Acetylenic C—H bonds

The C-H bond activation followed by addition to a double bond leads to the formation of alkylated compounds (Equation (1)). This reaction involves aromatic, aliphatic, olefinic, and acetylenic C-H bonds. 

The chemistry of the carbon-carbon triple bond is similar to that of the double bond. In this chapter, we see that alkynes undergo most of the same reactions as alkenes, especially the additions and the oxidations. We also consider reactions that are specific to alkynes some that depend on the unique characteristics of the C=C triple bond, and others that depend on the unusual acidity of the acetylenic =C—H bond. 

The protection of the acetylenic C—H bond has recently been reviewed . This reference contains a table of protecting groups, their stabilities under oxidative coupling and metalation conditions and the conditions for their removal. 

Acetylenes. Evidence for H bonding of acetylenic C—H bonds is good, but it is not voluminous. Stanford and Gordy found systematic frequency shifts of the acetylenic C— H stretching mode of phenyl acetylene dissolved in bases (1932). Another type of evidence concerns the solubilities of acetylenic compounds in many bases. McKinnis, for example, presumes that the solubility of acetylene is dominated by the H bond interaction, and is able to correlate the solubility with electronegativity of the base atom B (1308). (See also 563.)... 

The s character of the vinylic C-H bond of cyclopropene is midway between that of sp and sp hybridized bonds and some similarity in behaviour to the acetylenic C—H bond is noted, particularly with regard to the acidity of the proton. Molecular orbital calculations at the ab initio level support an enhanced acidity of C(l)-H over C(3)-H in cyclopropene and correctly predict the preference for a 1-lithio derivative. A bis-lithio derivative is expected to have both lithium atoms bridging the C(l)-C(2) a bond. 

Calculated (B3LYP) enthalpies of addition and the effects of phosphine methylation (substitution of PMca for PH3) in reactions of some iridium complexes with C-H compounds (as well as with molecular hydrogen) are summarized in Table VI. 3 [46], It can be seen that the addition of aryl and especially acetylene C-H bonds is thermodynamically more favorable than the addition of simple alkyl C-H bonds. Addition of an aryl C-H bond has been found to be at least 16 kcal mol less exothermic than H2 addition. However, on the basis of the Bryndza-Bercaw relationship [47]... 

During the course of studying the ene reaction, these authors identified an interesting reaction between benzyne and tert-butylacetylene, in which two products were observed (Scheme 12.57). When a 1 4 ratio of the substrates 200 and 201 was employed, product 202 from direct addition of the acetylenic C-H bond across the benzyne was obtained as the major product. Only -5% of another product, 203, generated from the Diels-Alder reaction between benzyne and product 202 was obtained. On the other hand, if the substrate ratio of 200 to 201 was switched to 2 1, the anthracene 203 was formed predominantly in 92% yield. 

A triple bond is described in terms of the overlap of sp hybrid orbitals of adjacent carbon atoms to form a cr bond, the overlap of parallel 2p orbitals to form one tt bond, and the overlap of parallel 2p orbitals to form a second tt bond (Figure 1.22). In acetylene, each carbon-hydrogen bond is formed by the overlap of a Is orbital of hydrogen with an sp orbital of carbon. Because of the 50% s-character of the acetylenic C—H bond, it is unusually strong (see Table 1.11 and related text). 

Draw a line at 3000, and look for vinylic or acetylenic C—H bonds to the left of the line. 

The radical character of the arylacetylene curing reaction was already well established (7,8) and our results confirm it The initiation step could be seen as a monomolecular process (27,28) corresponding to the homolytical cleavage of the terminal acetylene C-H bond. However such a mechanism could not explain the formation of the 1,2,3-trisubstituted aromatic trimer , even by considering the possibility for the monoradicals to rearrange. It could not either explain the delay between the first 20% of i consumption and the appearance of the first formed product 6 and IQ (Figure 4). 

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