P-hydride elimination from

The initially formed tetra-alkylferrate(II) represents the reactive intermediate in both reactions that undergoes a carboferration of the triple bond in eq. 2, Scheme 29. Transmetallation from Fe to Mg yields a vinyl-magnesium species, which liberates the desired olefin upon hydrolysis within the acidic work-up procedure. In the above two reactions, a competing p-hydride elimination from the ferrate yields the unreactive Fe-H species and hence is considered to be the deactivation step in the catalytic cycle. 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

One of the most defining characteristics of the late metal a-diimine polymerization systems is the uniquely branched polyolefins that they afford. This arises from facile p-hydride elimination that late transition metal alkyl complexes undergo. The characteristics of the isomerization process have been the subject of much investigation, particularly with the more easily studied Pd(II) a-diimine system. The process is initiated by P-hydride elimination from the unsaturated alkyl agostic complex 1.17, followed by hydride reinsertion into olefin hydride intermediate 1.18 in a non-regioselective manner (Scheme 5). In doing so, the metal center may migrate... 

A number of substituted allylarenes were synthesized under mild conditions using Et3N as an additive (Table 9.5). Curiously, only products of endocyclic P-hydride elimination (from the homobenzylic position) were observed, though mixtures of E/Z isomers were obtained from substrates bearing aliphatic chains (entry 4, Equation 9.3). In entry 3, the substrate cannot undergo a 1,5-hydride shift hence, the product resulting from a 1,6-hydride shift was obtained. 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

This combination of reagents has been used to oxidize terminal vinyl groups to methyl ketones and is known as the Wacker oxidation. The nucleophile is simply water, which attacks the activated alkene at the more substituted end in an oxypalladatlon step. P-Hydride elimination from the resulting o-alkyl palladium complex releases the enol, which is rapidly converted into the more stable keto form. Overall, the reaction is a hydration of a terminal alkene that can tolerate a range of functional groups. 

Presumably, the oxidative cyclization of 1 commences with direct palladation at the orfAo-position, forming o-arylpalladium(II) complex 3 in a fashion analogous to a typical electrophilic aromatic substitution (this notion is useful in predicting the regiochemistry of oxidative cyclizations). The mechanism of the second formal C—H bond functionalization step is not fully elucidated, but may occur either via (a) an intramolecular carbopalladation reaction (migratory insertion) followed by czHft-P-hydride elimination from 4 (Path A) (b) by o-bond metathesis (through a four-centered transition state) followed by reductive elimination (Path B) (c) by electrophilic aromatic substitution followed by C—C bond-forming reductive elimination (PathC) [9]. 

The complex prepared using (/ ,/ )-diaminocyclohexane (76) was four times less active and produced 1-phenylethanol in only 60% ee (S). The use of ethylenediamine (75) was even less active and decomposed after 40% conversion, possibly due to P-hydride elimination from a metal amide intermediate (Section 1.1.4). The diamino-cyclohexane moiety is quite flat and the ethylenediamine backbone is small, both leading to less steric interaction with the incoming ketone substrate. The rates for catalysts 77 and 78 with diethylphosphino substituents are lower than those with dia-rylphosphino substituents. These may form less active catalysts with less acidic NH groups (cf. Scheme 6). The active forms of complexes 67 and 74 appear to have optimum hydridicity and NH acidity resulting in high ATH activity. 

Fig. 5. Acetone TPD traces for the main isotopomers expected from the reaction of CD3CHICD3 with oxygen on Ni(lOO) surfaces. The exclusive formation of perdeutero acetone in this case indicates the high selectivity towards a P-hydride elimination step from the 2-propoxide intermediate, and rules out a mechanism where an initial P-hydride elimination from 2-propyl groups on Ni sites is followed by oxygen incorporation.

