Vinylic C-H bonds

There are three sorts of C-H bonds in cyclohexene, and Table 5.3 gives an estimate of their relative strengths. Although a typical secondary alkyl C-H bond has a strength of about 400 kj/mol (96 kcal/mol) and a typical vinylic C-H bond has a strength of 445 kj/mol (106 kcal/mol), ail allylic C-H bond has a strength of only about 360 kj/mol (87 kcal/mol). An allylic radical is therefore more stable than a typical alkyl radical with the same substitution by about 40 kj/mol (9 kcal/mol). 

Alkenes show several characteristic stretching absorptions. Vinylic =C—H bonds absorb from 3020 to 3100 cm-1, and alkene C=C bonds usually absorb near 1650 cm-1, although in some cases the peaks can be rather small and difficult to see clearly. Both absorptions are visible in the 1-hexene spectrum in Figure 12.14b. 

An allylic C-H bond of propene is broken with greater ease than even the 3C C-H bond of isobutene and with far greater ease than a vinylic C-H bond. 

The 13C—H coupling constants of methyl (213 Hz23 ) and phenyl (216 Hz55 ) cyclopropenone are in the order of those obtained for cyclopropene vinylic protons (200/201 218 Hz/221 Hz174 ) and reflect an s-contribution of more than 40% in the carbon hybrid orbital of the vinyl C—H bond. 

Vinyl C—H bonds are more acidic than the C—H bonds in saturated hydrocarbons because of their higher s-character and the polarizability of the double bond, but the corresponding carbanions are essentially localized. Allylic C—H bonds have the s-character of saturated hydrocarbons, but the resulting carbanions now have the possibility of additional stabilization by delocalization. Allylic positions are thus generally the most acidic in alkenes. 

C-H Insertions into vinylic C-H bonds are also a common reaction of electrophilic carbene complexes. Insertions into aromatic or heteroaromatic C-H bonds can proceed via cyclopropanation and rearrangement (Figure 4.6). 

This selectivity was attributed to the perpendicular geometry of the vinylic hydrogen atom to the olefinic plane. In such a conformation, the vinylic hydrogen is activated considering the large a -jt interactions between the vinyl C—H bond and the reacting C=C double bond (Scheme 25). 

SCHEME 25. a -n interactions between the vinyl C—H bond and the reacting C=C double bond... 

Both stereoisomers of a 4-(a-arylethylidene)-5(477)-oxazolone 441 and 443, undergo stereospecihc hydrolysis-methanolysis to furnish the corresponding (Z) and (E) isomers of 2-acetylamino(or benzoylamino)-3-aryl-2-butenoic acid or methyl ester, 442 and 444, respectively (Scheme 7.146). ° The requisite starting oxazolones were prepared by condensation of an acetophenone with an acylglycine or by methylene insertion into the vinyl C—H bond of a 4-arylidene-5(4//)-oxazolone. 

Unlike higher alkanes, ethane contains only primary C—H bonds, and the dehydrogenation product ethene contains only vinylic C—H bonds. As shown in Table I, these are strong bonds. Thus one would expect that, compared to other alkanes, the activation of ethane would require the highest temperature, but the reaction might be the most selective in terms of the formation of alkene. Indeed, this appears to be the case. 

Recoil UC atoms have been produced by nuclear transformations and allowed to react with ethylene.15 Both Q1/)) and C(3P) atoms are formed, and both add to the double bond and insert into the vinylic C—H bond. The resulting hot singlet adducts relax primarily to allene and methylacetylene, whereas the hot triplet adducts decompose to acetylene or are stabilized as carbenes, which mainly add to more ethylene to yield various C5 products. 

The OH radical reaction occurs at the >C=C< bond, and H-atom abstraction from the vinyl C-H bond is neglected. There are three Cl atom substituents around the >C=C< bond, and the "base" structural unit is the -CH=C< group (Table 14.2). The effects of the three Cl atom substituents are taken into account using the C(-Cl) value from Table 14.3. The rate constant, ktotal, for trichloroethene is given by ... 

If an alternative collapse could involve H abstraction, the same intermediate would be involved in propagation and vinyl chain transfer. The heat of formation of the complex then should be subtracted from the vinyl C-H bond energy as the radical now could spend most of its time associated with the double bond. 

Finally, the unusual ruthenium hydrido complex HRu(PPh3)3[HC=C(Me)C(0)0C4H9] is formed in the reaction between H2Ru(PPh3)4 and n-butylmethacrylate77). The Ru atom has oxidatively added to a vinylic C-H bond, and this compound represents the first example of such a structure. 

One of the oldest ruthenium-catalyzed C=C bond coupling reactions deals with the selective dimerization of functionalized alkenes, especially the dimerization of acrylates [ 1,2]. It usually involves either an initial hydrometallation process, oxidative coupling, or vinyl C-H bond activation (Scheme 1). 

Activation of vinyl C-H bonds with RuH2(CO)(PPh3)3 catalyst has allowed the formal insertion of a,/l-unsaturated ketones or esters into the C-H bond of vinylsilanes and led to a regioselective C-C coupling at the -position [9] (Eq. 6). Activation of the sp2 C-H bond occurred with the aid of chelation of a coordinating functional group and provided vinylruthenium hydride 14. Insertion of olefin afforded the tetrasubstituted alkene 13. The ruthenium activation of a variety of inert C-H bonds has now been performed by Murai [10]. 

Functionalized exo-methylenecyclopentanes can also be obtained by ruthenium-catalyzed intramolecular C-H bond activation [15]. l-(2-Pyridyl)-, l-(2-imidazolyl)-, and l-(2-oxazolyl)-l,5-dienes proceeded in a regiospecific manner to give five-membered ring products (Eq. 10). The proposed mechanism initially involves the activation of the vinylic C-H bond of the exocyclic C=C bond assisted by preliminary coordination of the nitrogen atom, followed by intramolecular insertion of the other C=C bond (see Eq. 6). 

When an oxidative coupling or addition takes place in the presence of carbon monoxide, CO insertion occurs leading to ketones. The Ru3(CO)12-catalyzed reaction of alkenylpyridyl or Af-(2-pyridyl)enamines and ethene performed under an atmosphere of carbon monoxide leads to the selective formation of a,/3-unsaturated ketones [16] (Eq. 11). After activation of the vinyl C-H bond, insertion of both carbon monoxide and ethylene takes place to give 25. 

Selective addition of alkenes and alkynes to aromatic compounds has also been performed by ruthenium-catalyzed aromatic C-H bond activation. Carbon-carbon bond formation occurs at the ortho positions of aromatic compounds, assisted by the neighboring functional group chelation. The reaction, catalyzed by RuH2(CO)(PPh3)3, was efficient with aromatic and heteroaromatic compounds, with various functional groups, and a variety of alkenes and alkynes [ 121 ] (Eq. 90). Activation of vinylic C-H bonds can occur in a similar manner. 

Guti6rrez-Puebla E, Monge A, Nicasio MC, Perez PJ, Poveda ML, Rey L, Ruiz C, Carmona E. Vinylic C-H bond activation and hydrogenation reactions of TpTr(C2H4)(L) complexes. Inorg Chem 1998 37(18) 4538-4546. 

Marciniec B, Walczuk-Gusdora E, Pietraszuk C. (2001) Activation of the vinylic =C-H bond of styrene by a rhodium sUoxide complex the key step in the sUylative coupUng of styrene with vinylsilanes. OrganometaUics 20 3423-3428... 

Monomerchain transfer constants are generally less than 10 . Reaction (6-79) involves breaking the strong vinyl C—H bond and the products of reaction (6-80) are not appreciably more stable than the reactants. 

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