Carbon—hydrogen bonds alkene reactions

Another important reaction principle in modem organic synthesis is carbon-hydrogen bond activation [159]. Bergman, Ellman, and coworkers have introduced a protocol that allows otherwise extremely sluggish inter- and intramolecular rhodium-catalyzed C-H bond activation to occur efficiently under microwave heating conditions. In their investigations, these authors found that heating of alkene-tethered benzimidazoles in a mixture of 1,2-dichlorobenzene and acetone in the presence of di-//-... 

The various modes of bonding that have been observed for alkenes to the trinuclear osmium clusters are shown in Fig. 7 [see (88)]. The simple 77-bonded structure (a) is relatively unstable and readily converts to (c) the vinyl intermediate (b) is obtained by interaction of alkene with H2Os3(CO)10 and also readily converts to (c) on warming. Direct reaction of ethylene with Os3(CO)12 produces (c), which is considered to be formed via the sequence (a) — (b) — (c) and (d). Both isomers (c) and (d) are observed and involve metal-hydrogen and metal-carbon bond formation at the expense of carbon-hydrogen bonds. In the reaction of Os3(CO)12 with C2H4, the complex 112088(00)902112, (c), is formed in preference to (d). Acyclic internal olefins also react with the carbonyl, with isomerization, to yield a structure related to (c). Structure (c) is... 

Under pyrolytic conditions at temperatures above 300°C, generally within 500-800°C, the pyrolysis reaction forms alkenes by carbon-hydrogen bond scissions. An early experiment, where propane was heated to 575°C for 4 min in a silica flask, yielded propylene by dehydrogenation [Eqs. (2.19)-(2.21)] at a somewhat slower rate than it yielded methane and ethylene by cracking 54... 

Treatment of an alkene with mercuric acetate in aqueous THF results in the electrophilic addition of mercuric ion to the double bond to form an intermediate mercuri-um ion. Nucleophilic attack by H2O at the more substituted carbon yields a stable organomercury compound, which upon addition of NaBH4 undergoes reduction. Replacement of the caiton-mercury bond by a carbon-hydrogen bond during the reduction step proceeds via a radical process. The overall reaction represents Markovnikov hydration of a double bond, which contrasts with the hydroboration-oxidation process. 

Particular interest has been shown in the reactions of photochemically generated arylcarbenes. A study of the photodecomposition of phenyldiazomethane in 2-chloropropane has revealed that carbon-chlorine bond insertion of singlet phenylcarbene predominates at low temperature in solution, whereas carbon-hydrogen bond insertion is preferred in a rigid matrix. The principal products of irradiation of diazoalkane (63) are phenylacetylene (64) and the cyclobutene (65), even in the presence of alkenes. At lower temperatures, however, carbene addition predominates as shown, for example, with isobutene as shown in Scheme 7. 

With some metals the process of olefin coordination and insertion may continue, leading to polymerization, but with palladium the metal is expelled from the molecule by a P-hydride elimination reaction and the product is an alkene, plus a Pd(ll) complex. For the whole process to be catalytic, this Pd(ll) product of P-hydride elimination must be converted to a Pd(0). This occurs in the presence of base, which removes HX from the palladium(ll) species. This is another example of reductive elimination one that forms a hydrogen halide rather than a carbon-carbon or carbon-hydrogen bond, as you saw earlier. 

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