Transmetallation product

Yields in the above reactions can often be improved by the addition of 1 mole of triphenylphosphine directly to the trifluoroacetic acid solution of the reactants immediately before final work-up. It would appear that the triphenylphosphine functions as a scavenger for TTFA released in the metal-metal exchange reaction, thus protecting the final phenol from further electrophilic thallation and/or oxidation. Validation of the metal-metal exchange mechanism was obtained indirectly by isolation and characterization of an ArTlX2/LTTFA complex directly from the reaction mixture. NMR analysis revealed that this complex still possessed an intact aryl-thallium bond, indicating that it was probably the precursor to the transmetallation products, an aryllead tristrifluoroacetate and TTFA. 

Apart from lead, hydrogen, phenol and transmetallation products, the dimer and the polymer are considered the only stable reduction products of ArPb(OAc)3. 

Selective transfer of the homoenolate moiety characterizes the transmetalation from tin to titanium described in the previous section. A very facile transmetalation of the same kind occurs for 3-stannylketoxime, Eq. (50) [48]. Reaction of the ( )- 3-(tributyltin)ketoxime with dichlorobis(benzonitrile)palladium in CH2C12 at 0 °C for 30 min yields 78% of the transmetalation product. The... 

W (Scheme 53),M a transmetallation product 186 is derived in 58% overall yield from compound 185 (R = SiMe3, M = W) (Scheme 77).202 The ethoxy silyl(l-alkynyl) derivative lv forms a stable Co2(CO)6 adduct 187 to the C=C bond, as does the corresponding phenyl complex lb (see Scheme 53).144a... 

At higher temperatures (from —10 to +10 °C) the transmetallation products 145 undergo a gradual conversion to the O-aUcylation products 146 by chlorine migration from carbon to germanium. Further increase in temperature (up to 60 °C) results in isomerization to the A-alkylation products 142-144. The latter reaction is similar to Chapman rearrangement, although it occurs under remarkably mild conditions. The half-lives for conversion of imidates 146 (R, R = (CH2)n, n = 4, 5) are 13 and 25 min, respectively... 

The reaction of ClCH2SnMe2Cl with A-TMS amides and lactams requires more drastic conditions (o-xylene, 80-100°C) than the analogous reactions of ClCH2SiMe2Cl and ClCH2GeMe2Cl (Section VI.A.l.c) and, in general, leads to a mixture of the O-and A-alkylation products " . A multistage reaction scheme established by NMR monitoring includes the transmetallation products 359 followed by subsequent formation of O-stannyhnethyl and finally Af-stannyhnethyl derivatives, 360 and 361, respectively (Scheme 18). 

Further treatment of Wp-(SiMe2) -Cl with Na[W(CO)3Cp] leads to the transmetallation product only, with no difference in the solvent used (Methylcyclohexane or THF/nheptane respectively), according to the work of Malisch [2]. Attempted reduction with LiAlH4 at -40 °C leads to cleavage of the Si-W bond. 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

The metal-free condensation of fr(2-formylphenyl)telluride with a series of diamines afforded the macrocyclic tellurium Schiff base macrocycles attempted complexation with Pt(II) and Hg(II) afforded transmetalated products <04JOM1452>. Reduction of the Schiff base components of these chalcogenaza macrocycles gave rise to more robust and flexible macrocycles, which form the desired Pd(II) Tie,iV,jV, Tie-complex <04CC322>. The related Se-derivatives are also therein reported. 5,10-Diphenyl-15,20-di(4-methoxyphenyl)-21-tellura-porphyrin was prepared (18%) by the acid-catalyzed condensation of 2,5-6w(l-phenyl-l-hydroxymethyl)tellurophene, pyrrole, and 4-methoxybenzaldehyde, followed by oxidation with /j-chloranil <04OM4513>. 

The reaction of pincer triflato complex 37 with 2-(tributylstannyl)furan allowed isolating the transmetallation product 38 as a stable compound (Scheme 1.25) [216]. Furthermore, by performing the reaction at low temperature, intermediate 39 was observed in which the furan is -coordinated to the palladium center. 

Several new species involved in the transmetalation step were now detected the radical cation 159 derived from a neutral species and the cationic homolog 160. Tin-containing species 161 was also observed and assigned as another transmetalation product. At m/z = 657, species 162 might proceed from neutral species [ArPd... 

This transmetallation product, when applied to aldol couplings as shown below, exploits the stronger chelation of the cerium ion (relative to lithium) in the... 

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. 

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