1,2-Azaphosphol-5-ene Complexes through Metal-Assisted Synthesis

Thermal ring opening of {[2-bis(trimethylsilyl)methyl-3-phenyl-2H-azaphosphirene-κP]pentacarbonyltungsten(0)} (1) in toluene in the presence of 1-piperidino carbonitrile (2) and various mono- and
disubstituted alkenes, i.e., ethyl acrylate (4), E-stilbene (6), fumaronitrile (7), and maleic anhydride (10),
was investigated. Special emphasis was placed on the dependence of the regio- and stereoselectivity of
the transiently formed nitrilium phosphane-ylide complex 3 on the electronic properties of the alkenes.
Three-component reaction of 1, 2, and 4 yielded regioselectively a 1:1 mixture of the diastereomeric
4-ethoxycarbonyl-substituted 1,2-azaphosphol-5-ene complexes 5a,b; the constitution of 5a was unambiguously established by single-crystal X-ray diffraction. Whereas E-stilbene (6) gave no reaction with complex
3, fumaronitrile (7) reacted with transient 3 to furnish the diastereomeric 1,2-azaphosphol-5-ene complexes
8a,b together with the 2H-1,3,2-diazaphosphole complex 9; the product ratio of 2:1:1 was determined by
31P{1H} NMR spectroscopy. Reaction of 1, 2, and maleic anhydride (10) resulted in a complicated product
mixture, whereby the constitution of the spirocyclic complexes 11a,b (bearing a 1,3,2-oxazaphosphol-3-ene moiety) is based on their NMR data. Further examination showed that the C−O double bond
system of 10 did not participate in a [3+2] cycloaddition reaction if the thermal decomposition of complex
1 was carried out in neat benzonitrile, thus showing the preference of the transiently formed C-phenyl-substituted nitrilium phosphane-ylide complex 12 for the C−C double bond of 6:  In this case, the bicyclic
complexes 13a,b were formed exclusively.