Hydride metallacycle

The catalytic cycle, which is supported by stoichiometric and labeling experiments, is shown in Scheme 38. Loss of 2 equiv. of N2 from 5 affords the active species a. Reaction of a with the 1,6-enyne gives the metallacycle complex b. Subsequently, b reacts with H2 to give the alkenyl hydride complex c or the alkyl hydride complex d. Finally, reductive elimination constructs the C-H bond in the cyclization product and regenerates intermediate a to complete the catalytic cycle. 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Codimerization of butadiene with dicyclopentadiene (example 8, Table II) was shown to proceed via a crotyl-nickel complex (62). Ring contraction of cyclooctadiene (example 10, Table II) appears to be a hydride promoted reaction. The hydride-promoted dimerization of norbomadiene to -toly 1 norbornene (example 9, Table II) appears to be quite different from dimerization via a metallacycle (see Table I, example 16). 

Carbonylation is an exceedingly broad subject, but the main reaction patterns can be easily rationalized by recalling the classification used earlier for coupling reactions involving (a) metallacycles (b) hydride-promoted reactions and (c) oxidative addition of organic halides to zero-valent nickel. In fact, one or other of these steps is necessary to form a species able to undergo carbonylation. 

Scheme 7 comprises the following patterns First, a metallacycle gives rise to ketones by CO insertion and reductive elimination. Next, a nickel hydride inserts an unsaturated substrate L, followed by CO. The acyl intermediate can give rise to reductive elimination with formation of acyl halides or acids and esters by hydrolysis, or it can insert a new ligand with subsequent reductive elimination as before. Alternatively, there may be a new insertion of carbon monoxide with final hydrolysis. Third, an intermediate R—Ni—X is formed by oxidative addition. It can react in several ways It can insert a new ligand L, followed by CO to give an... 

The reaction is thought to proceed via an iridium hydride, with the olefin group acting as a directing group. Metallacycle intermediates have also been implicated in this reaction (Scheme 22).96... 

A rhodium-catalyzed allenic Alder ene reaction effectively provides cross-conjugated trienes in very good yields (Scheme 16.70) [77]. The reaction most likely involves ft -hydride elimination of an intermediate rhodium metallacycle to afford an appending olefin and ensuing reductive elimination of a metallohydride species to give the exocyclic olefin. 

The mechanism was proposed to involve the formation of a nickel metallacycle by the oxidative cyclization of Ni(0) with the aldehyde and alkyne, followed by conversion of the metallacycle to product by a transmetalation/reductive elimination sequence. If R possesses a P-hydrogen, then P-hydride elimination after the transmetalation step generates the product with R = H in some instances. The mechanism was shown to be ligand dependent, and the mechanism depicted below is undoubtedly oversimplified. ... 

In particular, hydrolysis of the y-chelate would give add end groups and Pd-H, while the protonation of the enolate, in equilibrium with the p metallacycle via hydride shift (vide infra) would give keto end groups and Pd-OH (Scheme 7.4) [13c]. 

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

A plausible mechanism that diverges at a common intermediate and may account for the product distributions is shown in Scheme 1. Reaction between the Ni catalyst and cyclopropylen-yne 1 would ultimately afford eight-membered metallacycle 6, which could result from either initial oxidative coupling between an alkene and alkyne (5a) [8-10] or initial isomerization of the VCP (5b). Ultimately, (i-hydride elimination from 6 and reductive elimi-... 

Sixteen-electron ruthenium(O) species of type (rj6-arene)(L)Ru(0) and containing two-electron ligands are probable intermediates for C—H bond activation and formation of metallacyclic complexes (Section II,A,3,c). A variety of 18-electron complexes of general formula (arene)(L1)(L2)Ru(0) have been prepared by H. Werner and co-workers either by deprotonation of hydride ruthenium(II) complexes or by reduction of cations RuX(L)2-(arene)+. Some of these Ru(0) complexes have already been discussed with the formation of alkyl or hydridoruthenium complexes (Sections... 

Finally, a Nicholas-type reaction is presumably responsible for an unexpected result reported by Alcaide. During their work devoted to the application of the PKR in the field of -lactams and azetidines they reacted complexed azetidine 91 with TMANO, isolating a mixture of the expected PK product 92 and by-product 93. The formation of 93 is believed to be a consequence of the ionization of the propargylic C - N bond at the cobalt-acycle step. The crowded metallacycle formed after the insertion of the olefin (93), would prompt the cleavage of the C - N bond, forming an ionic species (94) that would trap a hydride, possibly from a cobalt hydride giving 95, which then would follow the usual pathway towards the cyclopen-tenone (Scheme 27) [124],... 

In addition to a- and p-C-H activation, the possibility occasionally arises for y- or even 8-functionalization. This is particularly common for aryl phosphine and phosphite ligands that may undergo metallation of the ortho-C-H bond of an aryl substituent. This process may be reversible however, if a suitable co-ligand is present which can undergo subsequent reductive elimination of the hydride, stable metallacyclic organyls are obtained (Figure 4.31). The formation of metallacyclic alkyls may confer some stability, as does the possibility of increased hapticity, e.g. in the case of xylyene ligands (see also Chapter 6). 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

Metallacyclic (see Metallacycle) complexes of niobium and tantalum play an important role in understanding several catalytic and stoichiometric transformations of organic compounds. Some group 5 metallacycles are formed from the inter- or intramolecular hydride abstraction reactions. Most of the Nb and Ta metallacycles are prepared, however, from reductive coupling (see Reductive Coupling) of unsaturated organic substrates. To be included in this section, the metallacyclic ligand must have at least one M-C bond. 

Complexes bearing Pt-Si and Pt-Ge bonds have been invoked as potential intermediates in the formation of polysilanesanspolygermanes. The metallacycle [(Et3P)2Pt(/r- -H2CO)Ge(N(SiMe3)2)2] takes part of a photochemical cycloreversion reaction with Pt hydride formation, releasing CO. ... 

The crystal structure of yellow metallacyclic-osmium(iv) complex 9 was also reported <20030M414>. The distribution of ligands around the osmium atom can be described as a four-legged piano-stool geometry with the carbon atom C-5 of the metallated phenyl group disposed transoid to the hydride ligand. The bidentate carbon donor... 

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