Thallium , direct bonding

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. 

Treatment of ZnMe2 with the thallium salt of a bis(pyrazolyl)hydroborate gave the methylzinc complex [BpZnMe] 104 in which zinc is three coordinate.162 This complex reacted with paraformaldehyde, Scheme 73, with the direct insertion of the CH20 moiety into the B-H bond to produce the methylzinc complex 105 (Figure 51), in which zinc is tetrahedrally surrounded by the methyl group and the newly formed N,N,O-ligand.163... 

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

The latter on treatment with thallium trifluoroacetate, under carefully defined conditions followed by aqueous work-up, led directly to narwedine (291) [74] in 51% yield. The remarkable transformation is believed to occur via the cationic intermediate 292, formed by an oxidative process, undergoing a 1,2 C-C shift to generate the carbenium ion 293. Aromatisation involving the cleavage of C-Pd bond, followed by hydrolysis of the resulting sulphonium salt 294 liberated the phenol, which smoothly underwent Michael addition to the proximate enone system, furnishing ( )-narwedine (291) in 44% overall yield. 

In contrast to direct electrophilic substitution in which the astatine attacks one of the C—H bonds, astatination through demetalation can be used to label a substrate at a preferred site in a regiospecific manner not affecting other sensitive sites of the molecule. Since the C—M bond is more sensitive to electrophilic attack than C—H, higher yields can be obtained in short reaction times under milder experimental conditions56. Organometallic compounds of mercury, thallium and tin have been used so far for astatination via demetalation. 

Yamamura and coworkers [158] developed an oxidative cyclization method to construct biphenyl ether bonds by thallium trinitrate (TTN) oxidation of the corresponding O,0 -dihalophenols followed by zinc reduction. The antibiotic piperazinomycin 266) was synthesized using this method as a key cyclization step [159]. As shown in Scheme 89, the diketopiperazine 263 was subjected to TTN oxidation in MeOH to afford an inseparable mixture containing plausible intermediate 264, which was directly reduced with zinc powder in AcOH-THF to give rise to the strained 14-membered biphenyl ether 265 in 19% yield together with two other isomers. 

Recently, the formation, structure, equilibrium, and kinetics of a family of cyano compounds containing a direct and not supported by ligands platinum-thallium metal-metal bond have been reported. The complexes are synthesized according to the reaction ... 

Multinuclear NMR- ( C, Pt, T1), 1R-, Raman-spectroscopy. Electron Spectroscopy for Chemical Analysis (ESCA), X-ray, and Extended X-ray Absorption Fine Structure (EXAFS) studies confirm direct, short (2.60-2.64 A) Pt-Tl bonds. Figure 1. shows a typical ° T1 NMR spectrum of [(N C)5Pt-Tl( CN)] together with the structure determined by EXAFS. The spin-spin coupling pattern is consistent with 4 -h 1 - -1 equivalentligands (I = 1/2), respectively and one Pt nucleus (natural abundance 33.8%, 1=1 /2). The spectrum has been selected to illustrate the usefulness of T1 NMR spectroscopy in studies of the inorganic chemistry of thallium. The compounds are diamagnetic, and... 

The conversion of a carbon-thallium bond into a carbon-carbon bond has been performed directly only in a relatively limited number of cases formation of aiylcyanides, reaction with nitroalkane salts as well as the reaction with copper acetylides. Other carbon-carbon bond formation reactions have been reported with a carbon-thallium substrate. However, they all involve palladium catalysis, and this is beyond the scope of this book. 147... 

However, some monovalent metals can form strong metal-phosphorus bonds if the phosphorus bonds are bound simultaneously to chalcogen atoms [3]. We used this observation to prepare the corresponding thallium compounds. Direct synthesis from the. elements yielded compounds of the compositions TIPS, TlPSe, and TlPTe. 

The thallium(in)-promoted rearrangement can be carried out chemoselectively at the double bond of an 1,2-dUiydronaphthalene moiety without oxidation at the reactive 3-position of an indole ring. Using this approach, the alkaloid fraKi-trikentrin A 51, which displays antimicrobial activity, has been synthesized. To avoid problems of epimerization, the ring contraction product was reduced in situ with NaBHq, giving directly the desired alcohol 50,... 

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