Cyclopropane olefins compared with

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

Reactions of rhodium porphyrins with diazo esters - According to Callot et al., iodorhodium(III) porphyrins are efficient catalysts for the cyclopropanation of alkenes by diazo esters [320,321], The transfer of ethoxycarbonylcarbene to a variety of olefins was found to proceed with a large syn-selectivity as compared with other catalysts. In their study to further develop this reaction to a shape-selective and asymmetric process [322], Kodadek et al. [323] have delineated the reaction sequences (29, 30) and identified as the active catalyst the iodoalkyl-rhodium(III) complex resulting from attack of a metal carbene moiety Rh(CHCOOEt) by iodide. 

Other terminal olefins were transformed to the corresponding cyclopropane esters with Z-menthyl and d-menthyl diazoacetates with high stereoselectivity up to 98% ee (Scheme 3). Intramolecular reaction of the phenyl-allyl ester 9 was carried out to give the bicyclic compound 10 with 86% ee and 93% yield. The enantioselectivity for intramolecular cyclopropanation of the 3-methylbutenyl ester 11 was compared with chiral Cu(I), Rh(II), and Ru Pybox catalysts Rh>Ru>Cu [26]. 

Cu(ll) complexes ofbisoxazohnes 42a, 42b and 43 (Figure 7.12) have been reported to catalyze the asymmetric cyclopropanation of olefins with diazo-compounds in ionic liquids (Table 7.11). The catalytic activities increased in ionic liquids compared with that in organic solvent (compare entry 1 with 9 entry 2 with entries 3 and 6) [63]. One important finding here was that catalytically less-active but cheaper and moisture-stable CuCfi could be activated in ionic liquids, in which the anion of the ionic liquid may exchange with chloride to generate the more reactive... 

General Methods.—The preparation of cyclopropanes by addition of the Simmons-Smith reagent [from CHjla + Zn (Cu)] to olefinic substrates has been a cornerstone in synthesis for almost two decades. Kawabata and co-workers have now described a seemingly more convenient procedure which uses copper in place of the familiar zinc-copper couple and produces cyclopropanes in comparable yields. In addition, the procedure can be employed with trihalogenomethanes and dibromo-acetates leading to monohalogenocyclopropanes and cyclopropane carboxylates, respectively, both with 5> n-stereoselectivity (Scheme 2). 

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... 

We now look at the change associated with two carbons in cyclopropane and cyclobutane being sp2 instead of sp3, i.e. we consider cyclopropene (16) and cyclobutene (14). The difference of their enthalpies of formation is (120.4 2.9) kJ mol"1. Is this still larger change due to destabilization of cyclopropene and/or stabilization of cyclobutene (cf species 15 with n = 3 and 4) One way of appraising this is to look at the olefination enthalpies (equation 37) of cyclopropane, cyclobutane and butane. These three numbers are (223.8 2.6), (128.3 1.6) and (118.5 1.2) kJ mol"1 showing that cyclobutane is comparatively normal (i.e. more like the unstrained, acyclic propane) while cyclopropane is considerably different. 

Stereocontrol in intermolecular cyclopropanation also depends on the structure of the unsaturated substrate. Early work concerning the influence of substrate on stereoselectivity has been summarized by Doyle2. In general, cyclopropanation of ciy-disubstituted alkenes results in higher stereoselectivity than with monosubstituted alkenes and the steric bulk of the olefinic substituent enhances the stereoselectivity. However, the stereocontrol appears not simply to be caused by a steric factor. In comparable cases, the presence of halogen as an alkene substituent may cause a reversal of the normal stereoselectivity. A few examples which illustrate these effects are shown in equations 124167 172, 74. 

For R = H and Me, the derived values are [321.3 ( >1.9)] and [323.3 ( >1.4)] klmoF , respectively. A value of [326 ( > 4)] kJmoF for AHf(g, 1,2,3-butatriene) is thus credible. What is found for the cyclopropanation enthalpies of butatriene There are seemingly no relevant data for either of its monocyclopropanation products, dimethylenecyclopropane (25a) or vinylidenecyclopropane (25b). The two dicyclopropanation products have comparable enthalpies of formation dicyclopropylidene (26), 286.6 (1) and 324.3 (g), and meth-ylenespiropentane (27), 287.0 (1) and 320.9 (g), respectively". The 2-6 kJ moF decrease in enthalpies of formation for gaseous dicyclopropanated products is not particularly in accord with the 3 kJ moF increase per alkyl substituent of cyclopropanation of simple olefins. However, in that the allene —> methylenecyclopropane —> spiropentane (3 23 7) enthalpy of formation changes are still enigmatic, and error bars are absent for the dicyclopropanated products, we do not fret. But we eagerly await more thermochemical data. 

DBU and silica gel in 1,2-dichloroethane, respectively.Paclitaxel and docetaxel analogs with 7a-F are not superior to the parent compounds in both in vitro and in vivo, and those with 7,19-cyclopropane and 6,7-olefin were comparably active. ... 

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