Hydride complexes abstraction reactions

The hydride ion abstraction reaction can be reversed by adding sodium borohydride to the w-tropylium complexes 109,110), e.g., [C7H7Cr(CO)3]+ + H C7HgCr(CO)3. Other anions react with the TT-tropylium-chromium... 

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. 

Reactions of the tetrahedral Ni° complex (994) have already been discussed in Section 6.3.5.4.2397 A nickel carbonyl cation (1062) containing a cyclophosphenium ligand has been assembled through a hydride abstraction reaction according to Equation (37).2552... 

A related dienyl species may be obtained by reaction of the diene complex with trityl ion, when hydride ion abstraction occurs from the diene to yield a dienyl group coordinated to one metal center, [(C6H7)Os3(CO)10]+ (153). 

Secondly, instead of a pure and simple electron transfer, the redox reaction can be coupled to a chemical reaction in such a way that the electron transfer takes place either after incorporation of the substrate or an intermediate into the inner coordination sphere of a metal ion ( inner-sphere electron transfer), by formation of a charge transfer complex, or in form of a hydrogen or hydride atom abstraction, respectively. In these cases the reaction between redox catalyst and substrate does not directly depend on the difference of the two standard potentials (see Sect. 2.3). 

Hydrogen abstraction reactions from the (NR2)- ligand have been observed even at room temperature (equation 12).284 With diisopropylamidolithium, the reaction is more complex the only product isolated (=10%) was the unusual methylene- and hydride-bridged compound (7 equation 13). Labeling experiments gave evidence that the hydride originates from the... 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition... 

Although tricarbonylbutadieneiron (1) was prepared by Reihlen et a/.1 in 1930, some considerable time passed before the corresponding cyclohexadiene complex (2 equation 1) was reported.2 Fischer and Fischer described the conversion of (2) to the cationic cyclohexadienyliron complex (3 equation 1) by reaction with triphenylmethyl tetrafluoroborate in dichloromethane.3 This particular complex is extremely easy to prepare and isolate as the hydride abstraction reaction proceeds the product (3) crystallizes out. Precipitation is completed by pouring the reaction mixture into wet diethyl ether, the small amount of water present serving to destroy any excess triphenylmethyl tetrafluoroborate by conversion to triphe-nylmethanol. Filtration, followed by washing the residue with ether, gives pure dienyl complex. 

With methyl-substituted cyclohexadienes, very little selectivity is observed during the preparation of the complexes and the hydride abstraction reaction. Dihydrotoluene, on heating with pentacaibonyliron, gives a mixture of complexes (37) and (38 Scheme 4). These cannot be easily separated using standard chromatographic procedures, and little is known about hydride abstraction from the individual complexes. Treatment of the equimolar mixture with trityl tetrafluoroborate gives a mixture of all three possible products (39-41 Scheme 4). 

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. 

Anionic chromium hydride complexes proved to be efficient hydrogen atom donors. Newcomb determined PPN+ HCr(CO)5 to be an efficient radical initiator and reducing agent for radicals and determined the kinetics of the hydrogen abstraction reaction [214]. In line with the observation that 3d metal complexes are much more prone to radical pathways than the corresponding 4d and 5d complexes, an increase of the extent of competing S -pathways for the bromide abstraction was found for molybdenum and tungsten complexes compared to the chromium complex. 

In the cases of tungsten and molybdenum, the complexes are believed to be intermediates in the oxidation of alcohols to carbonyl compounds (a hydride abstraction reaction).85 Figure 2.28 illustrates some of the oxygen transfer... 

The green bis(trimethylsilyl)amido complex can be prepared by allowing [RhCl(PPh3)3] to react with LiN(SiMe3)2 in THF.59 It will be noted that neither of these anions can undergo / -hydride abstraction reactions that destroy simpler amido complexes. 

Monohydrido transition metal complexes are the most active catalysts in double-bond migration reactions. These complexes form alkyl complexes when allowed to react with alkenes. The relatively long lifetimes of alkyl complexes in these systems allows them to undergo yS-hydride abstraction reactions before they can react with the other reagents present. The mechanism of the reaction is shown in Scheme 2. 

Markovnikov addition of hydrogen to the alk- 1-ene forms a 2-alkyl complex. Thermodynamically it is less likely that this 2-aUcyl complex will revert to the original alk-1-ene than be converted to an alk-2-ene as a result of competing /3-hydride abstraction reactions. 

If more than two deuterium atoms are added to cy clooctene, then isotope exchange must have taken place, and if more than four atoms of deuterium are added, then double-bond migration must also have occurred. The incorporation of excess deuterium can readily be explained by the participation of intermediate alkyl complexes that undergo -hydride abstraction reactions. 

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. 

Although Able et al. (2, 8) had originally set out to prepare 7r-cyclo-heptatrienyl complexes of metals, the cycloheptatriene complexes they actually obtained served as key intermediates in forming the former complexes. In 1958 Dauben and Honnen (61) reported that cycloheptatriene-molybdenum tricarbonyl reacted with triphenylmethyl tetrafluoroborate in methylene chloride solution with abstraction of hydride ion from the molybdenum complex. The reaction products, obtained in nearly quantitative yields, were triphenylmethane and the 7r-cycloheptatrienyl complex [(7r-C7H7)Mo(CO)3]+BF4 . 

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