Hydride anion donors

The allylic pyrophosphate (2.23), formed in the above reaction, then reacts with TPN-H. This is a coenzyme which is similar to NADPH and, like NADPH, serves as a hydride anion donor. The hydride ion adds to the newly formed double bond, displacing it in the direction of the pyrophosphate group, which is then lost by cleavage of the carbon-oxygen bond to produce squalene. Squalene is a triterpenoid hydrocarbon containing six double bonds and a tail-to-tail fusion in the centre of the molecule. It occurs widely in nature, shark liver oil being a particularly rich source. 

Rather interesting is the possibility of transition from benzyl-type cations to ben-zenium ions. The benzyl cations can, just as other carbenium ions, act as hydride-anion acceptors turning, in so doing, into an aromatic hydrocarbons which, in a strong acid, are protonated to form the benzenium ions. Similar transformations occurred in the case of the 2,3,4,5,6-pentamethylbenzyl cation, the hydride-anion donors being the aliphatic hydrocarbons, 9,10-dihydroanthracene and even the molecular hydrogen ... 

Nicotinamide is an essential part of two important coenzymes nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP ) (Figure 18.19). The reduced forms of these coenzymes are NADH and NADPH. The nieotinamide eoenzymes (also known as pyridine nucleotides) are electron carriers. They play vital roles in a variety of enzyme-catalyzed oxidation-reduction reactions. (NAD is an electron acceptor in oxidative (catabolic) pathways and NADPH is an electron donor in reductive (biosynthetic) pathways.) These reactions involve direct transfer of hydride anion either to NAD(P) or from NAD(P)H. The enzymes that facilitate such... 

The list of ligands covered by Ref. 1 (vol. 2) starts with the exotic mercury ligand (Dean, p. 1). Silicon and a series of metals (Ge, Ti, Pb) (Harrison, p. 15) also reveal ligand properties. Hydrogen and a variety of hydride anion complexes (Crabtree, p. 689), as well as the complexes formed by anions with a carbon-donor center (cyanides, fulminates, etc.) (Sharpe, p. 25) are briefly discussed. 

Reaction of Hydrides. The donor semi-bridging interaction of the carbonyls in 1 might render them more susceptible to nucleophilic attack by such reagents as hydrides. We reasoned that the formation of n2-formyl species, e.g., IV, or alternatively, anionic metal hydrides, e.g., V, might result. The actual reaction of l with Et3BH is consistently more complex than anticipated (-3.31. 

Earlier in this chapter, we saw how a carbonyl group may be reduced by a hydride ion donor. In the examples given, the hydride anion came from either a complex metal hydride such as LiAlH4, or... 

NAD+ serves as a donor or acceptor of hydride anions in biochemical reactions. NAD+ serves as a coenzyme for oxidoreductases. 

The basis of the catalysis of the splitting of the disulfide is presumably the formation of a charge-transfer complex between the two-electron donor NADPH (equivalent to a hydride anion) and the acceptor flavin combined with proximity effects. Both coenzymes, NADPH and FAD, are bound to the protein by adenosine phosphate-protein interactions, the substrate is loosely bound at the cleft between the units of a protein dimer (Fig. 9.6.12) (Schulz, 1983 Douglas, 1987). 

In this event, a boron-complex that operates as a Lewis acid bringing along its hydride anion (see 296) gave rise to the onti-ether in a 24 1 ratio (anti/syn), while the separation of hydride donor (triphenylsilane) and Lewis acid (BFj) yielded a 5 1 synlanti ratio of 298-syn. 

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

This reaction of aromatic aldehydes, ArCHO, resembles the Cannizzaro reaction in that the initial attack [rapid and reversible—step (1)] is by an anion—this time eCN—on the carbonyl carbon atom of one molecule, the donor (125) but instead of hydride transfer (cf. Cannizzaro, p. 216) it is now carbanion addition by (127) to the carbonyl carbon atom of the second molecule of ArCHO, the acceptor (128), that occurs. This, in common with cyanohydrin formation (p. 212) was one of the earliest reactions to have its pathway established— correctly —in 1903. The rate law commonly observed is, as might be expected,... 

Sn—Sn bond formation can be achieved by indirect electrolysis considering the relative ease of SnH-bond activation. Tributylin hydride is a known H atom donor. It is attacked by radicals like Mn(CO)3P(OPh)3]2, electrogenerated from the anion Mn(CO)3P(OPh)3]2p. The kinetics of hydrogen transfer and coupling of Ph3Sn and Mn(CO)3P(OPh)3]2 was studied188. 

The reaction of [Ni(ethene)3] with a hydride donor such as trialkyl(hydrido)-aluminate results in the formation of the dinuclear anionic complex [ Ni(eth-ene)2[2l 11 [22]. The nickel(O) centers in this complex are in a trigonal planar environment of two ethene molecules and a bridging hydride ion, with the ethene carbons in the plane of coordination. The two planes of coordination within the dinuclear complex are almost perpendicular to each other, and the Ni-H-Ni unit is significantly bent, with an angle of 125° and a Ni-Ni distance of 2.6 A [22],... 

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