Reactions ionic hydrogenation

Hydrogen reacts direcdy with a number of metallic elements to form hydrides (qv). The ionic or saline hydrides ate formed from the reaction of hydrogen with the alkali metals and with some of the alkaline-eartb metals. The saline hydrides ate salt-like in character and contain the hydride, ie,, ion. Saline hydrides form when pure metals and H2 react at elevated temperatures (300—700°C). Examples of these reactions ate... 

A considerable extension of the synthetic utility of the hypoiodite reaction is achieved if the steroid hypoiodite (2) is generated from the alcohol and acetyl hypoiodite and then decomposed in a nonpolar solvent. In this case ionic hydrogen iodide elimination in the 1,5-iodohydrin intermediate (3) is slow, thereby allowing (3) to be converted into an iodo hypoiodite (5). 

Lunsford and Leland (42) studied the reaction of hydrogen-deuterium exchange on crystals of MgO containing V-centers. As known, a Y-center in an ionic crystal, being a cationic vacancy with a hole localized near it, plays the role of an acceptor. These authors have found that the photocatalytic effect is intensified as the concentration of V-centers in a crystal increases, which is in accord with the experiments carried out by Kohn and Taylor. 

Alkenes to Alkanes. The ionic hydrogenation of many compounds containing carbon-carbon double bonds is effected with the aid of strong acids and organosilicon hydrides following the n-route outlined in Eq. 2. A number of factors are important to the successful application of this method. These include the degree and type of substituents located around the double bond as well as the nature and concentrations of the acid and the organosilicon hydride and the reaction conditions that are employed. 

Trisubstituted Alkenes. With very few exceptions, trisubstituted alkenes that are exposed to Brpnsted acids and organosilicon hydrides rapidly undergo ionic hydrogenations to give reduced products in high yields. This is best illustrated by the broad variety of reaction conditions under which the benchmark compound 1-methylcyclohexene is reduced to methylcyclohexane.134 146,192 202 203 207-210 214 234 When 1-methylcyclohexene is reduced with one equivalent of deuterated triethylsilane and two equivalents of trifluoroacetic acid at 50°, methylcyclohexane-... 

Kursanov, D. N. Parnes, Z. N. Kalinkin, M. I. Loim, N. M. Ionic Hydrogenation and Related Reactions-, Harwood Academic Publishers Chur, Switzerland, 1985. 

The success of stoichiometric ionic hydrogenations is due to achieving a fine balance that favors the intended reactivity rather than any of several possible alternative reactions. The acid must be strong enough to protonate the unsaturated substrate, yet the reaction of the acid and the hydride should avoid producing H2 too quickly under the reaction conditions. The commonly used pair of CF3C02H and HSiEt3 meets all these criteria. 

Another potential mechanistic complication is capture of the intermediate carbenium ion by the conjugate base of the acid. When CF3C02H is used as the acid, this would lead to trifluoroacetate esters. Kursanov et al. showed that, under the reaction conditions for ionic hydrogenations, trifluoroacetate esters can be converted to the hydrocarbon product (Eq. (3)). 

Stoichiometric ionic hydrogenation of the C=C bond of a,/ -unsaturated ketones by HOTf and [Cp(CO)3WH] results in the formation of -ketone complexes of tungsten [32]. As exemplified in Eq. (17), hydrogenation of methyl vinyl ketone gives a 2-butanone complex of tungsten. The bound ketone is displaced by the triflate counterion, giving the free ketone. Similar reactions were reported for hydrogenation of the C=C bond of a,/ -unsaturated aldehydes. 

The kinetics of the ionic hydrogenation of isobutyraldehyde were studied using [CpMo(CO)3H] as the hydride and CF3C02H as the acid [41]. The apparent rate decreases as the reaction proceeds, since the acid is consumed. However, when the acidity is held constant by a buffered solution in the presence of excess metal hydride, the reaction is first-order in acid. The reaction is also first-order in metal hydride concentration. A mechanism consistent with these kinetics results is shown in Scheme 7.8. Pre-equilibrium protonation of the aldehyde is followed by rate-determining hydride transfer. 

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]. 

Molybdenum and tungsten carbonyl hydride complexes were shown (Eqs. (16), (17), (22), (23), (24) see Schemes 7.5 and 7.7) to function as hydride donors in the presence of acids. Tungsten dihydrides are capable of carrying out stoichiometric ionic hydrogenation of aldehydes and ketones (Eq. (28)). These stoichiometric reactions provided evidence that the proton and hydride transfer steps necessary for a catalytic cycle were viable, but closing of the cycle requires that the metal hydride bonds be regenerated from reaction with H2. 

D.N. Kursanov, Z.N. Parnes, M.I. Kalin-kin, N.M. Loim, Ionic Hydrogenation and Related Reactions, Harwood Academic Publishers, New York, 1985. 

Ionic hydrogenation mechanisms involve the sequential transfer of hydride and proton to the substrate [67]. This was suggested by the Leitner group for the hydrogenation of C02 with the catalyst precursor RhH(dppp)2 (Scheme 17.7) [50]. Spectroscopic evidence for each of the three intermediates was obtained by studying the steps as stoichiometric reactions. However, catalyst precursors that generate the highly active RhH (diphosphine) species in solution were subsequently found to operate by a more conventional insertion mechanism [20]. 

With hydrogen, the alkali metals form the mono-hydrides MeH, having salt-like properties and a partially ionic, Me H, NaCl-type structure. They are colourless crystalline solids having a fairly negative AH of formation. The mono-hydrides react with water. They may be prepared from hydrogen and the metal (heated at 700-800°C for Li, 350-400°C for the others) or through the reaction of hydrogen with the alkali mono-oxide, nitride, etc. 

Ionic hydrogenation of the same bicyclic diene 382 by Et3SiH in the presence of CF3COOH at room temperature or at 80 °C via ions 387 and 388 is accompanied by transannular cyclizations (equation 139)192. The behavior of diene 382 under Ritter reaction conditions (MeCN, H2SO4) reveals new possibilities to control the transannular cyclizations (equation 140)193. Depending on the sulfuric acid concentration, the reaction temperature and the presence of a nucleophilic solvent, these transformations can be directed to the formation of either the bicyclic amides 389 and 390 having the precursor structure or the tricyclic products 391193. 

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