P-Hydrogen eliminations

The reverse of reaction 23.37 is a -elimination step. It involves the transfer of a 3-H atom (structure 23.38) from the alkyl group to the metal and the conversion of the a-alkyl group to a 7r-bonded alkene, i.e. a C—H bond is activated. The first step is thought to involve a cyclic intermediate 23.39 with an agostic M—H—C interaction. 

P-Elimination is responsible for the decomposition of some metal alkyl complexes (equation 23.38), but the reaction may be hindered or prevented by  

Abstraction of a second a-H atom gives a carbyne complex (e.g. reaction 24.46). Other routes to carbenes and carbynes are described in Section 24.12. 

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The general catalytic cycle for the coupling of aryl-alkenyl halides with alkenes is shown in Fig. 9.6. The first step in this catalytic cycle is the oxidative addition of aryl-alkenyl halides to Pd(0). The activity of the aryl-alkenyl halides still follows the order RI > ROTf > RBr > RC1. The olefin coordinates to the Pd(II) species. The coordinated olefin inserts into Pd—R bond in a syn fashion, p-Hydrogen elimination can occur only after an internal rotation around the former double bond, as it requires at least one /I-hydrogen to be oriented syn perpendicular with respect to the halopalladium residue. The subsequent syn elimination yields an alkene and a hydridopalladium halide. This process is, however, reversible, and therefore, the thermodynamically more stable (E)-alkene is generally obtained. Reductive elimination of HX from the hydridopalladium halide in the presence of a base regenerates the catalytically active Pd(0), which can reenter the catalytic cycle. The oxidative addition has frequently assumed to be the rate-determining step. 

Wherever possible, we have sought a direct comparison of the reactivities of structurally related Crni and q-II alkyls with ethylene. For example, after having established the catalytic activity of complexes of the type [( Cri (L)2R] (see above), we showed that the isostructural neutral compounds Cp Crn(L)2R did not polymerize ethylene instead facile P-hydrogen elimination was observed. [3) This difference in reactivity was not due to the charge of the complexes. Thus, we have subsequently shown that neutral Cr J alkyls are also active polymerization catalysts. For example, Cp Cr I(THF)Bz2 and even anionic Li[Cp Cr H(Bz)3] (Bz = benzyl) polymerized ethylene at ambient temperature and pressure, while the structurally related CpCrD(bipy)Bz proved inert.[5]... 

The alkyls Tp Cr-R are the best test case yet of the catalytic activity of CrU alkyls (see Section 1). However, they did not react with ethylene, even at elevated temperature. On the contrary, Tp - Cr-Et eventually decomposed by an apparent P-hydrogen elimination yielding Tp - Cr-H and ethylene. Thus our notion that divalent chromium alkyls are not the chain propagating species in polymerization catalysis receives further support... 

As reported in Scheme 1 the process involves a series of steps. The alkylpalladium species 1 forms through oxidative addition of the aromatic iodide to palladium(O) followed by noibomene insertion (4-7). The ready generation of complex 2 (8-11) from 1 is due to the unfavourable stereochemistry preventing P-hydrogen elimination from 1 (12). Complex 2 further reacts with alkyl halides RX to form palladium(IV) complex 3 (13-15). Migration of the R group to the... 

An additional prerequisite in this reaction, however, is inhibition of a premature P-hydrogen elimination. Reaction of 6/4-56 and 6/4-57 led to 6/4-58 with 41 % yield. Again, one can assume that first a Ni-complex 6/4-59 is formed, which gives the bicyclic 6/4-60 followed by formation of the triquinane skeleton 6/4-58 via 6/4-61 with a P-hydride elimination being the last step (Scheme 6/4.15). 

Epoxides can also be reductively opened to form a radical. An example of an intramolecular cyclization of such a radical has recently been reported <06TL7755>. Treatment of 40 with Cp2TiCl generates an intermediate alkoxy radical, which then adds to the carbonyl of the formate ester. The product, 41, is formed as a 2 1 mixture of isomers at the anomeric carbon. This reaction is one of the first examples of a radical addition to an ester. The major byproduct of this reaction is the exo-methylene compound, 42, arising from a P-hydrogen elimination. 

A wide variety of five-membered zirconacydes 8 may be formed by the formal co-cycliza-tion of two 7i-components (3 and 6 alkene, alkyne, allene, imine, carbonyl, nitrile) on zir-conocene ( Cp2Zr ) (Scheme 3.2) [2,3,8]. The co-cydization takes place via the r 2-complex 5 of one of the components, which is usually formed by complexation of 3 with a zircono-cene equivalent (path a) ( Cp2Zr itself is probably too unstable to be a true intermediate) or by oxidation on the metal (cyclometallation/p-hydrogen elimination) (path b). Two additional routes to zirconocene r 2-complexes are by the reverse of the co-cyclization reaction (i. e. 8 reverting to 5 or 9 via 7), and by rearrangement of iminoacyl complexes (see Section... 

