Mirror image conversions of cyclic conjugated non-planar allenes, C 9 H 7 X (XZH, F, Cl, Br) M.Z. Kassaee * , M. Koohi Department of Chemistry, Tarbiat Modarres University, P.O. Box 14155–4838, Tehran, Iran Received 19 January 2005; revised 8 June 2005; accepted 30 July 2005 Available online 10 October 2005 Abstract Ab initio studies show the possibility of involvement of cyclic, planar, singlet and/or triplet 2-X-2,4,6,8-cyclononatetraenylidenes, carbenic transition states (1  XÿS and/or 1  XÿT ), in the conversion of cyclic conjugated non-planar allenes, 1-X-1,2,4,6,8-cyclonona-pentaenes (1 X ), to their corresponding mirror images, 1 0 X (where XZH, F, Cl and Br). q 2005 Published by Elsevier B.V. Keywords: Cyclononapentaene; Cyclononatetraenylidene; Racemization; Carbene; Allene; C 9 H 8 ,C 9 H 7 Br, C 9 H 7 Cl, C 9 H 7 F, Ab initio; Calculations 1. Introduction ‘Symmetric’ and ‘dissymmetric’ allenes may exist in two enantiomeric forms (R) and (S), which are not super- imposable on each other [1–5]. Mechanisms of allene (R) and (S) stereo-inversions by imidozirconium complexes are recently reported [6]. The ring opening of trans- and cisK 2,3-dimethylcyclopropylidenes to their corresponding enan- tiomeric (R) and (S) allenic products are elucidated by quantum mechanical computations [7]. The stability of cyclic allenes is crucially dependent upon the ring size and the presence of un-saturations within the ring. Waali and Allison are the first for generating 1,2,4,6,8-cyclono- napentaene, 1 H , through the reaction of alkyllithiums with 9,9-dibromobicyclo[6.1.0]nona-2,4,6-triene (2)(Scheme 1), [8]. They presume formation of a cyclopropylidene carbene 3, which can isomerize to 1 H . Rapid electro-cyclic ring closure of 1 H to 4, followed by thermally allowed 1,5- sigmatropic hydrogen shift of 4, gives 5 (Scheme 1) [9]. Interestingly, unsaturated, fully conjugated C 9 H 8 carbene 1  XÿS , was also first generated by Waali and Wright, through treatment of lithium chlorocyclononatetraenide (6), with silver nitrate, which gives 9,9 0 -bisbicyclo[4.3.0]nona-2,4,7- triene (7)(Scheme 2) [10–12]. They suggest that the facile removal of chloride ion from 6, by silver ion, results in the generation of singlet 1  XÿS , which goes through intersystem crossing to a more stable triplet state, 1  HÿT , whose hydrogen abstraction gives the cyclononatetraenyl radical 8. Dimeri- zation of 8 to 9, followed by electro-cyclic ring closures of 9, gives 7. Indene (5), an important product of 1 H , in Scheme1, is not observed in their work [12]. Lloyd and Glidewell carried out MNDO calculations to ascertain the preferred geometric and electronic configur- ations of cyclo-C 9 H 8 intermediates [13,14]. They argued against Waali’s generation of both 1 H and 1  HÿS , on the basis of thermodynamic stabilities. Waali, and one of us, questioned such a thermodynamic control of the isomer distributions, which ignores the possibility of any kinetic control [15]. Moreover, we reported that a planar, C 2v constrained version of singlet 1  HÿS (a p 2 carbene) is much less stable than 1 H [15]. The vibrational force constant calculation for C 2v singlet 1  HÿS shows one negative force constant. Hence, it is not an intermediate. It transforms to lower energy C 2 structure (an allene 1 H ) with no activation energy. The theoretical and experimental reports on the possible racemization of cyclic conjugated non-planar allenes, through their potential valance isomeric, planar carbenes have made the chemistry of cumulated alkadienes of even greater interest. It has already been shown that cyclic conjugated planar singlet carbene 10  HÿS is not an energy minimum, at the MNDO level (Scheme 3) [15–18]. Journal of Molecular Structure: THEOCHEM 755 (2005) 91–98 www.elsevier.com/locate/theochem 0166-1280/$ - see front matter q 2005 Published by Elsevier B.V. doi:10.1016/j.theochem.2005.07.031 * Corresponding author. Tel.: C98 911 200392; fax: C98 21 8006544. E-mail addresses: kassaeem@modares.ac.ir (M.Z. Kassaee), drkas- saee@yahoo.com (M.Z. Kassaee).