Rearrangement cyclopropanation

The strength of the carbon-carbon bond adjacent to a difluoromethylene group IS hardly affected by the presence of fluorine on the cyclopropyl ring When heated, 2,2 difluoromethylenecyclopropane undergoes methylene-cyclopropane rearrangement [/2 ] Under kinetic control, 2,2 difluoro 1 methylenecyclopropane and (difluoromethylene)cyclopropane are formed in a 2 1 ratio, although the latter IS slightly more stable [129] (equation 27)... 

Alkoxycarbene complexes with unsaturation in the alkyl side chain rather than the alkoxy chain underwent similar intramolecular photoreactions (Eqs. 10 and 11) [60]. Cyclopropyl carbene complexes underwent a facile vinyl-cyclopropane rearrangement, presumably from the metal-bound ketene intermediate (Eqs. 12 and 13) [61]. A cycloheptatriene carbene complex underwent a related [6+2] cycloaddition (Eq. 14) [62]. 

Heterocyclic syntheses take advantage of a heteroatom version of the vinyl-cyclopropane rearrangement — an iminocyclopropane rearrangement. For example, the aminocyclopropanol 76, available analogously to the hemiketal 58, readily... 

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

Figure 12.6. a) Cope rearrangement of 1,5-hexadiene (6) oxy-Cope rearrangement (c) divinyl-cyclopropane rearrangement (d) degenerate rearrangements of bullvalene. 

The alkyl carbonium ions which result from these reversible, relatively unselective hydride abstractions then undergo a series of 1,2- (Wagner-Meerwein) or 1,3- (protonated cyclopropane) rearrangements which eventually result in the formation of the thermodynamically most stable products. The number of different reaction sequences by which one may rationalize the formation of a given products is, of course, necessarily large. A variety of independent pathways are generally available for the interconversion of the isomers of a given species by successive alkyl shifts. 

As for the cyclopropane rearrangement, the reaction probably involved the formation of a cationic silver cationic species followed by regioselective o-bond migration, leading preferentially to the more stable carbocation intermediate that is finally intramolecularly trapped by the organosilver species (Scheme 3.25).39... 

Bicyclo[5.3.0]deca-l,4-dienes 25 have been prepared via a cyclopropanation-rearrangement sequence using acyclic dienes 24.Direct rearrangement was observed when ( )-dienes were utilized. In contrast, thermolysis at approximately 220 °C of those divinylcyclopropanes derived from (Z)-dienes (e.g. 26) was required following the cyclopropanation step. 

The rate constant is 9.2 s. (a) What is the half-life of the reaction (b) How long does it take for the concentration of cyclopropane to reach one-quarter of the initial value Plan (a) The cyclopropane rearrangement is first order, so to find ti /2 we use Equation... 

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