Naoki Haga

Acid-catalyzed rearrangement of O-(2-arylphenyl)hydroxylamines to aryldihydroazepinones

Abstract Acid-catalyzed rearrangement of O-(2-arylpheynl) hydroxylamines followed by ring enlargement afford 7-aryl-2, 3-dihydro-1H-azepin-2-ones. 2-Amino-2-phenyl-3, 5-cyclohexadienone, an intermediate of the reaction, was trapped as the N-trifluoroacetamide.

Photoinduced electron transfer between acenaphthylene and 1,4-benzoquinones. Formation of dimers of acenaphthylene and 1 ∶ 1-adducts and effect of excitation mode on reactivity of the charge-transfer complexes

Photochemical reactions of acenaphthylene (ACN) with 1,4-benzoquinones (BQs) of varying reduction potentials in solution have been investigated in order to determine final products and quantum yields of the reactions and to get an insight into the factors which govern their reactivities. The products were the two isomeric dimers of ACN and three types of 1 ∶ 1-adducts (cyclobutanes, furans, and oxetanes) between ACN and BQs. However, the product distribution varied widely with the substitution on the BQ skeleton. Two modes of excitation, that is, selective excitation of the charge transfer (CT) complex (the CT mode; typical wavelength: 546.1 nm) and direct excitation of ACN or BQs (the direct mode; typical wavelength: 435.8 nm), essentially gave similar product distributions when the reaction took place through both modes. However, the direct mode showed higher quantum yields for the reactions, Φ435.8, than the CT mode, Φ546.1. Moreover, Φ546.1 tended to increase with increase of free energy gap, −ΔGBET, between the ground state CT complexes and the resulting radical ion pair (RIP). These observations can be rationalized by a mechanism involving a distinctive RIP as an intermediate generated by photoinduced electron transfer in each mode. Thus, the solvent-separated radical ion pair (SSIP) produced in the direct mode excitation will undergo dissociation to the free radical ions (FRIs), ACN+˙ and BQ−˙, affording final products competing with backward electron transfer (BET). In contrast, the contact radical ion pair (CIP) produced in the CT mode excitation much more rapidly deactivates to the ground state than dissociates to FRIs via SSIP due to faster BET, whose rate depends on −ΔGBET. The 1 ∶ 1-adducts can be formed from either a cage reaction inside the RIP (CIP or SSIP) or reaction between FRIs, whereas the dimers of ACN should be formed from reaction of ACN+˙ with the ground state ACN.

KINETICS AND MECHANISM OF PHOTOCYCLOADDITION OF DEOXYURIDINES TO 2,3-DIMETHYL-2-BUTENE

The mechanism of photocycloaddition of 2′‐deoxyuridine (1a) and thymidine (1b) to 2,3‐dimethyl‐2‐butene (Bu) in acetonitrile by UV irradiation has been studied. The reciprocal quantum yield for the cycloaddition increased linearly with reciprocal concentrations of Bu in acetonitrile to give limiting quantum yields at infinite concentration of Bu as 0.030 and 0.0096 for 1a and 1b, respectively. This shows that the cycloaddition proceeds in a two‐step mechanism between the triplet state of 1 and Bu through biradical intermediates. Addition of cis‐1,3‐pentadiene quenched the reaction obeying the Stern–Volmer equation. The above quenching experiments and laser transient spectroscopy revealed that the triplet state of 1a reacts with Bu with much larger rate constant (1.3–1.6 × 109 M−1 s−1) than that of 1b (4–5 × 107 M−1 s−1) reflecting larger steric hindrance exerted in the reaction of 1b than that of 1a.

The factor which determines whether excitation of charge-transfer complexes leads to final net products in comparison with the reactivity on excitation of one of the components

Selective excitation of charge-transfer complexes of indene or acenaphthylene with various electron acceptors does or does not afford final net reaction products, depending on the free energy of the resulting radical ion pairs over the ground state, -deltaGBET, with threshold values. A similar factor governs the efficiency of the reaction on direct excitation of either the donor or the acceptor of their components, except that it does not fall to nil below the threshold and the reaction affords higher quantum yields than the selective excitation of the charge-transfer complex.

Exclusive production of a cycloadduct from selective excitation of the charge-transfer complex between acenaphthylene and tetracyanoethylene in the crystalline state in contrast to failure of reaction in solution

Selective excitation of the charge-transfer complex between acenaphthylene and tetracyanoethylene in the crystalline state exclusively affords a 1:1 [2 + 2] cycloadduct, in contrast to the excitation in solution, which gives no product.

Control of reaction course of the excited state of charge-transfer complexes by the free energy of backward electron transfer

Selective excitation of charge-transfer (CT) complexes between acenaphthylene and various electron acceptors gives net reaction products when the resulting radical ion pairs lie at sufficiently higher energy than the ground state (large –ΔGBET); however, with decrease of –ΔGBET, these tend to become less reactive and finally non-reactive.