Alkyl complexes, hydride abstraction reactions

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. 

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. 

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. 

The next step in complexity are systems in which alkylation competes with hydride transfer to give dimeric alkyl cations which, when hydride abstraction occurs, yield dimeric saturated hydrocarbons (equation 14). This reaction path for cyclic aliphatic alcohols and olefins is often accompanied by some rearrangement (Deno etal., 1964 Pittman, 1964). 

Normally, hydride can be readily a-abstracted from alkyl complexes when the metal is in a low oxidation state and bound to good n-accepting ligands. In complexes with p-hydrogen, P-elimination might become the main reaction, specially when highly substituted alkenes are formed (Figure 3.10). 

However, with substrates prone to form carbocations, complete hydride abstraction from the alkane, followed by electrophilic attack of the carbocation on the metal-bound, newly formed alkyl ligand might be a more realistic picture of this process (Figure 3.38). The regioselectivity of C-H insertion reactions of electrophilic transition metal carbene complexes also supports the idea of a carbocation-like transition state or intermediate. 

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]. 

Alkyl ligands containing 0 hydrogens can also decompose in a similar way to yield hydrides (equations 42-43). Normally this reaction is so rapid when an empty coordination site is available (equation 43) that to obtain such an alkyl complex requires that all the ligands be firmly bound (e.g. CpFe(dpe)Et in equation 42). The empty site is required to abstract the / hydrogen (equation 44). 

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... 

The usefulness of 1,3-cyclohexadiene complexes is enhanced by their conversion to stable cationic complexes. The if -cationic complex 102 is prepared as a stable salt by the hydride abstraction from the neutral complex 66 via 101, and its highly regio- and stereoselective reaction with nucleophiles is used for synthetic purposes. Complex 102 reacts with nucleophiles such as amines, active methylenes, alkyl copper or alkoxides at C(l) or C(5) from the uncomplexed exo side. In other words, the nucleophilic attack occurs regioselectively at a dienyl terminus, and stereoselectively anti to Fe(CO)3 to give 103. Hydride abstraction from 103 affords 104, which reacts with a nucleophile to form 105. Decomplexation of 105 produces the 5,6-disubstituted-l,3-cyclohexadiene 106. 

Other methods for obtaining complexes of ethylene and other alkenes include ligand substitution reactions, reduction of a higher valent metal in the presence of an alkene, and synthesis from alkyl and related species [reductive elimination, of an allyl or hydride, for example hydride abstraction from alkyls protonation of sigma-allyls from epoxides (indirectly)] [74a],... 

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. 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

The ease of reversal of alkene insertion is evident from the numerous syntheses of transition metal-hydride complexes using main group metal alkyls as the source of hydride. The hydride in the products of such reactions usually arises from -hydride abstraction or elimination from intermediate unstable transition metal alkyls. This idea is reinforced by the greater effectiveness of secondary alkyls such as isopropyl or cyclohexyl compounds. However, it has been shown that in at least one case the hydride results from hydrolysis of a Pt-Mg bond, not from the alkyl formed from reaction of a Pt-Cl bond with a Grignard reagent. Several of the reactions listed in Table 1 are spontaneously reversible. Reactions where -hydride elimination has been used in the synthesis of hydrides are listed in Table... 

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