Homobenzvalene

Homobenzvalene (HB) is an electron-rich donor (IP = 8.02 eV) owing to the presence of a strained ring system, and thus readily forms a charge-transfer complex with TCNE. Charge-transfer irradiation of the [HB, TCNE] complex leads to rapid bleaching of the yellow color, and the formation of a mixture of isomeric cycloadducts208 (equation 73). 

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

The irradiation of methyl phenylglyoxylate, benzil, benzophenone as well as 1,4-benzoquinone in the presence of homobenzvalene 103 gave, as products of the Paterno-Buchi reaction, oxetane derivatives which contain the tricyclo[4.1.0.02,7] heptane subunit as well as ring-opened products (Sch. 31) [100]. 

Whereas the ring-enlargement of benzvalene to homobenzvalene and the second dibromocarbene addition posed no problems, the final insertion step provided... 

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. 

Cyano-substituted alkenes can also act as traps for biradical intermediates. This behaviour is exemplified by the formation of the three products 182-184 that are obtained on the irradiation of homobenzvalene 185 . 

Cycloaddition of TCNE to homobenzvalene yields various tetracyanocycloadducts. The photochemical process yields products that differ from the ones obtained by thermal cycloaddition ". Other polycyclic unsaturated hydrocarbons were also reacted to tetracyano cycloadducts. Hence the dihydrobullvalene 393 yielded 394. Pyrolysis of 394 gave 393 rather than elimination products" ". ... 

Convenient new routes to tricyclo[4,1,0,0 ]hept-3-ene and its derivatives have been disclosed. Acetone-sensitized irradiation of bicyclo[3,2,0]hept-6-en-2-one affords the ketone (624), whose enol phosphate is reduced by lithium-ammonia to give the parent alkene (625). " A second route is also described, starting from the 7,7-di-bromonorcarane derivatives (626) and (627). Reaction of (626) with methyl-lithium in ether at 0°C afforded a 3 1 mixture of the tricycloheptenes (628) and (629) similar reaction of (627) gave (630 40%), but reactions of the parent dibromide were unsuccessful. Catalytic Ag ion causes the rearrangement of (628) and (629) to, respectively, 3-methyl- and 1-methyl-cycloheptatriene. Initial Ag" attack at the least hindered edge bond is implicated. Attempted preparation of the tetrahedrane dimer (631) by the addition of dibromocarbene to homobenzvalene followed by treatment of the adduct so obtained with excess methyl-lithium in ether at 0°C afforded instead 5-ethynyl-cyclohexa-1,3-diene. 

Homobenzvalene 118 on irradiation in the presence of ethyl phenylglyoxalate gives regio- and stereoselective eniio-product 119 due to absence of steric interaction. 

The irradiation of methyl phenylglyoxylate, benzil, benzophenone, and 1,4-benzoquinone in the presence of homobenzvalene 103 gave, as products of the Paternb-Biichi reaction, oxetane derivatives that contain the tricyclo[4.1.0.0 > ]heptane subunit as well as ring-opened products. In the case of benzophenone, the cycloaddition competes with the isomerization of 103 to cycloheptatriene. Exclusive isomerization was observed with acetophenone and acetone. Carbonyl compounds with triplet energies lower than 69 kcal/mol prefer the cycloaddition path. Cyclopent-2-en-l-one is an exception to this rule in spite of its triplet energy of 74 kcal/mol, [2-l-2]-cycloadducts were formed rather efficiently. 

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... 

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