodology provides the first access to the differently substituted triphosphabenzenes through cyclotrimerization.

1,3-Diphosphacyclobutadienes 4 are almost certainly intermediates in the conversion by hexachloroethane of phosphaalkyne dimer complexes 3 to the tetraphosphacubanes 5. We now describe trapping reactions of 4, generated in the same way, wilh phosphaalkyne 1 (→9), ynamines 10 (→13), and electron-poor alkynes 11 (→14). The cyclooligomerization of 1 initiated by t-Bu-N=VCl3·DME (15) leads to the azatetraphosphaquadricyclanes 20 while the reaction with the stronger Lewis acid t-Bu-N=VCl3 (21) furnishes the 1,3,5-triphosphabenzenes 23 in high selectivity.

A small variation in the vanadium reagent leads to a completely different product: The cyclooligomerization of phosphaalkynes 2 with tBuN = VCl3 · DME (DME = 1,2-dimethoxyethane) proceeds with incorporation of the imido fragment to give the azatetraphosphaquadricyclanes 1. In contrast, with the corresponding vanadium compound containing no Lewis base this fragment is not incorporated, and the 1,3,5-triphosphabenzenes 3 are obtained.

For the first time selective cyclodi-, tri-, and tetramerization of phosphaalkynes are induced by an unique class of transition metal complexes by changing the reaction conditions.

Keywords: Cyclooligomerisierungen, Hafniumverbindungen, Heterocyclen, Phosphaalkine, Phosphoverbindungen