Metallacycle mechanism

Metal ion discharge, chemical identity of adsorbed intermediates, 38 19-20 Metal ion-exchanged zeolites, 31 13 Metallacycle mechanism, 41 323-324 Metallathiabenzenes formation of, 42 421 reaction with hydrogen gas, 42 420 Metallic catalysts, 27 3 see also specific catalysts... 

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

The metallacycle mechanism can also be considered a concerted mechanism. It is analogous to the one proposed for metal peroxo complexes and is based on the assumed formation of a cyclic intermediate that includes the peroxo group, the reactant molecule, and the metal ion (Mimoun, 1982, 1987 Huybrechts et al., 1992). For alkene epoxidation, the sequence of events would be represented as follows [Eq. (31)] ... 

The mechanism of the unprecedented chromium-catalysed selective tetramerization of ethylene to oct-1-ene has been investigated. The unusually high oct-1-ene selectivity of this reaction apparently results from the unique extended metallacyclic mechanism in operation. Both oct-1-ene and higher alk-l-enes were formed by further ethylene insertion into a metallacycloheptane intermediate, whereas hex-1-ene was formed by elimination from this species as in other trimerization reactions. Further mechanistic support was obtained by deuterium labelling studies, analysis of the molar distribution of alk-l-ene products, and identification of secondary co-oligomerization reaction products. A bimetallic disproportionation mechanism was proposed to account for the available data.120... 

Fig. 21, no absorption characterizing CH3 terminal groups is evident, even in the early stages of polymerization. Analogously, no spectroscopic evidence of carbene species could be found. Therefore, the chains were suggested to have a cyclic structure and to be very long even after short polymerization times (205,233), so that the metallacycle mechanism (I in Scheme 11) was considered the most probable. 

In contrast, cyclodimers and cyclooligomers are obtained from strained monoolefins by oligomerizations with Ni(0) and other Group 8-10 metal catalysts . Methylenecy-clopropane is dimerized by [Ni(cod>2] to a mixture of the cyclobutane derivative 2 and the cyclopentane derivative 3 (Scheme 1). By modifying the catalyst with phosphines, the cyclodimerization is suppressed and a mixture of trimers forms in which, depending on the nature of the phosphine, the linear trimer 4 or the cyclotrimer 5 prevails . A metallacycle mechanism has been developed for these reactions. The intermediate A may react further with methylenecyclopropane to form the metallacyclic precursors to the... 

In the case of the cyclic substrate 51, the intervention of a C-H insertion pathway reveals itself in terms of the diastereoselectivity, not regioselectivity. Thus, exposure of enyne 51 to the standard Ru catalyst at ambient temperature produced the transfused bicyclo[5.4.0]undecene 52 (Equation 1.60, path a) [55]. If a metallacycle mechanism was operative, coordination of the metal with both the alkene and alkyne must occur to form the cis-fused product. On the other hand, coordination of the Ru with the Lewis basic bridgehead substituent directs it to abstract an allylic C-H on the same face as the substituent, which then leads to the trans-fused product as observed. On the other hand, cycloisomerization using a Pd(0) precatalyst does indeed lead to the Z-fused bicycle (Equation 1.60, path b). 

Fig. 25 Potential energy surfaces for the most feasible two-state reaction pathways for ethylene dimerization catalyzed by Cr(II)OH (If), via either a Cr-carbene mechanism or a metallacycle mechanism determined at the M06 level of theory. Also shown are the crossing points optimized at CASSCF level. The triplet metallacycle reaction pathway is depicted in blue, and the triplet Cr-carbene reaction pathway is shown in dark red. The quintet parts are in black. Energies are in kcal moP and relative to il. Bond lengths are in angstroms. Angles are in degrees...

Why is the metallacycle mechanism able to give selective trimerization while the insertion/elimination mechanism found for A1-, Zr-, or Ni-catalyzed ethylene... 

Figure 6.16.6 Generally accepted metallacycle mechanism for the selective ethylene trimerization as first proposed by Briggs (Briggs, 1989).

In ethylene oligomerization, oxidative addition plays no role. In the insertion-elimination mechanism, the metal oxidation state is constant throughout the catalytic cycle. In the metallacycle mechanism, the oxidative step is an oxidative coupling reaction (see below). 

Oxidative coupling is the decisive step in the Cr-based ethylene oligomerization according to the metallacycle mechanism. As shown in Scheme 6.16.7, two ethylene units of the ligand sphere of the Crmetallacyclic intermediate. Assuming a cationic Cr-complex in the first place (with the MAO anion as very weakly coordinating counter-ion), the oxidation state of chromium increases in the process from +I to +III. 

The mechanism of metathesis catalysis has been widely investigated and several possible reaction pathways have been advanced. Among the mechanisms proposed, the cyclobutane mechanism [82,90-92], the metallocyclopropane mechanism [93], and the carbene metallacycle mechanism [86,94-96] remain prominent. It is accepted that the olefin metathesis reaction occurs via metallacylcobutane intermediates formed by reacting olefin with the metal-alkylydene complex (M=CHR). Whether this happens on a heterogeneous catalyst, metal-alkylydene... 

The metallacycle mechanism accounts for the catalysts of the third family. 

An ingenious method has been described for differentiating between a metallacycle mechanism (path a in Scheme 13) and the classical Cossee-Arlman mechanism (path b) for the [Ti(OBu")4] + Et3Al)-catalyzed copolymerization of H C= CH + H C= CH (4 96). The proton-coupled C mutation NMR spectrum shows that the polymer contains the C atoms linked by a double bond as predicted by path b and not a single bond as predicted by path... 

As shown by reactions 7.2.1.1 and 7.2.1.2, selective dimerization of ethylene may take place either by the Cossee mechanism or by the metallacycle mechanism. With the Ni catalysts, the Cossee mechanism operates. The experimental evidence for this is discussed later. 

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