Insertion of ((di-iso-propyl)amino)(chloro)phosphenium cation into phenylcyclopropane ring: Stereochemistry, regioselectivity and mechanism

dThe reaction of ((di-iso-propyl)amino) (chloro)phosphenium cation [i-Pr2N- P-Cl](+) (1) with phenyl cyclopropane afforded cis- and trans-1-diisopropyl amino-2-phenyl-phosphetane-1-oxide (3a-cis and 3a-trans), and -1-thion (4), respectively. The cis-1-diisopropylamino-2-(p-substituted-phenyl)-3-methyl -phosphetane-1-oxides (3b-3d) were obtained in excellent yields unexpectedl y from the similar reactions of trans-2-methyl-l-( p-substituted-phenyl)cyc lopropane; The six new compounds have been characterized on the basis of el emental analysis, IR, H-1, P-31 NMR and MS. The configurations of 3a-cis, 3 a-trans and 3c were elucidated by 2D NOESY. Molecular structure of 2-p-meth ylphenyl-3-methyl-phosphetane-1-oxide (3c) has been established by X-ray cr ystallography: in 3c, 1-diisopropylamino group is cis to 2-p-methylphenyl g roup as well as the dihedral angle between the planes C-(1)-P-C-(3) and C-( 1)-C-(2)-C-(3) is 15.09 degrees, which is the least in the found values. A carbocation mechanism including the attack of P+ on less hindered carbon on cyclopropane ring and the cation-pi interaction between the positive P+-N of 1 and benzene ring portion of 2 is proposed. The single-crystal data: pr imitive orthorhombic, space group Pbcn(#61), a = 20.053(2), b = 13.955(2), c = 12.526(4) V = 3505(1) Angstrom(3), Z = 8, D-c = 1.09 g cm(-3), F (000) = 1280.00, lambda(Mo-K alpha)= 0.71069 Angstrom, mu(Mo-K alpha) = 1.50 cm(- 1). Refinement with 182 parameters and 1138 independent reflections with I > 3.00 sigma (I) in the range 2 degrees < theta < 25.0 degrees gave R = 0.0 51 and R-W = 0.055.