The Mechanism of Interaction of Triplet 3‐Methylcyclohex‐2‐en‐1‐one with Maleo‐ and fumarodinitrile: Evidence for Direct Formation of Triplet 1,4‐Biradicals in [2 + 2] Photocyclo‐ additions without the Intermediacy of Exciplexes

Abstract

which has elsewhere['41 manifested a special coordination ability. Irradiation (d = 289 nm) of [Cr(C0),(q4-nbd)] and ECO in 2-methylpentane at -40°C affords mer-and juc-[Cr(C0),(q4-nbd) (q2-eco)], which are easily distinguished on the basis of their CO stretching bands (rner complex, f [cm-'] = 1997(m), 1925 (sst); fac complex < [cm-'] = 1968, 1904, 1876). The mer complex, which is more stable, is the only observed product at room temperature. In order to determine the quantum yield (@ = 0.18 at 300 nm, 0.1 1 at 365 nm), its formation was followed quantitatively by IR spectroscopy. If this reaction is assumed to serve as a model for the generation of the catalytically active [Cr(CO),] complex (Qind), then the relation eta, = Gind x (turnovers) gives a rough estimate of 10 to 20 for the quantum yield of the photocatalytic hydrogenation of norbornadiene (Qca,). The experimentally determined values (1.9,['] ca. 0.3[,') are considerably lower, probably because the active [Cr(CO),] complex generated in situ is rapidly transformed into inactive [Cr(CO),(q4-nbd)] by the CO still present in the system. The stability of [M(C0),(q4-nbd)(q2-olefin)] complexes increases markedly in the series Cr < Mo < W. As a result, the catalytically active species is trapped by excess substrate (NBD) as [M(CO), (q4-nbd)(q2-nbd)] in the cases of tungsten and molybdenum.['0] The analogous chromium complex, by contrast, has not yet been detected. Thus, the drastic decrease in the catalytic activity in the series Cr $ Mo > WI3] is readily explained.In analogy to the hydrogenation, norbornadiene can also be hydrosilylated with silanes (R,SiH), both photocatalytically with [Cr(CO), (q4-nbd)] and in the dark with 1. As we will report in detail elsewhere, both variants afford endo 1,2-, exo 1,2-, and homo 1,4-adducts with comparable product distributions. Experimental ProcedurePreparation of 1: [Cr(C0),(q4-nbd)] (1.00 g, 3.9 mmol) was irradiated in 200 mL of ethene-saturated pentane at -50°C in an immersion lamp apparatus (Solidex glass, i . 2 280 nm; Philips HPK 125-W Hg lamp) for about 3 h, until the starting material (Cco [cm-'1 = 2043, 1961, 1946, 1917) had largely reacted. The temperature was maintained at I -30°C during workup. After addition of about 2 g of silica gel (to absorb readily decomposable by-products), the solution was stirred for several minutes and then evaporated to dryness under vacuum. The residue was extracted into 3 x 10 mL of ethene-saturated pentane, transferred to a column (silica gel 60, Merck; 2 x 15 cm), and eluted with additional solvent. The first of the two yellow fractions was concentrated under vacuum to about 50mL and cooled to -78°C whereupon pure mer-[Cr(C0),(q4-nbd)(q2-ethene)] (0.46 g, 46 %) crystallized out. M.p. = 46°C; correct elemental analysis (C, H, Cr); IR (n-hexane): F[cm-'] = 2005(m), 1933 (sst) (CO). 'H NMR (400 MHz, C,D,, 243 K): 6 = 0.85 (m, 2 H), 1.85 (s, br, 4 H, ethene), 3.00 (s, br, 2 H), 3.16 (s, br, 2 H), 3.89 (s, br, 2 H). "C NMR (100.6 MHz, C,D,, 243 K): 6 = 47.41 (CH), 52.78 (ethene)...

Photocycloaddition of 4,4-Dimethylcyclohexenone to 1,1-Diphenylethylene. Evidence for a Triplet Exciplex Intermediate

“…Evidence for triplet exciplexes in photocycloadditions has been indirect, and none has been observed directly in a cycloaddition system. Schuster has questioned the need for these intermediates in cyclohexenone photoannelation, − and Peters , has interpreted the quenching of ketone triplets by electron rich alkenes as direct formation of a biradical. Biradical-trapping studies − have led Weedon to conclude that exciplexes are not a requirement in explaining the regiochemical outcome of enone−alkene photoannelations.…”

Photochemistry of substituted cyclic enones. Part 11.1 Synthesis and photophysics of 5-arylalkyl-3-phenylcyclopentenones

“…The extinction coefficient of the transient under investigation (ε T ) was determined using eqn. (3).…”

“…2 There is also interest in the mechanism of such reactions and there has been considerable discussion on the possible roles of exciplexes and biradical intermediates. 3,4 In this context 3-phenylcyclopent-2-enones are of interest as they undergo similar reactions and it has been shown that on excitation the parent compound 1 affords a triplet state which has an exceptionally long lifetime compared with those of other enones. 5 This may be caused by conjugation between the phenyl group and the enone which inhibits twisting in the excited state.…”

Mechanism of acid-catalyzed photoaddition of methanol to 3-alkyl2-cyclohexenones

“…10 for triplet energy transfer to acyclic 1,3-dienes in related systems are typically about lo8 M-' s-' (8). Rudolph and Weedon (1) also showed a plot of the inverse quantum yield for deconjugation of l a in benzene vs. the inverse concentration of acetic acid.…”