Hydride transfer reaction promoted

Pellet el al. reported on the reaction of propene over SAPO-n materials. Though the main products over ZSM-5 type aluminosilicate are aromatics and lower alkanes, oligomerization proceeds selectively over SAPO-11 and SAPO-31, indicating that these materials do not possess strong acid sites which are capable of promoting hydride transfer reactions. 

There are also reactions in which hydride is transferred from carbon. The carbon-hydrogen bond has little intrinsic tendency to act as a hydride donor, so especially favorable circumstances are required to promote this reactivity. Frequently these reactions proceed through a cyclic TS in which a new C—H bond is formed simultaneously with the C-H cleavage. Hydride transfer is facilitated by high electron density at the carbon atom. Aluminum alkoxides catalyze transfer of hydride from an alcohol to a ketone. This is generally an equilibrium process and the reaction can be driven to completion if the ketone is removed from the system, by, e.g., distillation, in a process known as the Meerwein-Pondorff-Verley reduction,189 The reverse reaction in which the ketone is used in excess is called the Oppenauer oxidation. 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

A mechanism possibly involving intermolecular hydride transfer in this promoted ruthenium system is thus very different from the reaction pathways presented for the cobalt and unpromoted ruthenium catalysts, where the evidence supports an intramolecular hydrogen atom transfer in the formyl-producing step. Nevertheless, reactions following this step could be similar in all of these systems, since the observed products are essentially the same. Thus, a chain growth process through aldehyde intermediates, as outlined earlier, may apply to this ruthenium system also. 

The essential features of the catalytic cycle (see Figure 10) involve the binding of NAD, the displacement of the water molecule by alcohol, the deprotonation of the coordinated alcohol to give a zinc alkoxide intermediate, the hydride transfer from the alkoxide to NAD to give a zinc-bound aldehyde, the displacement of the aldehyde by water and the release of NADH. The principal role of the zinc in the dehydrogenation reaction is, therefore, to promote deprotonation of the alcohol and thereby enhance hydride transfer... 

The major problem of the application of zeolites in alkane-alkene alkylation is their rapid deactivation by carbonaceous deposits. These either strongly adsorb on acidic sites or block the pores preventing the access of the reactants to the active sites. A further problem is that in addition to activity loss, the selectivity of the zeolite-catalyzed alkylation also decreases severely. Specifically, alkene formation through oligomerization becomes the dominant reaction. This is explained by decreasing ability of the aging catalyst to promote intermolecular hydride transfer. These are the main reasons why the developments of several commercial processes reached only the pilot plant stage.356 New observations with Y zeolites reconfirm the problems found in earlier studies.358,359... 

Iodine atom transfer reactions between alkyl radicals and iodocarbonyls are very rapid (107 M-1 s-1 to 109 M-1 s-1).130 This means that, even when these iodides are cyclized by the tin hydride method, iodine atom transfer may supersede hydrogen transfer, and the reductively cyclized product will ultimately be derived from the reduction of a cyclic iodide. Tin hydride cyclizations of halocarbonyls also often require very low concentration to avoid reduction of the initial radical prior to cyclization. For these reasons, reductively cyclized products are best formed by atom transfer cyclization at high concentration, followed by reduction of the product in situ. In a recent full paper, we have described in detail the preparative and mechanistic features of these cyclizations,19 and Jolly and Livinghouse have reported a modification of our reaction conditions that appears to be especially useful for substrates that cyclize very slowly.131 Cyclizations of a-iodocarbonyls can also be promoted by palladium.132... 

Normally, Oppenauer oxidations are performed employing Al3+ cations as catalyst because aluminium alkoxides possess a good balance of a desired high hydride transfer capability versus a low propensity to promote undesired base-induced reactions, like aldol condensations and Tischtschenko reactions. In the reaction, as originally described by Oppenauer, aluminium t-butoxide is used as catalyst,4 because its high basicity allows a very favourable equilibrium towards the formation of the aluminium alkoxide of the alcohol whose oxidation is desired. However,... 

Although the Mukaiyama oxidation is not in the top list of the most frequently used alcohol oxidants, the authors of this book have decided to pay full attention to this procedure because it succeeds in very sensitive organometallic compounds, where most other oxidants fail. The Mukaiyama oxidation operates via a somehow unique mechanism involving a hydride transfer from a metal alkoxide to a very good hydride acceptor, which resembles the Oppenauer oxidation. In variance with the Oppenauer oxidation, the Mukaiyama protocol involves much milder conditions and it does not promote as easily base-induced side reactions. 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

The -catalyzed hydride transfer from BNAH to Q is known to proceed via a -promoted ET from BNAH to Q, followed by a proton transfer from the resulting BNAH" to the Q" /(Mg ) complex and the subsequent fast ET from BNA to QH /Mg (117, 143). The change in the type of reaction depending on the Lewis acidity of the metal ion can be explained well by the ET mechanism in Scheme 23. The initial rate-determining ET from BNAH to Q results in the formation of radical ion pair (BNAH + and Q ), where Q forms 1 1 and 1 2 complexes with Sc. This result is followed by fast radical couphng between Q and BNAH to give the zwitterionic intermediate that is eventually... 

In the case of metal ion-promoted hydride transfer and cycloaddition reactions described above, binding of two metal ions to radical anions of electron acceptors can accelerate ET from electron donors to acceptors, leading to more efficient... 

The nature of the cation is unimportant in aqueous or other highly polar solutions of borohydrides, but influences the rate of reaction in isopropanol or pyridine, where the reagent exists mainly as associated ion-pairs [44]. Lithium borohydride is more reactive than the sodium compound in these solvents Li+ can probably associate more closely than Na+ with the carbonyl oxygen, promoting polarisation of the C=0 group, and so aiding hydride transfer from the anion. Other cations [e.g. Ca +j and solvents e.g. dimethylformamide) provide variations in reactivity which can have valuable uses for selective reduction of carbonyl groups [42]. 

On the basis of Kiyooka s working hypothesis for the aldol reaction mechanism, the reduction proceeds via by an intramolecular hydride transfer this is accelerated by matching between the chirality of the promoter and that of the newly formed aldol (Eq. 50). An alternative mechanism without chelation is also possible, and involves hydride delivery to the preferred O-silyl oxocarbenium ion conformer (Eq. 51). 

Cycloheptatriene and derivatives thereof donate hydride readily to a variety of carbonium ion acceptors. The position of the end equilibrium depends on the thermodynamics of the exchange. " These reactions are prototypes of a broad area of carbonium ion chemistry wherein carbonium ions equilibrate via intra- and inter-molecular hydride shifts between a donor C—H bond, usually jp hybridized, and a carbonium ion acceptor. This chemistry is often achieved with heterogeneous catalysts and is of great industrial significance it lies outside the emphasis of this review, however. Excellent treatises are available, and a review has appeared on the use of carriers like adamantane to promote hydride transfer in hydrocarbons under strongly acidic conditions. ... 

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