Di-tert-butyl(N-pyrrolyl)phosphane,-sulfide, and-selenide, studied by multinuclear magnetic resonance

 Di-tert-butyl(N-pyrrolyl)phosphane (1),-sulfide (2) and-selenide (3) were studied by ¹H, ¹³C, ¹⁵N, ³¹P and ⁷⁷Se NMR spectroscopy in solution. Restricted rotation about the P–N bond was observed for 1 (ΔG^≠ _C≈58.5 kJ/mol) and 2, 3 (ΔG^≠ _C≈45 kJ/mol) by ¹H and ¹³C NMR at variable temperature. The fairly high pyrrolyl rotation barrier in 1 is ascribed to the expected steric effects exerted by the tert-butyl groups and to repulsion between the lone pairs of electrons at the phosphorus and nitrogen atoms. These repulsive forces are reduced in 2 or 3, or in di-tert-butyl(phenyl) phosphane (ΔG^≠ _C≈44 kJ/mol). Signs of coupling constants ⁿJ(³¹P, ¹³C) and ⁿ⁺¹J(³¹P, ¹H) (n=2, 3) were determined by two-dimensional (2D) ¹³C/¹H heteronuclear shift correlations. The preferred orientation of the pyrrolyl group in 1 is evident from the coupling constants ²J(³¹P, ¹³C(2))=+35.4 Hz and ²J(³¹P, ¹³C(5))=−9.3 Hz, typical of C(2) in syn and C(5) in anti position with respect to the assumed axis of the phosphorus lone pair. Hahn-echo extended (HEED) polarization transfer pulse sequences served for measuring ¹J(³¹P, ¹⁵N) and isotope induced chemical shifts ¹Δ^15/14N(³¹P) from ³¹P NMR spectra. Received: 4 April 1996/Revised: 6 May 1996/Accepted: 11 May 1996