Reaction mechanism, metallacycle

A careful investigation of the reaction kinetics and detailed trapping experiments allow the conclusion that in this case a a-bond metathesis reaction mechanism applies. The polymerization reaction of PhSiH3 by CpCp Hf(SiH2Ph)Cl has been monitored by H-NMR spectroscopy. The data k(75 °C) = 1.1(1) x 10-4 M 1 s AH = 19.5(2) kcal mol" AS = -21(l)euandkH/fcD = 2.9(2) (75 °C) are in good agreement with the proposed mechanism with a metallacycle as transition state [164],... 

Scheme 10 Deuterium-labeling experiment providing evidence for a metallacyclic reaction mechanism [17]...

This protocol is also effective for the cyclization of an allenylaldehyde, the synthetic utility of which has been demonstrated in the synthesis of (+)-testudinariol A (Scheme 16.89) [97]. Cyclization of an allenylaldehyde provides a ris-cyclopentanol bearing a 2-propenyl group at the C2 position. The reaction mechanism may be accounted for by coordination of Ni(0) with both the aldehyde and the proximal alle-nyl double bond in an eclipsed fashion with a pseudo-equatorial orientation of the side chain, oxidative cyclization to a metallacycle, followed by Me2Zn transmetalla-tion and reductive elimination. 

SiCaT reaction of 82 (R=H, X=Y=C(C02Et)2) with PhMe2SiH, in the presence of this rhodium catalyst, afforded 86 (R=H, X=Y=C(C02Et)2) as the exclusive product. Interestingly, none of the CO-SiCaT/CO-CaT product 83 was formed in this reaction. Thus, it is clearly indicated that the CO-CaT reaction involves metallacycle intermediates, which makes a sharp contrast to the mechanism of CO-SiCaT reaction. 

Theoretical studies published since 1993 reporting computationally optimized structures for four-membered boracycles and metallacycles are listed in Table 1. Many of these investigations, however, maintain a specific focus on molecular transformations (i.e., reaction mechanisms) and no longer explicitly consider the details of metallacyclobutane structure. The most significant theoretical investigations of boracyclobutene derivatives were conducted sufficiently long ago to have been reviewed in CHEC-II(1996) <1996CHEC-II(lb)887> and are not discussed further. 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

The understanding of the reaction mechanism is directly related to the role of the catalyst, i.e., the transition metal. It is universally accepted that olefin metathesis proceeds via the so-called metal carbene chain mechanism, first proposed by Herisson and Chauvin in 1971 [25]. The propagation reaction involves a transition metal carbene as the active species with a vacant coordination site at the transition metal. The olefin coordinates at this vacant site and subsequently a metalla-cyclobutane intermediate is formed. The metallacycle is unstable and cleaves in the opposite fashion to afford a new metal carbene complex and a new olefin. If this process is repeated often enough, eventually an equilibrium mixture of alkenes will be obtained. 

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

A dinickel(I) compound 17 was made from the reaction between metallacyclic Ni(n) carboxylate ( nickelalactone ) and bis(diphenylphosphino)methane (dppm) (Scheme 10.7) [11]. The Ni(I)-Ni(I) bond length in 17 is 2.563(1) A (Entry 4, Table 10.2), and features three different bridging ligands (dppm, carboxylate, diphenylphosphido). The formation of 17 was proposed to proceed via the mechanism depicted in Scheme 10.8, and is remarkable because it acts as a model for the key step in the formation of acrylic acid from COj and ethylene. 

Two important points to note are that the basic reaction mechanism involves the formation of 4.39, a five-membered metallacycle from 4.38 (see Section 2.3.3). This may be considered an oxidative addition reaction as the formal oxidation state of M is zero and 2+ in 4.38 and 4.39, respectively. Second, conversion of 4.39 to 4.40 is the famiUar CO insertion into a metal-carbon bond, which is followed by reductive elimination of cyclopentenone. 

A considerable advance in this area was a discovery that styrene can be cat-alytically alkylated with various alkyl tosylates in the presence of (cyclohexyl) ethylmagnesium bromide to give the new Grignard reagent 8. Its reaction with oxygen furnishes after hydrolysis the alcohols 9 (Scheme 4). Authors claim that the reaction mechanism does not proceed through a metallacycle formation, instead that the ate-complex 10 is the species responsible for the alkylation (Scheme 5). This procedure enables addition of various linear, branched. 

Recently, it has been shown that the titanocene dicarbonyl (rj -C5H5)2Ti(CO)2 113 is a convenient complex that catalyzes a number of cyclization reactions. Among suitable substrates are enynes that can be easily cyclized into the corresponding vinylmethylenecycloalkanes 114 (Scheme 48) in the presence of this compound. Representative examples are shown in Table 21 [62]. The reaction mechanism is outlined in Scheme 49 and proceeds through the formation of the titanacyclopentene 115, which is followed by j3-hydrogen elimination to 116 and, finally, by reductive elimination to 114. Formally, it resembles cyclization of dienes via metallacycle formation. 

With regard to the mechanism of the cycloisomerization, Fiirstner et al. found strong evidence of a metallacyclic intermediate. By labeling the allylic position of enynes 46 and 48, they showed that reactions yielding traws-annulated rings 47 transferred the deuterium atom to the exocychc double bond (eq. 1 in Scheme 10), whereas c -annulated rings 49 formed with complete preservation of the position of the deuterium atom (eq. 2 in Scheme 10). This corresponds well to a metallacycUc... 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

Two essentially different mechanisms, (i) oxidative cyclization of two 7T-components (formation of metallacycle) and (ii) oxidative addition of reducing or alkylating agents followed by insertion of 7t-components, can operate in these three-component reactions.426 However, the aforementioned phenomena such as the reversal of regiochemistry and the crossover from reductive to alkylative manifolds remain unsolved. 

The olefin binding site is presumed to be cis to the carbene and trans to one of the chlorides. Subsequent dissociation of a phosphine paves the way for the formation of a 14-electron metallacycle G which upon cycloreversion generates a pro ductive intermediate [ 11 ]. The metallacycle formation is the rate determining step. The observed reactivity pattern of the pre-catalyst outlined above and the kinetic data presently available support this mechanistic picture. The fact that the catalytic activity of ruthenium carbene complexes 1 maybe significantly enhanced on addition of CuCl to the reaction mixture is also very well in line with this dissociative mechanism [11] Cu(I) is known to trap phosphines and its presence may therefore lead to a higher concentration of the catalytically active monophosphine metal fragments F and G in solution. 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

A proposed mechanism of this reaction was reported by Magnus and Principle [10], which is nowadays widely accepted (Scheme 1). Recently, negative-ion electrospray collision experiments have confirmed this mechanism in detail [11]. Starting with the formation of the alkyne-Co2(CO)6 complex 2, olefin 3 coordination and subsequent insertion takes place at the less hindered end of the alkyne. The in situ formed metallacycle 4 reacts rapidly under insertion of a CO ligand 5 and reductive elimination of 6 proceeds to liberate the desired cyclopentenone 7. It is important to note that all the bond-forming steps occur on only one cobalt atom. The other cobalt atom of the complex is presumed to act as an anchor which has additional electronic influences on the bond-forming metal atom via the existing metal-metal bond [12]. 

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