Reaction hydride transfer

1a Hydride Transfer Reaction The hydride transfer reaction is 

According to H. Eyring et al. (1936), the relative rate of the hydride transfer reaction, which is proportional to the collision cross section, should increase monotonically with a my, where a is the polarizability and my is the reduced mass. Using C2D5+, C3D7+, and C4D9+ as ions, Ausloos et al. (1966) have found confirmation of the theory for a number of alkanes and cycloalkanes. 

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That the reduction with formic acid proceeds by a hydride transfer reaction was proposed by Lukes and Ji2ba 100) and finally proven by Leonard and Sauers 63). The use of variously deuterated formic acid allowed Leonard and Sauers to determine that (1) protonation or... 

Of course, these conclusions do not rule out completely the occurrence of other reactions such as those listed above, but their contribution to the overall mechanism must be very small in the production of the oligomers. The dark colour of these products was attributed to hydride transfer reactions, similar in nature to those encountered in the cationic polymerization of 2-vinyl furan [see Section III-B-l-c)]. The subsequent process which transforms these oligomers into cross-linked resins was not investigated. 

Nickel-tin alloys electroplating, 6,14 Nicotinamide zinc complexes, 5,952 hydride-transfer reactions, 5, 954 Nicotinic acid... 

Field and Lampe (23) established the occurrence of the hydride transfer reaction in the gas phase in 1958 by detecting secondary ions of mass one unit lower than the parent compound. Subsequently, Futrell (24, 25) attempted to account for most lower hydrocarbon products formed in the radiolysis of hexane and pentane by assuming that hydride transfer reactions play a dominant role in radiolysis. More recently, Ausloos and Lias (2) presented experimental evidence which indicated that some of the products in the radiolysis of propane are, in... 

As a result of the conclusions reached in these studies, a simple competition method was devised 12, 32) to determine relative rates of hydride transfer reactions rather accurately. For example, to obtain relative reaction rates of ethyl ions with various additives, a suitable source of fully deuterated ethyl ions such as C3D8 or iso-C4Di0 was irradiated in the presence of a perprotonated additive (RH), leading to the formation of C2D6 and C2D5H by Reactions 2 and 3. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

The relative probabilities of Reactions 24, 25, and 26 were, respectively, 1.00, 0.25, and 0.12 at a hydrogen pressure of about 1 atmosphere (9). These numbers could be derived either by analyzing the stable alkanes formed in the unimolecular decompositions (Reactions 24-26) or from the products of the hydride transfer reactions between C5Hi2 and the alkyl ions. Elimination of H2 from protonated pentane may also occur, but it is difficult (although not impossible) to establish this reaction through neutral product analysis. 

Another milestone discovery in the held of silyl cations chemistry was achieved by Reed and co-workers in the same year, 1993, when Lambert published his Et3Si study. Reed synthesized his t-PrjSi (CBuHsBrg), (2+ (CBnH5Br5), by the hydride transfer reaction of /-PrjSiH and Ph3C (CBnH6Br6) in toluene,taking advantage of the very low nucleophilicity of the carborane anion" (Scheme 2.9). 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

Metal hahdes in imidazolium ionic hquids offer unique enviromnents able to facihtate dehydration reactions. Under such conditions certain metal halides are able to catalyze formal hydride transfer reactions that otherwise do not occur in the ionic liquid media. We have now discovered two systems in which this transformation has been observed. The initial system involves the conversion of glucose to fractose followed by dehydration the second system involves the dehydration of glycedraldehyde dimer followed by isomerization to lactide. CrCls" anion is the only catalyst that has been effective for both systems. VCI3" is effective for the glyceraldehyde dimer system but not for glucose. 

The difficulty in controlling the regiochemistry during radical-denitration of allylic nitro compounds is well known. The migration of the double bond is a serious problem, as shown in Eq. 7.96. This problem is overcome by a hydride transfer reaction in the presence of a palladium catalyst (Eq. 7.97).140... 

