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 11a−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 11a,b using potassium tert-butanolate (KOtBu) showed instantaneous reactions and color change from yellow to dark red; an intermediate was detected by 31P 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 31P 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 11a−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 11a−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.

The synthesis of 1,2-azaphospholes, 1,2-azaphosphorines and 1,2-azaphosphepines

“…When another carbonitrile derivative such as Ph 3 P=NCN was used, the thermal decomposition of complex 86 in xylene furnished the 2H-1,2-azaphosphole complex 92 through [3+2] cycloaddition reaction of nitrilium phosphane ylide complexes 91 formed with dimethyl acetylenedicarboxylate (Scheme 26). [59][60][61] 2.14 Reaction of l,3,2-diazaphosphole-4,5-dicarbonitriles with alkynes The regioselective cyclization of l,3,2-diazaphosphole-4,5-dicarbonitriles 93 with symmetrical and non-symmetrical alkynes in chloroform at room temperature yielded the l,2-azaphosphole-5-carbonitriles 94 (Scheme 27). In some cases of non-symmetrical alkynes, the other isomers 95 were also formed (Scheme 27).…”

Phosphines and Related Tervalent Phosphorus Systems

“…783 Thermolysis of (278, R¼CH(SiMe 3 ) 2 ) in o-xylene results in the formation of the 2,3-dihydro-1,2,3-azadiphosphete complex (259) and other products, via the intermediate formation of a phosphinidene. 784 The generation of phosphinidene intermediates by thermolysis of 2H-azaphosphirene complexes (or 7-phosphanorbornadiene complexes) and subsequent reactions with alkynes and other reagents has provided routes to a variety of heterocyclic compounds, including 2H-1,2-azaphospholes, 785 The chemistry of phospholes and related phospholide anion complexes remains a very active area, which also continues to attract the attentions of the theoretical community. Among recent theoretical contributions are a consideration of the stability, structure and bonding in lithium-and beryllium-pentaphospholide systems, 789 the aromaticity of the pentaphospholide anion (and its arsenic analogue) as probed by ring currents, 790 the remarkable influence of fluorine-substitution (either at phosphorus or at a ring carbon) on the electronic and thermochemical properties of phospholes, 791 and the effects of methyl and vinyl substitution at various positions on the geometries, relative stabilities and Diels-Alder reactivities of phospholes.…”