Transmetallation mechanisms

Scheme 2. Possible mechanisms of the rapid slippage of two molecular rings a, conventional mechanism b, transmetallation mechanism. Note that these schemes give only the topology of the pathways. Thus the second transmetalation in mechanism b might happen between any Pd-N bond.

Generally, the Lewis acid-promoted allylation of aldehydes with allyltrimethylsilanes proceeds via carbonyl activation. However, the SnCU-promoted allylation with allylsilanes bearing a coordinate site is rationalized by a transmetallation mechanism, in which allyltrichlorostannanes work as the actual allylating agents (Equation... 

In the presence of a copper salt, aromatic aldehydes, a,/3-unsaturated aldehydes, formates, and formamides react with 38 at or below room temperature to give the corresponding insertion products in good yield.289 This catalytic reaction shows similar stereo- and regiochemistry as does the thermal reaction of formamides. A transmetallation mechanism via an organocopper intermediate has been proposed for the Cu-catalyzed insertion. 

There are two plausible mechanisms one is a transmetallation mechanism via intramolecular carbopallada-tion, and the other involves nucleophilic addition of the internal silyl enolate to a Pd(ll)-coordinated alkene moiety. 

Relatively less acidic ketones compared to 1,3-dicarbonyl compounds are also suitable substrates for the palladium catalyzed coupling. a-Aryl ketones are obtained as products. In the early examples, masked ketone enolates such as silyl enol ethers [42] and enol acetates [43-45] were used in the presence of a tin source. These reactions involve tin enolates or acylmethyltins as intermediates and thus proceed by transmetalation (mechanism B in Scheme 1). 

The mechanism for this transformation is not clear, but one could speculate that the reaction proceeds via a transmetallation pathway similar to fluoride-induced silicon to palladium transmetallation mechanism. 

In contrast with the above Lewis acid-catalyzed asymmetric aldol reactions, chiral Pd and Pt cationic complexes have been found to catalyze the asymmetric process by a transmetalation mechanism involving a metal enolate intermediate (Section 10.2.1.3). 

The Tol-BINAP complexes of CuFa, CuF, and CuOt-Bu also work as efficient chiral catalysts of the aldol reactions of aromatic and a, -unsaturated aldehydes wifh dienolate 64 (Scheme 10.62) [164]. IR spectroscopy has revealed fhat the stoichiometric reaction of 64 wifh Cu(Ot-Bu)(S)-Tol-BINAP forms a Cu(I) enolate, and fhat subsequent reaction wifh an aldehyde gives a copper aldolate. The copper enolate is also obtained by stepwise treatment of 64 with Bu4NPh3SiF2 and Cu(C1O4)(S)-To1-BINAP. These results, with the known reduction of Cu(II) to Cu(I) by SEE, indicate that fhe Cu-catalyzed aldol reactions proceed fhrough a transmetalation mechanism involving a chiral Cu(I) enolate. 

Lectka et al. have reported that Tol-BI NAP-coordinated CUCIO4 catalyzes fhe addition of SEE to a-N-tosylimino esters wifh high enantioselectivity (Scheme 10.81) [227]. The use of a-substituted SEE gives anti adducts wifh high diastereoselectivity. The Tol-BINAP complex is superior to fhe corresponding BINAP complex in diastereo- and enantioselectivity. It has been proposed that this asymmetric Mannich-type reaction proceeds by a Lewis acid-catalyzed mechanism, not a transmetalation mechanism as reported by Sodeoka et al. 

In the Lewis acid-promoted allylation with allylsilanes, the promoter is usually regarded as activating electrophiles. Thomas [374] and Dias [375] have, however, described SnCLj-promoted allylation reactions proceeding via a transmetalation mechanism (Scheme 10.130). In the latter equation of Scheme 10.130, the formation of the allylstannane species has been observed by NMR spectroscopy [375 b]. 

Yamamoto et al., on the other hand, reported that a Ti-allylpalladium-TBAF cocatalyst system enables allylation of aromatic imines with allyltrimefhylsilane at room temperature [376]. As described in Scheme 10.131, this allylation would proceed via a transmetalation mechanism involving the bis-Ti-allylpalladium intermediate 121. The use of chiral 7r-allylpalladium complex 131 enables asymmetric synthesis of homoallyl amines wifh good enantioselectivity (Scheme 10.150). 

