Vinylcyclopropane mechanism

The vinylcyclopropane rearrangement is of synthetic importance, as well as of mechanistic interest—i.e. the concerted vs. the radical mechanism. A reaction temperature of 200 to 400 °C is usually required for the rearrangement however, depending on substrate structure, the required reaction temperature may range from 50 to 600 °C. Photochemical and transition metal catalyzed variants are known that do not require high temperatures. 

Studies carried out by different research groups for more than 30 years have shown that this reaction is very general and usually takes place with high chemical and quantum yields affording vinylcyclopropanes that in many instances are difficult to obtain, or not available, by alternative routes. A biradical mechanism has been proposed to account for the rearrangement as... 

The nature of vinylcyclopropane radical cations was elucidated via the electron transfer induced photochemistry of a simple vinylcyclopropane system, in which the two functionalities are locked in the anri-configuration, viz., 4-methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 39). Substrates, 39 and 47 are related, except for the orientation of the olefinic group relative to the cyclopropane function trans for 39 versus cis for 47. The product distribution and stereochemistry obtained from 39 elucidate various facets of the mechanism and reveal details of the reactivity and structure of the vinylcyclopropane radical cation 19 . 

Vinylcyclopropane, when irradiated with benzophenone or benzaldehyde, gives a mixture of two types of products. Suggest the mechanism by which product of type C is formed. 

Scheme 21.1. The four possible stereoisomeric cyclopentenes-ii2 that could be formed by Cl—C2 scission of the indicated single enantiomer of vinylcyclopropane-ii2- One possible mechanism, involving interconverting achiral biradicals, is also depicted. The pairs of letters under each product isomer indicate the stereochemical changes between reactant and product. Note that in an actual experiment, the reaction would be complicated by competitive Cl—C3 scission. In this scheme s = superfacial, a = antarafacial, r = retention, and i = inversion.

C. Doubleday, Mechanism of the Vinylcyclopropane-Cyclopentene Rearrangement Studied by Quasiclassical Direct D3mamics, J. Phys. Chem. A 2001, 105, 6333. 

Whereas the parent difluoro-vinylcyclopropane isomerizes to difluorocyclopentene under pyrolysis conditions, the corresponding alkyl compounds also lead to acyclic dienes. The activation energy for the difluoro-vinylcyclopropane isomerization is practically identical with that observed for the unsubstituted hydrocarbon [211, 212], If the alkyl group is oriented cis to the vinyl substituent, only dienes are isolated, and the process occurs at much lower, temperatures. Presumably these stereoisomers rearrange by a different mechanism (a 1,5-homodienyl hydrogen shift [213]). When the dichlorocyclopropane XVII is subjected to flash vacuum pyrolysis it isomerizes to 9,9-dichloro-bicyclo[5.3.0]dec-l(7)-ene [214],... 

All of the experimental and theoretical work on the stereomutations of cyclopropanes and vinylcyclopropanes covered above seems consistent with and understandable in terms of kinetically significant involvements of Cj(ts), Cs(ts) and EF(ts) structures and partitionings of EE trimethylene intermediates resulting in the formation of klt k2 and kl2 products at comparable rates. For trans-1,2-disubstituted cyclopropanes, neither the Smith mechanism (one-center stereomutations only) nor any two-center-only formulation can be correct, as demonstrated by Crawford and Lynch in 1968143 and reinforced by numerous subsequent studies (Figures 2 and 3). 

Feldman239,240 and Oshima and Utimoto241 have introduced a very sophisticated sequence of reactions that effects a radical annulation starting from a vinylcyclopropane. Scheme 59 provides two examples and a likely mechanism for this sequence, which involves fragmentation, addition and cyclization. The... 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

Eq. 52 and 53 demonstrate remarkable characteristics of this [3 + 2]-cycloaddition starting with a pure diastereomer 130, two stereoisomeric cyclopentanes 131 are obtained. This stereorandom outcome is most simply rationalized assuming a stepwise mechanism with a 1,5-zwitterion as an intermediate in the cycloaddition. The vinylcyclopropane 132 only gives five-membered ring products 133 and no cyclo-heptene derivative, which would result from a conceivable [5 + 2]-cycloaddition. Less activated olefins or cyclopropanes do not undergo a similar [3 + 2]-cycloaddition. Due to the specific substitution pattern, the cyclopentane formation from these siloxycyclopropanes is of no preparative value. 

Scheme III/8. The vinylcyclopropane rearrangement as a tool for alkaloid synthesis a proposed mechanism [31].

The potential energy surfaces (or free energy surfaces) governing the thermal behavior of such relatively small molecules, one may conjecture, might well be amenable to complete and reliable assessments by modern computational methods, and this possibility has been pursued tenaciously for nearly 30 years. On the experimental side there have been equally determined efforts to define as completely as possible the relative importance of conceptually distinct and possibly distinguishable mechanisms for these reactions. Numerous review articles on the thermal chemistry of cyclopropanes " and of vinylcyclopropanes provide convincing testimony to the significance this topic has been accorded by chemists in recent decades. 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

The term di-ir-methane rearrangement is meant to describe the photoisomerization of 1,4-dienes (i.e. two TT-systems separated by a methane unit) leading to vinylcyclopropanes. The reaction can be generalized in schematic form as in equation (1). As will be discussed more specifically in the section dealing with mechanism, such a simplistic presentation is not intended as the actual reaction path, but it gready helps in predicting the reaction products. 

An extension of the simplistic but practical diradical mechanism of equation (1) leads to the generalized di-TT-methane process given in equation (43). Thus, the initial 1,3-diradical, instead of cyclizing across C-3 and C-5 to the final vinylcyclopropane, bonds across C-2 and C-5 to generate a cyclopro-pyldicarbinyl diradical, which on opening of the C-2/C-4 bond affords the isomerized 1,4-diene. A num-... 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

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