Transmetalating agents

The same group found that heteroaromatic halides can be smoothly phenylated using NaBPh4 as the transmetallating agent and ligand-free Pd(OAc)2 as a catalyst with water or AT-methylformamide (NMF) as the solvent (Scheme 21) [39], The experiments were all performed using power control. 

The reaction of CO2 with 1,3-butadienes in the presence of Ni catalysts usually gave an isomeric mixture of carboxylic acids 89 and 90 after hydrolysis (Scheme 32).47,48 The oxa-7r-allylnickel complexes 87 and 88 might be the reaction intermediates, which could be formed through oxidative cyclization of Ni(0) with C02 and the dienes. When Me2Zn was used as a transmetallation agent to react with the oxa-7r-allylnickel intermediates under a C02 atmosphere, further carboxylation took place at the 7r-allylnickel unit. Thus, the 1,4-diesters 95 were obtained after acidic hydrolysis and treatment with diazomethane as shown in Scheme 32.47... 

In the case of 1,3-cyclohexadiene with Me2Zn, the dicarboxylation afforded the /rdicarboxylic acid 96 (Scheme 33). In contrast, when Ph2Zn was used instead of Me2Zn as a transmetallation agent, the phenylative carboxylation occurred in high regio- and stereoselectivity to give the l,4-m-substituted-2-cyclohexene 97, as a result of reductive elimination from the phenyl-7r-allylnickel intermediate, prior to the second carboxylation reaction (Scheme 33).47... 

Oxidative nickel-catalyzed coupling of aldehydes and alkynes to generate allylic alcohols. Intermolecular and intramolecular examples are both effective, and the transmetalating agent (MR" ) may be an organosilane, organoborane, organozinc, or alkenylzirconium. ... 

Mowery and DeShong reported on the use of siloxanes 72 (Figure 16) as versatile transmetallation agents for Pd(dba)3-catalyzed couplings with aryl halides and allylic alcohol derivates, in the presence of TBAF and at high temperature (95 They later used aryl silatrane 73 (Figure 16) as a suitable partner for the fluoride-promoted... 

In spite of its abundant use there is very little known about the actual mechanism of transmetalation. In certain cases the transmetalating agent has to be activated (made more nucleophilic) to achieve the desired conversion. Typical examples are boron- and siliciumorganic compounds, where the presence of hydroxide or fluoride ions is often necessary for a successful transformation. 

Because of their frequent use, some late transition metal catalyzed carbon-carbon bond forming reactions evolved into name reactions. The most prominent examples are cross-coupling reactions, where distinction is usually made on the basis of the transmetalating agent used. The common mechanism of cross-coupling reactions and its name variants are discussed in Chapter 2.1. 

Cyanamides are pseudo-halide nitrogen ligands that are readily coordinated to metals. A novel compound is 2-cyanaminofluoren-9-one (HL4)24. Its thallium salt T1+(L4)-(Scheme 7) is useful as a transmetallating agent in a reaction with trimethyltin chloride to produce the corresponding tin cyanamide complex [SnMe3L4] (Scheme 8). 

In the formation of RMX from group-IIB halides the choice of halide, especially when the transmetallating agent is itself made in situ from an organic halide is important, e.g., use of different halides in these two steps may result in mixed-halide products unless separation is possible. 

When the transmetallating agent is made in situ from the metal, filtration is performed to remove unused metal before the metal halide is added. 

Organic derivatives of other metals may also act as transmetallating agents. 

It is now possible to couple chlorophenols, haloanilines, and halovinyl alcohols with organozinc reagents without concern that protic groups found in R -X will react with the transmetalating agent. Equation 12.83 outlines this procedure, which also shows another variation in the synthesis of organozinc compounds— direct insertion of Zn into a C-X bond in the presence of LiCl.171... 

The bis-NHC-Ag(I) transmetallation agent generated in this reaction is unstable and significant decomposition occurs within hours. On the other hand, the resulting bis-NHC-Rh(I) complex is stable in air for months. The stability of metal-NHCs in general can be highly dependent on the N-substituents and on the identity of the metal. For example, a related bis-NHC-Rh(I) (where N-substituent is ra-butyl instead of r-butyl) decomposes in solution within a few minutes when exposed to air.10 Unlike the unstable bis-NHC-Ag(I) transmetallation agent presented here, many Ag(I) complexes of NHCs have been successfully characterized.11... 

In addition to the catalyst and base (if required), the use of metal-salt additives is very conunon for enhancing cross coupling and other reactions. Cul is the most common additive and in many cases it may operate as an intermediate transmetallating agent RM RCu —> RPdR. In other cases, it has been said to remove excess phosphine, thus increasing the reactivity of the palladium. Ag20 is sometimes used and may act by transmetallation or as a halide trap. 

A general mechanism for the coupling between thioether and aryl boronic acid is illustrated below, using Pd(PPh3)4 as the catalyst, in the presence of a stoichiometric amount of copper (I) 2-thiophenecarboxylate as the transmetalation agent. 

Summary The synthesis of a,(0-bis[tris(trimethylsilyl)silyl] alkanes was achieved by the reaction of ct,(0-ditosylalkanes with tris(trimethylsilyl)silyl potassium. Reaction of these with one equivalent of potassium /er/-butoxide resulted in clean formation of monopotassium silyl anions. Addition of another equivalent of the transmetallating agent led to the formation of the dipotassium compounds in cases where the alkyl spacer contained at least three methylene units. Partial hydrolysis of the dipotassium compounds induced an intramolecular reaction yielding a cyclic silyl potassium compound. 

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