Trifluoromethylative difunctionalization and hydrofunctionalization of unactivated alkenes have been developed into powerful synthetic methodologies. On the other hand, methylative difunctionalization of olefins remains an unexplored research field. We report in this paper the Cu-catalyzed alkoxy methylation, azido methylation of alkenes using dicumyl peroxide (DCP), and di-tert-butyl peroxide (DTBP) as methyl sources. Using functionalized alkenes bearing a tethered nucleophile (alcohol, carboxylic acid, and sulfonamide), methylative cycloetherification, lactonization, and cycloamination processes are subsequently developed for the construction of important heterocycles such as 2,2-disubstituted tetrahydrofurans, tetrahydropyrans, γ-lactones, and pyrrolidines with concurrent generation of a quaternary carbon center. The results of control experiments suggest that the 1,2-alkoxy methylation of alkenes goes through a radical-cation crossover mechanism, whereas the 1,2-azido methylation proceeds via a radical addition and Cu-mediated azide transfer process.

Rhodium(I)-catalyzed cyclization of allenynes with a tethered carbonyl group was investigated. An unusual insertion of a CO bond into the C(sp(2) )-rhodium bond of a rhodacycle intermediate occurs via a highly strained transition state. Direct reductive elimination from the obtained rhodacyle intermediate proceeds to give a tricyclic product containing an 8-oxabicyclo[3.2.1]octane skeleton, while β-hydride elimination from the same intermediate gives products that contain fused five- and seven-membered rings in high yields.

Rhodium­(I)-catalyzed enantioselective hydroacylation of 4-allenals was found to proceed smoothly, giving six-membered ketones in good yields (up to 84% yield) with high enantiomeric excess (up to 96% ee) even from racemic allenes as substrates. Mechanistic studies revealed that racemization of the allene moiety in the substrate would occur via a dynamic kinetic resolution (DKR) process during the hydroacylation.

Rhodium(I)-catalyzed cyclization of allenynes with a tethered carbonyl group was investigated. An unusual insertion of a C=O bond into the C(sp 2 )-rhodium bond of a rhodacycle intermediate occurs via a highly strained transition state. Direct reductive elimination from the obtained rhodacyle intermediate proceeds to give a tricyclic product containing an 8-oxabicyclo[3.2.1]octane skeleton, while bhydride elimination from the same intermediate gives products that contain fused five-and seven-membered rings in high yields.Transition-metal-catalyzed [2+2+2] cycloadditions of two C À C multiple bonds with C = O bonds, as in aldehydes and ketones, are useful methodologies for the construction of oxygen-containing polycyclic compounds. [1][2][3][4] Intramolecular variants are particularly attractive reactions that enable us to easily access polycyclic compounds from acyclic substrates in one pot [Scheme 1, Eq. (1)]. [2a,g,h, 3] These cycloadditions begin with the formation of the metalacycle intermediate A through oxidative cycloaddition of two multiple CÀC bonds to a low-valent transition-metal complex, and cyclized products are produced through insertion of a C=O bond into the MÀC bond (a) of the intermediate A followed by reductive elimination from the intermediate B. In these reaction processes, if insertion of a C = O bond into the M À C bond (b) of A occurs, the intermediate C would be produced [Scheme 1, Eq. (2)].However, most transition-metal-catalyzed cycloadditions proceed through intermediate B, [2a,g,h, 3] and there has been no report on cyclization through the intermediate C, probably owing to the highly strained transition state A''.Recently, we have reported a Rh I -catalyzed [6+2] cycloaddition of 4-allenals with alkynes or alkenes [5] in a tether [Scheme 2, Eq. (1)]. [5a] During ongoing investigation of this cycloaddition, it was found that reaction of 1 a with [Rh-(dppe)]ClO 4 (10 mol %) instead of [Rh(IMes)(cod)]ClO 4 did not produce the expected product 2 a, which contains fused five-and eight-membered rings, but produced the bicyclic alcohol 3 a, which contains fused six-and seven-membered rings, in 27 % yield. [6][7][8] The formation of 3 a could not be explained by the mechanism of the reported [6+2] cycloaddition, but it might be formed via the above-mentioned unknown intermediate C followed by b-hydride elimination [Scheme 2, Eq. (2)]. This unexpected result prompted us to investigate Rh I -catalyzed cycloaddition of allenynes with tethered aldehydes giving the product 3 a. Scheme 1. Intramolecular cycloadditions of CÀC multiple bonds with C=O bonds.Scheme 2. Rh I -catalyzed cycloadditions of 1 a with [Rh(IMes)(cod)]ClO 4 or [Rh(dppe)]ClO 4 . IMes = 1,3-di(2,4,6-trimethylphenyl)imidazolin-2-ylidene, cod = cyclooctadiene, dppe = bis(diphenylphosphanyl)ethane.

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