Quinone reactions with biradicals

The 1,5-biradicaI BRi formed during the Patemo—Biichi reaction of excited benzo-quinone B with quadricyclane Q (for the formulas, see Chart 9.4) provides one of the extremely rare examples of a short-chain biradical that produces CIDNP of the radical pair type. Extracts of the CIDNP spectra are displayed in Fig. 9.7. The occurrence of both absorption and emission in the same product is clear evidence for this mechanism of polarization generation. 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

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. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

By a detailed CIDNP investigation [117a] of the Patemo-Biichi reactions of anetholes 31 with quinones 30 in polar medium earlier mechanistic hypotheses were disproved. Stationary and time-resolved experiments showed the mechanism to have the following novel features (cf. Chart XIV) Spin-correlated radical ion pairs (i.e., 30 31,+) are key intermediates for cycloadduct formation free radical ions do not play a significant role. In the singlet state, these pairs undergo back electron transfer geminate reaction of triplet pairs leads to triplet biradicals, which are the precursors to the photoproducts. 

The photoenolization of the quinone (286) can be carried by irradiation at 313 or 365 nm in acid solution. The steady state irradiation has identified the product as the unstable hydroxylated compound (287) which is formed via the enol (288). The presence of this intermediate was detected in a laser flash study of the reaction. The quinones (289) undergo cyclization when irradiated with visible light.The mechanism by which the compounds (289) are transformed into the derivatives (290) involves the production of an excited state that is either a zwitterion or a biradical. After the transfer of a hydrogen the intermediate (291) is formed. It is within this species that cyclization occurs to give the final products. (2+2)-Cyclo-adducts such as (292) and oxetanes can be obtained by the photochemical addition of quinones to homobenzvalene. Interest in the photo-SET in quinone systems has led to the synthesis of the pyropheophytin substituted naphthoquinone dyads (293). A pulse radiolysis study of vitamin K in solution has been reported. 

Quantum mechanical calculations on the structure of the triplet intermediates in the photochromic reactions of phenoxy quinones show that photochemically induced phenyl migration occurs non-adiabatically with generation of the spiro form of the triplet biradical. Details of a range of new photochromic compounds have appeared in both the scientific and patent literature. ... 

Quinones. - Goerner has demonstrated that the yield of semiquinone radicals is low in the absence of hydrogen donating solvents. He notes that the quantum yield for decomposition is substantial in aqueous solution. A detailed account of the photohydroxylation reactions of 1,4-benzoquinone in aqueous solution has been published.The photochemical cyclization of some Diels-Alder adducts of benzo-l,4-quinones has been described. This has provided a path to complex molecules such as 3-bromotetracyclo[5.3.1.0 . 0 ]undec-10(12)-ene-9,l 1-dione. A biradical has been identified as the key intermediate in the photocyclization of phenylbenzoquinone. " A charge-transfer complex is formed initially on irradiation of 2-chloro-5-methoxybenzo-l,4-quinone in the presence of triethylamine and various solvents.2-Chloro-5-methoxyben-zo-l,4-quinone also undergoes addition to arylalkynes to afford oxetenes. The mechanism of this addition reaction was studied.A hydrogen abstraction is involved in the photochemical reactions of chloranil with 3-diketones. ... 

Goez, M. and Frisch, I., Rearrangements of photogenerated 1,5-biradicals in the Patemo-Buchi reaction of quinones with norbornadiene or quadricyclane, /. Inf. Rec., 25, 287-293, 2000. 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

Styrene derivatives are commonly used addends in the photocycloaddition studies of 1,4-quinones. With Z- and -anethole, 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 9,10-anthraquinone in acetonitrile solvent yield spiro-oxetanes in which the trans-isomer (e.g., 4 from naphthoquinone) predominates. The process has been studied in detail by CIDNP techniques from which it is deduced that product formation proceeds from triplet radical ion pairs to the triplet biradical, and that there is no significant contribution from direct conversion of exciplex intermediates into the biradicals. Spiro-oxetane formation between simple alkenes and BQ generally has low regioselectivity but this is markedly improved with alkylidene cyclohexanes (Figure 87.3) such that the major isomer can be used as a new access to useful synthetic building blocks. For the BQ/homobenzvalene 5 system, however, where the difference in stability between the intermediate biradicals can be expected to be considerably less, the selectivity ratio for the spiro-oxetanes 6 and 7 is reduced to 3 1, respectively, and the addition to NQ yields only the cyclobutane derivative 8. Quadricyclane and norbornadiene undergo the same photocycloaddition reaction to BQ, affording the oxolane 9 and the spiro-oxetane 10. Evidence from CIDNP... 

---