www.elsevier.nl/locate/jorganchem Journal of Organometallic Chemistry 586 (1999) 166 – 170 Stereospecific cyclodimerization of 1-methylphosphole 1-oxide: a theoretical study Gyo ¨ rgy Miklo ´ s Keseru  a, *, Gyo ¨ rgy Keglevich b a Department of Chemical Information Technology, Technical Uniersity of Budapest, H-1521 Budapest, Hungary b Department of Organic Chemical Technology, Technical Uniersity of Budapest, H-1521 Budapest, Hungary Received 18 February 1999 Abstract PM3 semiempirical calculations on the transition states for the cyclodimerization of 1-methylphosphole oxide (1,Y =Me) suggest the selective formation of product A (2,Y =Me) that is one of the eight possible isomers. Ab initio calculations at the 3-21G* level of theory go even further by justifying the specific formation of isomer A (2,Y =Me) that is the exclusive product of the syntheses. The dimerization seems to be directed by kinetic factors. Calculations on the HOMO – LUMO orbital interactions also confirm the preference for the formation of isomer A (2,Y =Me) where the phosphole rings are joined with the endo fusion and where at both phosphorus atoms the oxygen of the phosphoryl group is projected to the center of the system. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Phosphole oxide; Cyclodimerization; Stereospecificity; Semiempirical and ab initio calculations 1. Introduction It is well known that the P(IV) derivatives of phos- pholes undergo dimerization to the [4 +2] cycloadduct. The product containing the 7-phosphanorbornene ring system attracted considerable attention due to the unique properties of its P-center [1–3]. On the basis of the pioneering observations of Westheimer [4,5], Mathey and Quin explored the possibilities for the synthesis of the dimers of phosphole derivatives includ- ing oxides, sulfides and quaternary salts. The dimeriza- tion is regio- and stereospecific, and in no reported case has an isomeric mixture been formed, even when a number of isomers are possible. The Diels – Alder cy- cloaddition of 1-substituted phosphole oxides (1) could, for instance, result in the formation of eight isomers, while the cyclodimerization of the 3-methyl-phospholes (3) may, theoretically, give as many isomers as 64. Conversion of the 3-methylphosphole oxides (3) to dimer 4 is not only stereospecific, but also regiospecific. The only isomers (2 or 4) isolated from the dimeriza- tion had the endo -fusion of the rings with the P(7)O function located anti to the newly formed double-bond and with the P(1)O function directed under the bi- cyclic framework (Scheme 1). Stereostructure of the dimers (2 and 3) was confirmed by a series of single- crystal X-ray analyses. The same geometry was sug- gested for dimers with the tert -phosphine oxide [6,7], the phosphinic ester [8], or the phosphinic amide func- tion [9]. Scheme 1. * Corresponding author. 0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0022-328X(99)00258-2