Vinylcyclopropanes products

A further point of interest is the reaction regiochemistry. For the reaction in Scheme 2 we might consider two possible vinylcyclopropane products. Thus the intermediate cyclo-propyldicarbinyl diradical might open in two ways. However, only one product is formed. In Scheme 3 this is shown more definitively where the two modes of unzipping the cyclopropyl dicarbinyl diradical are depicted. A general rule is that the less stabilized diradical center utilizes its odd-electron density to open the three-membered ring. This leads onwards to the more stabilized Diradical II 3. 

The di-7r-methane rearrangement, takes place on structures that possess two -groups connected with a single carbon (the central carbon), upon direct and/or sensitized irradiation of a C=C double bond chromophore, affording vinylcyclopropane products. Zimmerman proposed a stepwise cyclization of the excited --system via a 1,4-diradical of... 

In selected cases, l-/ -alkyl-3,4,5-t -allyl complexes can be obtained via nucleophilic attack of carbanions at an internal carbon of pentadienyl ligands. Thus, in contrast to a previous communication, nucleophilic attack of the malonate carbanion does not occur at the terminal position of a pentadienyltricarbonyliron cation complex 1 to give a diene complex product 2 as the major product. Instead, the internal position is attacked to give an alkyl-allyl complex 3, which upon oxidative decomposition aifords a vinylcyclopropane product X ... 

Butadienylsulphonium salts (140) undergo condensation with stabilized carbanions to give vinylcyclopropanes in good yields (37—65%) by two routes (paths a and b Scheme 16). The operation of both pathways has been established by using butadienes substituted at C-3 which result in two distinct vinylcyclopropane products. The... 

An efficient carboannulation proceeds by the reaction of vinylcyclopropane (135) or vinylcyclobutane with aryl halides. The multi-step reaction is explained by insertion of alkene, ring opening, diene formation, formation of the TT-allylpalladium 136 by the readdition of H—Pd—I, and its intramolecular reaction with the nucleophile to give the cyclized product 137[I08]. 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

In contrast to the behavior of homoallylic alcohol (70a) when treated with methanesulfonyl chloride is pyridine, heating A -19-methanesulfonate (68b) in pyridine gives the 5)5,19-cyclo-6-ene (72). Vinylcyclopropane (72) is inert to the conditions used for converting vinylcyclopropane (73) to the A ° -B-homo-7)5-ol (70a). The latter results are only consistent with the existence of two discrete isomeric carbonium ion intermediates which give rise to isomeric elimination products. °... 

Reaction of the cyclopropyl-substituted pivalate (25) with dimethyl benzylidenema-lonate in the presence of a palladium catalyst gave a mixture of alkylidenecyclo-propane (26) and vinylcyclopropane (27). The ratio of these two adducts is found to be quite sensitive to the choice of ligand and solvent. While triisopropyl phosphite favors the formation of the methylenecyclopropane (26), this selectivity is completely reversed with the use of the bidentate phosphite ligand dptp (12). Interestingly there was no evidence for any products that would have derived from the ring opening of the cyclopropyl-TMM intermediate (Scheme 2.8) [18]. 

The rearrangement proceeds from the Si-state of the 1,4-diene 1. The Ti-state would allow for different reactions like double bond isomerization. Rigid systems like cyclic dienes, where EfZ -isomerization of a double bond is hindered for steric reasons, can react through the Ti-state. When the rearrangement proceeds from the Si-state, it proves to be stereospecific at C-1 and C-5 no -isomerization is observed. Z-l,l-Diphenyl-3,3-dimethyl-l,4-hexadiene 5 rearranges to the Z-configured vinylcyclopropane 6. In this case the reaction also is regiospecific. Only the vinylcyclopropane 6 is formed, but not the alternative product 7. ... 

The vinylcyclopropane rearrangement is an important method for the construction of cyclopentenes. The direct 1,4-addition of a carbene to a 1,3-diene to give a cyclopentene works only in a few special cases and with poor yield. The desired product may instead be obtained by a sequence involving the 1,2-addition of a carbene to one carbon-carbon double bond of a 1,3-diene to give a vinylcyclopropane, and a subsequent rearrangement to yield a cyclopentene ... 

Vinylcyclopropane reacts with HBr to yield a rearranged alkyl bromide. Follow the flow of electrons as represented by the curved arrows, show the structure of the carbocation intermediate in brackets, and show the structure of the final product. 

Ferrate 38 also turned out to catalyze [5 + 2]-cycloadditions. In this context, various vinylcyclopropane derivatives 52 were converted into the corresponding cycloheptadiene derivatives 53. The products were obtained in good to excellent diastereoselectivites favoring the 1,2-trans-disubstituted isomer 53a. 

