Alkynals internal redox reactions

In 2012, the Sun group developed an efficient NHC-catalyzed internal redox reaction of alkynals bearing a y-leaving group. This process provides a new route to a range of allenoates in good yields. Preliminary results demonstrate that the enantioselective variant can also be achieved (one example with 89% yield and 30% ee). Albeit with low enantioselectivity, this first successful exploitation of an allenolate intermediate for allenoate formation provides a versatile platform for further development of new NHC-catalyzed reactions of alkynals. Without the use of transition metals and the production of phosphine oxide waste, this process serves as a green alternative for the synthesis of allenoates (Scheme 7.92). 

Scheme 7.92 NHC-catalyzed internal redox reaction of alkynals with a y-leaving group reported by Sun.

Nickel-Catalyzed Couplings via Internal Redox Reactions In contrast with the reductive couplings discussed earlier, where two tt-systems are joined via C—C bond formation and a net two-electron reduction takes place by the action of a reducing agent, Montgomery et al. observed that an alkyne and an enal can be coupled via an internal redox process where the aldehyde is oxidized and the alkyne is reduced [60]. A three-component... 

Analogous trans-a,dditions of perfluoroalkyl iodides to terminal or internal alkynes in the presence of sodium dithionite and mediated by ultrasonic irradiation or catalysed by triethylborane have been reported. 1,2-Dibromotetrafluoroethane reacts with terminal acetylenes in DMF in the presence of the redox system (NH4)2S208/HC02Na.2H20 to yield a mixture of mainly (E)- and some (Z)-olefins 237, the initial adducts having undergone reductive debromination under the reaction conditions... 

Rhodium(lll)-catalyzed redox—neutral coupling of N-phenoxyacet-amides and alkynes led to benzo[l)]furan derivatives (13AGE6033). Furo[2,3-l)]pyran-6-one derivatives were prepared via rhodium(II)-catalyzed reactions of diazo compounds and ethynyl compounds (13T9294). Copper-mediated oxidative annulation of phenols and unactivated internal alkynes afforded benzo[l ]furan derivatives (13CS3706). E t-kaurane maoecrystalV was produced via C-H functionalization (13JA14552). Rhodium-catalyzed intramolecular C-H... 

Annulation reactions between unactivated alkynes and a range of heteroatom-containing substrates have facilitated the preparation of a host of heterocycles. For example, the preparation of isoquinolones was achieved through a rhodium-catalyzed annulation of internal acetylenes with 0-methyl hydroxamates (Scheme 3.78 and Example 3.12) [81]. The reaction conditions were quite mild, and most reactions were complete within 16 h. The overall process was redox neutral and used a catalytic amount of cesium acetate to promote the reaction. Additionally, the process was not appreciably sensitive to the electronic composition of the 0-methyl hydroxamates. One of the more attractive aspects of this chemistry was the observation that the rhodium-catalyzed reaction... 

By introduction of an oxidizing directing group, Li and Wang demonstrated a regioselective ruthenium(ll)-catalyzed isoquinolone synthesis through oxidative cyclization of JV-methoxybenzamides with internal alkynes (Eq. (7.24)) [31]. This redox-neutral strategy circumvents the use of wasteful metal oxidants, and the reaction could take place at much milder conditions with broad substrate scope. 

Matsunaga, Kanai, and coworkers developed a Cp Co -catalyzed redox-neutral annulation reaction of an Af-carbamoyl-indole bearing a morpholinyl group and an internal alkyne to afford a pyrrololndolone derivative (Scheme 10.12) [32]. The annulation reaction employs a catalytic system consisting of a cationic... 

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