Hydrogen-transfer processes double bond hydrogenation

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Ene and Carbonyl-Ene Reactions. Certain double bonds undergo electrophilic addition reactions with alkenes in which an allylic hydrogen is transferred to the reactant. This process is called the ene reaction and the electrophile is known as an enophile A When a carbonyl group serves as the enophile, the reaction is called a carbonyl-ene reaction and leads to [3,-y-unsalurated alcohols. The reaction is also called the Prins reaction. 

Consequently, it is I ICo(CN)s3 that functions as a catalyst in hydrogenation processes. In the first step of the process shown in Figure 22.9, the alkene coordinates to HCo(CN)s3 as one hydrogen atom is added to the molecule so that only one double bond remains. The monoene is bonded to the cobalt in rf fashion. In the second step, another HCo(CN)53- transfers hydrogen to the alkene, which undergoes reductive elimination and leaves, having been converted to 1-butene. 

The low induction for the acetoacetates was attributed to a transfer hydrogenation process within an enol form of the substrate, coordinated through the carbon-carbon double bond, CH3C(OH)=CH—C02R, rather than hydrosilylation of the carbonyl moiety (285). 

Addition of 0- to double bonds and to aromatic systems was found to be quite slow. Simic et al. (1973) found that O- reacts with unsaturated aliphatic alcohols, especially by H-atom abstraction. As compared to O, HO reacts more rapidly (by two to three times) with the same compounds. In the case of 1,4-benzoquinone, the reaction with O consists of the hydrogen double abstraction and leads to the 2,3-dehydrobenzoquinone anion-radical (Davico et al. 1999, references therein). Christensen et al. (1973) found that 0- reacts with toluene in aqueous solution to form benzyl radical through an H-atom transfer process from the methyl group. Generally, the O anion-radical is a very strong H-atom abstractor, which can withdraw a proton even from organic dianions (Vieira et al. 1997). 

These reactions are most common for polar double bond as reactants (carbonyls and imines) than for non-polar substrates (alkene and alkyne). Hence, hydrogen-transfer processes are a very interesting option in order to perform polar double bond hydrogenation since they allow mild conditions, high selectivity,... 

It was found that most synthetic processes that are employed to prepare the diimide reagent generate trans-diimide, but ds-diimide undergoes faster hydrogen atom transfer to a double bond than does the trans isomer. It follows that a fast trans-cis isomerization precedes reduction. The transfer of hydrogen atoms takes place in a synchronous process188 via the transition state 19 ... 

This stabilization may also be interpreted in terms of oxygen anions, which, due, to the vacancy, are initially double bonded to Mo. One electron is transferred to the catalyst in this reaction step. To form acrolein, a second hydrogen atom is transferred (to form water) and an oxygen atom is bonded to the allyl radical. In this (rather complex) process, another three electrons are transferred to the catalysts and doubtless distributed over several Mo ions. Reoxidation takes place at the bismuth cations, where oxygen molecules are attracted by the free electron pair. The intermediate result is a surface bismuth with an oxygen coordination similar to that in the bulk, viz. 

Interestingly, when a fi-substituted alcohol is used in the Barton-McCombie reaction and if a [3-elimination process occurs faster than the hydrogen transfer step, then the formation of a double bond is observed. We have just seen such an example with a dixanthate (see Section 3.1.3). Many others are known as in [3-hydroxy sulfides [231] and [3-hydroxysulfones [232,233] in a modified Julia synthesis of olefins. 

When unsaturatcd polymers have hydrogen or halogen atoms in a-position to the double bonds, they are especially sensitive to chain transfer by a free radical attack. Therefore in these cases, the graft copolymerization may involve a combination of two initiation processes which occur simultaneously and compete with each other, one by chain transfer, the other by addition copolymerization. The relative importance of both processes is again dependent on the nature of the polymerizing monomer and of the backbone polymer involved in the reaction. 

A further possibility to interpret cis addition is the so-called cA-concerted mechanism74,145. It assumes that the addition of the two hydrogen atoms takes place in a single step in a concerted fashion on a single 3M site possessing three coordinative unsaturations. The transfer of the two hydrogens to the double bond through a concerted process, where the interaction with the catalyst removes the symmetry restrictions imposed by the Woodward-Hoffman rales, leads directly to alkane formation. 

Co(CN)6]3 or [Fe(CN)6]3. 89 The double biimidazole-NCRu hydrogen bond is evident in both the solid state and in CDC13 solution, where binding constants of over 103 - 105 were measured. Interaction with hexacyanoferrate(III) brings about a photoinduced electron transfer process which entirely quenches the Ru(II) luminescence. 

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