Cycloadditions with vinylcyclobutanes

Initial attempts to effect the [6+2] cycloaddition with simple vinylcyclobutanes were unsuccessful. Either the substrates were unreactive or complex mixtures resulted. The work of Liebeskind (Scheme 13.9) [20] suggested that a vinylcyclobutanone might be a preferred 6 C component. Such systems are more strained and would be expected to encounter heteroatom-facilitated insertion. 

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. 

The second problem in the addition of 1,1-dichlorodifluoroethylene to 1,3-butadiene is regiospecificity. Carbon 1 of 1,3-butadiene may become attached either to the carbon holding two chlorines or to the carbon with two fluorines. The cycloaddition of fluorinated alkenes is usually not a concerted four-center reaction in which the bonds are formed simultaneously or nearly so. Instead, the reaction is a stepwise biradical process in which the first step is formation of a free-radical intermediate with a single electron at that end of the double bond that can better accomodate it. That happens at the carbon linked to two chlorine atoms. Thus, a biradical is formed that cyclizes to form 2,2-dichloro-l,l-difluoro-3-vinylcyclobutane M [118, 119, 720]. 

It is well documented that fluoroalkenes lacking additional electron-withdrawing groups more readily undergo [2-F 2]- than [4-f 2]-cycloaddition reactions.For example, tetrafluoro-ethene (1) undergoes a [2 -F 2]-cycloaddition reaction with buta-1,3-dicne to give 1.1,2,2-tetra-fluoro-3-vinylcyclobutane (2) as the only isolated product. ... 

Kinetic modeling based on the scheme revealed that vinylcyclobutane was formed with logk = 15.4 — 12300/23RT which places this pathway, which presumably involves a cisoid biradical, 6.3 kcal/mol higher in enthalpy (and free energy) than the retro 4 + 2 cycloaddition. This difference is 5 kcal/mol than the thermochemical estimates of Scheme 7.105, which is based on a temperature roughly 500°C lower. 

The homodienyl-1,5-hydrogen shift in cw-2-alkylvinylcyclobutane was initially observed in the 2-ethyl case (see Chapter 7, Section 4 - vinylcyclobutane) where it constituted roughly two-thirds of the total reaction. Kinetic studies with 2,2-dimethylvinylcyclobutane revealed that the hydrogen shift had log A = 10.8 — 38 380/2.3/ r, but the retro 2-h 2 cycloaddition had log A = 15.09 — 47440/ 23RT and was faster than the hydrogen shift by a factor of three even at the lowest temperature (Scheme 8.61). ... 

MINDO/3 calculations of the cycloreversion of vinylcyclobutane to ethylene and butadiene show a biradical transition state to be involved, and the cycloaddition of cyclobutadiene and acetylene, affording Dewar benzene (4), was also examined. The equilibrium geometry of the latter, predicted by semi-empirical INDO calculations, was in good agreement with experimental results and the calculated dipole moment of < 0.04 D suggests that the likelihood of obtaining a microwave spectrum is marginal at best. ... 

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