Most of the catalytic methods reported so far are the chiral metal compound-catalyzed allylation reactions using allylic silanes or allylic stannanes. Two types of catalytic mechanism, Lewis acid mechanism and transmetallation mechanism, can be considered for these reactions as illustrated in Scheme 2. If the chiral metal catalyst acts as a Lewis acid in the asymmetric allylation, an al-... 

Scheme 10 Proposed transmetallation mechanism for aryl transfer...

The mechanism by which the tin-carbon bond of the grafted organometalHc is cleaved by chemisorbed hydrogen is not yet fully understood but the analogy with mechanisms of molecular chemistry in solution suggests that there is a four-center transmetallation mechanism as in Scheme 18.8. 

In this study, the full catalytic cycle for the recently reported asymmetric Suzuki-Miyaura coupling between 1 -bromo-2-methylnaphthalene (2) and 1 -naphthal eneboronic acid (3) catalyzed by a [Pd(bis-hydrazone)] (1) complex [47], was theoretically investigated by means of DFT calculations. Importantly, the results derived from this study revealed that the transmetalation reaction does not occur in just one step, but occurs in three steps. This is owing to the relative lability of the bis-hydrazone ligand, which can easily dissociate one of the N atoms coordinated to the Pd catalyst. Very recently, this variant for the transmetalation mechanism has been reported for the Suzuki-Miyaura coupling catalyzed by a diimine chelated palladium complex [15]. 

In this case, two different transmetalation mechanisms, labeled as cyclic and open, have been proposed to explain the experimental data, and they are both shown in Fig. 11.8. The existence of the cyclic mechanism was proposed by Espinet and coworkers [49] to account for the evidence that some Stille processes take place with retention of configuration at the transmetalated carbon atom. This retention of configuration implies a transition state, or intermediate, where the X and R ligands are bridging the two metal atoms (see Fig. 11.8, upper pathway). The open mechanism was proposed for those cases where the product presents inversion of configuration. This mechanism can produce either cis or trans geometries of the L2Pd(R)(R ) species (Fig. 11.8, lower pathway). The open pathway seems to be... 

Mukaiyama aldol reactions of silyl enol ethers are generally rationalized by a Lewis acid activation of the carbonyl group by in situ formation of a complex that reacts with the silyl enol ether or the silyl ketene acetal [99,167]. Transmetallation mechanisms according to which silicon is replaced under formation of a metal enolate have been discussed as well for catalytic versions of the reaction [168], in particular for late transition metals [169]. 

The reactions of polynuclear Cu(II) substrates with the labile Zn(II), Ni(II), and Co(II) complexes of substituted hydrazine-carbodithioates, [M(NS)2], are very effective transmetalation routes to the production of mixed polymetallic complexes. Modeling these processes by the reaction with [Cu(acac)2] to establish possible details of the transmetalation mechanism, Davies and co-workershave measured the equilibrium constants and stopped flow and conventional rate kinetics. For Ni(II), the kinetics indicate formation of two labile precursors, with formation constants which have quite different temperature dependences,and each of which convert to successor complexes at rates which can be resolved the successor complexes then dissociate to the transmetalated product when subjected to the subsequent chromatographic separation. Data are accumulating to build free-energy, enthalpy, and entropy profiles for the process and to identify the shifts in these for different [M(NS)2] reagents. 

When considering the kinetic stability of a lanthanide chelate, dissociation and trans-metallation reactions are the main processes that must be taken into account. For complexes formed with EDTA, the proton-assisted dissociation dominates over direct attack by another metal ion. Indeed, the structure does not leave space for direct attack, because none of the acetate arms remains exposed [7]. This can be contrasted with the DTPA case where the two residual negative charges on Ln(DTPA) make the direct attack reaction more likely. A study by Briicher et al. [8] demonstrated that at physiological pH the transmetallation mechanism dominates and that the kinetics of the process largely depends on the attacking... 

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