Cyclopropylcarbene rearrangement

A caveat in cyclopropylcarbene rearrangements is the possibility that, at least in certain systems, rearrangements may occur in stepwise fashion via diradical intermediates.113 Whether such complications intrude with the simplest cyclopropylcarbenes (83-86) is not yet clear. 

Hypothetical (carbene)gold(i) structures of intermediates and reaction coordinates have been calculated (B3LYP/ 6-31G and LAN2DZ levels) for (H3P)Au+-catalyzed cyclization reactions of terminal enynes. The endocyclic skeletal rearrangement reactions were found to proceed exclusively via cyclopropylcarbene complexes.240... 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

The rearrangement of cyclopropylcarbene to cyclobutene was studied theoretically using B3LYP at the 6-311G(d) level.58 The results show that the rearrangement of cyclopropylcarbene to cyclobutene is an electrophilic rearrangement, very different from the traditional mechanism of nucleophilic rearrangement for carbocations. 

Endo-skeletal rearrangements also take place with 1,6-enynes, bnt the proposed mechanism is just a variation of the exo-single-cleavage rearrangement. Formed by endo cyclization, bicyclo[4.1.0]hept-4-ene derivatives arise in some cyclizations of 1,6-enynes by proton loss and protodemetalation ofthe endo cyclopropylcarbene. " That is the case from 1,6-enynes tethered as sulfonamides and in the intramolecular cyclization of 1,6-enol ethers with alkynes (equation 40). ... 

It has been suggested on the basis of kinetic results that the mechanism involves the trapping of the first-formed biradical by the second alkene unit (equation 53), although another possibility is that the biradical rearranges to a cyclopropylcarbene before it reacts further. 

A similar rearrangement and geometric isomerization occur in the case of 2,4-hexadienemonooxirane. Irradiation of the spirooxirane 135 leads to cumulene, 135a, allyl alcohol 135b, and oxetane derivatives 135c. ° Cumulene formation is interpreted as occurring via a cyclopropylcarbene. 

The sodium salt of cyclopropanecarboxaldehyde tosylhydrazone (157) behaves similarly on heating to 125 135°C. Labelling experiments using the lithium cyclopropanecarboxaldehyde-p-tosylhydrazone salt has proved the occurrence of a cyclopropylcarbene-cyclobutene C3 - C4 ring expansion instead of a simple intramolecular carbene insertion in one of the four C-H bonds and bicyclobutane ring-opening . For other examples and discussion of such a rearrangement see Ref. 189. In an apparently related reaction, when cyclopropylmethyl tosylate (158) is treated with potassium t-butoxide in dimethyl sulphoxide at room temperature for 1 h, a quantitative mixture of cyclobutene and methylenecyclopropane is produced in equal amounts (equation 110) °. 

However, the intermediacy of a cyclopropylcarbene from a-elimination was ruled out by labelling experiments. The rearrangement is assumed to proceed by a bimolecular eliminative mechanismFor a recent convenient preparation of practically pure cyclobutene from cyclopropylmethanol, see Ref. 191. 

Mueck-Lichtenfeld, C. Theoretical Prediction of the Stability and Intramolecular Rearrangement Reactions of Heteroanalogues of Cyclopropylcarbene 2-Oxiranyl-, 2-Aziridinyl-, and 1-Aziridinylcarbene. J. Org. Chem. 2000, 65, 1366-1375. 

Recently, diazirine rearrangement in the excited state (RIES) that mimics the result of cyclopropylcarbene fragmentation has been postulated. For examples, see (a) Ref. 128d,e (b) Thamattoor, D.M., Jones Jr., M., Pan, W. and Shevlin, P.B. (1996). Tetrahedron Lett. 37, 8333-8336... 

Most cyclopropylmethyl to cyclobutyl rearrangements are cationic in nature and these reactions are covered in Section 7.A.3.2. For cyclopropylcarbene to cyclobutene rearrangements see Section 8.A.3. 

Cycloheptadienones The cyclopropylcarbene-tungsten complex 1 reacts with diphenylacetylene at 100° to form a cycloheptadienone 2, which rearranges to the more... 

The oxymercuration of 1-substituted (i,e, H, Me, and C02Me) tricyclo[4,l,0,0 ]-heptanes with mercuric acetate affords norcaranyi- and norpinyl-mercury compounds. A synthesis of 3,4-benzotricyclo[4,l,0,0 ]heptene (692) has been reported in which the usual cyclopropylcarbene C—H insertion process is employed. Isomerization of (692) with silver perchlorate gave benzocycloheptatriene which is also formed in the thermal isomerization of (692). Reaction of (692) with n-ally 1 palladium(ii) chloride dimer yielded 2-methylene-l T-naphthalene which rearranged readily to 2-methylnaphthalene at room temperature a carbenoid mechanism appears to be involved. 

In order to take these results into account, a different mechanistic pathway was proposed relying on the generation of a (cyclopropylcarbene)palladium intermediate 42. Successive [2+2] and retro-[2+2] rearrangements lead to the formation of carbene 43. A 1,2-hydride shift and elimination complete the catalytic cycle to produce the dienes 44 and 45 (Scheme 7.23). 

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