In this proposed process, p-hydride elimination first yields a putative hydride olefin rc-complex. Rotation of the -coordinated olefin moiety about its co-ordination axis, followed by reinsertion produces a secondary carbon unit and therefore a branching point. Consecutive repetitions of this process allows the metal center to migrate down the polymer chain, thus producing longer chain branches. Chain termination occurs via monomer assisted p-hydrogen elimination, either in a fully concerted fashion as illustrated in Figure 2b or in a multistep associative mechanism as implicated by Johnson1 et al. 

In addition to the favorable reaction cycle, the P-hydrogen elimination from Z7 leading to the formation of vinylborane side-products is also found to be competitive (Figure 7). In other words, side products are difficult to avoid in the associative reaction pathway. 

Side reactions are exchange of organic groups, followed by homo coupling, P-hydrogen elimination with alkene formation, isomerisation followed by coupling, or benzene formation and alkene liberation. Examples are shown in Figure 13.22. 

The product dichloride undergoes decomposition via p-hydrogen elimination. An intermediary iron dichlorohydride is formed alongside 2-methylpropene ... 

Unlike ethene, the largely predominant path to chain termination in styrene/CO copolymerisation consists of a fast p-hydrogen elimination from the last inserted... 

The Chevron/Gulf process also uses triethylaluminum but in a catalytic reaction at higher temperature. Reaction conditions exert a strong effect on product distribution. Under the proper conditions (200-250°C, 140-270 atm) the rates of insertion and chain transfer (displacement) are comparable, ensuring frequent p-hydrogen elimination. A broader product distribution compared with that of the two-step ethyl process is obtained. 

Watson et al.124-1261 studied the polymerization of ethylene and propylene with Lu(n5-C5Me5)2(CH3) ether in toluene or cyclohexane at 30-80 °C. The Lu complex produced polymers of Mn = 10M04 for ethylene, and oligomers for propylene. In the oligomerization of propylene an unusual chain transfer reaction due to 0-alkyl elimination was found together with P-hydrogen elimination from Lu-alkyls as chain-terminating processes 125). 

The molecular derivatives of platinum group metals are usually rather well soluble in organic solvents and volatile in vacuum. At normal pressure they demonstrate very low thermal stability and easily decompose producing fine metal powders. This decomposition occurs more easily for the derivatives of branched radicals as it is based on a P-hydrogen elimination process. An important feature of the chemical behavior of these alkoxide complexes is their rather high stability to hydrolysis. Some derivatives can even form outer sphere hydrates when reacted with water in organic solvents. This stability to hydrolysis can at least partially be due to the kinetic inertness of the complexes of this group. 

Scheme 2 shows the mechanism generally accepted for the catalytic arylation of olefins with aryl iodides in the presence of a tertiary phosphine-coordinated palladium catalyst and a base (4). Oxidative addition of aryl iodide (Arl) to a Pd(0) species (A), which is most commonly generated from palladium diacetate and a tertiary phosphine ligand, forms an arylpalladium iodide complex (B). Coordination of olefin on B followed by insertion of the coordinated olefin into the Pd-Ar bond forms a a-alkylpalladium species (C), which undergoes p-hydrogen elimination reaction to give the arylation... 

Scheme 6 shows the two possible modes for coordination of 2,3-dihydrofuran to the [PdPh (R)-BINAP ]+ species. As suggested from the molecular model in Figure 1, dihydrofuran may coordinate to the palladium center more easily in mode (a) than (b). The olefin-coordination in mode (a) followed by olefin-insertion and p-hydrogen elimination reactions forms the phenylation product having (/ -configuration, that is the observed configuration in major regioisomer 2 in the actual catalytic reactions. 

The amination chemistry depends on the absence of irreversible P-hydrogen elimination from the amido complexes before reductive elimination of amine. At the early stages of the development of the amination chemistry, it was remarkable that the unknown reductive elimination of arylamines could be faster than the presumed rapid [57,58] P-hydrogen elimination from late metal amides. In fact, directly-observed P-hydrogen elimination from late metal amido complexes was rare, and no examples were observed to occur irreversibly from a simple monomeric amido species [69], At this point, it is clear that C-N bond-forming reductive elimination of amines and ethers can be rapid, and that P-hydrogen elimination can be slow. 

P-Hydrogen elimination from amido complexes is a process that people assumed was rapid, but that had not been observed directly with monomeric amido complexes until recently. Fryzuk and Piers have studied the related insertion of imines into a dimeric, bridging hydride of Rh1 [69]. Their results showed that imine insertion was reversible when the imine was isoquinoline, suggesting that insertion and elimination processes are nearly thermoneutral. 

Selectivity Reductive Elimination versus P-Hydrogen Elimination... 

Two studies have been conducted that outline the effects of ligand steric and electronic properties on the relative rates for reductive elimination of amine and P-hydrogen elimination from amides. One study focused on the amination chemistry catalyzed by P(o-C6H4Me)3 palladium complexes [111], while the second focused on the chemistry catalyzed by complexes containing chelating ligands [88]. 

Scheme 3.4 Catalyst activation via P-hydrogen elimination from 2-propanol...

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