Cyclopropanes 1,2-migration rearrangement

Oxaspiropentanes substituted in the cyclopropane part rearrange with preferential migration of the more substituted carbon atom. This is exemplified by the lithium iodide induced rearrangement of 4,4-dimethyl-l-oxaspiropentane which predominantly yielded 3,3-dimethylcy-clobutanone (3).44... 

BUTENE. As shown in Figure 38, a group attached to C-1 can migrate from position 1 to 3 (1,3 shift) to produce an isomer. If it is a methyl group, we recover a 1-butene. If it is a hydrogen atom, 2-butene is obtained. A third possible product is the cyclopropane derivative. The photochemical rearrangement of 1-butene was studied extensively both experimentally [88]... 

It is believed that this process involves migration through a pentacoordinate protonated cyclopropane in which an alkyl group acts as a bridge in an electron-deficient carbocation structure. The cyclohexyl- methylcyclopentyl rearrangement is postulated to occur by rearrangement between two such structures. 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

The dominant pattern for the thermal fragmentation of thietane dioxides involves extrusion of sulfur dioxide leading to a 1,3-diradical (i.e. 242) which closes to final products, mainly cyclopropanes, accompanied by rearrangement products resulting from hydrogen migration within the diradical191,1930 230,256-258 (equation 92). 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

An important example of a 1,2-C migration (C—C insertion) is the ring expansion of chlorocyclopropylcarbene (54) to 1-chlorocyclobutene (Scheme 7.20). The 1,2-C shift takes precedence over 1,2-H shift. Chloromethylenecyclopropane (55), the putative product of a 1,2-H shift, is not formed. Interaction of the electron-rich bent cyclopropane C—C bond(s) with the vacant p orbital of carbene 54 leads to a more favorable rearrangement pathway than the alternative Oc-H/p interaction leading to 55. 

Oxaspiropentanes generally rearrange with inversion at the migrating terminus (see Section 3.2.2.2.). However, if a primary cation is involved, cyclopropylmethyl to cyclopropyl-methyl rearrangement with formation of a more stable secondary cation may precede the ring enlargement, and the stereochemistry of substituents in the cyclopropane part of the oxaspiropentane may be lost. This was found to be true for /ram-4,5-dimethyl-l-oxaspiropentane (5) where a considerable amount of m-2,3-dimethylcyclobutanone (m-6) was formed.47... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

While the cyclopropane to propene isomerization is not observed in the thermal rearrangements of gem-difluorocyclopropanes (vide supra), this is the principal reaction of polyfluorocy-clopropanes in the presence of Lewis acids.23 Perfluoro(alkylcyclopropanes) 27 isomerize to perfluoroalkenes 28 in high yield, despite the harsh conditions (100°C) required in some cases. It should be noted that double-bond migration under these reaction conditions occurs readily (see Section 5.1.2.2.). 

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