P-H-elimination

C-C and C-E (E = heteroatom) bond formations are valuable reactions in organic synthesis, thus these reactions have been achieved to date by considerable efforts of a large number of chemists using a precious-metal catalysts (e.g., Ru, Rh, and Pd). Recently, the apphcation range of iron catalysts as an alternative for rare and expensive transition-metal catalysts has been rapidly expanded (for recent selected examples, see [12-20, 90-103]). In these reactions, a Fe-H species might act as a reactive key intermediate but also represent a deactivated species, which is prepared by p-H elimination. 

The stoichiometric insertion of terminal alkenes into the Cu-B bond of the (NHC)Cu-B(cat) complex, and the isolation and full characterisation of the p-boryl-alkyl-copper (I) complex has been reported. The alkyl complex decomposes at higher temperatures by P-H elimination to vinylboronate ester [67]. These data provide experimental evidence for a mechanism involving insertion of alkenes into Cu-boryl bonds, and establish a versatile and inexpensive catalytic system of wide scope for the diboration of alkenes and alkynes based on copper. 

The hydrido(ethoxo) complex carrying an electron-donating q -CsMes (= Cp ) ligand, [Cp IrH(OEt)(PPh3)] (4), was prepared by a metathesis reaction between [Cp Ir Cl2(PR3)] (3) and NaOEt followed by P-H elimination from the intermediate diethox-ide complex (Eq. 6.4) [7]. Several other iridium alkoxide analogs [Cp IrH(OR)... 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

It was assumed that C—C bond cleavage passes through an elementary step of p-alkyl transfer. The mechanism of hydroisomerization passes also by a p-alkyl transfer step, but in this case the P-H elimination-olefin reinsertion occurs rapidly and a skeletal isomerization also occurs. 

The presence of tin atoms regularly distributed on the platinum surface isolates the platinum atoms by increasing the distance between two adjacent platinum atoms, as does the copper atoms on a nickel surface [108] or the tin atoms on a rhodium, platinum or nickel surface [106, 109-111]. The presence of tin would thus avoid the hydrogenolysis reaction, leading to a more selective catalyst (Figure 3.37). Indeed, the formation of isobutene from isobutane involves only one platinum atom, with the reaction passing through a simple mechanism of P-H elimination after the first step of C-H bond activation (Scheme 3.26). 

The activation of (P-P)Pd" promoters in MeOH proceeds via formation of Pd"-OMe (Eq. (1)) that can straightforwardly initiate the catalysis cycle or generate Pd"-H via P-H elimination, yielding formaldehyde (Eq. (2)) [16]. The fast kinetics under real copolymerisation conditions do not allow for the spectroscopic detection of Pd-H initiators. However, their formation has been unambiguously assessed by end-group analysis, isotopic labelling experiments and model reactions [Ij. 

By means of in situ NMR spectroscopy combined with deuterium incorporation experiments, van Leeuwen has elucidated the mechanism of termination by protonolysis, showing that the fl-chelates are in equilibrium with their enolate form by a p-H elimination/hydride migration process (Scheme 7.19). The enolate intermediates are regioselectively protonated at the C2 carbon atom by either MeOH or H2O to give Pd-OMe or Pd-OH and keto terminated copolymer. The enolate formation has been reported to be rate determining in the chain transfer [19]. 

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry has contributed remarkably to unravelling the termination and initiation steps of the styrene/CO copolymerisation catalysed by the highly active bis-chelated complex [Pd(bipy)2](Pp5)2 in TFE [40]. Chain-end group analysis of the material produced in the absence of BQ showed that the termination by P-H elimination is accompanied by three different initiators two palladium alkyls from Pd-H formed by reaction of the precursor with CO and water (a and b) and a palladium carboalkoxy species formed by reaction of the precursor with the fluorinated alcohol and CO (c) (Chart 7.4). The suppression of the chain-transfer by alcoholysis was proposed to be responsible for the enhanced stability of the palladium acyl intermediates and hence for the high molecular weight of the copolymers produced. 

F and B NMR spectroscopy. The rate of propene polymerisation with this system was only three times faster than that of 1-hexene. This slow rate contributes to the high regioselectivity of the polymerisation no 2,1-propene misinsertions were detected. H and NMR spectroscopy also provided information about the chain termination mechanism here this occurred by p-H elimination in a first-order process. Polymer chain-end epimerisation, i.e. chirality inversion at the P-carbon of the polymer chain (Scheme 8.31), proceeded via a zirconium tert-alkyl (rather than tt-allyl) intermediate [96c]. 

The two intermediates depicted above differ fundamentally from each other. The COx-producing intermediate has a direct metal-carbon (M-R) bond whereas the C2-producing intermediate has a metal-oxygen-carbon (M-O-R) bond. From known organic decomposition pathways, the formation of selective oxidation products from the M-O-R intermediate is likely. An a-H elimination produces acetaldehyde and a P-H elimination produces ethylene. 

The results indicate that the p-H elimination of the M-R intermediate is more difficult than either the a-H or the P-H elimination of the M-O-R species. Thus, the M-O-R species can readily decompose to yield stable hydrocarbon products, while the M-R intermediate is relatively stable. Its long lifetime on the surface makes it susceptible to irreversible oxidation to CO,. 

The alkylpyridine ligands of these insertion products can be removed from Zr by normal hydrolysis or other electrophilic Zr—R bond cleavage reactions developed for Cp2Zr(R)Cl compounds (i). The five-membered metallacycles are quite resistant to y -H elimination. The P hydrogens cannot attain the correct orientation for transfer to Zr owing to the chelated structures, and, as for other Zr alkyls, y -H elimination is likely to be endothermic in any case 120). However, P-H elimination can be induced by ligands which open the chelate rings and can trap the cationic Zr... 

The final step of the catalytic cycle, base-assisted reductive elimination, has been addressed by Deeth et al. [14]. In their calculations, the authors investigated palladium complexes with the chelating diaminomethane H2N(CH2)NH2 and di-phosphinomethane H2P(CH2)PH2 ligands. Within this system, they found that the postulated hydrido-olefin complex, which is usually formed by p-H elimination of the y9-agostic insertion product, is in fact not a stable minimum structure in this particular case (eq. (11)) [14]. 

This result may reflect a hampered P—H elimination in the organonickel intermediates. With other phosphanes or phosphites the yields of 4 and 5 range from 56 to 86 %. The data collected in Table 1 reveal that there is no simple correlation between product distribution and basicity or cone angle 601 of L. 

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