Alkyne reduction with

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

A somewhat unusual sequence to generate azepanones 80 involved the intramolecular addition of hydroxylamines to alkynes 76 to form cyclic nitrones 77. A vinyl magnesium bromide addition at low temperatures and a reduction with TiCls followed by N-Boc protection led to the azepane 78. Double bond bromination and subsequent RUO4 oxidation gave the lactam 79. Several further steps allowed the generation of the lactam structure 80 proposed for d,/-aca-cialactam, but the spectral data of the synthetic material differed from that of the natural product (Scheme 16)] [23 a, b]. 

Starting from 63, the carbonylation may proceed via coordination and insertion of CO into the vinyl-C-Pd bond to provide an a,P-unsaturated acyl complex. This complex reacts with (ArY) 2, and subsequently the C-Y bond is formed by reductive elimination to give 64 (Scheme 7-14). Because the compound 64 could be directly converted into the corresponding enal 65 by the Pd-catalyzed reduction with BujSnH, this sequence is synthetically equivalent to the regio- and stereoselective thioformy-lation and selenoformylation of alkynes (Eq. 7.49) [53, 54]. 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

An interesting application of the Pd-catalyzed reduction was reported for the preparation of the chromiumtricarbonyl complexed phenylallene 27 (Scheme 3.14), which was obtained from the internal alkyne 26 with good regioselectivity [45]. 

Palladium-catalyzed reduction of propargyl acetates is possible with Sml2 in the presence of a proton source (Scheme 3.17) [51]. The allene/alkyne selectivity is greatly influenced by the choice of the proton source. Propargyl phosphates were also converted into hydridoallenes by Pd-catalyzed reduction with Sml2 [52],... 

It has been reported that a cationic iridium such as [Ir(cod)2]BARF (BARF = 3,5-(CF3)2C6H3 4B), when combined with l,l -bis(diphenylphosphino)ferrocene (DPPF), catalyzed a hydrogen-mediated reductive carbon-carbon bond formation [68]. Thus, the reaction of alkynes 150 with a-ketoesters 151 produces p.y-unsaturated-a-hydroxy ketones 152 (Equation 10.40). 

At ambient temperatures, the primary CV processes observed for Co2(CO)6 2 ifi-T] ijL-ri -RC2C2R) (R = Ph, Fc), which contain two chemically equivalent C02 (/z-alkyne)(CO)6 redox centers, are an apparent irreversible 2-e reduction, with an... 

Hydrogenation using Raney nickel is carried out under mild conditions and gives cis alkenes from internal alkynes in yields ranging from 50 to 100% [356, 357, 358, 359, 360]. Half hydrogenation of alkynes was also achieved over nickel prepared by reduction of nickel acetate with sodium borohydride (P-2 nickel, nickel boride) [349,361,362] or by reduction with sodium hydride [49], or by reduction of nickel bromide with potassium-graphite [363]. Other catalysts are palladium on charcoal [364], on barium sulfate [365, 366], on... 

Total synthesis of motuporamide C is achieved by alkyne metathesis as a key step/ Diyne 133 is readily transformed into macrocyclic alkyne 134 with either catalyst. Lindlar reduction of 134 gives cycloalkene 135, which is further derivatized to motuporamine C-2HC1 (Scheme 47). 

Alkyne metathesis is employed for constructing the a-chain of PGE2.methyl ester. Reaction of alkyne 144 and symmetrical alkyne 145 in a slight excess in the presence of the I4O/CH2CI2 catalyst produces the desired CM product 146 in 51% yield, which is then converted to PGE2.methyl ester by partial reduction with a Lindlar catalyst leading to (Z)-olefin 147 and subsequent deprotection (Scheme 49). 

The radical anion of /3-trimethylsilylstyrene also undergoes dimerization but coupling takes place at the carbons a to silicon 33). The kinetics of the alkyne dimerization, followed by ESR, showed the reaction to be second order in radical anion 43). With Li+, Na+, K+, or Rb+ as the counterions, the rate increases in the order Si < C < Ge 45). Consistent with the increased stability of the trimethylsilyl-substituted radical anion, the radical anion of 1,4-bis(trimethylsilyl)butadiyne, produced by reduction with Li, Na, K, Rb, or Cs in THF is stable at room temperature even on exposure to air, whereas the carbon analog, 1,4-di-r-butyl-1,3-butadiyne radical anion, dimerizes by second-order kinetics at -40° (42). The enhanced stability of the trimethylsilylalkynyl radical anions has been attributed to p-drr interactions (42). 

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