Hydride donors structure

Bicyclo[3.3.1]nonan-9-one is another ketone that exhibits interesting stereoselectivity. Reduction by hydride donors is preferentially syn to electron-attracting substituents at C-5 (X = EWG in the structure shown below) and anti to electron-releasing substituents (X = ERG below). These effects are observed even for differentially substituted phenyl... 

A-methoxy-A-methyl amides.87 LiAlH4 and DiBAlH have both been used as the hydride donor. The partial reduction is again the result of the stability of the initial reduction product. The A-methoxy substituent leads to a chelated structure that is stable until acid hydrolysis occurs during workup. 

Reduction of Ketones and Enones. Although the method has been supplanted for synthetic purposes by hydride donors, the reduction of ketones to alcohols in ammonia or alcohols provides mechanistic insight into dissolving-metal reductions. The outcome of the reaction of ketones with metal reductants is determined by the fate of the initial ketyl radical formed by a single-electron transfer. The radical intermediate, depending on its structure and the reaction medium, may be protonated, disproportionate, or dimerize.209 In hydroxylic solvents such as liquid ammonia or in the presence of an alcohol, the protonation process dominates over dimerization. Net reduction can also occur by a disproportionation process. As is discussed in Section 5.6.3, dimerization can become the dominant process under conditions in which protonation does not occur rapidly. 

The hydride donor ability of ZrH2(C5Me5)2 was demonstrated convincingly in a separate study (88b) when it reacted with (C5H5)2W(CO) to yield the zirconoxycarbene compound of structure (17b). 

In addition to their reactions with alkenes and carbanions as nucleophiles benzhydryl cations react with hydride donors.282 284 These hydride transfer reactions show the same linear dependence of log k upon E as the reactions with alkenes and the same constant relative selectivity, that is with slopes of plots close to 1.0, for structures ranging from cycloheptatriene to the... 

Because systematic variations in selectivity with reactivity are commonly quite mild for reactions of carbocations with n-nucleophiles, and practically absent for 71-nucleophiles or hydride donors, many nucleophiles can be characterized by constant N and s values. These are valuable in correlating and predicting reactivities toward benzhydryl cations, a wide structural variety of other electrophiles and, to a good approximation, substrates reacting by an Sn2 mechanism. There are certainly failures in extending these relationships to too wide a variation of carbocation and nucleophile structures, but there is a sufficient framework of regular behavior for the influence of additional factors such as steric effects to be rationally examined as deviations from the norm. Thus comparisons between benzhydryl and trityl cations reveal quite different steric effects for reactions with hydroxylic solvents and alkenes, or even with different halide ions... 

Chiral C2-symmetric boron bis(oxazolines) act as enantioselective catalysts in the reduction of ketones promoted by catecholborane.321 DFT calculations indicate that the stereochemical outcome is determined by such catalysts being able to bind both the ketone and borane reducing agent, activating the latter as a hydride donor, while also enhancing the electrophilicity of the carbonyl. X-ray structures of catalyst-catechol complexes are also reported. 

H +H +C-N-H / V H H MP2/6-31G // MP2/6-31G Geometry, energy, transition state structure and energy barrier to proton loss, isodesmic reaction with hydride donor, proton affinity of CH2NH2+ 51... 

Suitable Hydride Donors and Organometallic Compounds the Structure of Organolithium Compounds and Grignard Reagents... 

Fig. 10.14. Examples and structural requirements for the occurrence of Cram-selective additions of hydride donors to tt-chiral carbonyl compounds. In the three compounds at the bottom RUrge refers to the large...

The reverse conversions of carbonyl compounds into alcohols are typically achieved with the utilization of complex hydrides such as LiAlH4, NaBH4, etc. Activity and selectivity of these reagents can be attenuated within wide limits by varying the nature of the hydride donor. Thus, one can finely tune the selectivity to a particular structural pattern (see discussion in the next section). 

