6205
Barriers to Pyramidal Inversion at Phosphorus in Phospholes, Phosphindoles, and DibenzophospholeslV2 William Egan,3a Reginald Tang, Gerald Z O ~and, Kurt ~ ~ Mislow” Contribution from the Department of Chemistry, Princeton University, Princeton, New Jersey 08540. Received February 9, 1971 Abstract: Pyramidal inversion barriers (AG*)of variously substituted phospholes, a phosphindole, and a dibenzo-
phosphole have been measured and compared with the inversion barriers of model compounds. It has been found that alkyl or aryl substituents do not significantly influence the unusually low barrier to pyramidal inversion (ca. 15-16 kcal/mol) at phosphorus in the phosphole system, but that mono- and diannulation (with a benzene ring) of the phosphole ring results in a significant increase in barrier height (ca. 8 and 10 kcal/mol, respectively), relative to the parent system. It is suggested that the low barrier to pyramidal inversion in phospholes is largely determined by stabilization which manifests itself to a greater extent in the planar transition state than in the pyramidal ground state, and which is ascribable to cyclic ( 3 p - 2 ~ )delocalization ~ (“aromaticity”). The increase in barrier height which occurs upon annulation of the phosphole ring is attributed to a virtual disruption of the phosphole “aromaticity.”
T
of phosphole aromaticity had not explicitly considered he planarity of pyrrole in the ground state4 is atthese questions concerning the geometry (planar us. tributable to maximal delocalization of the 6 1 elecpyramidal) about phosphorus. trons (“aromaticity”) in the planar form, where the Nuclear magnetic resonance spectroscopy provides orbital axis of the lone electron pair on nitrogen is a convenient experimental technique for the investigaparallel to the orbital axes of the 2 p electrons ~ on cartion of these questions. First, the ground-state conforbon. In conformations of pyrrole containing a pyramimation of phospholes may be probed by incorporation dal nitrogen atom, p orbital overlap and T delocalizaof groups containing enantiotopic nuclei, which betion are reduced, the extent of reduction being related come diastereotopic in the chiral molecular environto the “degree of orthogonality” of the interacting orbiment that would be associated with appropriately subt a l ~ . These ~ considerations, together with the greater stituted pyramidal phosphorus. Second, if phosphole pyramidal stability of phosphines, relative to the correis nonplanar, dynamic nuclear magnetic resonance sponding amines: provided the impetus for the present (dnmr) spectroscopy may be used to measure the instudy. The questions to which we addressed ourselves version barrier, if the activation energies are in the were: first, will the increase in aromatic stabilization range of ca. 5-25 kcal/mol. To this end, variously associated with planar phosphole be sufficient to oversubstituted phospholes were prepared and analyzed come the energy required to flatten the phosphorus by pmr. The syntheses of these compounds and model pyramid, and thus lead to a planar ground-state consystems, the results of the investigation, and the disformation? Second, if the ground-state conformation cussion of the results are given in the following sections. of phosphole is not planar, i.e., pyramidal, and the planar conformation thus becomes the transition state for inversion at phosphorus, to what extent will the inSyntheses creased aromaticity of the transition state make itself The phosphorus heterocycles used in the present felt in a lowering of the inversion barrier? Surprisstudy were prepared by direct extensions of reported ingly, with a single exception,’ previous d i s c u s s i ~ n s * ~ ~ methods. Phosphole ring systems with I-phenyl substituents (1-3) were conveniently synthesized by the (1) This work was supported by the Air Force Office of Scientific cyclization reaction reported by Markl and P o t t h a ~ t , ~ ~ Research (AF-AFOSR-1188-B), by the National Science Foundation (GP-22542), and by Hoffmann-La Roche, Nutley, N. J. wherein substituted butadiynes and phenylphosphine (2) For a preliminary account of a portion of this work, see W. Egan, combine according to eq 1. The synthesis of l-benzylR. Tang, G. Zon, and I
