Biradicals 1,4-acyl-alkyl

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,... 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

With saturated, or nun-conjugated unsaturated, cyclic ketones, tv-cleavage takes place readily in solution on irradiation, usually in a iriplel-staie process. The resulting acyl-alkyl biradicals have a num-... 

The acyl—alkyl biradical obtained by ring-opening of a cyclic ketone is able to undergo intramolecular disproportionation in one of two ways. A hydrogen atom may be transferred to the acyl radical from the position adjacent to the alkyl group, and this produces an unsaturated aldehyde (4.21). Alternatively, a hydrogen may be transferred to the alkyl radical from the position adjacent to the acyl group, and this results in the formation of a ketene 4.22). Many ketenes are labile, and the use of a nucleophilic solvent or addend,... 

If all of the reactions just described do not occur due to adverse conformation effects, unfavorable substitution, ring strain, or other unknown factors, biradical [1] can recyclize to the starting ketone (kc c). It should be emphasized that the existence of a discrete acyl alkyl biradical intermediate in the photolytic transformations of cyclic aliphatic ketones, while it makes for facile mechanistic rationalization, has not been directly verified by classical trapping or spectroscopic observation. Several recent reports, however, have provided strong support for the existence of [1], although only for specific cyclic ketones (10). 

Using 2,2-dimethylcyclobutanone [26a], as a specific example, initial excitation to produce an excited state species followed by a-cleavage would produce the acyl alkyl biradical [30]. Subsequent decomposition of [30] would then afford ester [27a] (via ketene), cyclopropane [28a], and acetal [29a], the observed photoproducts. The intermediacy of biradical [30] was supported by (a) the nearly exclusive formation of methyl acetate (as opposed to methyl isobutyrate), (b) the exclusive formation of the 5,5-dimethyl substituted acetal [29a] (as opposed to its 3,3-dimethyl substituted isomer), (c) its role as a common intermediate for all products, and (d) analogy to the photochemistry of cyclopentanones and cyclohexanones. Recently, Wasacz and Joullie have reported that photolysis of oxacyclohexanone [32] affords a 3% yield of acetal [29a] (18). It is conceivable that the formation of [29a]... 

Dowd and coworkers have also published results which seemingly support the intermediacy of an acyl alkyl biradical (19). When cyclobutanone [21] was irradiated at -78°C in neat 1,3-butadiene, an equimolar mixture of 3-vinylcyclohexanone [33] and oxetane [34] was produced. The formation of [34] is a characteristic bimolecular reaction of aliphatic ketones and 1,3-dienes and has ample precedent (20). The formation of cyclohexanone [33], on the other hand, is novel and has been suggested to arise... 

The possible intermediacy of an acyl alkyl biradical (e.g., [30]) in the photochemical reactions of cyclo-butanones has intrigued several researchers. Unlike Hostettler s results with bicyclo-[3.2.0]-heptenone [15]/ Erman observed that a-cleavage occurred on both sides of the carbonyl group of photoexcited bicyclo-[3.2.0]-heptenone [36] (21). This result might be anticipated if... 

The corresponding values for cyclohexanone appear to be independent of exciting wavelength. Lee and coworkers (47,48 have interpreted the sharp decrease in for cyclobutanone to be due to a competing predissociation mechanism from vibrationally excited S. This facile predissociation is described as a-cleavage of the vibrationally and electronically excited ketone to form an acyl alkyl biradical which subsequently decomposes to products. Interestingly, the rate of this cleavage must be extremely... 

Any attempt to present a meaningful mechanistic scheme to describe the excited-state chemistry of cyclobutanone, therefore, must be capable of accounting for (a) the apparent photoreactivity of the state, (b) the regiospecificity of a-cleavage (i.e., that direction which would afford the most stable acyl alkyl biradical intermediate), (c) the apparent stereoselectivity of all reactions, and (d) the inherent inefficiency of all photoreactions, including that for ketone disappearance (see Table III). Consideration of all the available evidence suggests the mechanism set forth in Scheme IV. Excitation (n -> u ) of the ground-state ketone (SQ) initially produces the lowest electronically excited... 

The postulation that the "biradical-like" transition state [135] (and not a freely rotating acyl alkyl biradical intermediate) is the precursor of the oxacarbene intermediate [136] is made primarily to accomodate the fact that the ring-expansion reaction is stereoselective. Transition state [135] could also decay by S-cleavage and/or decarbonylation [both stereospecific (23), although definitive evidence concerning this point is not available in the solution phase. Finally [135] could decompose back to the starting cyclobutanone, which would explain the observed lack of efficiency in the previously described photolyses. (See Section II.E for a further discussion of this point.)... 

