Trans transmetallation reactions

The transmetallation reaction involves the transfer of the organic group from an organometallic species to a Pd(II) species and produces a trails Pd(II) species. Isomerization from the trans arrangement to a cis one is necessary prior to the reductive elimination step. Reductive elimination yields the coupled product and regenerates the transition metal catalyst. Because the reductive elimination is very fast, competing reactions leading to by-products are usually not observed. 

Related square-planar Rh(I) species RhRL3 and trans-Rh(R)L2CO (L = PR3) are more frequently accessible via transmetallation reactions. For instance, RhCl(PPh3)3 and MgRX (R = Me, Ph X = Br, I) yield RhR(PPh3)3. Such compounds are extremely susceptible to cyclometallation reactions via hydrogen transfer and only the orthometal-... 

Using this transmetalation reaction with ZcrZ-butyllithium we also succeeded for the first time in synthesizing cis- and trazw-l,2-dilithioethylene According to preliminary experiments the cir-isomer 14 seems to rearrange into the more stable trans-isomer 9, the results, however, have to be confirmed. 

B.iii.c. ttsns-ArPd(OAc)L2 Complexes (L = PPh ). Irani-ArPd(OAc)(PPh3)2 complexes are reagents for the transmetallation step in Suzuki cross-coupling when acetate ions are added as a base. Indeed, Ishiyama and co-workers have reported that trans-ArPd(OAc)(PPh3)2 complexes, formed by substitution of bromide by acetate in trans-ArPdBr(PPh3)2 (see Sect. undergo transmetallation reactions with diboronic esters... 

Fig. 4.2 Undesired transmetalation reactions in the Pd-catalyzed Negishi reaction between trans-[Pd(Rf)(Cl)(PPh3)2] andZnMe2 orZnMeCl [27]...

With the reaction mechanism for the transmetalation of 1 to the trans product 3 established, we next focused on investigating the transmetalation of 1 to the cis product 2. The reaction mechanism that we computed for this process is also concerted but, unlike the one that yields 3, it involves a Me by phosphine substitution followed by a phosphine by chloride substitution. The calculated Gibbs energy profile for the transmetalation reaction through this concerted mechanism is shown in Fig. 4.7. 

So far, the experimental and theoretical results obtained for the transmetalation reactions of 1 to 2 or 3 have been presented, but no comparison between them has been made yet. Hence, with this aim, the Gibbs energies obtained in the theoretical and experimental studies for these two transmetalations at 223 K have been summarized in the simplified reaction profiles depicted in Fig. 4.9. According to these results, the theoretical calculations reproduce qualitatively all the experimental observations. In particular, from the thermodynamic point of view, both computed transmetalation reactions are endergonic, in agreement with experiments. Furthermore, the order of stability of the reagents and products in equilibrium predicted by calculations is exactly the same that the one observed in the experiments 1 > 2 (cis) > 3 (trans). Therefore, calculations also conclude that 2 is the thermodynamic product. 

As far as the kinetics is concerned, the theoretical results show that the reverse reactions (i.e. retrotransmetalations from 2 or 3 to 1) are faster, which means that both transmetalation reactions are quickly reversible. This is consistent with the experimental observation that the addition of ZnCl2 to a solution of 2 or 3 ieads to the transformation of these species to 1, and accordingly, with the fact that the isomerization of 3 to 2 occurs via retrotransmetalation from 3 to 1 and subsequent transmetalation to 2. On the other hand, the computed energy barriers for both reaction pathways are low, which accounts for the fast transmetalation reactions observed in the experiments. More specifically, fhe global energy barrier for fhe fransmetala-fion reaction fo fhe trans produef 3 (lO.Skcal mol ) is lower tiian fhe one to the... 

Hence, according to the experiments, in the absence of added phosphine the observed rate is too fast for the transmetalation to occur on 1 consequently, the reaction must be going via the non-observed cationic intermediate 4+. Similarly to the reaction with ZnMeCl, the transmetalation reaction affords the trans product 3 at a higher rate than the cis product 2. Furthermore, the isomerization of 3 to 2 is also slow. On the other hand, in the presence of added phosphine, the cationic intermediate 4+ is not available and, accordingly, the transmetalation takes place on 1, but at a lower rate. 

The copper(I)-alkyne adduct undergoes a transmetallation reaction in which the alkyne group becomes bonded in a Pd(II) complex. The alkyne group is initially trans to the species to which it will be coupled. 

The transmetallation reaction produces a trans Pd(II) species. Isomerization to a cis arrangement is necessary prior to the reductive elimination of the coupled product. The Pd(0) catalyst is regenerated upon reductive elimination. The reductive elimination reaction is very fast. Therefore competing reactions leading to side products are not usually a problem. 

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... 

Direct transmetalation of organoboranes to organocopper reagents is not a general reaction. Because of dieir similar bond energies and electronegativities, diis trans-nietalation is linided to die preparation of alkenylcopper and unfiinctionalized... 

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

Although analogous to the direct coupling reaction, the catalytic cycle for the carbonylative coupling reaction is distinguished by an insertion of carbon monoxide into the C-Pd bond of complex A (see A—>B, Scheme 31). The transmetalation step-then gives trans complex C which isomerizes to the cis complex D. The ketone product E is revealed after reductive elimination. 

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