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, Scott R. Wilson

Research output: Contribution to journalArticle

Abstract

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 P=X 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 P=X 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.

Original languageEnglish (US)
Pages (from-to)8551-8563
Number of pages13
JournalJournal of Organic Chemistry
Volume61
Issue number24
DOIs
StatePublished - Nov 29 1996

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Ground state
Anions
Chalcogens
Nitrogen
Stabilization
Derivatives
X rays
Potential energy surfaces
Bond length
Lithium
Cations
Crystal structure
Chemical activation
Atoms
Geometry

ASJC Scopus subject areas

  • Organic Chemistry

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An ab initio study of the P-C bond rotation in phosphoryl- and thiophosphoryl-stabilized carbanions : Five- and six-membered heterocycles. / Kranz, Michael; Denmark, Scott E; Swiss, Kevin A.; Wilson, Scott R.

In: Journal of Organic Chemistry, Vol. 61, No. 24, 29.11.1996, p. 8551-8563.

Research output: Contribution to journalArticle

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abstract = "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 P=X 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 P=X 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.",
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N2 - 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 P=X 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 P=X 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.

AB - 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 P=X 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 P=X 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.

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