Alkyne, electron-donating effect

To see the effects of alteration in number of alkyne electron-pair donations, let us compare the four-electron ML2 Ir(HCCH)+ complex in Fig. 4.86(b) with the corresponding two-electron ML Au(HCCH)+ complex,... 

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 global utility of this H-NMR alkyne probe is decreased by the scarcity of terminal alkyne adducts relative to the abundance of internal alkyne adducts. Diphenylacetylene and dimethylacetylenedicarboxylate (DMAC) are two particularly popular alkyne ligands which have no acetylenic proton to monitor. An empirical correlation between alkyne irx donation and, 3C chemical shift for the bound alkyne carbons has been recognized (155) which partially fills this spectroscopic need. A plot of alkyne 13C chemical shifts, which span over 100 ppm (Tables II and III), versus N, the number of electrons donated per alkyne to fulfill the effective atomic number guideline, reveals both the advantages and the limitations... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

The title compounds can be synthesized in moderate to good yields. The substituents on the acid chloride 4 and the terminal alkyne 5 can be varied within a wide range from electron donating to electron withdrawing without a general effect on the yields. The only exception is trimethylsilyl acetylene which produces lower yield. Interestingly, the trimethylsilyl group is not cleaved under the employed conditions. 

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