Cyclopropanes olefinic character

The first nonintroductory section of the text starts with the observation made early in this century that cyclopropanes have significant olefinic character. That is, corresponding ethylene and cyclopropane derivatives have significant similarities. There are literature comparisons of the thermochemistry of direct counterparts such as the parent species (1 and 2, X = H), propene and methylcyclopropane with X = Me, and of methyl acrylate and methyl cyclopropanecarboxylate 8 with X = COOMe. But the chemistry of substituted eth-ylenes is far richer than just that of vinyl compounds. One can retrieve enthalpy of formation data for cumulenes ( cumulenated olefins) such as allene (3) and both cis- and trans-2,3,4-hexatriene (4a and 4b), and for conjugated olefins such as 1,3-butadiene (5) and both (Z)- and (E)-1,3,5-hexatriene (6a and 6b). For the cyclopropane chemist it is natural to... 

As noted above, it is long-established that cyclopropane has olefinic character. If this cyclopropane-ethylene analogy were strictly thermochemically correct, then the difference of the enthalpies of formation of vinyl-X and cyclopropyl-X would not depend on the affixed group X. Equivalently, the enthalpy of formation difference quantity <517 (Vi, Cypr, X) would be independent of X (equation 17). 

Relevant examples of 1,2-dialkylcycloalkenes with which to test our conjecture about reaction 12 are disappointingly few. The simplest example of this class of compounds is 1,2-dimethylcyclopropene (24, n = 3, R1 = R2 = Me), and indeed, the requisite thermochemical data are available. Using the derived enthalpy of formation of 1,2-dimethylcyclopropene from Reference 56, we find this reaction to be 32 kJmol-1 endothermic. A posteriori, we are not surprised that this reaction is endothermic. After all, if it is part of the folklore of cyclopropanes that they are said to have olefinic character, then cyclopropenes are also said to have acetylenic character. Indeed, the related transalkylation reaction involving acetylenes... 

Since cyclopropane rings can often participate in place of double bonds, pericyclic reactions have been extended to them also. Atleast they show olefine-like character and in their ability to transmit conjugation. 

In addition to the [2-I-2-I-1] carbocyclization, an analogous [3-I-2-I-1] version has been reported [23]. The high 7r-character of the rr-bond of the cyclopropane provides reactivity similar to that of an olefin thus when the 4-pentynyl cyclopropanes 42 were subjected to the PK reaction conditions, the bicyclo[4.3.0]nonenone 43, in addition to a small amount of the oxidized product 44, was afforded in modest yield (Eq. 7). The analogous transformation with the vicinally disubstituted cyclopropane 45 furnished regioisomers 46 and 47, in which cleavage at the less substituted bond is favored (Eq. 8). 

As cyclopropanes have olefin-like properties, the presence of a double bond in the three-membered ring adds some acetylenic character to the methine protons. X-ray crystallographic and gas-phase structural studies on cyclopropenes have been reviewed by Allen11 and recently by Boese5. 

The relative importance of a and r contributions to the overall bonding is unclear, but several different combinations of relative strengths lead to limiting case models. When there are 2 electrons in the forward (T-bond and 2 electrons in the ir-backbond, there are 2 bonding electrons for each metal-carbon bond. This is mathematically equivalent to 2tr-bonds and a metallocyclopropane structure (72). This model does not necessitate strict sp3 hybridization at the carbon atoms. Molecular orbital calculations for cyclopropane (15) indicate that the C—C bonds have higher carbon atom p character than do the C—H bonds. Thus, the metallocyclopropane model allows it interactions with substituent groups on the olefin (68). 

The fascinating carbenoid character of the epoxide anion is also manifested in an intramolecular cyclopropanation reaction, in which the anion adds across a tethered olefin to provide bicyclo[3.1.0]hexanols (Equation 52). The reaction is remarkably chemo- and diastereoselective. No C-H insertion is observed, and yields are generally very good. The stereochemical outcome is rationalized on the basis of a /ra r-lithiation, as well as the geometric constraints imposed by the [3.1.0] bicyclic system <2004JA8664>. 

The direct photolysis and benzophenone-sensitized decomposition of methyl diazomalonate has been shown to produce two different spin states of biscarbo-methoxycarbene. In the direct photolysis experiments stereospecific addition to olefins and a high yield of insertion products in the presence of alkanes demonstrate the singlet character of the carbene in the sensitized decomposition the yield of insertion products decreases, and addition to cis- or ra 5 -4-methyl-2-pentene yields about 88 % trnwj-cyclopropane. In this case, as with most carboalkoxy-carbenes, the addition reaction to olefins can proceed by two paths ... 

Another application of rhodium carbenoid chemistry relates to the synthesis of strained-ring nitro compounds as high energy-density materials. Nitrocyclo-propanes are the simplest members of this class of compounds and catalyzed additions of a nitrocarbene to an olefin have only been described recently [40], Detailed studies have shown that the success of the reaction is, as expected, dependent on both the alkene and the nitrodiazo precursor. Consistently with the electrophilic character of rhodium carbenoids, only electron-rich alkenes are cyclopropanated. The reaction has been extended to the synthesis of nitrocyclo-propenes but the yields are good for terminal acetylenes only [41]. 

Interestingly, cobalt porphyrin catalysts tend to prevent carbene dimerization reactions, and allow cyclopropanation reactions with electron-deficient alkenes. This feature illustrates the more nucleophilic behavior of the carbenoid species formed as compared to typical electrophilic Fischer carbenes. The enhanced nucleophilic character of the carbene reduces its tendency to dimerize and allows reactions with more electron-deficient olefins. 

Several copper-exchanged and CuCl2-supported solids, together with copper oxide, have been tested as catalysts in the benchmark cyclopropanation reaction of styrene with ethyl diazoacetate. The catalytic activity does not depend on the amount of copper but on the structure and pretreatment of the catalyst. The trans/cis selectivity also depends on the nature of the solid and with KlO-montmorillonite the cis-cyclopropane is predominantly obtained, so that the selectivity is reversed with regard to that observed with copper homogeneous catalysts. The use of several olefins confirms this tendency to reverse the selectivity obtained in solution and the electrophilic character of the reaction. The effect of the reaction conditions and the influence of the solvent are also analyzed. 

Since incomplete deuterium incorporation into the phenylcyclo-propane product (29% di) excludes a free carbonium (or diazonium) ion as its precursor, a partially-protonated (partially exchanged) diazo species (20) was proposed to react with olefin to give a hexacoordinate, carbenoid-type intermediate (21) which gives either cyclopropane or syn ion pair (22). Intermediate (20), although not a normal diazonium ion, is presumably electrophilic and should have diazonium ion character, in support of the biogenetic analogy. Also of interest in the above reaction is the product resulting from the combination of olefin alkylation and... 

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