Competing biradical reactions

Manganese(m) acetate oxidation (cf. Vol. 3, p. 34) of camphene gives (186) as a 95 5 mixture by carboxymethyl radical insertion no rearranged products were obtained, in contrast to /3-pinene which gave Wagner-Meerwein products only, and no free-radical insertion.279 The E- and Z-isomers of (187) probably result from a non-concerted biradical intermediate formed by benzyne addition to camphene.280 Benzyl-lithium adds to the aminocamphor (188) exclusively from the exo-side whereas only the competing enolization reaction occurs with more sterically hindered organometallics.281... 

Despite high probabilities for triplet formation and for triplet reaction, many overall photoreactions proceed in low quantum efficiency. In the absence of competing chemical reactions, the factor most often responsible for low quantum yields is the revertibility 8> of primary triplet reactions. Metastable intermediates such as radicals, biradicals, and charge transfer complexes (either excited or ground state) are the usual photoproducts from excited triplet reactions. These intermediates generally can revert to ground state reactant, thus providing a chemical path for radiationless decay , as well as proceed to stable products. Hence, the factor Pp is necessary to describe the probability that the intermediate will form product. 

The formation of a retained, label-scrambled material, SA, is suggestive of a significant contribution from a concerted 3,3-shift, but the formation of some inverted scrambled product, RA, requires the intermediacy of a cis,cis-hisa y ic biradical for its formation. This latter intermediate then is probably responsible for ca. 3% of the 3,3-shift product as well as 3% of inverted, unscrambled material, RV. However, most of the inverted, unscrambled material probably arises from a cis,trans-hisai y ic biradical which cannot give any other 4-vinylcyclohexene except starting material. Thus, biradical reactions appear to compete well with the 3,3-shift in this case, most likely because the 3,3-shift transition state must not only be boat-like, but suffer from steric interactions not unlike those in bicyclo[2.2.2]octane. 

A new theoretical description of photolytic [2 -I- 2] cycloadditions includes the suggestion that, as in the case of thermal reactions, biradical mechanisms have too frequently been invoked erroneously to account for ostensibly non-stereospecific reactions. The apparent non-stereospecificity may rather be due to competing concerted reactions, each one of which is differently stereospecific. 

Both the rate constants for competing triplet reactions and the partitioning of biradical intermediates may be affected by the environment. The major effects on triplet reactivity involve conformational changes induced by highly ordered media " and decreases in the efficiency of radical a-cleavage. The major effects on biradicals involve conformational restrictions that impede or promote coiling and solvation that inhibits disproportionation back to ketone. 

So far, the solid state type I reaction has been reliable only when followed by the irreversible loss of CO to yield alkyl-alkyl radical species (RP-B or BR-B) in a net de-carbonylation process. The type 11 reaction relies on the presence of a y-hydrogen that can be transferred to the carbonyl oxygen to generate the 1,4-hydroxy-biradical (BR C). The type-1 and type-11 reactions are generally favored in the excited triplet state and they often compete with each other and with other excited state decay pathways. While the radical species generated in these reactions generate complex product mixtures in solution, they tend to be highly selective in the crystalline state. 

DFT studies on heteroatom diallenes have found, in line with previous experimental data, that biradical cyclization to a five-membered heteroaromatic ring is the preferred reaction pathway, although protonation of the heteroatom has been found to promote a competing cyclization to a six-membered, initially biradical, ring.59... 

This suggests that the formation of quadricyclane during the thermal decomposition of 70 exo results from the competing [2s+2s+2s] retrocycloaddition process rather than via intramolecular trapping of the diradical intermediate by a C-C bond. Thus, the thermal decomposition of 70 exo resulted from [2s+2s+2s] cycloreversion reaction rather than from a biradical pathway. If the latter pathway was active, both exo and endo isomers should have furnished quadricyclane 72. In fact, 70 endo is geometrically incapable to participate in the cycloreversion process. 

Synthetic applications of the reaction are somewhat limited as the highly reactive biradicals and radical pairs tend to undergo reactions that compete with C—C bond formation. As in previous cases, the reaction may have synthetic value for the synthesis of strained structures involving small rings. For example, the preparation of the simplest [2]-ladderane 55 by photodecarbonylation of bicyclo[3.2.0]heptan-3-one 54 gave the bicyclic structure in 5% yield with a ring-opened 1,5-heptadiene being the dominant product (Scheme 2.14) [41]. 

The lowest excited state of most ketones has the (n, n ) electronic structure, which gives the carbonyl (C=0) double bond a 1,2-biradical character. Therefore, the electron-deficient oxygen atom of this moiety, obtained upon excitation, acquires a radical reactivity, similar to the alkoxy radical. This excited state property of ketones leads to intramolecular H-abstraction to form 1-hydroxy-l, x-biradicals. Depending upon the structure and reaction conditions of the carbonyl compound, two common competing reactions may follow ... 

Furthermore, it was shown that alpha- as well as ortho-substituents in such ketones retard the photocyclization due to other competing reactions. However, photocyclization showed interesting stereochemical trends, which were also strongly affected by the solvent polarity and the phase (Scheme 8.7). For example, the photocyclization of a-(o-ethyl phenyl acetophenone (27) either in benzene or as a solid favored the isomer with the methyl and phenyl group trans to R, due to the conformational preferences in the 1,5-biradical intermediate [9]. 

Studies aimed at the comparison of Rh(II) and Cu(II)-catalyzed onium ylide reactions of diazoketones 449 using 3 mol% of the former and 15 mol% of the latter led to the conclusion that the copper-catalyzed process provides the better yields and selectivities for [1,2]-rearrangement products 450 (Fig. 107) [490, 491]. In the rhodium-catalyzed process, 1,5-C-H insertion may compete. The diastereo-selectivity with both catalysts is in some cases similar, in others the Rh-catalyzed process is more selective. Analogous reactions of acetal 451 provided a mixture of stereoisomers 452a and 452b at the benzylidene position, supporting a stepwise process. The authors proposed that the involved intermediate was either a 1,6-biradical or the corresponding ion pair. 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Provided that reaction 61 competes favorably with the T of PhO, this secondary radical will also be polarized and this, indeed, was observed. Needless to say, the formation of the biradical III and its rearrangement to IV would afford a very challenging system for CIDNP studies. 

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