Transfer hydrogenation alkyne derivatives

The enantioselective version of the relay transformation by organic and metallic catalyses was successfully demonstrated by Gong and coworkers (Scheme 3.39) [83]. They accomplished the direct transformation of o propargylaniline derivatives into tetrahydroquinolines in a highly enantioselective manner through the hydroamina tion of alkynes/isomerization/enantioselective transfer hydrogenation (see Sec tion 3.3 for details) sequence under the relay catalysis of an achiral Au complex/ chhal phosphoric acid binary system. 

Soon after these initial reports, the groups of Antilla [92] and You [93] indepen dently applied the chiral phosphoric acid catalysis to the enantioselective hydro genation of a imino esters. The method provides an alternative route to the enantioselective synthesis of a amino esters. Antilla and coworkers employed a new type of axially chiral phosphoric acid (9) derived from VAPOL originally developed by his research group (Scheme 3.42), whereas lg was used in You s case. In both cases, excellent enantioselectivities were achieved. You and coworkers further applied the method to the enantioselective reduction of a imino esters having an alkynyl substituent at the a position (Scheme 3.43) [94]. Both alkyne and imine moieties were reduced under transfer hydrogenation conditions with an excess amount of... 

A novel protocol described the direct conversion of 2-(2-propynyl)aniline derivatives (128) into tetrahydroquinolines (129) in high enantioselectivities, in one operation, through a consecutive hydroamination of alkynes/asym-metric transfer hydrogenation reactions under the catalysis of an achiral Au complex and the chiral phosphoric acid (124) binary system (Scheme 31). ... 

Sol 2. (c) The reactant undergoes intramolecular ene reaction under thermal conditions. In this reaction, allyl hydrogen is transferred to alkyne system instead of the transfer of propargylic hydrogen to alkene part, which may give allene derivative. 

The scope of this reaction was expanded with the development of the asymmetric transfer hydrogenation of P-y-alkynyl a-imino esters. In this reaction, both imine and aUcyne undergo reduction to yield synthetically challenging franx-alkenyl amino acid derivatives [65]. It was shown that conjugate hydride transfer to the alkyne takes place first, as propargyl a-amino esters failed to undergo reduction under the reaction conditions. 

Reaction of acetylenic complexes with triosmium dodecacarbonyl leads to a variety of products involving one, two, or three acetylenic units. As with ruthenium, for the monosubstituted alkynes, hydrogen transfer can occur to the metal cluster. Thus, Os3(CO)12 and phenyl-acetylene (L) yield, in refluxing benzene, the derivatives Os3(CO)10L, Os3(CO)10L2, Os3(CO)9L, and HOs3(CO)9(L-H). The general chemistry is summarized in Scheme 2 (131). 

Each of the syntheses of seychellene summarized in Scheme 20 illustrates one of the two important methods for generating vinyl radicals. In the more common method, the cyclization of vinyl bromide (34) provides tricycle (35).93 Because of the strength of sjp- bonds to carbon, the only generally useful precursors of vinyl radicals in this standard tin hydride approach are bromides and iodides. Most vinyl radicals invert rapidly, and therefore the stereochemistry of the radical precursor is not important. The second method, illustrated by the conversion of (36) to (37),94 generates vinyl radicals by the addition of the tin radical to an alkyne.95-98 The overall transformation is a hydrostannylation, but a radical cyclization occurs between the addition of the stannyl radical and the hydrogen transfer. Concentration may be important in these reactions because direct hydrostannylation of die alkyne can compete with cyclization. Stork has demonstrated that the reversibility of the stannyl radical addition step confers great power on this method.93 For example, in the conversion of (38) to (39), the stannyl radical probably adds reversibly to all of the multiple bond sites. However, the radicals that are produced by additions to the alkene, or to the internal carbon of the alkyne, have no favorable cyclization pathways. Thus, all the product (39) derives from addition to the terminal alkyne carbon. Even when cyclic products might be derived from addition to the alkene, followed by cyclization to the alkyne, they often are not found because 0-stannyl alkyl radicals revert to alkenes so rapidly that they do not close. 

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