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We report herein a new strategy of the Rh(III)-catalyzed C-H activation/cyclization of indoles and pyrroles, for the divergent synthesis of privileged heterocycles. A simple derivation of indoles and pyrroles to N-carboxamides with oxidative bidentate directing group could enable rhodacycle formation and late-stage redox-neutral cyclization with alkynes, alkenes and diazo compounds, for access to five- and six-membered fused heterocycles, such as pyrimido[1,6-a]indol-1(2H)-one, 3,4-dihydropyrimido[1,6-a]indol-1(2H)-one, and 1H-imidazo[1,5-a]indol-3(2H)-ones. Kinetic isotope effect study was conducted, and a plausible mechanism was proposed. Furthermore, this protocol was applied to concise synthesis of 5-HT3 receptor antagonist in gram-scale.

Various dienes and a triene can be regioselectively diaminated at the internal double bond with good yields and high diastereoselectivity using di-tert-butyldiaziridinone (5) as nitrogen source and Pd(PPh3)4 (1–10 mol%) as catalyst. Kinetic studies with 1H NMR spectroscopy show that the diamination is first-order in total Pd catalyst and inverse first-order in PPh3. For reactive dienes, such as 1-methoxybutadiene (6g) and alkyl 1,3-butadienes (6a, 6j), the diamination is first order in di-tert-butyldiaziridinone (5) and zero-order in the olefin. For olefins with relatively low reactivity, such as (E)-1-phenyl-butadiene (6b) and (3E,5E)-1,3,5-decatriene (6i), similar diamination rates were observed when 3.5 equivalents of olefins were used. Pd(PPh3)2 is likely to be the active species for the insertion of Pd(0) into the N-N bond of di-tert-butyldiaziridinone (5) to form a four-membered Pd(II) complex (A), which can be detected by NMR spectroscopy. Olefin complex (B), formed from intermediate A via ligand exchange between the olefin substrate and the PPh3, undergoes migratory insertion and reductive elimination to give the diamination product and regenerate the Pd(0) catalyst.

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Diamination of olefins presents an effective strategy to access vicinal diamine moieties which are contained in various biologically active molecules and are widely used as chiral control elements in asymmetric synthesis.1 Various metal-mediated and metal-catalyzed diamination processes have been reported.1-8 In our earlier studies, we have shown that conjugated dienes can be regioselectively diaminated using Pd(0)9 and Cu(I)10 as catalysts and di-tert-butyldiaziridinone (2)11 , 12 as nitrogen source. The Pd(0)-catalyzed diamination occurred regioselectively at the internal double bonds of dienes,9 and the Cu(I)-catalyzed diamination occurred primarily at the terminal double bonds with 10 mol% CuCl-P(OPh) 3 (Scheme 1).10 The Cu(I)-catalyzed diamination was proposed to proceed via a stepwise radical mechanism involving a Cu(II) species. During our ongoing studies, it was a surprise to find that the Cu(I)-catalyzed diamination could occur regioselectively at the internal double bond under certain conditions (e.g. with CuBr as catalyst) (Scheme 1). The reaction is likely to proceed via a concerted mechanism involving a four-membered Cu(III) species. Herein, we wish to report our preliminary studies on this subject.

A Rh(III)-catalyzed selective coupling of N-methoxy-1H-indole-1-carboxamide and aryl boronic acids is reported. The coupling is mild and efficient toward diverse product formation, with selective C-C and C-C/C-N bond formation. Kinetic isotope effects studies were conducted to reveal a mechanism of C-H activation and electrophilic addition.