Sulphonium ylide rearrangement

Sulphonium ylides are in certain cases unstable and they undergo further transformation affording useful final products. In this way allylic sulphides and selenides were used to transfer an alkylthio- or alkylseleno-group onto the a-carbon of / -dicarbonyl compounds in the form of their ylides the sequence of reactions were a transylidation followed by [2,3]-sigmatropic rearrangement. 

Thermal [2,3] sigmatropic rearrangement of the sulphonium ylide (114) gives a 54% yield of a mixture of four isomeric products, the major (70%) of which is the allenic thioether (115), a species which contains the entire artemisyl carbon skeleton hydrolysis with mercuric chloride then leads to artemisia ketone in 80% yield. The second route involves methallylation of the methyl thioacetal monoxide (116), the unstable intermediate (117) on distillation being transformed into the trienic sulphide (118). The latter species on hydrolysis gives artemisia ketone in 57% yield. 

The details of the earlier work on the Stevens rearrangement of sulphonium ylides have now been published by Schollkopf et al. Using a series of ylides (10), it was found that the rearrangement in THF was characterized by very low p-values as the R group was varied, by the presence of coupling by-products, and by the exhibition of a CIDNP effect in the n.m.r. spectrum of the benzylic protons. Baldwin et al. found similar products and also detected a CIDNP effect when the rearrangement was conducted in... 

Numerous examples of 1,5-rearrangements of unsaturated sulphonium ylides have been reported. Hackler has discussed this rearrangement in detail and described its competition with the 1,3-rearrangement (Stevens), both being represented by the general mechanism (13) (14) (1,5-product)... 

Several groups earlier had demonstrated that even stabilized sulphonium ylides, e.g. phenacylides, would undergo 1,5-rearrangement. Baldwin and Erickson found that even the aromatic sulphonium salt (21), when treated with butyl-lithium at — 40 °C, would undergo 1,5-rearrangement leading to destruction of the aromatic system. 

Other reactions of sulphonium ylides include o /9 -elimination,metal-mediated carbene-transfer to olefins, insertion into aromatic C—H bonds or other carbenoid-type processes, formation of Pd" complexes, addition to enones (forming cyclopropyl ketones or heterocycles ), reaction with isoquinoline 2-oxide, and [2,3]-sigmatropic rearrangements " [as in the case of (12) 1 or... 

Foster R, Gilchrist TL (1991) Rearrangement of sulphonium ylides of the lH-l,4-thiazine type. J Chem Soc Perkin Trans I 2249-2254... 

Considerable attention has been focussed recently upon the reaction of car-benes with sulphur compounds, and there are now many examples of stable sulphonium ylides. The first example of a rearrangement within this series has been reported by Benati et aL, with formation of the dibenzothiepin (251). 

Sigmatropic rearrangement of the allylic sulphonium ylide (279a) in... 

Mageswaran S, Ollis WD, Sutherland lO. Base catalysed rearrangement involving ylide intermediates. Part 9. The rearrangement reactions of cyclic allylic ammonium and sulphonium ylides. 7. Chem. Soc., Perkin Trans. 1 1981 1953-1962. 

Reactions.—The reaction of sulphonium ylides with carbonyl compounds to form oxirans is now a standard reaction. Among the more unusual examples reported are an intramolecular version directed towards the preparation of arene epoxide (12) (not isolable), the synthesis of the epoxide of 8-phenylthio-methylenecycloheptane, and the direct conversion of a diketone by dimethyl-sulphonium methylide into a doubly homologated hydroxymethyl aldehyde (13) by the rearrangement shown. ... 

Sigmatropic rearrangements of allyl sulphonium allyl ylides (75) provide the basis of a repetitive one-pot ring-expansion sequence" for 2-vinyl thiocyclic compounds (Scheme 50). A paper" and a section in a review article" have described the utilization of allyl thiol dianions and allyl sulphide anions in carbonyl group umpolung . 

Bu OK in THF. Further studies of the formation of sulphides from rearrangements of sulphonium salts provide more details of the repression of the rearrangement by adjacent substituents capable of causing delocalization of charge from the intermediate ylide. 

The standard method for the conversion of a-thio-esters into a,/3-unsaturated esters involves oxidation to the corresponding sulphoxide followed by pyrolysis. An alternative approach, which avoids the use of an oxidant, is by formation of a sulphonium salt using CFjSOaCHzSiMea followed by ylide formation using caesium fluoride and rearrangement (Scheme 44). This method can run into difficulties if the starting thio-ester already contains an olefinic bond. 

Primary allylic bromides react with 1,3-dithian to give sulphonium bromides, the ylides from which undergo a smooth [2,3]sigmatropic rearrangement (Scheme 85) hydrolysis completes this new synthesis of Sy-unsatu-rated aldehydes. 

Reactions.—Further publications " have appeared on the [2,3] sigmatropic rearrangements of allylsulphonium ylides (71) in connection with ring-expansion processes (Scheme 52). For sulphonium allylides (71 R = CH=CH2), compet-... 

The Diels-Alder-type cycloaddition between 1,3-dithienium fluoroborate (106), obtained from 1,3-dithian and trityl fluoroborate, and 1,3-dienes formed the basis of a new and efficient synthesis of A -cyclopentenones. Simple 1,3-dienes such as butadiene, isoprene, or 2,3-dimethylbutadiene reacted rapidly with (lOQ to give (107) (107b) was obtained stereospecifi-cally from (106) and isoprene in 95% yield. Treatment of the sulphonium salts (107) with butyl-lithium at -78 °C gave an ylide which rearranged on warming to the cyclopropyl-dithian (108). The latt , v/hea heated further to 200 C, underwent the well-known vinylcyclopropane to cyclopentene transformation. Hydrolysis of the dithian thus obtained furnished cyclopen-tenones. The overall yield of 3-methylcyclopent-3-enone from (106) and isoprene was greater than 83%. 

Whereas sulphonium phenacylides are thermally decomposed to cyclopropanes, an oxysulphonium phenacylide gave (84) when heated in the presence of cupric sulphate. Heating (85) in the absence of any catalysts gave (86), probably via a 2,3-sigmatropic rearrangement and sulphoxide elimination to afford a diene, which was attacked at the a, 3-bond by a second molecule of ylide. ... 

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