Methylenecyclopropane rearrangement

At 150 °C, the analogous 2-methylenetricyclo[4.1.0.01,3]heptane (34) equilibrated with 35 and 36 as products of a methylenecyclopropane rearrangement and a subsequent retro-Diels Alder reaction, respectively, until at 180 °C an irreversible rearrangement to bicyclo[4.2.0]octa-l,5-diene and 3,4-dimethylenehexa-l,5-diene (39) via diradical 37 took place.223,224 Increasing pressure favored the formation of the bicyclic system 38 (50% yield at lOOTorr).223... 

In the light of experimental difficulties associated with the identification of intermediates, a MINDO/3 quantum-mechanical study of the singlet state ( a2) of the cyclobutylidene to methylenecyclopropane rearrangement has been carried out. It has been proposed that the whole process is initiated by electrophilic attack from the C3 methylene group of cyclobutylidene at the empty p atomic orbital on the Cl carbene site, so that a shift of electron density towards Cl can take place to give the bicyclobutane-like nonclassical carbene intermediate 4. Finally, the bicyclobutane intermediate 4 undergoes a symmetry-allowed conrotatory bond-fission process to generate methylenecyclopropane. The activation enthalpy calculated for the two steps is 8 kcal mol-1.2... 

Methylenecyclopropene (19) dimerizes to give (20) and (21) in a 92 8 ratio. Bicyclopropylidene (22) is converted in 35% yield to dimer (23) a competitive methylenecyclopropane rearrangement forming (24) is the major side reaction. 

The photochemical nitrogen extrusion from 3H-pyrazoles provides a major access to cyclopropenes. It has been shown that the 3H-pyrazoles are transformed into diazoalkenes in the first step of the decomposition, and the resultant diazoalkenes give the cyclopropenes by loss of nitrogen". Benzocyclopropenes have been prepared in a similar manner" The thermal or photochemical decomposition of 4-alkylidene-l-pyrazolines produces methylenecyclopropanes". The products obtained especially in the triplet sensitized photolysis are frequently derived by a methylenecyclopropane rearrangement. 

Wang, R., Ksebati, M. B., Corbett, T. H., et al. (2001) Methylene< m-difluorocyclopropane analogues of nucleosides Synthesis, cyclopropene-methylenecyclopropane rearrangement, and biological activity. J. Med. Chem., 44, 4019 4022. 

I.I. Methylenecyclopropane Rearrangements (Including Metal-Catalyzed Reactions)... 

The first example of such a methylenecyclopropane rearrangement was the isomerization of dimethyl rraM -3-methylenecyclopropane-l,2-dicarboxylate (the dimethyl ester of Feist s acid, 1) which occurred upon heating to about 200°C. It was not until 1952 that this reaction was formulated in proper structural terms. Thus, 1 rearranged to a 1 1 mixture of methyl (Z)- and ( )-2-(methoxycarbonylmethylene)cyclopropanecarboxylate (2). ... 

The kinetics of the methylenecyclopropane rearrangement have also been studied for deuterium-labeled methylenecyclopropanes " and various methyl-substituted derivatives including 1 -methyl-2-methylenecyclopropane, 1,1 -dimethyl-2-methylenecyclopropane, 1,1,2,2-tetra-methyl-3-methylenecyclopropane, and l,l-dimethyl-2-methylene-3-vinylcyclopropane(Table 1). Thermolysis of 2- [ H2]-methylene -l,l-diphenylcyclopropane (Table 1, = D ... 

When a partially deuterium-labeled starting material was used it became evident that the isomerization of 1 (R = R = H) to 3-methylenecyclopentene was accompanied by a competing methylenecyclopropane rearrangement that gave rise to an equilibrium between two equivalent methylenecyclopropane structures. 

Thermolysis of both (Z)- and ( )-l-(prop-2-enylidene)-2-vinylcyclopropane (1, R, R = CH = CHj, H) gave a mixture of 3-methylenecyclohepta-l, 4-diene (4) and 3-methylene-4-vinylcyclopentene (5) in the ratio of 80 20 and 86 14, respectively. This isomerization involves a methylenecyclopropane rearrangement to l-methylene-2,3-divinylcyclopropane, followed by a Cope rearrangement. 

In a similar manner, l-(buta-l,3-dienyl)-2-methylenecyclopropane rearranged to 3-methylene-5-vinylcyclopentene. ... 

Me) at 180°C led to a 7 1 mixture of 4,4-dimethyl-3-oxabicyclo[3.1.0]hexan-2-one (19) and 4,5,5-trimethylfuran-2(5//)-one (20). ° The formation of product 19 can be rationalized by intramolecular addition of the carboxy group across the exocyclic C — C double bond, whereas furanone 20 is probably the result of an intramolecular trapping of the diradical intermediate of the methylenecyclopropane rearrangement. This assumption was confirmed by pyrolysis of the isomeric add, 2,2-dimethyl-3-methylenecyclopropanecarboxylic acid (18, R = Me R = H), which led to the same products, but in a different ratio (2 7). ... 

Similarly, treatment of an allylidenecyclopropane 3 with hydrogen chloride gave the ring-conserved product 4 and the allylic rearrangement products 5 and 6 in a ratio of 3.5 1 1 (for additional information on methylenecyclopropane rearrangements see Section 2.4.1.1.). 

The Electron Transfer Photoinduced Degenerate Methylenecyclopropane Rearrangement... 

Since the time when the thermally induced methylenecyclopropane rearrangement (A - B) of Feist s esters was first observed by Ullman in 1959, rearrangements of a large number of methylenecyclopropane derivatives have been subjected to kinetic, stereochemical, and theoretical studies. The main objectives of these efforts were to understand the role and nature of the trimethylenemethane biradical intermediate (C ) in the rearrangement process. Considerable attention has focused on the theoretical and experimental elucidation of the relationship among structure, spin state, and reactivity of this simple non-Kekule molecule as well as on its applications as a synthetic and practicaP" intermediate. 

Although SET photochemistry is often a practical method to promote cation radical reactions, such a variant had not been probed until we recently discovered the SET-photoinduced degenerate methylenecyclopropane rearrangement of 2,2-diaryl-1-methylenecyclo-propanes." Below, we describe in detail the results of our studies of this process including the chemical and spectroscopic identification... 

The thermal degenerate methylenecyclopropane rearrangement of was reported by Gilbert and Butler in 1970. One intriguing finding in this effort is that pyrolysis of rf2-77c gives an equilibrium mixture of and in a ratio of 1.04 1.00, but it does not... 

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