Rearrangement sulfur ylids

Sulfur ylids containing a benzylic group (analogous to 18) undergo an analogous rearrangement. 

Sulfur ylids containing a Z group give an analogous rearrangement, often also referred to as a Stevens rearrangement. In this case too, there is much evidence... 

The sulfur ylid Me2S(0)CH2 is a very weak base, and, when coordinated to nickel in the complex [Ni( / -C2H4)2CH2S(0)Me2] (91), spontaneous decomposition to ethane, cyclopropane, and methane occurs (93). In another reaction, compound 92 rearranges under UV irradiation to yield, with insertion of iron into a phenyl-carbon bond, a cyclic carbene complex, 93 94). 

Reaction of the naphthothiin 229 with benzyne produces acenaphthylene in high yield via formation and Stevens rearrangement of the sulfur ylid 230 (Scheme 22).115... 

Although each of these ions reacts differently toward butyllithium, treatment with phenyllithium always gives predominantly the Stevens rearrangement, and treatment with amide ion in liquid ammonia gives predominantly the orfAo-rearrangement. Sulfonium and benzyl sulfides 59) on treatment with potassium amide undergo a similar c Ao-rearrange-ment which proceeds via the sulfur ylid. 

Honda and his co-workers synthesised methyl (+)-nonactate 179, setting up the C-6 to C-8 relationship by a chelation controlled allylsilane reaction on the aldehyde 169, and the C-3 centre by hydrogenation of the dehydro intermediate 176 carrying two methoxycarbonyl groups (37) (Scheme 24). The thiolactone 175 and dimethyl diazomalonate gave the dehydro intermediate 176 in the presence of dirhodium tetraacetate, by way of a sulfur-ylid rearrangement developed by these... 

There are many variations on the Pummerer rearrangement but they all involve the same steps a leaving group is lost from the sulfur atom of a sulfonium ylid to create a cationic intermediate that captures a nucleophile at the a carbon atom. Often the starting material is a sulfoxide. 

There can now be little doubt that the reaction of singlet carbenes and car-benoids with thiophene proceeds by attack of the carbene at the ring sulfur atom to generate the S,C-ylid. However, only in the special cases indicated above do these ylids enjoy any real stability. In the majority of cases, other products usually result from rearrangement of the intermediate ylids and the nature of the products formed is remarkably sensitive to both steric and electronic effects. Broadly speaking, six major reaction pathways have been observed (1) 2-substituted thiophene formation, (2) 2H-thiopyran formation, (3) formation of derivatives of 2-thiabicyclo[3.1.0]hex-3-ene, (4) 3-substituted thiophene formation, (5) oxathiocin formation, and (6) carbenic fragmentation. 

Trost and co-workers used this type of rearrangement to generate oxaspiropentane derivatives that were converted to cyclo-butane derivatives. In section 8.8.B.ii, oxaspiropentanes such as 61 were prepared by the reaction of ketones and sulfur stabilized cyclopropyl ylids. Treatment of 61 with aqueous tetrafluoroboric acid (HBF4) gave cation 62, which rearranged to 63. Loss of a proton gave the final product, ketone 64. ... 

Even thiophene itself will react with carbenes, at sulfur, to produce isolable thiophenium ylids, and in these the sulfur is definitely tetrahedral." The rearrangement of thiophenium bis(methoxycarbonyl)methylide to the 2-sub-stituted thiophene provides a rationalisation for the long known" reaction of thiophene with ethyl diazoacetate, which produces what appears to be the product of carbene addition to the 2,3-double bond, but which can now be viewed as being produced via initial attack at sulfur followed by S — C-2 rearrangement, then collapse to the cyclopropane. Acid catalyses conversion of the cyclopropanated compound into a thiophene-3-acetic ester." 2,5-Dichlorothiophenium bis(methoxycarbonyl)methylide has been used as an efficient source of the carbene simply heating it in an alkene results in the transfer to the alkene." ... 

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