Tris , transmetallation

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

NLO materials (16 and 17) (Fig. 13) have been obtained from polyurethanes by the incorporation of sidechains with boron chromophores.37 The dihydroxy ligand of an azobenzene ligand containing a dimesityl boron acceptor was reacted in a polycondensation fashion with the diisocyanate groups of the polyurethanes to yield the desired polymers. Halogen displacement and transmetallation reactions have been utilized in the development of extended ir-conjugated systems of tri-9-anthrylborane with dendritic structures.38 In one (18) (Fig. 14) of the novel compounds, three identical... 

While the transmetalation step is often the rate-determining step for Pd-catalyzed reactions with organometallics, the oxidative addition step is often the rate-determining step in the Heck reactions, although olefin insertion can be rate-limiting in some cases — this is why the Heck reactions of tri- and tetra-substituted olefins sometimes proceed slower than those of di-substituted and terminal olefins. 

The main products under all variations of R and R are the P2-chlorinated triphosphanes. Side reactions such as SiMe3/Cl exchange, P—P cleavage by LiP(SiMe3)R, or transmetallations take place only to a small extent. MeP(SiMe3)2 is an exception inasmuch as it reacts with P-chlorinated phosphanes to yield tri- and even tetraphosphanes. 

Hayashi proved the validity of this catalytic cycle by the observation of aU three intermediates and their respective transformations using NMR experiments (Scheme 3.5) [16]. Transmetallation of a phenyl group from boron to rhodium takes place by addi-hon of phenylboronic acid 2 m to hydroxo-rhodium complex 16 in the presence of tri-phenylphosphine to generate the phenylrhodium complex 17. The reaction of 17 with 2-cyclohexenone la gives oxa- j-allylrhodium 18, which is converted immediately into hydroxo-rhodium complex 16 upon addition of water, liberating the phenylation product 3 am. In this NMR study, triphenylphosphine was used to stabilize the phenylrho-dium(I) complex. In the absence of triphenylphosphine, the characterization of the phenyl-rhodium species was unsuccessful. 

The chemistry outlined in Scheme 24 was then put into effect catalytic hydrogenation of the tris-isoxazole (302) and recyclization with triethylamine gave a tricyclic ligand which was chelated with nickel ions to give (303). Introduction of the fourth nitrogen atom was accomplished by treatment of (303) with ammonium acetate, giving (304). Treatment with cyanide removed the nickel ion which was then replaced with zinc(II) to give (305). The reasons for this transmetallation step were two-fold firstly, zinc(II) corrins, as shown by Eschenmoser, can be readily demetallated, and this fact opens up many options later in the synthesis, but secondly, and more importantly, Eschenmoser s photochemical cyclization of seco-corrins (see Section 3.07.3.4.2.3) does not proceed with nickel complexes of seco-corrins, whereas zinc(II) seco-corrins can be cyclized in almost quantitative yield... 

Contrary to the tin series, transmetallation reaction with alkyl- or arylgermanium trichloride starting from zirconaindenes does not give monochloro 1-germaindene in good yield103. However, the required compounds were synthesized from 1,1-dichloro-1-germaindenes, by reaction of phenyl- or silyl-tris(trimethylsilyl)lithium (Scheme 18). 

The reagent (1) can be generated conveniently in situ by transmetallation of (trimethylsilylmethyl)tri-M-butyltin with n-butyllithium in THF/hexane at 0° (equation... 

Optimum conditions for the coupling of the alkenyl triflates 330 with the arylstannanes 331 have been studied. Ligandless Pd complexes such as Pd(dba)2 are most active in the reaction of the enol triflate. PI13P inhibits the reaction. NMP as a polar solvent gives the best results. The use of tri(2-furyl)phosphine and Ph As in the coupling of stannanes with the halides and triflates increases the rate of the transmetallation of the stannanes to Pd, which is thought to be the rate-... 

Products of the carbometallation 442 and 443 contain very reactive metal carbon bonds, and can not be isolated in many cases. They undergo further reactions such as protonation, transmetallation and nucleophlic attack. The preparation of tri- or tetrasubstituted alkenes 444 and 445 from alkynes via the alkenylmetals 443 is particularly useful. Only catalysed carbometallations are treated here. 

Synthesis of 4,4 -(5-cyanobenzene)-l,3-bis[l,2,3,5-diselenadiazolyl] 28 was reported (Scheme 53) <1993CM820>. The starting material, 5-cyanobenzene-l,3-bis[iV,iV,iV1-tris(trimethylsilyl)carboxamidine] 329, was prepared by treatment of 1,3,5-tricyanobenzene 328 with 2equiv of lithium bis(trimethylsilyl)amide, followed by transmetalation with trimethylsilyl chloride. Reaction of the bis(amidine) 329 with 4 molar equiv of SeCh affords the bis(l,2,3,5-diselenadiazolium) dication as its dichloride salt [28][Cl2]. Reduction of the dication with triphenyl-antimony affords compound 28 in 26% from the bis(amidine) 329. 

Preparation of 4-cyanophenylzinc bromide by transmetallation and its reaction with phosphorus trichloride preparation of tris (4-cyanophenyl)phosphine-borane complex8... 

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