2-Butenyl radical

EPR spectra have been widely used in the study of reactions to detect fiee-radical intermediates. An interesting example involves the cyclopropylmethyl radical. Much chemical experience has indicated that this radical is unstable, giving rise to 3-butenyl radical rapidly after being generated. 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

It has also been proposed that the ring-opened radicals may undergo ring-closure to a cyclobutane (Scheme 4.23).202,2 8 At this stage the only evidence for this pathway is observation of signals in the NMR spectrum of the polymer that cannot be rationalized in terms of the other structures. There is no precedent for 1,4-ring-closure of a 3-butenyl radical in small molecule chemistry and the result is contrary to expectation based on stcrcoclcctronic requirements for intramolecular addition (Section 2.3.4). However, an alternate explanation has yet to be proposed. The possibility of carbonium ion intermediates should not be discounted. 

This is less common than rearrangement of carbocations, but it does occur (though not when R = alkyl or hydrogen see Chapter 18). Perhaps the best-known rearrangement is that of cyclopropylcarbinyl radicals to a butenyl radical. The rate constant for this rapid ring opening has been measured in... 

Cyclopropylcarbinyl radicals (5) are alkyl radicals but they undergo rapid ring opening to give butenyl radicals." The rate constant for this process has been measured by picosecond radical kinetic techniques to be in the range of 10 M s for the parent to lO Af s for substituted derivatives. This process has been observed in bicyclo[4.1,0]heptan-4-ones. ... 

This second alternative is more consistent with the general behaviour of ionized alky-lamines, many of which isomerize rapidly to DIs. Thus, extensive exchange of the hydrogen atoms attached to nitrogen with those of the methyl groups precedes butenyl radical loss from ionized labelled neopentylamines116. 

The 3-exo cychsation of ester-substituted 3-butenyl radicals is important in the rearrangement of 2-methyleneglutamate to 3-methyl itaconate catalysed by a-methyleneglutamate mutase. Newcomb and co-workers have applied laser flash photolysis to cleverly designed precursors to show that an ester group at the 1-position of 3-buten-l-yl accelerates the 3-exo cychsation by a factor of about 3, but that the same substituent at the 3-position slows the process by a factor of about 50 [45]. 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec". - The various stereoelectronic effects of this rearrangement have been reviewed. The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

The radiolysis of liquid ethylene shows many of the same reactions as the gas-phase radiolysis, with the ethyl, vinyl, and hydrogen atoms being the predominant radical species . The vinyl radical plays a more important role in the liquid phase, as does the butenyl radical. 

Providing R has a facile reaction, usually with O2, such as (27) for butenyl radicals, then [allyl] [R], and the recombination of allyl radicals (19) remains by far the dominant termination step. With (19) the only termination, and low [O2] so that radical branching reactions are negligible, then the ratio ki/kip can be obtained from the initial rates of formation of HDE in the presence (Rhde) and absence (Rhde)o of RH and the use of equation (1.7)... 

Turning to butene-1 and trans-butene-2, a totally different abstraction mechanism becomes apparent. The high yields of butadiene (Table 1.28) indicate that the delocalized butenyl radicals formed via H abstraction... 

Although /c63 2 X 10 cm molecule" s at about 750 K is a factor of 50-100 lower than for alkyl + O2 reactions, it is about lO higher than k(,4 for allyl + O2, and so provides a good sink for butenyl radicals. 

Butenyl Radicals. Walling and Thaler have shown that the free butenyl radicals formed in chlorinating butenes by tert-huty hypochlorite are configurationally stable (57) ds-2-butene yielded no trans-2-butenyl chloride, and trans-2-hutene yielded no ds-2-butenyl chloride. Thus, hydrogen transfer from hydrido complex to butenyl radicals may account for the partial retention of structure observed in the reduction of butenyl chlorides (Table II). Menapace and Kuivila likewise postulate the formation of such radicals as intermediates in the reduction of the isomeric chlorides with triphenyltin hydride (26). 

At —180° the spectrum from ethylene indicates the presence of both vinyl and ethyl radicals, and at a higher temperature ( — 104°) the spectrum of the 3-butenyl radical, resulting from the addition of the vinyl radical to ethylene, is observed. The spectrum of the vinyl radical is particularly interesting it has been interpreted in terms of uncertaintybroadening which requires that the radical interconverts between the... 

Rearrangements of a-substituted cyclopropylmethyl radicals afford mixtures of the (E)-and (Z)-butenyl radicals usually with a preponderance of the Z-isomer. This stereoselectivity is probably a consequence of the higher proportion of the a /-conformer compared to the j n-conformer, the former being lower in energy for steric reasons i.e. nonbonded interactions between the substituent and ring hydrogens are less important. The -alkene is also thermodynamically more stable than the Z-alkene. ... 

The reaction wifh cyclopropylmefhyl bromide confirms the proposed pafhway. As might be expected, cyclopropylmefhyl and 3-butenyl units were introduced regioselectively giving rise to 143 as fhe sole dialkylation product in 50% yield (Scheme 3.147). This result, wifh fhe evidence fhat ring opening of cyclopropylmefhyl radical to fhe 3-butenyl radical is a rapid process which is much faster than fhe addition of primary radicals to styrene strongly supports the proposal that fhe first alkylation step is a radical process but that fhe second step is not. 

A complete analysis of the products reported in Fig. 1 requires some more comments on cyclopentadiene and benzene. Both are typical secondary products, and are mainly the result of successive addition and condensation reactions of alkenes and unsaturated radicals. Once a significant amount of ethylene and propylene is formed, vinyl and allyl radicals are present in the reacting system and form butadiene, via butenyl radicals. Successive addition reactions of vinyl and allyl-like radicals on alkenes and dialkenes sequentially explain the formation of cyclopentadiene and benzene. These reactions are discussed in-depth in the literature and will be also analysed in the coming paragraphs (Dente et al., 1979). It seems worthwhile mentioning that these successive reactions and interactions of small unsaturated radicals and species constitute the critical sub-mechanism for the correct evaluation of ethylene selectivity. In fact, once the primary decomposition of the hydrocarbon feed has largely completed, the primary products and mainly small alkenes can be... 

Similarly, the kinetic parameters of the decomposition reaction of the 5-hexen-l-yl radical to form C2H4 and the 3-butenyl radical... 

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