Alkenyl alane–pyridine complexes in a new synthesis of aryl alk-1-enyl sulfoxides G. Signore, a M. Calderisi, b C. Malanga a and R. Menicagli a, * a Dipartimento di Chimica e Chimica Industriale, Universit  a di Pisa, Via Risorgimento 35, 56126 Pisa, Italy b Laboratori ARCHA, via di Marmiceto, 6/C, 56014 Pisa, Italy Received 18 April 2006; revised 25 September 2006; accepted 12 October 2006 Available online 2 November 2006 Abstract—A new and an efficient synthesis of aryl alk-1-enyl sulfoxides is reported. The reaction between alane–pyridine complexes, tri- phenylphosphine, and sulfonyl chlorides affords the title products in good to excellent yields (70–94%) in short reaction times using mild conditions. The optimal ratio between reagents (alane–pyridine/PPh 3 /sulfonyl chloride 1.00/1.35/0.92) was obtained performing a chemio- metric analysis. A rationale for the reaction was advanced. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction a,b-Unsaturated sulfoxides are well known as useful re- agents in organic synthesis. 1 Despite their widespread inter- est, synthetic pathways to this class of compounds are tedious or time-consuming, or require commercially unavailable reagents. Among the extensively reviewed approaches to a,b-unsaturated sulfoxides, 2 the most general ones are: con- trolled oxidation of the corresponding sulfides, 3 condensa- tion starting from carbonyl compounds, 4 and alkylation of sulfinates with organometallic reagents. 5 In a previous work 6 we described an improvement of the Reinheckel procedure 7 to prepare aryl as well as alkyl alk- 1-enyl sulfoxides. The reported reaction 6 is rather general, but it is carried out under drastic conditions and affords the products in a 50% yield referred to the alkenyl alane; these drawbacks constitute a limitation to the synthetic ap- plicability of our previous protocol. Dialkyl alk-1-enyl alanes are useful reagents in organic syn- thesis when an unsaturated chain has to be transferred; 8 we thus considered their employment in the direct synthesis of alkenyl sulfoxides. In the context of a study on the chemistry of a,b-unsaturated sulfoxides, 6 multigram amounts of these intermediates were needed; an easy and cost-effective synthetic procedure was thus required. Use of aryl sulfonyl chlorides and of triphe- nylphosphine as reducing agents seemed to be reasonable in view of their low cost and commercial availability. In a preliminary experiment a low yield (35%) of p-tolyl hex- 1-enyl sulfoxide, recovered in a 1:1 ratio with the corre- sponding sulfone was obtained by reacting tosyl chloride and triphenylphosphine with an hexane/THF solution of alkenyl alane at room temperature. The ratio between products was found to be independent of the temperature (0–25  C) as well as the solvent (THF and CH 2 Cl 2 ). Taking into account that coordination of dialkyl alk-1-enyl alanes with amines seems to enhance 9 the hardness of the unsaturated chain, we decided to employ alkenyl alane– pyridine complexes 10 to increase the selectivity of the process. In a further reaction, tosyl chloride was added at room tem- perature to the preformed alane–pyridine complex 1, as shown in Scheme 1. In the reported conditions, the organometallic reagent 1 is unreactive toward sulfonyl chloride. An immediate reaction occurs when triphenylphosphine is added, giving p-tolyl hex-1-enyl sulfoxide, p-tolyl hex-1-enyl sulfone, and p-tolyl iso-butyl sulfoxide in a 2.5/2.5/1.0 ratio (overall yield 60%, Table 1, entry 1). Reaction temperature has in this case a remarkable effect on the selectivity in the desired product: alkenyl sulfoxide is re- covered as the sole product in a 50% yield when the reaction is performed at 0  C(Table 1, entry 2). No reaction occurs at temperatures lower than 30  C (Table 1, entry 3), and different alk-1-enyl alanes give * Corresponding author. Tel.: +39 50 2219281; fax: +39 50 2219260; e-mail: rime@dcci.unipi.it 0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2006.10.039 Tetrahedron 63 (2007) 177–182