Hydrogen elimination from

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... 

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. 

The mechanism of reversible (5-hydrogen elimination from square planar lr(l) alkoxide complexes with labile dative ligands, followed by associahve displacement of the coordinated ketone or aldehyde by incoming phosphine, which can be implied in TH reactions, was proposed by Hartwig and coworkers [36]. 

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

Ru(OCOCF3)2(CO)(PPh3)2-catalyzed hydrogen elimination from secondary... 

Reactions of Cr(NEt2)4, some examples of which have been discussed elsewhere, are presented in Scheme 119. The unusual reactions with C02, which produce the chromium(III) and chromium(II) carbamato complexes (287) and (288), are believed163 to proceed by C02 insertion into a Cr—N bond, which promotes /3-hydrogen elimination from a coordinated diethylamido ligand, and then reductive elimination of Et2NH produces a reactive chromium(II) species Crn(02CNEt2)(NEt2). The subsequent reaction is dependent upon the relative concentration of C02. 

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 parent silabenzene 24 was first matrix-isolated by our group in 198035 by pyrolysis of precursors 25 and 26, which yield the expected silabenzene by retro-ene fragmentation. Later, it could be shown that in analogy to carbon chemistry the hydrogen elimination from silacyclohexadiene 27 also gives the silaaromatic 2436. This reaction is allowed by the Woodward-Hoffmann rules. In accordance with the Woodward-Hoffmann rules, it could be demonstrated that silabenzene 24 is not accessible by pyrolysis of the conjugated silacyclohexadiene 28 (equation 8). 

After the successful preparation of silabenzene 24 by hydrogen elimination from 27, it was tempting to try to generate 1,4-disilabenzene 31 by pyrolysis of the easily accessible disilacyclohexadiene 3041 (equation 9). 

Where no specific chain transfer agent has been added to the polymerisation system, three chain transfer reactions are usually considered transfer with the monomer [scheme (36)], transfer with the trialkylaluminium activator [scheme (37)] and spontaneous transfer [scheme (38)]. The transfer with the monomer and the spontaneous transfer involve a reaction of /1-hydrogen elimination from the growing polymer chain, whereas the transfer with the activator is the exchange reaction of substituents at two metals [240,241]. 

All beta hydrogen elimination occurs from the 14-electron species Cp2ZrR+ where, by definition, R is branched. In contrast, beta hydrogen elimination from Cp2ZrR,+ is slow (because the alkyl group, n-hexyl, is linear) and the rate of this reaction is taken to be zero. 

In order to fit the data, it is not necessary to assume that beta hydrogen elimination from the monolefin complexes Cp2ZrR 0 or Cp2ZrRO+ occurs. The model therefore ignores these reactions. 

Competing 3-Hydrogen Elimination from Amido Complexes... 

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. 

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]. 

Most early transition-metal complexes of arynes have been prepared by thermally induced /3-hydrogen elimination from appropriately substituted aryl sigma complexes [Eq. (I)]2,8... 

A zircononium complex of cyclohex-3-eneyne (257) (two isomers) has been prepared by /3-hydrogen elimination from 256 [Eq. (38)].97 This reaction was studied in an attempt to prepare a zirconium complex of 1,2,3-cyclohextriene (255) but, in the absence of a suitable blocking group (Section IV,B), H2 was eliminated to the exclusion of H6 and only 257 was formed. 

Three zirconium/cycloheptadienyne complexes (231a-c) have been prepared by /3-hydrogen elimination from a mixture of cycloheptatrienyl complexes 269-271 (Scheme 33) and have been used as intermediates for the preparation of a zirconaazulene.87 The alkyne complexes are formed to the exclusion of the allene isomer 268. This is believed to be due to the proximity of the /3-vinyl hydrogen that is a result of both the shorter double bond and its forced coplanarity with the metal. Allene formation from 269 might be induced by blocking the vinyl position (see Sections IV,B and IV,C), but this has not been tested. 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

The amination chemistry depends on preventing irreversible y -hydrogen elimination from the amido complexes before reductive elimination of the 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 [71, 72] jff-hydrogen... 

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