Synthesis of 2<i>H</i>-1,2-Azaphosphole Complexes by [3 + 2] Cycloaddition of Nitrilium Phosphane−Ylide Complexes with Various Alkynes:  Studies of the C-Substituent and Metal Effects on the Reaction Course

Abstract

Thermal ring opening of [2-(bis(trimethylsilyl)methyl)-3-phenyl-2 H -azaphosphirene-κ P ]pentacarbonylchromium(0), -molybdenum(0), or -tungsten(0) ( 1a − c ) in the presence of three different alkynes, phenylacetylene, ethyl acetylenecarboxylate (EAC), and dimethyl acetylenedicarboxylate (DMAD) ( i − iii ), was investigated, using toluene ( a ) and benzonitrile ( b ) as solvents, whereby special emphasis was to determine the dependence of the [2 + 1]/[3 + 2] cycloaddition product ratio and the regioselectivity on the electronic properties of the acetylenes and the transiently formed nitrilium phosphane−ylide complexes. It is shown that the stability of the latter clearly depends on the donor abilities of the C-substituent of the C,N,P 1,3-dipole system. In toluene 1 H -phosphirene complexes 11a − c are obtained exclusively ( ia ), whereas when EAC ( iia ) and DMAD ( iiia ) were employed as trapping reagents, the metal-dependent formation of either a mixture of 1 H -phosphirene and 2 H -1,2-azaphosphole complexes (M = Cr, W; iia, 12a, c and 13a, c; iiia, 4a,c and 5c ) or a mixture of 1 H -phosphirene and a diphosphene complex was observed (M = Mo; iia, 12b and 14; iiia, 4b and 14 ). Reaction iia yielded 13a, c regioselectively. Exclusively in the case of DMAD ( iiia, b ), but for all 2 H -azaphosphirene complexes 1a − c, the further unidentified byproduct 15 was detected spectroscopically. In benzonitrile the reactions of complexes 1a − c led generally to decreased yields of 1 H -phosphirene complexes 11a − c ( ib ) but not to 2 H -1,2-azaphosphole complex formation in the case of ib . In the case of the reactions iib and iiib, significantly changed 1 H -phosphirene/2 H -1,2-azaphosphole complex ratios are observed in favor of the latter (complexes 13a, c and 5c ). Although the regioisomeric complexes 13a, c were formed predominantly, evidence was obtained spectroscopically, at least, for the other regioisomeric tungsten complex 18a . The dependence of the 1 H -phosphirene/2 H -1,2-azaphosphole complex ratios on the arylnitrile concentration and the electronic influence of the para aryl substituent was demonstrated by an 31 P NMR spectroscopic study ( iv ) for the 2 H -azaphosphirene complexes 1c and 16a, b . Further three-component reactions with 2 H -azaphosphirene complexes, different nitriles, and DMAD ( v ), EAC ( vi − viii ), and phenylacetylene ( ix ) are reported. Thus, thermolysis of complex 1c in acetonitrile or tert -butyl cyanide and with DMAD led to 5-alkyl-substituted 2 H -1,2-azaphosphole complexes 17c, d ( v ), and with acetonitrile and EAC the 2 H -1,2-azaphosphole complex 13d was obtained ( vi ). Thermolysis of 2 H -azaphosphirene complexes 1a − c in toluene with EAC as trapping reagent and dimethyl cyanamide ( vii ) or 1-piperidinonitrile ( viii ) selectively furnished 2 H -1,2-azaphosphole complexes 13e − h and 18b − e, the former being the preferred regioisomers. Using the 2 H -azaphosphirene complex 1c, dimethyl cyanamide or 1-piperidinonitrile, and phenylacetylene ( ix ), the last as solvent and trapping reagent, gave complicated product mixtures. These consist each of three different types of main products, the 2 H -1,2-azaphosphole complexes 21a, b and the two acyclic, isomeric complexes 22a, b and 23a, b, resulting from opposite 1,3-additions of the C−H function of phenylacetylene to the 1,3-dipole system of the intermediately formed C -dialkylamino-substituted nitrilium phosphane−ylide complexes 10 and 19c; reaction ix shows that stability and reactivity significantly depend on the C-substituent of the C,N,P 1,3-dipole system. The structures of the 2 H -1,2-azaphosphole complex 21a and of the [bis(trimethylsilyl)methyl](trimethylsiloxy)phosphane complex 24c were determined by single-crystal X-ray diffraction.