Hydride abstraction reactions

Reactions of the tetrahedral Ni° complex (994) have already been discussed in Section 6.3.5.4.2397 A nickel carbonyl cation (1062) containing a cyclophosphenium ligand has been assembled through a hydride abstraction reaction according to Equation (37).2552... 

The results from these experiments also allowed Hannon and Traylor to determine the primary and secondary hydrogen deuterium kinetic isotope effects for the hydride abstraction reaction. If one assumes that there is no kinetic isotope effect associated with the formation of 3-deutero-l-butene, i.e. that CH2=CHCHDCH3 is formed at the same rate (k ) from both the deuterated and undeuterated substrate (Scheme 25), then one can obtain both the primary (where a deuteride ion is abstracted) and the secondary deuterium... 

Dications 222+ and 232+ were synthesized by hydride abstraction reaction of the corresponding hydro derivatives as stable dark-brown powder. The p/CR+ values for these dications are also extremely high for doubly-charged systems (222+ 11.7 and 232+ 11.7). The electrochemical reduction of 222+ and 232+ exhibited a reduction wave at less negative potentials than that of dication 212+. This wave corresponds to the reduction of two cation units by a one-step, two-electron reduction to form thienoquinoid products. Chemical reduction of 222+ and 232+ afforded the closed-shell thienoquinoid compounds (22 and 23), which exhibited high electron-donating ability. The formation of the closed-shell molecules is in contrast with the result from reduction of dication 212+connected via a / -phenylenediyl spacer. 

Abstraction of a hydride from carbon is almost invariably an endothermic process. The rate of the reaction depends on the stability of the transition structure which closely resembles the product carbocation and is expected to be stabilized by the same factors, among them, substitution by X and C substituents. Nevertheless, initial interactions set the trajectory for the hydride abstraction reaction. The interaction of a C—H bond with a C substituent is shown in Figure 10.7. The feature relevant to the present discussion is that the HOMO which involves some admixture of the C—H bond has been raised in energy. Therefore, attack by electrophiles, while most likely at the n bond of the C substituent, is also possible at the C—H bond. The interaction of an X substituent with a CH bond is shown in Figure 10.7a. In general a single X or C substituent is not sufficient to activate the C—H bond toward hydride abstraction. 

The interaction of a C—H bond with a strong Lewis acid (low-energy LUMO) is shown in Figure 10.8a. The p orbital of a carbocation as the LUMO is shown by way of example. Examples of hydride abstraction reactions are shown in Scheme 10.1. 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

With methyl-substituted cyclohexadienes, very little selectivity is observed during the preparation of the complexes and the hydride abstraction reaction. Dihydrotoluene, on heating with pentacaibonyliron, gives a mixture of complexes (37) and (38 Scheme 4). These cannot be easily separated using standard chromatographic procedures, and little is known about hydride abstraction from the individual complexes. Treatment of the equimolar mixture with trityl tetrafluoroborate gives a mixture of all three possible products (39-41 Scheme 4). 

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

The hydride abstraction reaction of NO+ has been employed in a modified Ritter-type reaction51 1 (Scheme 5.50, routea) as well as in ionic fluorination512 of bridgehead hydrocarbons (Scheme 5.50, route b). 

The second class of TAM RE AC s inventory includes the reactions between the coordinated ligands and external organic reagents. We divide these reactions into nucleophilic and electrophilic attacks and consider them as acid-base interactions. Table III presents their general description. The nucleophilic attacks are either addition reactions to unsaturated coordinated ligands (Reactions 44-46) or abstraction reactions (usually a proton abstraction, Reactions 47-50). The electrophilic attacks are similarly addition reactions (Reactions 51 and 52) and abstraction reactions (usually a hydride abstraction, Reactions 53-59). Reactions 60 to 63 represent some other intermolecular reactions. 

In the cases of tungsten and molybdenum, the complexes are believed to be intermediates in the oxidation of alcohols to carbonyl compounds (a hydride abstraction reaction).85 Figure 2.28 illustrates some of the oxygen transfer... 

The green bis(trimethylsilyl)amido complex can be prepared by allowing [RhCl(PPh3)3] to react with LiN(SiMe3)2 in THF.59 It will be noted that neither of these anions can undergo / -hydride abstraction reactions that destroy simpler amido complexes. 

The final step of a synthesis of the alkaloid Monocrotaline [Scheme 3.94] required liberation of a hindered 1,2-diol encased in the methylene acetal 941.173 The task was accomplished by a hydride abstraction reaction using triphenyicar-benium tetrafluoroborate to give an intermediate of unknown structure whose hydrolysis afforded Monocrotaline in 86% yield. Triphenylcarbenium tetrafluoroborate can also be used to deprotect dioxolane derivatives of ketones174 and benzylidene derivatives.1755... 

The carboxonium ions may participate in chain transfer through hydride abstraction reaction ... 

Monohydrido transition metal complexes are the most active catalysts in double-bond migration reactions. These complexes form alkyl complexes when allowed to react with alkenes. The relatively long lifetimes of alkyl complexes in these systems allows them to undergo yS-hydride abstraction reactions before they can react with the other reagents present. The mechanism of the reaction is shown in Scheme 2. 

Markovnikov addition of hydrogen to the alk- 1-ene forms a 2-alkyl complex. Thermodynamically it is less likely that this 2-aUcyl complex will revert to the original alk-1-ene than be converted to an alk-2-ene as a result of competing /3-hydride abstraction reactions. 

If more than two deuterium atoms are added to cy clooctene, then isotope exchange must have taken place, and if more than four atoms of deuterium are added, then double-bond migration must also have occurred. The incorporation of excess deuterium can readily be explained by the participation of intermediate alkyl complexes that undergo -hydride abstraction reactions. 

The stability of the intermediate alkyl also depends upon the neutral ligands present. Its stability can be increased by using P(4-C6H4p)3 ligands in place of triphenylphosphine. These reduce the electron density on rhodium and make /3-hydride abstraction reactions less likely to occur. 

Metallacyclic (see Metallacycle) complexes of niobium and tantalum play an important role in understanding several catalytic and stoichiometric transformations of organic compounds. Some group 5 metallacycles are formed from the inter- or intramolecular hydride abstraction reactions. Most of the Nb and Ta metallacycles are prepared, however, from reductive coupling (see Reductive Coupling) of unsaturated organic substrates. To be included in this section, the metallacyclic ligand must have at least one M-C bond. 

Products from the hydride abstraction reactions of the zirconocene cycloalkylhydridoborate complexes 637 with B(C6Fs)3 are a function of the solvent and the -BR2 moiety. Thus, the reaction in poorly coordinating solvents such as... 

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