Gas-Phase Specific Reactivity of Isomeric 1,3-Benzodithiole Anions: Tandem Mass Spectrometry and DFT Theoretical Studies Yves Gimbert,* ,† Roger Arnaud,* ,‡ Edmond de Hoffman, § and J. C. Tabet | UniVersite ´ Joseph Fourier de Grenoble, Chimie Recherche - Equipe de Synthe ` se Organique (LEDSS), 38041 Grenoble Cedex, France, UniVersite ´ Joseph Fourier de Grenoble, Chimie Recherche - Equipe de Chimie The ´ orique (LEDSS), 38041 Grenoble Cedex, France, UniVersite ´ Catholique de LouVain, Laboratoire de Spectrome ´ trie de Masse, Place Pasteur, 1, B-1348 LouVain la NeuVe, Belgique, and UniVersite ´ Pierre et Marie Curie, Laboratoire de Chimie Structurale Organique et Biologique, 4, Place Jussieu, 75230 Paris Cedex 05, France ReceiVed: September 15, 2000; In Final Form: February 9, 2001 1,3-Benzodithiole dideuterated at the dithioacetal carbon atom has been ionized in the chemical ionization source of a tandem triple quadrupole mass spectrometer under NICI condition. Two isomeric anions are formed by abstraction of an o-phenylic proton or through abstraction of a deuteron. The fragmentation patterns occurring under dissociative collisions differ and confirm the coexistence of two nonconverting carbanions. DFT calculations show that the dithioacetal anion is more stable than the phenylic. The nucleophilic reactivity toward CS 2 has been studied by collision-induced reactions in the collision cell of the tandem instrument. Both isomeric anion forms display characteristic fragmentations of the resulting [(M-H(D)) + CS 2 ] adduct anions, demonstrating a difference in reactivity: the dithioacetal anion reacts regioselectively at C and the o-phenylic at S. DFT calculations, performed with the aim of rationalizing this observed difference and understanding the formation of the diagnostic fragments, have been successful. Introduction The continuing interest in the experimental reactivity (uni- molecular and bimolecular processes) and fundamental proper- ties of anions derived from organosulfur compounds is due to their importance in synthetic organic chemistry. 1 The particular reactivity of sulfur-containing anions differs from that character- izing the analogous oxygen-containing compounds. The mech- anism of the stabilization of the negative charge on a carbon atom adjacent to a third-row elementsespecially sulfurshas been a matter of discussion. On the basis of PMO theory, interaction between the carbanion lone pair and the antibonding σ* orbital of the S-R group has been suggested to contribute to the stabilization of the carbanion site. 2 It is well established that this stabilization by third-row substituents is due mainly to polarization and negative hyperconjugation and not to d-orbital effects, as suggested in a few publications. Carbanions derived from thioacetals are widely used in organic chemistry as masked acyl anion equivalents, 3 mainly with 1,3-dithiane, in contrast to the carbanion derived from dithiolane which has been described as impossible to produced because of immediate decomposition into ethylene and dithioformate anion. Other such anions, such as 2-lithio-1,3-benzodithioles (1 Li , Scheme 1), have also been recommended as masked acyl anions. 4,5 In addition to reversing the carbonyl group reactivity, anions from 1,3- dithioles are also very useful as building blocks for new sulfur heterocycles. Researchers have investigated 6,7 the reactions of these species with CS 2 as a way to an interesting synthon and a redox-active ligand (Scheme 1). Surprisingly, the anion from 4,5-dimethyl-1,3-dithiole does not react with carbon disulfide in the predicted way (eq 1). 6 On the other hand, under the same conditions (Scheme 1, eq 2), the anion from 1,3-benzodithiole 1 leads to the expected dianion, lithium (1,3-benzodithiol-2-ylidene)methanedithiolate. 7 This dramatic difference in the reaction with CS 2 of these apparently similar dithioles led us to investigate a series of 1,3- dithioles in order to gain some insight into the relationship between the structure of the dithiole carbanion and its reactivity * Corresponding authors. Tel: (33) 4-76-51-46-91. Fax: (33) 4-76-51- 43-82. E-mail: Yves.Gimbert@ujf-grenoble.fr (Y.G.) and Roger.Arnaud@ujf- grenoble.fr (R.A.). † Universite ´ Joseph Fourier de Grenoble, Chimie Recherche - Equipe de Synthe `se Organique (LEDSS). ‡ Universite ´ Joseph Fourier de Grenoble, Chimie Recherche - Equipe de Chimie The ´orique (LEDSS). § Universite ´ Catholique de Louvain. | Universite ´ Pierre et Marie Curie. SCHEME 1 5221 J. Phys. Chem. A 2001, 105, 5221-5231 10.1021/jp003294j CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001