Electron-rich acetylides

The electron-rich acetylide anion produced from deprotonation of 1-alkynes with Na in NH3 (liq.) is reluctant to accept an electron, allowing the selective reduction of an internal triple bond in the presence of a terminal one. Reduction to the corresponding 1-alkenes can be achieved in the presence of ammonium sulfate. ... 

The reaction proceeds with isolated double bonds and electron-rich alkynes. Electron-withdrawing groups in the acetylene moiety decelerated the reaction. A plausible mechanism implies the activation of the olefin by coordination of the metal triflate followed by nucleophilic attack of the acetylene or acetylide (Scheme 31). 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Silver acetylides also reacted with epoxides yielding propargylic alcohols.27 As for related alkylations with silver acetylides (see Section 10.6.1), this reaction required zirconocene dichloride and catalytic amount of silver triflate. This method proved useful for both electron-rich and electron-deficient alkynes and compatible with various acid- and base-sensitive functional groups (Table 10.2). 

The synthesis of alkylidyne complexes by y -addition of electrophiles to vinylidene and acetylide ligands is now well established (5,6). Pombeiro and co-workers synthesized several new rhenium alkylidyne complexes by protonation of the electron-rich vinylidene complexes 13 [Eq. (18)] (55). The mechanism of formation of the benzylcarbyne complex 14 (R = Ph)... 

An enolate anion is a nucleophile and, unlike Grignard reagents and organolithium reagents, reacts easily with the electrophilic carbon of an alkyl halide. Although both the oxygen and the carbon are nucleophilic (sec. 9.3.B), the carbon is usually the most nucleophilic site in the enolate. As with other carbon nucleophiles, such as acetylides (sec. 8.3), the electron rich carbon attacks the electrophilic carbon of the halide to form a new carbon-carbon bond, with lithium iodide (Lil) as the byproduct. 

Equations 13.10-13.12 show three examples of the synthesis of vinylidene complexes by reactions of metal-acetylide complexes with acid or base. The molybdenum(II) acetylide complex in Equation 13.10 reacts with acid to protonate the p-carbon and generate a cationic vinylidene complex. In this case, the vinylidene complex is thermodynamically unstable. Warming to 0 °C leads to rearrangement of this species to the tautomeric alkyne complex. In contrast, the more electron-rich molybdenum-acetylide complex in Equation 13.11 containing three phosphite donors generates a vinylidene complex upon addition of a proton from alumina to the 3-carbon of the acetylide. The vinylidene form of the complex is apparently more stable than the alkyne complex in this case. 

The reaction of alkynyliodonium salts with electron-rich transition metals usually results in an oxidative addition under formation of a metal-acetylide complex. Although this type of intermediates has been postulated in many catalytic reactions, this section is limited to the cases in which the metal complexes could be isolated and characterized. 

The complexes [Fe2(CO)6GM-C=CR )(/i-PPha)] (R = Ph or Bu ) are attacked by nucleophiles at the a- or /S-carbon atom of the bridging acetylide to give dipolar imminium derivatives. Isomers (19) and (20) have been isolated from the reaction with cyclohexylamine, and dicyclohexylphosphineaffords (21). Electron rich olefins [ ==CNR (CHa)gNR 2] (R = Me, Et, or Bz) yield (22) via carbene acetylide coupling. ... 

A fascinating series of reactions involving the coordination of 7] -C=CR acetylides to electron-rich triruthe-... 

More recently, we demonstrated the first alkynylation of benzylic C-H bonds not adjacent to a heteroatom with 1 mol% of a CuOTf-toluene complex in the presence of 1.5 equiv. of DDQ. Various allq nes were successfully coupled with diphenylmethane derivatives (Scheme 1.8). Aromatic allq nes were smoothly converted and the use of electron-rich derivatives resulted in a slightly improved jdeld, rationalized by the nucleophilicity of the substrates. However, aliphatic allq nes [i.e., n-heiq ne) did not give the corresponding CDC product under standard conditions. The mechanism was proposed to proceed via the generation of radical intermediates, which were converted into a benzylic cation in the presence of DDQ through two successive SET steps. The resulting hydroquinone subsequently then abstracted the acidic proton from the allq ne to form the copper acetylide, which added to the benzylic cation to afford the desired product. 

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