Dedicated to Professor Xiyan Lu on the occasion of his 80th birthdayTransition-metal-catalyzed aryl C sp2 ÀN bond formation has been intensively studied over the past decade because of the importance of the amines and their derivatives in chemistry related fields.[1-9] The Pd-or Cu-catalyzed formation of anilines from electrophilic aryl halides and nucleophilic primary or secondary amines, pioneered by Buchwald and co-workers and Louie and Hartwig, as well as others, [10][11][12][13][14] is a hallmark reaction in this field. In 1998, Chan et al., Evans et al., and Lam et al. independently developed Cu-mediated oxidative amination of nucleophilic arylboronic acids with amines or other nucleophiles, [16][17][18][19][20] and the methods were later improved to include catalytic versions. [15,17,21,22] However, electrophilic amination of organometallic reagents by R 1 R 2 N + synthons, [23][24][25][26][27][28][29][30][31] though conceptually feasible, has received much less attention. [32][33][34][35][36][37] The putative catalytic cycle of this transformation is illustrated in Scheme 1: (1) reaction of R 1 R 2 N À X (as an electrophile) with the metal catalyst (M 1 ) would introduce the amine group to the metal center to form R 1 R 2 N-M 1 -X; [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48] (2) transmetalation of RM 2 (M 2 = B(OH) 2 ) with R 1 R 2 N-M 1 -X to furnish R 1 R 2 N-M 1 R; and (3) final reductive elimination to form the CÀN bond.The groups of Liebeskind and Johnson have studied this type of CÀN bond formation employing NÀO derivatives. [32][33][34][35][36][37] N-Chloroamides, though attractive because of the ease of preparation [49][50][51] and the high activities of the N À Cl bond, [23,50,51] have seldom been utilized in catalytic electrophilic amination reactions. To the best of our knowledge, Göttlich et al. has primarily studied the Cu-mediated reaction of N-chloroamines with alkenes. [38,39,52] Herein, we communicate our results regarding ligandless copper-catalyzed electrophilic amination of arylboronic acids by N-chloroamides under mild conditions.Our initial experiments of N-chloro-N-phenylacetamide [49] (1 a) and phenylboronic acid (2 a) were carried out by using palladium catalysts. In all attempts, biphenyl was formed instead of the desired N-arylation product (3 a).
The oxidation of bacteriopyropheophorbide with ferric chloride hexahydrate or its anhydrous form produced the ring-D oxidized (ring-B reduced) chlorin in >95% yield. Replacing the five-member isocyclic ring in bacteriopyropheophorbide- a with a fused six-member N-butylimide ring system made no difference in regioselective oxidation, and the corresponding ring-B reduced chlorin was isolated in almost quantitative yield. When the oxidant was replaced by 2,3-dichloro-5,6-dicyano-p-benzoquinone, which is frequently used at the oxidizing stage of the porphyrin synthesis, the ring-B oxidized (ring-D reduced) chlorins were obtained. With both ring-B reduced and ring-D reduced chlorins in hand, their photophysical and electrochemical properties were examined and compared for the first time. The ring-B reduced chlorine 20, with a fused six-member N-butylimide ring, exhibits the most red-shifted absorption band (at lambda(max) = 746 nm), the lowest fluorescence quantum yield (4.5%), and the largest quantum yield of singlet oxygen formation (67%) among the reduced ring-B and ring-D chlorins investigated in this study. Measurements of the one-electron oxidation and reduction potentials show that compound 20 is also the easiest to oxidize among the examined compounds and the third easiest to reduce. In addition, the 1.62 eV HOMO-LUMO gap of 20 is the smallest of the examined compounds, and this agrees with values calculated using the DFT method. Spectroelectrochemical measurements afforded UV-visible absorption spectra for both the radical cations and radical anions of the examined chlorins. The ring-B reduced compound 20, with a fused six-member N-butylimide ring, is regarded as the most promising candidate in this study for photodynamic therapy because it has the longest wavelength absorption and the largest quantum yield of singlet oxygen formation among the compounds investigated.
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