Biradicals decarbonylation

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

That resonance stabilization of intermediate biradicals is important in determining the efficiency of decarbonylation follows from the following examples yielding benzyl radicals upon loss of carbon monoxide(57) ... 

Five- or six-membered saturated cyclic ketones can also react by another pathway that does not involve decarbonylation. In these reactions, the biradical initially formed by a-cleavage undergoes internal disproportionation without loss of carbon monoxide, resulting in the formation of either an unsaturated aldehyde or a ketene. Methanol is usually added to convert the reactive ketene to a stable carboxylic-acid derivative (Scheme 9.2). 

It was originally reported that irradiation of 2-methylcyclohexa-none gave solely franj-5-heptenal,321 in accord with the then-postulated concerted singlet mechanism. Now, however, it has been reported that 2,6-dimethylcyclohexanone yields both cis- and /rstep process.328 The reaction seems to resemble type I cleavage very closely, with the difference that disproportionation in the biradical primary product is much faster than decarbonylation, except in high vibrational levels of the excited singlet state, or when both ends of the biradical so produced are resonance stabilized.329... 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

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.)... 

From the above results it is evident that a lower energy content of the decomposing molecule favours the formation of the unsaturated aldehyde at the expense of decarbonylation. The explanation of this fact has been attempted on the basis of both the concerted and the biradical mechanisms. 

U.v. irradiation of the unsaturated A-seco-5-ketone (324) gave none of the expected oxetan (325), but instead produced the cyclobutanols (327) as major products, along with a little of the B-seco decarbonylation product (328). Cyclobutanol formation proceeds through hydrogen transfer from C-2 to the carbonyl oxygen, which is followed by cyclization of the 2,5-biradical (326). Similar reactions occur with the alkynyl-ketone (329) and with the saturated analogue (330). ... 

Photo-decarbonylation of the cyclohexanone (32a) is efficient with a quantum yield of 0.9. The reaction yields the two products (33) and (34) in a ratio of 1 2. The cyclopentanone (32b) also decarbonylates photochemically but is less efilcient with a quantum yield of 0.5. A laser flash study has been carried out on these systems and has identified the biradicals produced by the Norrish type I process. The lifetime of the biradicals (35a) and (35b) are O.Ojxs and 0.5 xs respectively. In a related study the photodecarbonylation of cIs- and frans-2,6-diphenylcyclo-hexanone has been shown to yield a mixture of cis- and 2-diphenyl-cyclopentane and cIs- and... 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

The ketone (13) does not undergo loss of CO on irradiation in the crystalline phase. In benzene solution, however, decarbonylation does occur to give biradicals that disproportionate to yield (14) and (15). The more hindered ketone (16) behaves differently and decarbonylates in both the crystal and solution with different results. Thus (17) and (18) are formed in solution, while only the latter... 

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