The selective eletron-transfer quenching of the radical anions of
dicyanoanthracene, phenazine, and
anthraquinones in the excited state by a quencher such as fumaronitrile
or dicyanobenzene is investigated in
N,N-dimethylformamide solution at room
temperature using the pulse radiolysis−laser flash
photolysis
combined method. The radical anions generated by pulse radiolysis
do not change upon irradiation with a
laser flash at 532 nm. The radical anions in the excited state
decay into the ground state within the laser
flash (5 ns). Lifetimes of approximately 4 ns are estimated for
three radical anions in the excited state assuming
a diffusion-controlled rate constant for the electron-transfer
quenching. The shorter lifetimes of 1.0−1.4 ns
for methyl and chloro substituents on anthraquinone are discussed in
terms of internal conversion from the
excited to the ground state of the radical anions accelerated by
rotation of the substituents. The energy gap
between the excited and ground states of the radical anions is a
significant factor for the rate of the internal
conversion. The quencher radical anion−neutral molecule pair is
suggested as an intermediate in the electron-transfer quenching of the radical anions during the excited state by the
quencher and is discussed with respect
to separation and back electron transfer in the pair.
Optically active 1,3-dioxolan-2-yl cation intermediates were generated during enantioselective dioxyacetylation of alkene with chiral hypervalent iodine(III). Regioselective attack of a nucleophile toward the intermediate resulted in reversal of enantioselectivity of the dioxyacetylation.
Ongoing efforts have been dedicated to the development of reaction processes controlled by chiral hypervalent iodine reagents with high enantioselectivity. [1][2][3][4][5][6][7][8][9][10][11][12] The oxidation of sulfides into sulfoxides, [2] the a-oxygenation of ketones, [3,4] the dioxygenation of alkenes, [4,5,12] and the dearomatization of phenols [6-8, 9b] have been reported, and most of these reactions resulted in an encouraging level of enantioselectivity. Kita and co-workers reported dearomatizing spirolactonization of naphthols (78-86 % ee) using a spirocyclic iodine(III) reagent.[6] Ishihara and co-workers recently reported that higher enantioselectivities were obtained for the spirolactonization by using a chiral iodine compound derived from lactic acid. [8] Our studies with optically active hypervalent iodine compounds have been focused on mechanisms of the reaction concerned [11] as well as synthetic applications.[12] Asymmetric oxidation of 4-acyloxybut-1-ene into 3-acyloxytetrahydrofuran (up to 64 % ee) was achieved by using chiral hypervalent iodine(III) reagents, 1 and 2, which have a lactate moiety as a chiral source.[12] During the course of these studies for the asymmetric oxidative cyclization of alkenes, we found that oxidation of ortho-alk-1-enylbenzoate with the hypervalent iodine reagent regio-and diastereoselectively gave 3-alkyl-4-oxyisochroman-1-one in a practically useful degree of enantiomeric purity (90-98 % ee); the isochromanone framework is a biologically relevant building block of natural products. [13,14] Herein, we report the synthetic utility of such enantiodifferentiating endo-selective oxylactonizations.The series of optically active hypervalent iodine (III) reagents 1-6 employed in this report is shown in Scheme 1. On the basis of reagents 1 and 2 reported previously, [12] the structures of the iodine reagents were tuned for improved enantioselectivity. The X-ray crystallographic structures of 1-4 showed a typical T-shape orientation around the iodine center, where the two acetoxy ligands occupied apical positions (see the Supporting Information).2-Ethenylbenzoic acid (7 a) was subjected to the reaction conditions with the optically active hypervalent iodine(III) reagent. The reaction was carried out in the presence of paratoluenesulfonic acid (TsOH) to activate the iodine reagent, and the tosylate also worked as a nucleophile to give lactones 8 a and 9 a ( Table 1). The reaction proceeded regioselectively to give the d-lactone product 8 a as the major product. The reaction with 6 gave a higher ee value of 8 a with high regioselectitvity (Table 1,
entry 5).It is remarkable that the oxylactonization proceeds with endo selectivity [15,16] in addition to the high enantioselectivity. For elucidation of the mechanism of the endo selectivity and its synthetic applications, we varied the nucleophile and the substrate in the oxylactonization. When boron trifluoride diethyl etherate was employed as an activator in the presence Scheme 1. Optically active hypervalent iodine(III) rea...
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