Intermolecular reactions sulfur nucleophiles

Kinetic data are very scarce and available mostly for amine, phosphane [337], and sulfide [338] radical cation reactions with nucleophiles. In intermolecular reactions, phosphane radical cations react with sulfur nucleophiles [339], k = 10 -10 s , ... 

Application of an organocatalytic domino Michael addition/intramolecular aldol condensation to the preparation of a series of important heterocycles has recently received much attention [158] with methods being disclosed for the preparation of benzopyrans [159-161], thiochromenes [162-164] and dihydroquinolidines [165, 166]. The reports all use similar conditions and the independent discovery of each of these reactions shows the robust nature of the central concept. A generalised catalytic cycle which defines the principles of these reports is outlined in Fig. 10. Formation of iminium ion 102 is followed by an intermolecular Michael addition of an oxygen, sulfur or nitrogen based nucleophile (103) to give an intermediate... 

Several mechanisms have been proposed for the intriguing interconversions of sulfur (or selenium) rings. These include the formation of (i) radicals by homolytic S-S bond cleavage, (ii) thiosulfoxides of the type S =S via ring contraction (an intramolecular process) or (iii) spirocyclic sulfuranes (or sele-nanes) via an intermolecular process. A fourth alternative (iv) invokes nucleophilic displacement reactions. Generic examples of mechanisms (ii)-(iv) for homoatomic sulfur or selenium rings are depicted in Scheme 12.1. 

It was hypothesized that propargyl allenyl isomerization produces an allenic intermediate 246 first which cycloisome-rized to the major product 246 (path a, Scheme 68). Alternatively, an intermolecular nucleophilic attack of the sulfur atom at the central carbon atom of the allene can transform it into the thiirenium ion 247, which then is transformed into the minor product 245 via Ad -E 247 —> 248 or through a direct SN2-Vin-type of process (path b. Scheme 68). This reaction was successfully applied for synthesis of furans and pyrroles (from thiopropargylimines) (Scheme 69 Table 10). 

As was introduced earlier, once the sulfoxide is activated, the resulting sulfonium salt 102 can undergo nucleophilic attack at the sulfur atom This is known as an interrupted Pummerer reaction and is outlined in Scheme 20.24. The nucleophilic substitution on the sulfur atom yields a new sulfonium salt 103. Depending on the structure of R, Nu, and in 103, this can have three a electrophilic positions, which means that a nucleophilic substitution of sulfide will produce the loss of one of the three fragments. Usually, the first nucleophile is present in the same molecule, reacting in an intramolecular way. However, some exanples using interrupted Pummerer reactions in intermolecular processes are described in the literature, and inportantiy, the reaction can be used to activate relatively unreactive positions in heterocyclic rings. 

The synthetic and kinetic regularities of the amino-Claisen rearrangement were studied for the transformation of 2,5-dimethyl-Af-(pent-3-en-2-yl)aniline." The products are obtained via conversion of a binary r-complex formed by the reaction of N-alkenylaniline hydrochloride with the hydrochloride of the solvent (2,5-dimethylaniline). Spirocyclic oxindoles have been prepared from iodoindoles via a sequential intramolecular Ullmann coupling and Claisen rearrangement." Nucleophilic ort/io-propargylation of aryl sulfoxides has been reported to occur by intermolecular delivery of the nucleophile to sulfur followed by an intramolecular relay to carbon in a... 

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