Rearrangement to alkenes

Carbenes can rearrange to alkenes in many cases. A 1,2-hydrogen shift leads to an alkene, and this is often competitive with insertion reactions.Benzylchlorocarbene (42) rearranges via a 1,2-hydrogen shift to give the alkene.Similarly, carbene (43) rearranges to alkene (44), and replacement of H on the a-carbon with D showed a... 

Carbenes can rearrange to alkenes in many cases. A 1,2-hydrogen shift leads to an... 

Johnson has shown that a-halo-A -tosylsulfoximines with an a -hydrogen atom, on treatment with KOH in refluxing methanol, undergo rearrangement to alkenes by analogy with the Ramberg-Backlund... 

A detailed recent study has been made to try and elucidate whether metallacyclobutanes rearrange to alkenes by p elimination or by a elimination to give an intermediate ylide complex, which can rearrange to an alkene complex. Using deuterium-labeled platinum(IV) platinacyclobutanes it is concluded that the pathway involves a [1,3] H shift (a elimination) rather than a [1,2] H shift ()3 elimination). Platinacyclopentanes have also been formed by an alkene coupling between Pt(cod)2 and butadiene. Addition of PMcs gives complex (66 equation 2(K)). ... 

Singlet carbenes can rearrange to alkenes by a 1,2-hydrogen shift if hydrogen atoms are located on adjacent carbon atoms. Evanseck and Houk determined from ab initio calculations that the activation energy for the rearrangement of methylcarbene to ethene (equation 5.42) is only 0.6 kcal/mol. ... 

Furthei-more, the cyclization of the iododiene 225 affords the si.x-membered product 228. In this case too, complete inversion of the alkene stereochemistry is observed. The (Z)-allylic alcohol 229 is not the product. Therefore, the cyclization cannot be explained by a simple endo mode cyclization to form 229. This cyclization is explained by a sequence of (i) e.vo-mode carbopallada-tion to form the intermediate 226, (ii) cydopropanation to form 227. and (iii) cyclopropylcarbinyl to homoallyl rearrangement to afford the (F3-allylic alcohol 228[166]. (For further examples of cydopropanation and endo versus e o cyclization. see Section 1.1.2.2.)... 

FIGURE 5 7 The first formed carbocation from 3 3 dimethyl 2 butanol is secondary and rearranges to a more stable tertiary carbocation by a methyl migration The major portion of the alkene products is formed by way of the tertiary carbocation... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

Carbocation Rearrangements in Hydrogen Halide Addition to Alkenes... 

CARBOCATION REARRANGEMENTS IN HYDROGEN HALIDE ADDITION TO ALKENES... 

Our belief that carbocations are intermediates m the addition of hydrogen halides to alkenes is strengthened by the fact that rearrangements sometimes occur For example the reaction of hydrogen chloride with 3 methyl 1 butene is expected to produce 2 chloro 3 methylbutane Instead a mixture of 2 chloro 3 methylbutane and 2 chloro 2 methylbutane results... 

The synthesis of dehydro-hnalool (28) rehes on the basic chemicals acetone and acetjiene. Addition of a metal acetyUde to acetone yields methylbutynol (33). Semihydrogenation affords the alkene (34) which is reacted with /-propenylmethyl ether. A Cope rearrangement of the adduct yields methyUieptenone (35). Addition of a second mole of metal acetyUde to dehydro-linalool (28) is followed by a second Cope rearrangement to yield... 

Rahman and Clapp decomposed dinitromethane derivatives in DMF in the presence of alkenes to obtain 2-isoxazolines. Without any alkene present, an acid and KNO2 were obtained. They proposed a mechanism which proceeded via a three-membered ring or a nitrocarbene which rearranged to a nitrile oxide (76JOC122, 75MI41612). 

Irradiation of the A-bromo- or N- chloro-azetidin-2-ones (71) in the presence of alkenes, alkynes or radical donors induces rearrangement to the /3-haloalkyl isocyanates (72) via a... 

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