4 - Iranian Journal of Organic Chemistry Vol. 2, No. 1...

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Iranian Journal of Organic Chemistry Vol. 2, No. 1 (2010) 290-296 S. M. Habibi-Khorassani et al. 290 Quantum mechanical calculation for determination of more stable isomer of phosphorous ylide involving an indole Sayyed Mostafa Habibi-Khorassani, a * Ali Ebrahimi, a Malek Taher Maghsoodlou, a Hojjat Ghasempour a , Majid Moradian b a Department of Chemistry, University of Sistan and Baluchestan, P. O. Box 98135-674, Zahedan, Iran b Department of Chemistry, Islamic Azad University of Ghaemshahr, Ghaemshahr, Iran Abstract: Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:1:1 addition reactions between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of NH-acid, such as indole. These stable ylides exist in solution as a mixture of two geometrical isomers as a result of restricted rotation around the carbon-carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. In the recent work, the stability of the Z - and the E - isomers were undertaken for the two rotamers of phosphorus ylides involving an indole [namely dimethyl 2-(indole-1- yl)-3-(triphenyl- phosphanylidene)- butanedioate( 4a )] by means of atoms in molecules AIM, CHelpG and NBORead methods. Keywords: Stable phosphorus ylides; Dialkyl acetylenedicarboxylates; Z and E rotamer; AIM method. Introduction Phosphorus ylides are important reagents in synthetic organic chemistry, especially in the synthesis of naturally occurring compounds with biological and pharmacological activity [1]. The synthesis of phosphorus ylides is an important reaction in organic chemistry because of the application of these compounds in the synthesis of organic products [2-36]. Phosphorus ylides most often are prepared by treatment of a phosphonium salt with a base. Most of the phosphonium salts are usually made from the reaction of phosphine and an alkyl halide [4-8], though they can be obtained by Michael addition of phosphorus nucleophiles to activated olefins [3,4]. A facile synthesis for the reaction between triphenylphosphine 1 , dialkyl acetylendicarboxylates 2 and indole 3 (as a NH-acid) has been earlier reported [26]. The reaction is shown in Fig 1. The Z and E isomers were optimized for all ylide structures at HF/6- 31G(d,p) level of theory by Gaussian 98 package program. The energy of both geometrical isomers have been calculated at B3LYP/6-311++G** level. Atoms in molecules AIM, CHelpG and NBORead methods at HF/6-31G(d,p) level of theory have been performed in order to gain a better understanding of most geometrical parameters of both *Corresponding author. Fax: +(98) 5412446565; E-mail: smhabibius@yahoo.com the E - 4(a, c) and the Z - 4(a, c) of phosphorus ylides. The numbers of critical points and intramolecular
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4 - Iranian Journal of Organic Chemistry Vol. 2, No. 1...

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