Alkynes C-H bond

Our previously reported synthetic strategies (30-32), based on oxidative addition of terminal alkyne C-H bonds to RhW centers, could be extended to include bifunctional alkynes HCsC-(C6H4-4-)n-CsCH (n = 0-2). Thus, reaction of 2 equiv. of [(PMe3)4Rh]Cl with 5 or 6a,b in THF suspension gave the dinuclear hydrido-acetylides cis-cis-[ Rh(PMe3)4(H))2 CsC-(C6H4-4-)n CsC-)]2+[Cl2]2 (n = 0, 12a n = 1,12b n = 2,12c) in high yields as white precipitates. 

Scheme 2. Formation of vinylcarbamates via terminal alkyne C-H bond-activation with ruthenium catalysts.

The first example involves the dimerization of terminal alkynes. It takes place via initial activation of the alkyne C-H bond, but several examples involve a vinylidene intermediate. In most cases, conjugated enynes are obtained by ruthenium-catalyzed tail-to-tail dimerization [84,85], as in the following example [85] (Eq. 63). 

You may be confused the first time you see the IR spectrum of a terminal alkyne, R-OC-H, because you will see a strongish sharp peak at around 3300 cm"1 that looks just like an N-H stretch. The displacement of this peak from the usual C-H stretch at about 3000 cm"1 cannot be due to a change in the reduced mass and must be due to a marked increase in bond strength. The alkyne C-H bond is shorter and stronger than alkane C-H bonds. 

Catalytic addition of carbodiimides to terminal alkyne C-H bonds and amine N-H bonds provides a straightforward and efficient method for the synthesis of propiolamidines and substituted guanidines, respectively (Equations 8.38 and 8.39), which are widely used as ancillary ligands for stabilization of various metal complexes. 

Fig. 7 Plot of relative Rh-R bond strength in Tp Rh[P(OMe)3](R)H vs. C-H bond strength of hydrocarbon substrates. Experimentally determined D(Rh-C) vs. D(C-H). The solid line is fit to the hydrocarbons blue filled box, y = 1.550lx — 174.59), and the dashed line is fit to the -CH2X substrates red filled triangle, y = 1.4535x — 158.06). Experimental C-H bond strengths were used for all substrates except the alkynes. Alkyne C-H bond strengths were calculated (B3LYP) since experimental values are unavailable [13]. The vertical separation of the lines at >c-h = 100 kcal/ mol is 6.9 kcal/mol. Reproduced with permission of the RSC from Jiao et al. [20]...

Use the IR tables to locate the absorption bands of the stretching frequencies of the alkyne C—H bond, the alkyne C=C bond, and the alkene C—H bond. Using these data, explain how you would distinguish between 1-butyne, 2-butyne, and 2-butene. 

Much interest has been focused on the catalytic, direct conversion of alkyne C-H bonds through C-C bond-forming reactions without the stoichiometric generation of acetylides." One of the mostwidely used procedures for such an... 

Hydrogenation of Activated Alkenes and Alkynes (C-H Bond Formation)... 

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