Sulfonium salts cyclopropanes

Ring closures with sulfoxonium and sulfonium salts Cyclopropane ring... 

Placing anion stabilizing groups on the cyclopropane greatly facilitates generation of cyclopropyl nucleophiles. The diphenylsulfonium ylide has proven to be an exceptionally versatile conjunctive reagent25). The sulfonium salt 9, available from either 3-chloro-l-iodopropane (Eq. 24a) or 3-chloro-l-propanol (Eq. 24b), is a nicely crystalline stable salt that can be stored indefinitely26,Z7). While substituted... 

These reactions rapidly found wide use and success, and many other sulfur ylides have been prepared and exploited [194, 195, 203, 204]. Various experimental procedures are to be found in the detailed monograph by Trost and Melvin [204] for sulfonium salts, ylides, epoxidations and cyclopropanations. 

More reactive than Helquist s reagent, it can be used at room temperature for cyclopropanation of alkenes. Thus, ds-cyclooctene was converted to the cyclopropane in an 85% yield in 3 h at 22°C [472], Although probably oversimplified, the pathway given here accounts for the cyclopropanation property of the sulfonium salts [471]. 

Recently, Tang, Wu and co-workers have reported the synthesis of vinylcyclopro-panes using an alternative catalytic cycle for sulfur ylide-catalyzed cyclopropanation (see Scheme 10.22) [98]. Sulfonium salt 41a or 41b was deprotonated by CS2CO3 to form an ylide which then reacted with chalcones 37 to form cyclopropanes and a sulfide. The sulfonium salt was regenerated in situ through reaction... 

A class of compounds which have drawn some interest are the sulfonium salt complexes 416, prepared by alkylation of sulfide derivatives (415) (Table IV). These coordinated ylides have found use as stable, stereospecific cyclopropanating reagents yielding compounds of the class 417. 

The most widely explored method is the formation of tri- and tetrasubstituted cyclopropanes by reaction of vinylsulfonium salts with methylene compounds activated by ester, ketone, nitrile, or sulfone substituents. A series of examples is collected in Table 25. A variant of this method, where cyclopropanedicarboxylates cis-1 were actually obtained by intramolecular cyclization of the sulfonium salt Cj building block is also shown.Alkylthiocyclopropane derivatives 8 were obtained by the reaction of a ketene dithioacetal monosulfonium salt with carbanions derived from doubly activated methylene compounds. ... 

The reaction of ylides with alkenes proceeds in two steps according to the MIRC reaction mechanism. Dimethylsulfonium trimethylsilylmethylide (2), generated in situ from the corresponding sulfonium salt 1 with ec-butyllithium, reacts with various a,/l-unsaturated ketones to cyclopropanate the activated C-C double bond (see Table 2). The resulting l-acyl-2-trimethylsilylcyclopropanes 3 are isolated as mixtures of diastereomers. 

A modification of this general procedure was used in the synthesis of (prop-2-enylidene)cyclo-propane (allylidenecyclopropane, 12).Allyl bromide was first alkylated in 81 -90% yield with l-lithio-l-(phenylsulfanyl)cyclopropane (10) in the presence of copper(I) iodide or cop-per(I) iodide-dimethyl sulfide. The adduct was then methylated with methyl fluorosulfonate or with dimethyl sulfate, and the sulfonium salt cleaved with powdered potassium hydroxide in dimethyl sulfoxide, to give the product in 50-70% overall yield for the second step. 

Although probably oversimplified, the pathway given here accounts for the cyclopropanation property of the sulfonium salts [471],... 

Cyclopropanes from sulfonium salts under mild conditions... 

Sodium hydroxide benzyhriethylammonium chloride Cyclopropanes from sulfonium salts... 

Sodium hydride dimethyl sulfoxide Ring closures with sulfoxonium and sulfonium salts Synthesis of oxido compounds from oxo compounds Cyclopropanes from ethylene derivatives... 

A catalytic cycle was proposed as shown in Scheme 20.20. First, the sulfide 2a reacts with cinnamyl bromide to afford the sulfonium salt 21a, which is deproto-nated by CS2CO3 to generate ylide 22. Then, the reaction of ylide 22 with a,p-unsaturated ketones give the vinyl cyclopropanes and regenerates the sulfide. 

The camphor-derived P-hydroxyl-sulfonium yhde proved to be unusually efficient for the cyclopropanation of electron-deficient alkenes. For example, the reaction of salt 21b with a,P-unsaturated nitrile affords the desired cyclopropane in 61% yield and 96% ee. In comparison, the corresponding tetrahydrothiophene and dimethyl sulfide-derived sulfonium salts (23 and 24), which lack the hydroxyl group, only gave a trace amount of the desired cyclopropane (Scheme 20.21). 

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