Positive hydride abstraction

Direct oxidation of diphenylmethanes is of little practical value as color formers. In liquid sulfur dioxide, leuco diphenylmethane 12 (Scheme 2) undergoes hydride abstraction by triphenylcarbenium perchlorate at the benzylic amine position to form immonium ion7 13, whereas in acetonitrile... 

Since alkyllithium compounds and their carbanions have an isoelectronic structure with alkoxides, their reaction behavior with carbenes is expected to be similar to that of alkoxides, showing enhanced reactivity in both C-H insertion and hydride abstraction.35 In this reaction, the hydride abstraction cannot be followed by recombination and, therefore, can be differentiated from the insertion. Indeed, the reaction of alkyllithium compounds 70 or nitrile anions (see Section IV.B) with ethyl(phenylthio)carbenoid, which is generated by the reaction of 1-chloropropyl sulfide 69 with BuLi, takes place at the -position of 70 more or less in a similar manner giving both insertion product 71 and hydride abstraction products 72 and 73, respectively. This again supports a general rule C-H bonds at the vicinal position of a negatively charged atom are activated toward carbene insertion reactions (Scheme 22). 

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. 

Kinetic resolution (enantiomer differentiation) of cycloalkenyl diazoacetates has been achieved (for example, according to Eq. 3) [34]. In these cases one enantiomer of the racemic reactant matches with the catalyst configuration to produce the intramolecular cyclopropanation product in high enantiomeric excess, whereas the mismatched enantiomer preferentially undergoes hydride abstraction from the allylic position [35] to yield the corresponding cycloalkenone. With acyclic secondary allylic diazoacetates the hydride abstraction pathway is relatively unimportant, and diastereoselection becomes the means for enantiomer differentiation [31]. 

Irradiation of y-ketoamide 41 affords stereoisomeric lactams 42a and b via hydride abstraction from e-position to the keto group (88H(27)133, 98T2529). The observed stereoselectivity in dichloromethane (88% 42a) is significantly higher than that obtained in ferf-butanol. An additional stereocenter in j5-position to the keto group of amide 41 inverts the dia-stereoselectivity (98MI1). 

Sommer (130, 130a) and Hall (131) have independently described the low-temperature H/D exchange of isobutane on zeolites. The traditional mechanism involves a five-coordinate carbonium ion intermediate yet no exchange occurred for the methine position, and this is inconsistent with a carbonium ion. This surprising result was explained by Sommer with a reaction sequence beginning with hydride abstraction by an unknown route... 

Concerning the mechanism of Cr(VI) oxidations, initial attack of Cr03 to form a symmetric intermediate was proposed.677 Hydrogen atom or hydride abstraction from the allylic position leads to resonance-stabilized allylic radical or carbocation, respectively, which is eventually converted to the unsaturated carbonyl compound. 

The nitrosonium ion does not react toward aromatics except in activated systems. It forms a Jt-complex with aromatics with deep color.533,534 However, it is a powerful hydride-abstracting agent in the case of activated benzylic or allylic positions. Olah and Friedman535 have demonstrated that isopropylbenzenes undergo hydride abstraction to cumyl cations 222 [Eq. (4.151)] which further reacts to give various condensation products. The reaction has been employed to prepare a variety of stable carbocations.536... 

The complex Ru(tpy)(bpy)02 [tpy = 2,2,2"-terpyridine, bpy = 2,2 -bypyridine] oxidizes organic substrates by hydride abstraction or oxo transfer. This complex, and its derivatives, cleave DNA by oxidation of the sugar at the V position and oxidation of guanine. Oxidation at the V position leads to the release of free bases and a furanone product. The kinetic parameters for the oxidation of D-ribose, 2-deoxy-D-ribose, and nucleotides by Ru(tpy)(bpy)02 were determined in phosphate buffer (pH 7). The increased reactivity of DNA as compared to RNA was rationalized on the basis of deactivation of the sugar oxidation product by the polar effect of the 2 -hydroxyl group.160... 

In the initial phases of this effort a means for introducing appropriate functionality in the C-l/C-2 positions from an unsubstituted precursor such as 80 that would eventually lead to an aziridine ring in the final target compound(s) was needed. When considering the oxidation of simple hexahydro-8-oxo-1 //-pyrrolo[ 1,2-a] indoles using DDQ, two mechanistic pathways are possible leading to either of the carbocationic intermediates 81 or 82 via hydride abstraction at C-5 or C-1, respectively (Scheme 12).69 Proton loss from 81 would ultimately lead to the phenolic product 83 whereas nucleophilic trapping of intermediate 82 by alcohols would afford the C-l-substituted ethers 84. Oxidative transformations at... 

The C-H bonds at the P position relative to R3M- (M = Si, Ge, Sn, Pb) substituents are activated towards attack by electrophilic reagents. Two types of electrophilic attack at a P C-H bond are considered in Scheme 3 Path 1 involves hydride abstraction by the electrophile, resulting in the formation of a carbenium ion intermediate, a process that is assisted by the metal P-effect. Such a pathway might be expected to be followed by strongly Lewis acidic reagents, such as carbenium ion reagents. 

Hydride abstraction from 225 gives the dibenzannuleted tropyne complex 226 (Scheme 27).85 As in the case of 223, 226 reacts rapidly with HBr to give 227, which, in solution, isomerizes slowly to 228. KBEt3H and bis(dicyclohexylphosphine)ethane attack the 4-position of the tropyne to give alkyne complexes 225 and 229, respectively. 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

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). 

This reaction describes the entrance of a nucleophile into the allylic position of an olefin. In aqueous medium this reaction is of minor importance but in nonaqueous medium, particularly under the conditions of acetoxylation, it attracts broad interest. As already mentioned above and outlined later (see Section 3.3.14.6), higher and cyclic olefins give exclusively allylic esters. Two mechanisms have been proposed. One possibility is according to eq. (17) hydride abstraction through the palladium of an oxypalladation moiety by -elimination from the adjacent C-atom whieh had not been added to the nucleophile [9]. 

---