Figure 10. Field dependence of the C1DNP intensities for acyl-alkyl biradicals (open symbols) and a bisalkyl biradical (filled symbols) produced in the photoreactions of a a,a, a -tetramethylated cycloalkanones. Triangles, 1,15-biradical open squares, 1,12-biradical diamonds, 1,10-biradical circles, 1,8-biradical filled squares 1,7-biradical (resulting from decarbonylation of the 1,8 acyl-alkyl biradical). [Reproduced with permission from G. L. Closs, M. D. E. Forbes, and P. Piotrowiak, J. Am. Chem. Soc., 114, 3285 (1992). Copyright 1992 American Chemical Society.]...

Singlet-state ntt reactions that occur in the gas phase and in solution may not be observed in crystals because the acyl-alkyl radical pair (E) is subject to the perfect cage effect . Radical-radical recombination in the singlet-state acyl-alkyl biradical (from E =>A) may occur within the time scale of a single bond vibration. However, because many decarbonylation reactions have been documented in crystals, conditions may be satisfied for a-cleavage to compete with excited-state decay and for decarbonylation to compete with recombination of the acyl-alkyl radical pair. In this chapter, we review various aspects of the a-cleavage and decarbonylation steps in the reaction, describe recent theoretical advances, and conclude with examples that reflect our current understanding of the reaction in the solid state. 

FIGURE 48.5 Reaction coordinate (RC) derived from computational studies of the Norrish type I reaction. The RC shows a-cleavage from both Sj and Tj surfaces to yield the acyl-alkyl biradical/radical pair. Subsequent decar-bonylation of the acyl radical generates the alkyl-alkyl biradical/radical pair. 

Miranda etaL reported that irradiation (254-nm mercury lamp) of [3.2]PCP-2-one 16 in benzene led to [2.2] PCP 15 as the only product via acyl-alkyl biradical 32 and biradical 33. A laser-flash photolysis study (266 nm) of 16 in cyclohexane indicated that biradical 33 is detectable at room temperature and does not cleave to p-xylylene 35 but cycKzes to [2.2]PCP 15 or dimerizes to [2 ]PCP 34. Irradiation (laser, 308 nm) of a cyclohexane solution of 15 containing a triplet quencher (cyclooctadiene) showed two absorption maxima, which correspond to p-xylylene 35 and the hiradical 33. The cyclophanes [2 ]PCP 21 and [2 ]PCP 34 were formed by irradiation (laser, 248 or 266 nm) of cyclohexane solutions of [2.2]PCP 15. While the formation of 34 agrees well with the intermediacy of 33, the detection of [2 ] PCP 21 provided further evidence for the intermediacy of p-xylylene 35, which can couple either its tiimeric or tetrameric products. 

Smaller-ring ketones, especially cydobutanones and more rigid cyclopentanones or cyclohexanones, give biradicals that follow the fourth of the pathways in which carbon monoxide is not tost. In this process a new oxygen-carbon bond is formed by attack of the oxygen of the acyl radical on the alkyl radical centre this generates a carbene which can subsequently react with a nucleophilic solvent such as methanol (4.28). 

This mechanistic sequence (Sch. 4) wherein the triplet excited enone adds to the alkene, either via an exciplex intermediate or directly, to afford triplet 1,4-biradicals, which (after undergoing intersystem crossing) either cyclize to product(s) or revert to ground state reactants, is confirmed by both semi-empirical and ab-initio calculations [21-24], The origin of regioselectivity is supposed to stem from the primary binding step, the enone triplet being considered as a (nucleophilic) alkyl radical at C(3) linked to an (electrophilic) ot-acyl radical at C(2) [25], Thus additions of C(2) to the less substituted terminus of electron rich alkenes and of C(3) to the least substituted terminus of electron deficient alkenes should occur preferentially [26],... 

Alternative intramolecular hydrogen abstractions are possible in a, 3-unsaturated carbonyl compounds. Irradiation of the 2-(N-acyl-N-alkyl-amino)cyclohex-2-enones (115), for example, gave the lV-alkyl-1-azaspiro[3.5]nonane-2,5-diones (116), presumably via 1,4-biradical intermediates (117), The 2-(dialkylamino)-3-acetylenyl-1,4-naphthoquinones... 

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