Allylation of Phosphorus, Arsenic, and Antimony Trihalides by Allylic Stannanes. Synthesis, Spectroscopic Characterization, and Quantum Chemical Investigations of Allylic Phosphines, Arsines, and Stibines.

Abstract

The reaction between an allylic tributylstannane and a phosphorus, arsenic, or antimony trihalide led to the corresponding allylic phosphine, arsine, or stibine dihalides. With phosphorus derivatives, only the gamma-regioselection was observed, as shown by the formation of (1-methyl-2-propenyl)- (2e,f)or (1,1-dimethyl-2-propenyl)dihalophosphines (2g,h) starting from crotylstannanes 1c,c' or prenylstannane 1d, respectively, and PCl(3) or PBr(3). On heating at 80 degrees C, some of these phosphines led to the corresponding thermodynamic products. Allylic dichloroarsines 3a-d were also prepared and kinetic compounds 3c,d completely rearranged at room temperature into the corresponding crotyl- (3e,e') and prenyldichloroarsines (3f). For antimony derivatives, even at low temperature (-90 degrees C), crude mixtures containing only the thermodynamic products were observed. While allylic dihalophosphines and -arsines are not efficient allylation reagents of electrophiles, allylic dichlorostibines 4a,c reacted with benzaldehyde to lead to the corresponding homoallyl alcohols. Syn and anti products were mainly produced starting from crotyldichlorostibines (4e,e'). The primary allylic phosphines 7a-f, arsines 8a-f, and stibines 9a,bhave been prepared by the chemoselective reduction of the corresponding allylic dihalophosphines, -arsines, or -stibines with LAH in tetraglyme or with Bu(3)SnH as reducing agent and characterized by (1)H and (13)C NMR spectroscopy and mass spectrometry. The primary allylic arsines and stibines are the first elements of new classes of compounds. Several allylic phosphines and arsines were investigated by ab initio quantum chemical methods and photoelectron spectroscopy. The most stable structure of these compounds is when the C-E (E = P, As) bond is out of the plane of the allyl system. This conformation is stabilized by the hyperconjugation between the pi-orbital and the C-E sigma-bond. Due to this geometrical arrangement, the phosphorus (arsenic) lone pair interacts strongly and unprecedently with the pi-system. The strength of this interaction is due to the close proximity of the pi and the n(P) (n(As)) levels.