Carbon-Hydrogen Bond Formation

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

NADH or aldehyde) in excess of the other reactant. Under these conditions, the chemical conversion of aldehyde to alcohol occurs with a (saturated) apparent first-order rate constant of 200 to 400 sec i. This process, as measured either by the disappearance of NADH or by the disappearance of chromophoric aldehyde, has been shown by McFarland and Bernhard (80) to be subject to a primary, kinetic isotope effect ksjkj) =2 to 3 when stereospecifically labeled (4-R)-deuterio NADH is compared to isotopically normal NADH. Shore and Gutfreund (84) earlier had investigated substrate kinetic isotope effects on the pre-steady-state phase of ethanol oxidation. Their studies demonstrated that the rate of the presteady-state burst production of NADH is subject to a primary kinetic isotope effect, kulkj) 4—6 when 1,1-dideuterio ethanol is compared to isotopically normal ethanol, and that there is no primary kinetic isotope effect on the steady-state rate. It can be concluded from these studies (a) that the rate of interconversion of ternary complexes e.g., Eq. (19) above], as already mentioned, is rapid relative to turnover, and (b) that the transition-state for the rate-limiting step in the interconversion of ternary complexes involves carbon-hydrogen bond scission and/or carbon-hydrogen bond formation. 

Several mechanistic pathways have been proposed for nickel-catalyzed aldehyde/ alkyne reductive couplings, and an overview of the possible mechanisms has been provided elsewhere [3]. Therefore, this description will focus on what is generally believed to be the operative mechanism. The key features of this mechanism are the oxidahve cyclizahon of a zero-valent nickel aldehyde-alkyne complex to form a five-membered oxametallacycle, followed by reductive cleavage of the nickel-carbon bond, and carbon-hydrogen bond formation via reduchve elimination (Scheme 8.30). 

The diversity of results of studies of hydrogenation and hydroformylation catalysis have prompted a report on the factors governing the different mechanisms of binuclear elimination reactions of complexes leading to carbon-hydrogen bond formation. 

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