Transmetallation reaction mechanisms

A mechanism has been proposed for this, and related transformations, involving a chelation assisted C-H bond functionalization. Following hydride addition to the solvent, acetone, and a transmetallation reaction, reductive elimination yields the ketimine. Hydrolysis of the latter affords the ketone (Equation (131)).114 114a... 

Synthetically useful allylstannanes are provided by palladium-catalyzed carbostan-nylation using hexamethylditin (Scheme 16.58) [63]. The reaction mechanism can be rationalized by transmetallation between ditin and a Jt-allylpalladium complex produced by reaction of an allene with an arylpalladium iodide. In this process, hexamethylditin is added to the reaction mixture slowly via a syringe pump to suppress its high reactivity towards the arylpalladium species leading to an arylstannane. 

Thus ethyl groups may be added to a-carbons as long as benzylic hydrogens are available for replacement. The mechanism which has been proposed by Pines et al. 19) for the reaction consists of the addition of the benzylic carbanion formed by reaction of the aromatic and the catalyst with the olefin followed by a transmetalation reaction with more of the aromatic [Reaction (4)]. 

This addition is energetically favored because a more stable primary carb-anion is formed from a less stable tertiary carbanion. The addition step in the side-chain alkylation reaction is probably not energetically favored because a primary carbanion is formed from a resonance-stabilized benzylic carbanion. However, as the rapid and energetically favored transmetalation reaction following it restores the benzylic carbanion, the over-all process takes place readily. An alternative radical combination mechanism has also been proposed by Morton and Ward 39) for the alkylation reaction. 

Alkylpolysilanes are cleaved slowly with alkali metals, as shown by Stolberg609. Complicated conversion and transmetalation reactions also occur, since higher chain (up to n = 7) and cyclic methylsilanes appear after interruption of the reaction by addition of me3SiCl. A silylene insertion mechanism also seems to be active here. 

Interestingly, 1,2-disubstituted epoxides led to a,a-disubstituted propargyl alcohols, indicating that 1,2 shifts and therefore epoxide-to-aldehyde rearrangements occurred during the reactions (Scheme 10.12). Mechanistic studies by NMR validated this rearrangement and also supported the transmetallation-abstraction mechanism as already proposed (Scheme 10.13).8... 

Higher temperatures and polar solvents are considered to switch the reaction mechanism of transmetalation from a four-centered transition state (Se2 (cyclic)) to a back-side attack of the palladium(II) complex (Se2 (open)) (Fig.1). 

Although coordinative saturation (or unsaturation) is not a property of metals themselves, its effect on transmetallation may be discussed here. In few cases have the mechanistic details of transmetallation reactions been established. One mechanism proposed for various transmetallation reactions is ... 

Because it is not the case that every line represents exactly two electrons in a drawing of an organometallic or inorganic compound, it follows that the curved-arrow convention for showing the movement of electrons cannot be applied unambiguously when reaction mechanisms inolving transition metals are drawn. For this reason, in mechanisms involving transition metals, the name of each individual mechanistic step (insertion, transmetallation, oxidative addition, etc.) is indicated in place of curved arrows. You may use curved arrows to show electron movement in some steps if you wish, but it is more important for you to name every step. 

The SN2(Sn)(coord) mechanism, however, is familiar in the synthetically important Sn/Li transmetallation reaction that occurs between tetraalkyltin compounds and alkyl lithiums. This process is discussed in Section 19.1.1, and its applications in synthesis in Section 22.1. 

Hiyama and Mori have reported that homo-coupling reactions of vinyl-, aryl-, and alkynylsilanes are effectively promoted by an equimolar amount of CuCl and air (Scheme 10.218) [565]. The reaction mechanism probably involves formation of organocopper species by Si-Cu transmetalation and subsequent oxidative dimerization by air. The strong tendency of CuCl to activate alkynylsilanes has been used for coupling reactions with 1-chloroalkynes (Scheme 10.218) [566]. 

The arrows on the transmetallation step are intended to indicate which group joins to which rather than a mechanism. We have seen other transmetallation reactions of this sort - Li exchanged for Cu or boron or tin for example in chapter 16 in particular. They may well go through a four-centred transition state in which the aryl group (in this case) is bonded to both metals and the halide also. Organometallic mechanisms are not always known in detail. 

The reaction mechanism proceeds via a transmetallation giving an aryl-copper-nitrogen species, followed by a reductive elimination, but the difference from the aryl hahde reaction is that this generates a species (Cu(0)) that cannot enter into a catalytic cycle. However, methods that are catalytic in copper have been developed, using an oxidant to regenerate Cu(ll), although here there is the possibility for a variation in... 

An equally plausible mechanism involves simply a transmetalation reaction between RCH2CH2Li and ethylene—that is, RCH2CH2Li + CH2 = CH2 — RCH2CH3 -f- CH2 = CHLi. Studies involving sodium alkyls have shown that metalation can occur at vinylic or allylic positions (17). 

The kinetics of the ring-contraction transmetallation reaction in Scheme 111 (M = Cu") is first order in both (45) and Cu", suggesting a mechanism which involves either rate-determining transmetallation followed by ring contraction, or ring opening and closure initiated by metal coordination to meso nitrogens. 

The general reaction mechanism has been shown to involve typical steps for cross-coupling [98, 113]. Oxidative addition of an aryl halide generates a Pd(II) species that undergoes transmetalation to form a Pd(II)-thiolate. C-S reductive elimination provides the aryl sulfide and regenerates the Pd(0) catalyst. More recently, Hartwig reported a detailed mechanistic analysis of the Pd/Josiphos system derived from different Pd precursors. The dominant Pd species were found to be off the catalytic cycle, which accounted for differences in rates between stoichiometric and catalytic reactions [114]. Thioketones are also effective thiolate nucleophiles for C-S bond formation. The reaction involves tandem Pd-catalyzed thioenolate alkylation, followed by 5-arylation (8) [102]. Presumably, the arylation process proceeds by a similar mechanism to related Pd-catalyzed transformations. 

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