Synthesis of 2H-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-2H-azaphosphirene-κP]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 1H-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 1H-phosphirene and 2H-1,2-azaphosphole complexes (M = Cr, W; iia, 12a,c and 13a,c; iiia, 4a,c and 5c) or a mixture of 1H-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 2H-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 1H-phosphirene complexes 11a−c (ib) but not to 2H-1,2-azaphosphole complex formation in the case of ib. In the case of the reactions iib and iiib, significantly changed 1H-phosphirene/2H-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 1H-phosphirene/2H-1,2-azaphosphole complex ratios on the arylnitrile concentration and the electronic influence of the para aryl substituent was demonstrated by an 31P NMR spectroscopic study (iv) for the 2H-azaphosphirene complexes 1c and 16a,b. Further three-component reactions with 2H-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 2H-1,2-azaphosphole complexes 17c,d (v), and with acetonitrile and EAC the 2H-1,2-azaphosphole complex 13d was obtained (vi). Thermolysis of 2H-azaphosphirene complexes 1a−c in toluene with EAC as trapping reagent and dimethyl cyanamide (vii) or 1-piperidinonitrile (viii) selectively furnished 2H-1,2-azaphosphole complexes 13e−h and 18b−e, the former being the preferred regioisomers. Using the 2H-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 2H-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 2H-1,2-azaphosphole complex 21a and of the [bis(trimethylsilyl)methyl](trimethylsiloxy)phosphane complex 24c were determined by single-crystal X-ray diffraction.