Substituent effects alkyne carbons

Studies reporting substituent effects on the palladium- and copper-catalysed Sonogashira coupling reaction between an aryl iodide and an alkyne the 5 2 allylic substitution reactions between benzyl amine and racemic allyl carbonates substituted with a j -X-Ph- group on C(l) in the presence of a Rh(15,15, 2/ ,2/ -tangphos)(COD)Bp4 catalyst the stereoselective 5 2 reactions between a-substituted linear 0-ketoesters and meta- and /Jura-substituted cinnamyl carbonates generating vicinal quaternary and tertiary stereocenters in the presence of an Ir-V-arylphosphoramidite catalyst, TBD, and LiOBu-t identity vinyl halide reactions the S N... 

Alkynes have chemical shifts between 63 and 95 ppm. The chemical shifts for some alkynes are presented in Table 4.13. The effect of carbon substituents on alkyne chemical shifts are presented in Table 4.14. It may be noted that the resonance of the non-hydrogen-bearing carbon in alkynes is less intense due to lower NOE and is therefore usually readily recognized. 

After extensive screening of various aldehydes to optimize the reaction conditions, it was found that aromatic aldehydes were able to serve as a carbon monoxide source, in which the electronic nature of the aldehydes is responsible for their ability to transfer CO efficiently [24]. Consequently, aldehydes bearing electron-withdrawing substituents are more effective than those bearing electron-donating substituents, with pentafluoro-benzaldehyde providing optimal reactivity. Interestingly, for all substrates tested the reaction is void of any complications from hydroacylation of either the alkene or alkyne of the enyne. Iridium and ruthenium complexes, which are known to decarboxylate aldehydes and catalyze the PK reaction, demonstrated inferior efficiency as compared to... 

The ability to harness alkynes as effective precursors of reactive metal vinylidenes in catalysis depends on rapid alkyne-to-vinylidene interconversion [1]. This process has been studied experimentally and computationally for [MC1(PR3)2] (M = Rh, Ir, Scheme 9.1) [2]. Starting from the 7t-alkyne complex 1, oxidative addition is proposed to give a transient hydridoacetylide complex (3) vhich can undergo intramolecular 1,3-H-shift to provide a vinylidene complex (S). Main-group atoms presumably migrate via a similar mechanism. For iridium, intermediates of type 3 have been directly observed [3]. Section 9.3 describes the use of an alternate alkylative approach for the formation of rhodium vinylidene intermediates bearing two carbon-substituents (alkenylidenes). 

The catalyzed reduction was effective for selective reduction of carbonyl groups of enones and ynones without affecting the C-C unsaturated bond (Scheme 43). Acyclic enones 544,1107 endocyclic enones 545,1108,1109 exocyclic enones 546 and 547,1110,1111 and ynones 548-5511112-1115 were enantioselectively reduced under catalytic conditions. The reductions of ynones, for example, 551, to give anti-product 552 via a cyclic chelate 553 were sensitive to the steric size of the distal group of alkyne and to the nature of substituents on the carbonyl carbon. 

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