Trimethylenemethane biradical

In addition to serving as substrates with which to further study the effects of various substituents on the photochemical transformations of cyclobutanones, 3-methylenecyclobutanones are capable of generating the theoretically interesting trimethylenemethane biradical species (32) by way of the photodecarbonylation reaction. Indeed, Dowd and Sachdev (33) observed a triplet ESR spectrum upon photolysis of 3-methylenecyclobutanone [69]... 

As a final suggestion for future research, cyclobutanones have also provided the organic photochemist with the opportunity of investigating the existence of unusual and reactive intermediates oxacarbenes, trimethylene biradicals, trimethylenemethane biradicals, acyl alkyl biradicals, and ketenes. Evidence for the intervention of oxacarbenes in the ring-expansion reaction is quite compelling however, their unusual behavior relative to "typical" carbenes (e.g., failure to form cyclopropane adducts with some olefinic substrates) makes them prime subjects for further study and characterization. Unlike oxacarbenes, the existence of acyl alkyl biradicals (e.g., [30]) is tenuous at best. Ideally,... 

Trimethylenemethane biradicals have been proposed as intermediates in the photodecomposition of fluorine-substituted 4-methylenepyrazolines both in the gas phase and in solution.Evidence for the intermediacy of two trimethylenemethane biradicals on direct irradiation of the bi-(l-pyrazolin-4-ylidene) (29) has also been reported. Benzophenone-sensitized irradiation of the same pyrazoline, however, takes a different course and affords the allene dimer (30), presumably via the tetramethylene-ethane biradical (31). 

In 1932 Kon and Naji reported that pyrolysis of trans-2,3-ethoxycarbonyl-methylenecyclopropane ( Feist s ester ) resulted in an isomerization to give compounds later identified as the stereoisomeric 2-carbethoxy-l-(carbethoxy-methylene)cyclopropanes (Figure 36). Subsequent studies by Ullman " using the optically resolved reactant showed that the products were not racemic and that the planar trimethylenemethane biradical could not, therefore, be the sole intermediate in the... 

Since the time when the thermally induced methylenecyclopropane rearrangement (A - B) of Feist s esters was first observed by Ullman in 1959, rearrangements of a large number of methylenecyclopropane derivatives have been subjected to kinetic, stereochemical, and theoretical studies. The main objectives of these efforts were to understand the role and nature of the trimethylenemethane biradical intermediate (C ) in the rearrangement process. Considerable attention has focused on the theoretical and experimental elucidation of the relationship among structure, spin state, and reactivity of this simple non-Kekule molecule as well as on its applications as a synthetic and practicaP" intermediate. 

Nanosecond laser flash photolyses of 77a under TCNB- and NMQ BF -sensitized conditions provide strong spectroscopic evidence not only for the intermediacy of cation radical 82a in the SET-sensitized photoreactions but also for the trimethylenemethane biradical intermediate 82a. Laser excitation (308 nm) of TCNB with 77a in acetonitrile gives rise to two intense transient absorptions with maxima at 350 and 500 nm whereas in dichloromethane only the shorter wavelength transient absorption is observed. In contrast, under the NMQ BF -sensitized conditions in acetonitrile the longer wavelength transient absorption predominates but the shorter wavelength band is extremely weak (Table 5). 

Because both 1-methylenespiropentane (89) and 1-cyclo-propylidenecyclopropane (95) include methylenecyclopropane moiety in their structures, the methylenecyclopropane-type reversible interconversion between 89 and 95 is expected to occur upon pyrolysis, involving a trimethylenemethane biradical intermediate. However, such a rearrangement formally does not take place, though 95 rearranges to 89. Instead, on pyrolysis at 320 C, 89 rearranges to dimethylenecyclobutanes (91 and 93) through the tetramethylene-ethane biradical 90 and the vinylic-allylic biradical 92, respectively. Presumably, biradical 94 formed by C-2-C-5 bond fission... 

Crandall first reported formation of 1,2-dimethylenecyclopropanes in the pyrolysis of vinylidenecyclopropanes in the per-methyl series.The rearrangement would appear to involve cleavage of the C2-C3 bond to give a trimethylenemethane biradical species which reforms the cyclopropane ring between different carbons (Scheme 6.8). 

Vinylmethylenecyclopropane was first prepared by Shields and Billups who also reported its thermal rearrangement to 3-methylenecyclopentene (Scheme 7.71). Subsequently, the kinetics of the rearrangement was determined in a stirred flow vapor phase reactor to give log k = 11.48 - 25 00/23RT The reaction would appear to involve the trimethylenemethane biradical which undergoes an allylic rearrangement. 

It was proposed that both allylidenecyclopropanes gives the 2-vinylmethylene-cyclopropanes via a trimethylenemethane biradical, and this species can also be responsible for formation of the cyclopentenes. 

The relatively low-temperature conversion of MSD to MBD can reasonably be attributed to the formation of a trimethylenemethane biradical, presumably in its singlet state, which is further stabilized by a cyclopentadienyl moiety (Scheme 9.29). Of interest is the extent to which this species might be stabilized by electron transfer to form a species reminiscent of the oxaallyl zwitterion from cyclopropa-none (see Chapter 5, Section 2). Of further interest is the fact that the dimers formed are not those from a trimethylenemethane triplet state. 

It appears as if two different trimethylenemethane biradical species give rise to the two products albeit in different amounts presumably because the less stable cyclopropyl radical is required in the intermediate giving rise to the minor product. 

The rearrangement appears to be an intramolecular cyclization of the trimethylenemethane biradical which could be produced in the ring opening of the methylenecyclopropane moiety (see Chapter 5, Section 2). 

Methylenecyclopropanes. An extensive ab initio study of all, even remotely plausible, transition states for the degenerate rearrangement of methylenecyclopropane shows the orthogonal methylene-allylic trimethylenemethane biradical to be the most stable of all possible midpoints for the reaction. There is a secondary potential minimum at this point, making this an interesting target for experimental detection. ... 

---