P-hydride elimination to produce Morita-Baylis-Hilman type products [74]. In addition, Ru - H species also were found to work as the same catalyst [75]. For example, the coupling of vinyl methyl ketone and propanal (200 mol %) was catalyzed with RhH(PPh3)4 (lmol%) and RuH2(PPh3)4 (lmol%) at 40 °C for 40 h without solvent to form the unsaturated ketone 169 in good yields, 78% and 82%, respectively (Scheme 43). It was proposed that P-hydride elimination from metal-aldolates could release the a, -imsaturated P -hydroxy ketones, which were Morita-Baylis-Hilman type products. 

Deuterioformylation experiments carried out at partial substrate conversion has proved to be the best way to investigate the reversibility of die above step [11]. As shown in Figure 5, when a deuterated alkyl species undergoes a P-hydride elimination process, the elimination of Rh-H is favored over that ofRh-D one, because of the well documented kinetic isotope effect observed in this kind ofprocess [26]. Thus P-hydride elimination from the linear alkyl species gives rise to an alkene deuterated at the carbon atom in position 2, whilst the analogous process for the branched alkyl intermediate generates an alkene deuterated at the terminal position of the double bond. 

Even simple alkenes, such as 1-hexene 9.270, can yield ir-allyl complexes on reaction with vinyl halides under Heck conditions (Scheme 9.75). This is only apparent when a nucleophile is present, such as a secondary amine or a malonate anion. The ir-allyl complex 9.279 is generated by isomerization of the corresponding T -complex 9.278 that is itself generated by a -hydride elimination-reinsertion sequence from the initial insertion product 9.275. Nucleophilic attack on the Tr-allyl complex gives the three component coupling product 9.273, while the Heck product, diene 9.272, can arise from dissociation from t -complex 9.276 or p-hydride elimination from the -intermediate 9.278. Again, the reaction is particularly effective in an intramolecular sense. °°... 

The overall mechanism is presented in Fig. 16 and is suggested to proceed via a metal hydride as the key intermediate with (1) reaction of the hydride with the amine-borane to form an amido borane, (2) p-hydride elimination from the amine-borane to form the unsaturated species R2N=BH2, (3) the insertion of R2N=BH2 into the M-N bond of the amido-borane complex, and (4) turnover from the corresponding metallocyclic species via either p- or 5-hydride elimination. 

Scheme 2.23) [36]. Formation of cyclopentene byproducts is attributed to P-hydride elimination from a nickelacycle followed by reductive elimination. Maleate and fumarate also reacted similarly. A comparison of the stereochemistry of thermal and catalyzed reactions was performed by studying the results of a deuterium-labeling experiment. All attempted reactions of bicyclo[3.1.0]hexane and bicyclo[4.1.0]heptane were unsuccessful. 

Similar to rhodium, copper mediates retro-allylation when it is complexed with an N-heterocyclic carbene ligand [31]. The allyl transfer takes place not only to aromatic aldehydes but also to aromatic imines (Scheme 5.43). Notably, secondary homoallylic alcohols transfer their allyl groups, retro-allylation dominating over P-hydride elimination from the copper alkoxide intermediates. 

When 35 was treated with AgBp4 and butyl acrylate under stoichiometric conditions, decomposition products resulting from reductive elimination from all of the key catalytic intermediates were observed. The stability of the catalyst under the actual reaction conditions was much higher, and was attributed to faster rates of p-hydride elimination from the arylated acrylate at the elevated reaction temperatures, which removed at least two of the decomposition pathways. However, it should be noted that these decomposition pathways could be responsible for the generation of catalytically active Pd complexes containing one or zero carbene ligands in this or other systems. 

Finally, a new method was developed for the amidation of amines with primary alcohols in the presence of in situ formed [(NHC)Ru(PR3)] complexes. According to the authors, the reaction proceeded via an aldehyde formed by the dehydrogenation of the substrate, which would stay coordinated to the ruthenium centre. Subsequent attack of the amine would give a coordinated hemiaminal and p-hydride elimination from this intermediate would result in the A-alkyl amide. 

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