The regiochemical control of Pd-catalyzed hydride transfer reaction is much more effective than that of the radical denitration, as shown in Eq. 7.98. The base-catalyzed reaction of nitroolefins with aldehydes followed by denitration provides a new synthetic method of homoallyl alcohols (Eq. 7.99).140 Exomethylene compounds are obtained by denitration of cyclic allylic nitro compounds with Pd(0), HC02H and Et3N (Eq. 7.100).140b... 

ELECTROCATALYTIC REDUCTION OF PROTONS AND HYDRIDE TRANSFER REACTIONS... 

In reaction (11) the metal-hydride addition suggests a protonation reaction whereas, in reaction (12) the addition appears to be a hydride transfer reaction. If the reaction is indeed a hydride transfer reaction then the introduction of p-electron donating substituents, which place more electron density at the carbonyl carbon, (the site of hydride attack) will inhibit hydride addition. The data in Table 2 show that the introduction of p-electron donating substituents reduces the turnover frequency. This is consistent with hydride attack at the benzaldehyde carbonyl carbon, (12). 

The r/zreo-3-deutero-2-trimethylstannylbutane that Hannon and Traylor158 used to determine the stereochemistry of the hydride transfer reaction and to shed light on the mechanism of this reaction was synthesized using the reactions in Scheme 22. Each of the reactions in Scheme 22 is stereo specific and the analysis showed that the product was at least 97% r/rreo-3-deutero-2-trimethylstannylbutane. If the elimination reaction from t/zreo-3-deutero-2-trimethylstannylbutane occurs with an awh -periplanar stereochemistry, the products shown in Scheme 23 will be obtained. Thus, if the elimination occurs by an awft -periplanar stereochemistry, all the fraws-2-butene will be monodeuterated while the ds-2-butene will not be deuterated. A syw-periplanar elimination from f/zreo-3-deutero-2-trimethylstannylbutane, on the other hand, would give the products shown in Scheme 24. If this occurs, the cw-2-butene will contain one deuterium atom and the fraws-2-butene will contain none. 

The primary hydrogen-deuterium kinetic isotope effect is obtained from the percent cw-2-butene obtained from the deuterated and undeuterated stannanes. This is possible because a hydride and a deuteride are transferred to the carbocation when the undeuterated and deuterated stannane, respectively, forms c -2-butene. The secondary deuterium kinetic isotope effect for the hydride transfer reaction is obtained from the relative amounts of fraws-2-butene in each reaction. This is because a hydride is transferred from a deuterated and undeuterated stannane when trans-2-butene is formed. 

The exo and the endo ring closures (the kc reactions) are in competition with the aryl radical-tributyltin hydride transfer (the ks or ku reaction). These workers162 used this competition to determine the primary hydrogen-deuterium kinetic isotope effect in the hydride transfer reaction between the aryl radical and tributyltin hydride and deuteride. 

Protein-Substrate Interactions During the Hydride Transfer Reaction... 

Hydride transfer reactions from [Cp2MoH2] were discussed above in studies by Ito et al. [38], where this molybdenum dihydride was used in conjunction with acids for stoichiometric ionic hydrogenations of ketones. Tyler and coworkers have extensively developed the chemistry of related molybdenocene complexes in aqueous solution [52-54]. The dimeric bis-hydroxide bridged dication dissolves in water to produce the monomeric complex shown in Eq. (32) [53]. In D20 solution at 80 °C, this bimetallic complex catalyzes the H/D exchange of the a-protons of alcohols such as benzyl alcohol and ethanol [52, 54]. 

The proposed mechanism shown in Scheme 7.11 is supported by stoichiometric proton- and hydride-transfer reactions of metal hydrides that were dis-... 

Intermolecular hydride transfer (Reaction (6)), typically from isobutane to an alkyl-carbenium ion, transforms the ions into the corresponding alkanes and regenerates the t-butyl cation to continue the chain sequence in both liquid acids and zeolites. 

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