Even the three-component coupling reaction of diphenyl diselenide, ethyl propiolate, and unsaturated compounds occurs under photo-irradiation (Table 5). Alkenes,198,199 1,3-dienes,198,199 vinylcyclopropanes,200 and isocyanides199,201 serve as the unsaturated coupling partner to afford the corresponding coupling products with high regioselectivity. 

The product possesses a homoallylic stannane moiety, which can be utilized as a useful synthon for cyclopropane formation (Scheme 68). Upon treatment of the homoallylstannane with HI, destannative cyclization takes place to give cyclopropylmethylsilane.271,272 A Lewis acid-catalyzed reaction with benzaldehyde dimethyl acetal affords vinylcyclopropane.273... 

The extrapolation of the vinylcyclopropane-cyclopentene rearrangement to a vinyl-cyclobutaiie-cyclohexene synthesis begins to create new insights into the synthesis of six membered ring natural products. The eudesmane sesquiterpene (—)-P-selinene, 217 illustrates such a strategy as summarized in Scheme 14 80). A suitable cyclohexene... 

The opening of cyclopropylcarbinols to homoallylic bromides constituted the first use of cyclopropyl compounds for the stereocontrolled synthesis of natural products. The cyclopropyl conjunctive reagents enhance the richness of this notion. The stereocontrolled opening of vinylcyclopropanes by a homopentadienyl proton shift provides an approach to trisubstituted olefins and thereby a new strategy. The fungal prohormone methyl trisporate B (224) as summarized in Scheme 15 illustrates this conceptual development97). 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

Rhodium-catalyzed [5 + 2]-cycloaddition of an allene and a vinylcyclopropane proceeds with complete chemo-, endo/exo- and diastereoselectivity, representing an effective general route to bicyclo[5.3.0]decane derivatives (Scheme 16.75) [81]. This cydoaddition protocol has been applied successfully to asymmetric total syntheses of natural products [82, 83]. 

Overberger and Borchert (1960) were the first to report that the P3u olysis of vinylcyclopropane yielded cyclopentene as the major product. Independently Flowers and Frey (1961b) studied this isomerization and found that it was homogeneous and kinetically first order and almost certainly unimolecular. The Arrhenius equation for the isomerization was found to be... 

Since the isomerization to the cyclopentene is made more difficult by the substitution, but the isomerizations to the dienes are tmaffected, the result is that (unlike the case of vinylcyclopropane itself) the dienes now constitute major products of the reaction. 

A particularly interesting example is that of 1,2-dicyclopropylethylene. In this case, the rearrangement of one of the cyclopropyl rings leads to a product which still contains a vinylcyclopropane system, and hence... 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

As a consequence of the above observations, the intramolecular capture of the vinylcyclopropane radical cation was attempted next. Thereby, chrysanthemol and homochrysanthemol both yielded the expected cyclization products (Scheme 35) [55],... 

Since the pioneering work by Sarel and co-workers on the iron carbonyl promoted transformation of vinylcyclopropanes and related compounds [1], a variety of transition metal complexes have been examined to achieve effective activation of the vinylcyclopropane-cyclopentene rearrangement which usually requires pyrolytic conditions. These reactions have been applied to natural product synthesis in some cases and have already been reviewed in several excellent articles [2-4]. 

Thermal rearrangement of propadienylcyclopropanes to methylenecyclopentenes has been examined in several cases however, selective transformation to the product has not necessarily been easy due to the harsh reaction conditions required for the rearrangement. The first example of this type of reaction was reported by Dewar, Fonken, and co-workers in a paper on the kinetics of the thermal reaction of 3-cyclopropyl-l,2-butadiene (44), and the reaction was found to proceed much faster (activation energy difference 8.2 kcal) than that of the corresponding vinylcyclopropane [25]. Several examples have appeared since this initial work, most of which have dealt with the mechanistic aspect of the reaction, but none of them has reached a synthetically useful level [26]. For example, thermal reaction of 3-(2-methylcyclopropyl)-1,2-butadiene (45) gives a mixture of five products, as shown in Scheme 20 [27]. 

Another unusual two-step [3 + 2] cycloaddition involves the ring expansion of tert-bu-tyl-l-vinylcyclopropane-l-carboxylate 148 to the a-ethylidenebutyrolactone 149 (Scheme 14.18) [108]. When the reaction is catalyzed by boron tribromide the monocycHc product 149 is formed, but when the Lewis acidic oxidant VOCl2(OEt) is used, a very unusual dimeric product (150) is formed. 

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. 

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