An Ab initio Study of the P-C Bond Rotation in Phosphoryl- and Thiophosphoryl-Stabilized Carbanions: Five- and Six-Membered Heterocycles Michael Kranz, Scott E. Denmark,* Kevin A. Swiss, and Scott R. Wilson Roger Adams Laboratory, University of Illinois, Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801 Received February 9, 1996 (Revised Manuscript Received September 18, 1996 X ) The potential energy surfaces for the P-C bond rotation in the 2-oxo- and 2-thioxo-2-methyl-1,3,2- diazaphosphorinane and -1,3,2-diazaphospholidine anions have been investigated at MP4(SDQ)/ 6-31+G*//HF/6-31+G* + ZPE. Four stationary points have been found for the six-membered ring species. The lowest energy structures exhibit a completely or nearly planar carbanion with its substituents parallel to the PdX axis (X ) O, S). The transition state (TS) structures have a strongly pyramidalized carbanion in which the lone pair (LP) is approximately perpendicular to the PdX bond. Isodesmic equations, bond length comparisons, and orbital interactions indicate a superior ground state (GS) stabilization of the thioxo derivative and a favorable TS stabilization of the oxo species. Both effects cooperate to furnish the computationally and experimentally observed higher (ca. 2.5 kcal/mol in both cases) P-C rotational barrier for the 2-thioxo-1,3,2-diazaphosphorinane based anions. Coordination of a lithium cation to the chalcogen atom yields a distinct preference for the axial/equatorial orientation of the nitrogen substituents in the oxo species and for the diequatorial arrangement in the thioxo analog, in perfect agreement with X-ray crystallographic data. The X-ray crystal structure of lithio 2-(1-methylethyl)-1,3-dimethyl-1,3,2-diazaphosphorinane 2-sulfide‚3THF is reported and it is consistent with existing theoretical and experimental geometries. The five-membered ring analogs (1,3,2-diazaphospholidines) exhibit the same conformational preference for the carbanion in the GS and the TS. The activation barrier for P-C bond rotation is higher in the thioxo derivatives as well. Whereas only one nitrogen substituent changes its orientation in the diazaphosphorinanes during the P-C rotational coordinate, the ring backbone responds strongly in the diazaphospholidines. Introduction As part of a broadly-based program on the synthetic applications of asymmetrically modified phosphorus(V)- stabilized carbanions, 1 we have undertaken a thorough examination of the structures of these species. One of the more intriguing dimensions of this program has been the comparison of phosphoryl (PdO)- versus thiophos- phoryl (PdS)-type anion-stabilizing groups. To compli- ment ongoing studies and provide a foundation for the fundamental understanding of the relationship between structure and reactivity of phosphorus(V)-stabilized car- banions, we have initiated a comparison of phosphoryl- and thiophosphoryl-stabilized anions by computational methods. The basic features of bonding, hybridization, and rotational barriers in simple, acyclic phosphorus(V) derivatives have been detailed in an earlier account. 2 Herein we describe an expanded computational examina- tion of the synthetically more significant cyclic phosphon- amides and thiophosphonamides in five- and six-mem- bered rings. Background Preparative. In the early stages of our search for a general and highly selective chiral auxiliary for phos- phorus(V) anionic reagents, we noted a strong depen- dence of reaction selectivity on many structural variables such as heterocycle ring size, PdO versus PdS stabiliza- tion, nature of the nitrogen substituent, nature of the carbanion, and the electrophile. A selection of these results, which highlights the importance of ring size and anion-stabilizing group (constant N-substituent, and alkylating agent), is compiled in Figure 1. While the change in diastereoselectivity between phospholidine i and phosphorinane ii is noteworthy, a much greater effect is observed with the rigid phospholidine iii. 3 Most striking, however, is the enhancement in diastereoselec- tivity of benzylation in iii and iv 4 when changing from X Abstract published in Advance ACS Abstracts, November 1, 1996. (1) Denmark, S. E.; Chen, C.-T.; Reed, R. A. Adv. Carbanion Chem. Manuscript in preparation. (2) Kranz, M.; Denmark, S. E. J. Org. Chem. 1995, 60, 5867. (3) (a) Denmark, S. E.; Kim, J.-H. J. Org. Chem. 1995, 60, 7535. (b) Denmark, S. E.; Kim, J.-H.; Pansare, S. V. Manuscript in prepara- tion. (c) Brice, L. J. Unpublished results from these laboratories. (4) Denmark, S. E.; Chen, C.-T. J. Am. Chem. Soc. 1995, 117, 11879. Figure 1. Comparison of alkylation selectivities for various types of auxiliaries. 8551 J. Org. Chem. 1996, 61, 8551-8563 S0022-3263(96)00278-2 CCC: $12.00 © 1996 American Chemical Society