Tetrafluoroethylene, cycloaddition reactions

Although tetrafluoroethylene and other 1,1-difluoroalkenes readily undergo thermally-induced [2 + 2] dimerization, perfluoromethylenecyclopropane (105) does not dimerize on heating at 150°C for 24 h [28]. On the other hand, it readily undergoes [4 + 2] cycloaddition reactions (Table 10) [29]. 

The photochemical addition of simple olefins to a,/ -unsaturated ketones is a reaction of great current interest.113 The steroidal A16-20-ketone system is especially prone to cycloaddition under mild conditions. Sunder-Plassmann et al114,115 irradiated 3/ -acetoxypregna-5,16-dien-20-one (67) in the presence of tetrafluoroethylene and obtained a 2 1 mixture of the a- and / -face adducts (68) and (69) together with a third adduct which has been characterized as (70) by X-ray crystallographic analysis.1153... 

Indications of the occurrence of cycloaddition were first obtained from reactions of specifically deuterated allyl anions with tetrafluoroethylene. Assuming that no hydrogen/deuterium exchange occurs in the collision complex as shown for the allyl anions themselves (Dawson el al., 1979a), the results obtained (Nibbering, 1979) may be interpreted as indicating that 65% of the allyl anions react by a linear addition (51), 20% by a [2 + 2] atom cycloaddition (52) and 15% by a [2 + 3] atom cycloaddition. (53). It should be noted here that the precise mechanistic details of the losses of HF molecules from the collision complexes in eqns (51)—(53) are not known. However, in view of the nucleophilic aromatic substitution discussed in the previous section, it is quite likely that they occur in a stepwise fashion in which complexes solvated by fluoride anions play a role. 

The observations described prompted a study of the reaction between the 2-cyanoallyl anion and tetrafluoroethylene, since this system has the merit that the charge may be accommodated in the nitrile group of the reaction intermediate in the [2 + 3] atom cycloaddition (54). Elimination of two HF molecules indeed occurs, presumably in a consecutive way, although the product ions resulting from loss of one molecule of HF from the collision complex have not been observed (Dawson and Nibbering, 1980). However, reaction (54) has not yet been studied with specifically deuterated 2-cyanoallyl anions, so that at present an end-on addition process (55) cannot be excluded. 

If the reaction mixture is contained in an internal poly(tetrafluoroethylene) cell transmitting IR radiation (see Sec. 6.7.2), quantitative analysis via high-pressure spectroscopy (Figures 6.7-16 to 6.7-18)) yields precise values of the rate coefficients of these Diels-Alder reactions. As an example, the pressure dependence of the rate coefficient k of the cycloaddition of 2 (with R = CF3) and 3 in CH2CI2 solution at 45 °C (Buback et al., 1989) is shown in Fig. 6.7-19. 

The preferred sp3 hybridization for saturated fluorocarbons is a major driving force for the easy polymerization of TFE to PTEF with more exothermic enthalpy change (Scheme 1.34) [2]. The it-bond energy of tetrafluoroethylene is 52 kcal/mol, which is smaller than that of ethylene, 64-65 kcal/mol [3].The easy [2 + 2] cycloadditions of gem-difluoroalkenes (thermal concerted reactions of which are forbidden) proceed stepwise via biradical intermediates. The intermediacy of a biradical is supported by the fact that a 1 1 mixture of the cis- and trans-dideuteriocyclobutanes (7 and 8) was obtained in the [2 + 2] cycloaddition of TFE with cis- (4) and frans-dideuterioethylene (5) [4] (Scheme 1.34). 

Noteworthy, very few reports can be found on UV light-initiated cycloaddition of carbonyl compounds to tetrafluoroethylene (TFE). " The yields of oxetanes in these processes are generally low due to facile polymerization of TFE under the reaction conditions. 

Fluorinated alkenes have been reported to give cyclobutane derivatives by thermal additions to other alkenes. Olefins in their ground electronic states have not been observed to give stereospecific cycloaddition one with another whereas a carbon-carbon double bond which is part of an allene or keten will undergo stereospecific concerted cycloadditions. The separate reactions of tetrafluoroethylene with cis- and trans-[l,2- H2]ethylene, which should not be subject to an adverse steric effect, have been studied to establish whether these reactions are stereospecific. Identical mixtures of products were obtained from both reactions, as expected if they proceed through the... 

Further experimental results (see also ref. 164) on the stereospecificity of [2 -1- 2] cycloadditions are available. Tetrafluoroethylene adds to both cis- and tra s-2-butene and to ethylene in a non-stereospecific manner, interpreted as evidence of biradical intermediates. Some stereospecificity is observed in allene-keten [2 + 2] cycloadditions and a one-step [,2, + J cycloaddition is indicated. Further deuterium-isotope studies indicate that the cyclodimerization of allenes by a [2 q- 2] addition is non-concerted, but their reactions in [3 + 2] and [4 + 2] cycloadditions are concerted. ... 

The characteristic reactivity of tetrafluoroethylene, and of related polyfluoro-olefins and -dienes, in addition reactions, including 2+2 cycloadditions, has recently been discussed extensively in terms of hybridisation effects. Hybridi-... 

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