Multifunctional 2H-1,2-Azaphosphole Complexes: A Good Starting Point for the Synthesis of Functional 1,2-Azaphospholide Complexes?

Abstract

The thermolysis of P-functionalized 7-phosphanorbornadiene tungsten complexes 3a-d in o-xylene at 120 °C in the presence of various carbonitriles and dimethyl acetylenedicarboxylate (DMAD) has been investigated. The reaction of complex 3a, 1-piperidino carbonitrile, and DMAD yielded a product mixture, whereby 2H-1,2-azaphosphole complex 6 was formed in ca. 40-45% yield together with diazaphosphole complexes 7 (ca. 10-15%) and 8 (ca. 10-15%) and other nonidentified products (ca. 30%). Unfortunately, none of these products could be separated by column chromatography. In marked contrast, reactions of 7-phosphanorbornadiene tungsten complexes 3a-d with triphenylphosphane-imino carbonitrile and DMAD led to P-functionalized 2H-1,2-azaphosphole complexes 11a-d in good to excellent yields. Preliminary investigations of decomplexation reactions revealed a remarkable thermal stability of lla-d: no reaction occurred with bis(diphenylphosphino)ethane (DPPE) at 140 °C. Preliminary studies of exocyclic P-C bond cleavage reactions of 2H-1,2-azaphosphole complexes lla,b using potassium tert-butanolate (KOtBu) showed instantaneous reactions and color change from yellow to dark red; an intermediate was detected by 3 1 P NMR spectroscopy. Treatment with an excess of methyl iodide led to one major product, which, most probably, is the N-methyl-substituted 1H-1,2-azaphosphole complex 13; unfortunately, chromatography failed and it could only be observed by 3 1 P NMR spectroscopy. A comparative study on P-C bond cleavage/methylation reactions using monofunctional 2H-1,3,2-diazaphosphole complex 14 and the system KOtBu/methyl iodide showed the selective formation of the P-methyl-substituted 2H-1,3,2-diazaphosphole complex 16; complexes 14 and 16 were prepared using the Cu(I)Cl route. Complexes lla-d, 14, and 16 were characterized by elemental analyses, NMR, UV/vis, IR, and mass spectroscopy and, in addition, by single-crystal X-ray diffraction in the case of lla-d. Additionally, we investigated selected isomers of lithium and potassium 1,2-azaphospholides by ab initio calculations using HF and MP2/6-311++G(2d,p) level of theory; stability, structure, and aromaticity aspects are discussed.