C-H/acetylene coupling

This chelation-assisted C-H/olefin and C-H/acetylene coupling can be applied to a variety of aromatic compounds with a directing group such as ester, aldehyde, imine, azo, oxazolyl, pyridyl, and nitrile [7]. In this section, we describe the coupling reactions of aromatic carbonyl compounds with olefins using a transition metal catalyst. 

Although there are several examples of transition metal-catalyzed addition of C-H bonds to acetylenes, there is neither experimental evidence nor theoretical consideration in respect of a reaction mechanism for C-H/acetylene coupling. This coupling reaction is believed to proceed through a pathway similar to that proposed for C-H/olefin coupling. 

Woodgate et al. [51] applied the C-H/acetylene coupling to the ortho-selective alkenylation of terpene derivatives (Eq. 27). The basic feature of this reaction is the same as the alkenylation reaction of Murai et al. The combination of acetophenone and diynes provides a new entry for the copolymerization of aromatic ketones with acetylenes. Weber et al. [50] studied extensive reactions of ruthenium-catalyzed C-H/acetylene coupling with respect to the step-growth copolymerization of aromatic ketones and acetylenes (Eq. 28). These coupling reactions provide a new route to the preparation of trisubstituted styrene derivatives. 

This C-H/olefin coupling can be extended to coupling with acetylenes [6], The reaction of aromatic ketones with internal acetylenes gives the ortho alkenylated product in high yield (Scheme 2), but reaction with terminal acetylenes does not afford the coupling product. With terminal acetylenes, dimerization of acetylenes occurs as a predominant reaction. 

GLASER - C H 0 D KI E W C Z Acetylene Coupling Polyacetylenes from monoacetylenes in the presence of copper salts. 

The combination of these findings with the aforementioned results of Beckhaus, Rosenthal, and Mach may also be interpreted in terms of an alternative mechanism for the polymerization of acetylene, which differs from that of Alt [12] (Scheme 10.3). In the absence of coupling, as in 11, or of twofold C—H activation as found in 12, the steps after substitution of MeiSiC=CSiMci by HC=CH and formation of Cp2Ti(i]2-HC2H) are (i) oxidative addition to give the hydrido-acetylide Cp2Ti(H)(C=CH),... 

The coupling constants were found to be related to the conventional %s character for the C-H bonds of methane (25), ethene (31) and acetylene (50) 5... 

Another unusual three-component coupling reaction involving an imine as intermediate has been developed by Ishii who has shown that a C-H bond adjacent to the nitrogen atom of an imine can be activated by an iridium complex. Carbo-metallation reactions of acetylenic compounds may then be achieved, which lead to unsaturated imines 155 (Scheme 8.67) [122]. 

As hydrocarbons, terminal acetylenes enjoy a rich reaction chemistry [1]. This is in no small part because of a unique feature of terminal acetylenes that differentiates them from other hydrocarbons - the acidity of the terminal proton (p-K,= 25). It is suggested that the lability of the terminal C-H towards deprotonation results its being bound to an sp-hybridized carbon [2]. This characteristic has been recognized for some time and has led to a diversity of methods for generation of metal acetylides which can participate in coupling reactions. 

The same simple scheme gives the correct description of the dimeric hydrocarbons. Acetylene has 10 valence electrons and three bonds, H-C-C-H, leaving two odd couples, predicting a bond order of 3. Ethylene with 12 electrons, 5 bonds and 1 odd couple has bond order 2. Ethane is saturated. 

Hence, monoarylation, monoalkenylation and monoalkynylation products of acetylene can only be obtained via monocoupling with alkynes that bear a substituent at one terminus that may be cleaved off after the coupling. In this section, we learned about three alkynes where this procedure is practical even if this fact has so far not been mentioned. Examples of alkynes with such protecting groups were presented in Figure 16.30 (HO)H2 C-C=C—H), in Fig-... 

---