Vinylcyclopropanes pyrolysis

Upon flash vacuum pyrolysis or under silver nitrate catalysis, a variety of 2-ethenyl-sub-stituted cydopropylamines 146 cleanly undergo a vinylcyclopropane to cyclopentene rearrangement [129] and afford high yields (up to 95%) of 4-aminocyclopent-l-enes 147, some of which have unprecedented substitution patterns (Scheme 11.37) [130],... 

Vinylcyclopropanation cyclopentenes. The lithium dienolate of 1 adds to a,p-enones to form an a-keto vinylcyclopropane, which on pyrolysis (550°) provides a bicyclic keto acrylate (equation I).1... 

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

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],... 

The thermal isomerization of l-ethynyl-2-vinylcyclopropane 57 has been also reported. Thus, the dimer 59 was formed in the pyrolysis of the tosylhydrazone salt... 

Heteroatoms, depending upon their location, have varied effects. Thus, for instance, the pyrolysis of l,l-dichloro-2-methyl-2-vinylcyclopropane (180) at 200-275 C yielded exclusively l-methyl-4,4-dichlorocyclopentene (181) whereas its isomer, l,l-dichloro-2-(l-methylcyclopropyl)ethylene (182) was remarkably stable up to 400 °C (equation 123) . 

Formation of cyclopentene as one of the products of pyrolysis of vinylcyclopropane was first reported by Overberger and Borchert and then was quantified by Flowers and Frey °, who reported the activation parameters shown in Figure 20. 

In 1967 Willcott and Cargle reported the pyrolysis and rearrangement of trans-vinylcyclopropane-2-d. They found that stereomutation occurred at least five times faster than rearrangement, a result that might seem inconsistent with the intermediacy of a common biradical for the two processes. It is important to note, however, that there are two geometrical isomers of the biradical, both of which can cause stereomutation but only one of which can allow ring expansion to the cyclopentene (see Figure 24). It is unlikely that the... 

One reaction, mentioned in this section, combines some features of the meth-ylenecyclopropane rearrangement and the vinylcyclopropane rearrangement (Section III.C) it is the pyrolysis of 2-vinyl methylenecyclopropane. Here, too, studies with deuterium-labeled compounds have been valuable in helping to unravel the various mechanistic pathways involved. 

T. Hudlicky et al. achieved the short enantioselective total synthesis of (-)-retigeranic acid. " The C ring of the natural product was assembled via the thermai vinyicyciopropane-cyciopentene rearrangement for which the precursor was prepared by the vinyicyciopropanation of a bicyclic enone with a dienolate. The vinylcyclopropane was evaporated at 585 °C in high vacuum through a Vycor tube conditioned with PbCOs (flash vacuum pyrolysis) to afford the annulated product in good yield. 

Neureiter, N. P. Pyrolysis of 1,1-dichloro-2-vinylcyclopropane. Synthesis of 2-chlorocyclopentadiene. J. Org. Chem. 1959, 24, 2044-2046. Overberger, C. G., Borchert, A. E. Novel thermal rearrangements accompanying acetate pyrolysis in small ring systems. J. Am. Chem. Soc. 1960, 82, 1007-1008. 

Crawford, R. J., Cameron, D. M. Pyrolysis of 3-vinyl-1-pyrazoline and 3-vinyl-1-pyrazoline-5,5-d2, and its relation to the vinylcyclopropane to cyclopentene rearrangement. Can. J. Chem. 1967,45, 691-696. 

Gajewski, J. J., Squicciarini, M. P. Evidence for concert in the vinylcyclopropane rearrangement. A reinvestigation of the pyrolysis of trans-1-methyl-2-(1-tert-butylethenyl)cyclopropane. J. Am. Chem. Soc. 1989, 111, 6717-6728. 

Table 1. Fused Carbocyclic Ring Systems from Flash-Vacuum Pyrolysis of Vinylcyclopropanes...

Stereoisomeric vinylcyclopropanes 36 under flash-vacuum-pyrolysis conditions yield mixtures of cis- and trans- isomers of cyclopentenes 37 and 38 in good yield (see table below). 

A recent example of a cyclopentene annulation utilizing flash-vacuum pyrolysis led to hydrocarbons of medium molecular weight, e.g. 39 and 40. The practical limit of this particular method is defined by the volatility of the starting vinylcyclopropanes under high vacuum (approximately 400-500 MW at lO Torr). 

One example of a reverse vinylcyclopropane to cyclopentene rearrangement has been reported during the flash-vacuum pyrolysis of tricyclic alkene 41. ... 

The pyrolysis of vinylcyclopropanes 78 to 79 was accompanied by the competing vinyl-cyclopropane to cyclopentene rearrangement, which furnished sesquiterpene intermediates 80.8 ... 

Bridged bicyclic ketones 7 are obtained by the Cope rearrangement of silyl enol ethers or lithium enolate anions derived from ent/o-vinylcyclopropanes 4. i 11 The corresponding exo-isomers of 5 require flash-vacuum pyrolysis at temperatures in the range of 250-500°C to... 

Overberger and Borchert (1960) were the first to report that the pyrolysis of vinylcyclopropane 3nelded 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... 

A second example of this is provided by the pyrolysis of cis-l-methyl-2-vinylcyclopropane, which decomposes at about 120 °C lower than its trans isomer to give cis-hexa-1,4-diene (17, 18, 58) ... 

In an effort to determine the stereochemistry of the 1,3-shift of what at the time was thought to be a minimally perturbed vinylcyclopropane system, which also would not suffer from extensive geometric isomerization via an s-trans alkylallylic species, Olson prepared and pyrolyzed c 5-2,3-dideuterio-fran5 -l-(Z-2-deuterio-l- r -buty-lethenyl)cyclopropane." The H NMR of the epoxide of the major cyclopentene product from short-term pyrolysis revealed greater than 90% suprafacial inversion stereochemistry in the rearrangement despite the fact that the stereochemistry of the starting material was also compromised in a faster reaction, apparently via reversible formation of an s-cis alkylallylic species (Scheme 6.19). 

Cyclopentane Annulation. Condensation of l-lithio-l-(tri-methylsilyl)cyclopropane with ketones gives vinylcyclopropane intermediates, which upon pyrolysis undergo vinylcyclopropane rearrangement to give annulated cyclopentene derivatives (eq 5). ... 

So far the systems under consideration for the Diels-Alder reaction with MA were conjugated dienes or systems that could give rise to diene intermediates. The latter could be generated, as we saw, by isomerization, tautomeriz-ation, pyrolysis, or even as the result of DA reaction when in some cases the primary products are unstable. In addition, there are some cases where a double bond in the diene may be replaced by a cyclopropane ring. An example of this is the vinylcyclopropane derivative reported by Sarel and Breuer. They found a-cyclopropyl-styrene 184 to react with MA. Two main products were obtained. Product 185, obtained in a 40% yield, resulted from participation of the vinylcyclopropane unit in an apparent 1,5 addition process. The other product, 186, results from participation of the styrene unit in the fashion discussed earlier. 

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