Hydride abstraction reactions, group

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

Metallacyclic (see Metallacycle) complexes of niobium and tantalum play an important role in understanding several catalytic and stoichiometric transformations of organic compounds. Some group 5 metallacycles are formed from the inter- or intramolecular hydride abstraction reactions. Most of the Nb and Ta metallacycles are prepared, however, from reductive coupling (see Reductive Coupling) of unsaturated organic substrates. To be included in this section, the metallacyclic ligand must have at least one M-C bond. 

Abstraction of hydrides by Lewis adds has also been studied extensively. Several groups have measured the thermodynamics for hydride abstraction by hydride acceptors, such as triarylmethyl cation and l-benzyl-l,4-dihydronicotinamide (BzNADH). Examples of these hydride abstraction reactions are given in Equations 12.8-12.9. " ... 

Brounts RHAM, Buck HM (1983) Hydride abstraction reactions from cycloheptatriene and 1-carbamoylcycloheptatriene. Effect on the CONH2 group orientation on the reactivity difference of the exo and... 

For example, three hydride abstraction reactions were separated from the other reactions and put into one class, photochemical reactions were grouped in another, etc. The two largest classes contained 32 reactions each and were definably Michael additions, as in... 

The tri(azulenyl)methane derivative 24+ including a 6-azulenyl group was prepared by the reaction of azulene 6b with diethyl 6-formylazulene-l,3-dicarboxylate. Synthesis of 24+ was accomplished by hydride abstraction with DDQ. Cation 24+ was isolated as a hexafluorophosphate salt by treatment with aqueous HPF6. The new salt is a stable, deep-green colored crystals, that can be... 

The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(II) acetate. 

If chiral catalysts are used to generate the intermediate oxonium ylides, non-racemic C-O bond insertion products can be obtained [1265,1266]. Reactions of electrophilic carbene complexes with ethers can also lead to the formation of radical-derived products [1135,1259], an observation consistent with a homolysis-recombination mechanism for 1,2-alkyl shifts. Carbene C-H insertion and hydride abstraction can efficiently compete with oxonium ylide formation. Unlike free car-benes [1267,1268] acceptor-substituted carbene complexes react intermolecularly with aliphatic ethers, mainly yielding products resulting from C-H insertion into the oxygen-bound methylene groups [1071,1093]. 

We interpret the effect to indicate that the three enzymes must have close resemblance of the transition state(s) for the initial part of the reaction sequence, leading to the 4-ulose intermediate. When we talk about the 4-ulose intermediate, we do not intend to imply that in each instance actual formation of a 4-keto group occurs. The intermediate to initiate subsequent rearrangements may well be the remaining car-bonium ion after hydride abstraction. There is now no experimental evidence to distinguish these possibilities. 

The reaction path depicted in Scheme 5.14 involves Wagner-Meerwein shifts of the methyl group prior to cyclization followed by hydride shift to a number of cationic intermediates. The second scheme (Scheme 5.15) depicts ring closure before methyl migration. The first step involves protolysis of the C—H bond next to the methyl-bearing carbon. The corresponding ion can then rearrange by a 1,2-methyl shift and yield 1,16-dimethyldodecahedrane 28 by hydride abstraction from a hydride donor. 

Nonactivated C—H bonds in imines can be selectively monofluorinated in HF-SbF5 in the presence of CC14 to yield the corresponding fluoroketones when the reaction mixture is quenched with HF-pyridine534 (Scheme 5.54). The transformation is initiated by hydride abstraction with CC13+ from the most reactive carbon farthest from the functional group and involves dicationic intermediates 134 and 135. 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

The stereochemistry of the reaction in which C-4 is oxidized to give intermediate 41 is difficult to envisage, because the hydride abstraction and addition would have to take place from opposite sides of the carbonyl group. Various solutions to this problem have included (a) a double binding-site for the substrate, which can transfer from one site to the other as the intermediate 4-ulose,146... 

The desired bicycle 58 was obtained but oddly the enantiopuiity of the previous intermediate was lost, and the bicycle was a completely racemic mixture. It was an unexpected result in view of the previous work by Somfai describing the same reaction leading to enantiopure product from the Af-tosyl analogue (with a methoxy instead ethoxy group in the a-position) (Skrinjar et al. 1992 Somfai and Ahman 1992). Although there was no logical explanation for these discrepancies, they proposed that the racemization could go via hydride abstraction by the iminium ion. As a result, a different ( )-anatoxin-a synthesis was completed by deprotection (TMSI) (Manfre et al. 1992) in 65% yield (9% overall for nine steps). 

---