Studies of various complexes [MHL ]+ (M = Pt or Ni, L = phosphine or diphosphine) including crystal structures and hydride reactivity, concluded that [PtH(dmpe)2]+ is the best hydride donor of those studied, being a 5d-metal complex of a small-bite chelating ligand.260... 

The reactions of various analogs of NAD H with ketones and acridinium cations gave isotope effects on the rate constants for substrate reduction (ku/k ) that were different in magnitude from the isotope effects measured by isotope abundances in products compared to reactants (Yh/Td)- For example, Yh/Yd was found to be constant at around 6 in one series of reactions, while kn/k varied with the structure of the hydride donor from about 3.3 to about 5.7. A hydride-transfer mechanism therefore appeared to be excluded. 

In the Albery-Kreevoy-Lee approach, as outlined in Chart 4.2, a hydride-transfer transition state can be described by the structure-reactivity sensitivity factor or Bronsted coefficient a = d[ln(k o)]/d[ln(Kio)], where kio is the rate constant for one of a series of hydride-acceptors A,+ reacting with a standard hydride donor AqH, and K o is the equilibrium constant for the transfer. The Bronsted coefficient in turn is a sum of two terms (Chart 4.2, Eqs. (vii)-(x)). 

For example, two systems with different but related structures considered by Lee et al. [42] generated a values of 0.67 (for one system in which structural variation was in the hydride donor) and 0.32 (for a different system in which structural variation was in the hydride acceptor). It is tempting, since the sum of these values is near unity, to imagine that the two systems have identical transition-state structures, with the hydride ion liberated to the extent of about 70% from the donor and attached to the acceptor to the extent of about 30% and the hydride atom itself therefore bearing little or no charge. However, these systems had been thoroughly studied from the viewpoint of Marcus theory and estimates were available to permit the values of x to be calculated from Eq. (viii) of Chart 4.2. The calculations yielded x = 0-49 for variation of the hydride donor and x = 0.48 for variation of the hydride acceptor, suggesting that in fact both transition states were centrally located and of symmetrical structure about the hydride moiety. Furthermore... 

The X-ray crystal structures of two HLADH mutants revealed a correlation between the hydride transfer distance and the RS exponent [23], The high-tunneling F93W mutant was compared with the low-tunneling V203A mutant, each of which was crystallized with the nonreactive substrate-analog trifluoroethanol. It was observed that the distance between the hydride donor and acceptor (C-1 of alcohol... 

The two-electron reduced, or hydroquinone, isoalloxazine is pale yellow. It is an electron-rich heterocycle and, when planar, is antiaromatic according to Hiickel s rule. The ring system of the hydroquinone in some small-molecule structures and some protein structures is bent by as much as 30° along the N5—NIO axis, presumably to relieve the antiaromaticity. However, the majority of hydroquinones bound to proteins do not deviate from planarity much more than oxidized isoalloxazines. This is likely to be influenced by protonation of Nl, whose pA a is 6.7 in aqueous solution. Quantum calculations show that neutral hydroquinone adopts butterfly conformation but the anion is planar. A survey of crystal structures agrees with this correlation — anionic hydroquinones are generally nearly planar, while the few instances of the butterfly conformation belonged to neutral hydroquinones. Hydroquinones react as single-electron donors, as hydride donors, or as nucleophiles at N5 or C4a. 

This reaction fixes carbon but there is no net change in oxidation number. The CO2 is reduced to carboxyl but one of the carbon atoms in the RuBP is oxidized to yield the second carboxyl group. In subsequent steps, each mole of PGA reacts with a mole of NADPH in order to produce two moles of 3-phosphoglyceraldehyde, a product in which average oxidation number of carbon is 0. NADPH is the reduced form of nicotinamide adenine dinucleotide phosphate (see any biochemistry text for structures and further details). In biosynthetic processes, it functions as a hydride donor or reductant. A typical reaction is shown below. Note that NADPH + H is equivalent to NADP + H2. 

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