Thermal ring-enlargement

The differences between thermally and photochemically induced reactions of 2 are illustrated by the photolyses of cyclotrisilane 2 in the presence of disparate isocyanides. While in the thermally induced process ring-enlarged molecules can be isolated, the photolytic process results in the formation of 2,4-disila- and 3,4-disilacyclobutanediimines. A plausible explanation for the formation of these... 

The rearrangement of 70 to vinylketenes is thermally possible [37]. A variety of cyclobutenones are prepared by the transition-metal-induced carbon-carbon bond formation, which elevates the synthetic utility of their ring enlargement. 

A new selective thermal cascade ring-enlargement process of 4-chloro-substituted spiro[cyclopropane-l,5 -isoxazolidines], leading to a new method for the synthesis of the indolizine skeleton, has been reported (see Scheme 17). Apparently, the process is made possible by the presence of a chlorine substituent on the carbon a to the spirocyclopropane ring which facilitates a cyclopropyl-to-cyclobutyl ring enlargement mediated by a polar solvent. 

Table 1. Thermally Induced Ring Enlargement of Vinyl Cyclobutanes to Cyclohexenes...

The thermal rearrangement products of buta-l,3-dienyl substituted cyclobutanes are usually cyclohexenes.39 The vinyl group may be part of a phenyl ring, e.g. as in the case of 5,46,47 in which case rapid rearomatization follows the ring enlargement. 

The ring enlargement of 2,3-divinylcyclobutanones has been catalyzed by boron trifluoride or trifluoroacetic acid.110 This acid catalysis, however, usually gives lower yields than the thermal reaction and favors rearrangement to five- or six-membered rings. 

Nickel(O) or palladium(II) compounds in stoichiometric amounts promote the ring enlargement of simple alkyl-substituted 1,2-divinylcyclobutanes in benzene at room temperature to give 1 1 metal complexes of cycloocta-1,5-dienes.119 Destruction of the palladium complexes with potassium cyanide affords the free cycloocta-1,5-dienes. The stereochemistry observed is the same as in the thermal reaction at 150°C. 

Donor-acceptor-substituted vinylcyclopropanes such as (203) have been shown to undergo thermal ring enlargement to functionalized cyclopentene derivatives (204) at... 

Conversely, the photoinduced ring-opening of cyclobutenes allows the construction of medium-sized hetero- or carbocycles, starting from bicyclic frameworks. Alkenes can undergo thermal (metal-assisted) or photochemical [2 + 2]-cycloaddition to a variety of alkynes the subsequent two-carbon ring-enlargement can serve as a versatile method for the preparation of hydroazulenes and dioxacyclooctadiene derivatives (Scheme 9.44) [74, 75]. 

Mislin, G.L. and Miesch, M. (2003) Photochemical, thermal and base-induced access to hydroazulene derivatives via two-carbon ring-enlargement reactions of condensed electrophilic cyclobutenes. Journal of Organic Chemistry, 68, 433 141. 

The an/i-orientation of the deuterium at C5 on the spirocyclobutanone 218 was proven unambiguously by examination of the H NMR spectra of this ketone coordinated with tris-(dipivalomethano)europium effectively, two protons at C5 and C8 were markedly shifted downfield (A8 = 3 ppm) implying their syn orientation with respect to the carbonyl group. Therefore, the thermal ring enlargement 217 - 218 seemed to involve an intramolecular stereospecific cis-addition on the double bond (Fig. 2) which, consequently ruled out a concerted process41. ... 

The efficient formation of dihalocyclopropanes could be extended to allyl alcohol adducts 74, which are oxidized to 75. The resulting ketones are very useful synthetic building blocks. Reaction with alkoxide, for instance, affords dihydrofuran derivatives 76 via alkoxysubstituted cyclopropyl ketones 43). This ring enlargement might be a purely thermal process (1,3-sigmatropic shift), but other mechanistic possibilities could also be conceived (cf. Scheme 2). 

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