No abstract

[reaction: see text] The solution and chelation properties of 2-thienyllithium reagents with potential amine and ether chelating groups in the 3-position and related model systems have been investigated using low temperature 6Li, 7Li, 13C, and 31P NMR spectroscopy, 15N-labeling, and the effect of solvent additives. In THF-ether mixtures at low temperature 3-(N,N-dimethylaminomethyl)-2-thienyllithium (4) is ca. 99% dimer (which is chelated) and 1% monomer (unchelated), whereas 3-(methoxymethyl)-2-thienyllithium (5) is <10% dimer. Compound 5 crystallizes as a THF-solvated dimer, but there is no indication that the ether side chain is chelated in solution. Both 4 and 5 form PMDTA-complexed monomers almost stoichiometrically, similar to the model compound 2, in sharp contrast to phenyl analogues, which show very different behavior. The barriers to dimer interconversion are ca. 2 kcal/mol lower and chelation is significantly weaker in the 2-thienyllithium reagents than in their phenyl analogues.

The lithium±metalloid exchange reaction is widely used to prepare synthetically valuable organolithium reagents. The full synthetic potential of one of the most important of these reactions, the Li/I exchange, was not realized (e.g., in the synthesis of alkyllithium reagents) until mechanistic studies showed that it was a well-behaved polar process without a significant radical component. [1a, b, 2] Ate complexes, first proposed by Wittig and Schˆllkopf, [3] are presumptive intermediates in these reactions [Eq. (1)]. Ate complexes of iodine, [1a,b, 4] tellurium, [1b, 5] selenium, [1c, 5] tin, [1d, 6] and silicon [7] having only alkyl and aryl ligands have been characterized in solution and in the solid state, and probed by high-level calculations. [8] Their presence, however, is not proof that they are intermediates in the exchange. No direct evidence has been reported for any Li/M exchange which shows that all of the exchange proceeds along path k ate , and none by the direct exchange path k d , which bypasses the ate complex [Eq. (1)]. To perform such an experiment with dynamic NMR (DNMR) spectroscopy, the three reactants (monomeric ArLi, ArM, and Ar 2 MLi) in the triangular equilibrium of Equation (1) must be present in detectable concentrations under conditions for which the rates k d , k ate , and k 0 ate can each be measured. A number of Li/I and Li/Te exchanges we investigated failed to provide a clear answer. Exchanges of PhI and Ph 2 Te with PhLi could not be analyzed because no solvent of appropriate polarity was found such that all species were detectable. In THF both monomer and dimer were observable, but the equilibrium constant K eq [Eq. (1)] for the ate complex was so high that insufficient PhLi and/or PhI/Ph 2 Te (whichever was stoichiometrically limiting) was present. [1b] In less polar, mixed solvents, the concentration of PhLi monomer was too low for proper line-shape analysis [1e, 9] (the dimer of PhLi is not active in the exchange process [1b] ). A problem which prevents kinetic analysis of Li/I exchanges is the fast reaction between ArI and the ate complex, in which the latter ZUSCHRIFTEN 3586

The lithium±metalloid exchange reaction is widely used to prepare synthetically valuable organolithium reagents. The full synthetic potential of one of the most important of these reactions, the Li/I exchange, was not realized (e.g., in the synthesis of alkyllithium reagents) until mechanistic studies showed that it was a well-behaved polar process without a significant radical component. [1a, b, 2] Ate complexes, first proposed by Wittig and Schˆllkopf, [3] are presumptive intermediates in these reactions [Eq. (1)]. Ate complexes of iodine, [1a,b, 4] tellurium, [1b, 5] selenium, [1c, 5] tin, [1d, 6] and silicon [7] having only alkyl and aryl ligands have been characterized in solution and in the solid state, and probed by high-level calculations. [8] Their presence, however, is not proof that they are intermediates in the exchange. No direct evidence has been reported for any Li/M exchange which shows that all of the exchange proceeds along path k ate , and none by the direct exchange path k d , which bypasses the ate complex [Eq. (1)]. To perform such an experiment with dynamic NMR (DNMR) spectroscopy, the three reactants (monomeric ArLi, ArM, and Ar 2 MLi) in the triangular equilibrium of Equation (1) must be present in detectable concentrations under conditions for which the rates k d , k ate , and k 0 ate can each be measured. A number of Li/I and Li/Te exchanges we investigated failed to provide a clear answer. Exchanges of PhI and Ph 2 Te with PhLi could not be analyzed because no solvent of appropriate polarity was found such that all species were detectable. In THF both monomer and dimer were observable, but the equilibrium constant K eq [Eq. (1)] for the ate complex was so high that insufficient PhLi and/or PhI/Ph 2 Te (whichever was stoichiometrically limiting) was present. [1b] In less polar, mixed solvents, the concentration of PhLi monomer was too low for proper line-shape analysis [1e, 9] (the dimer of PhLi is not active in the exchange process [1b] ). A problem which prevents kinetic analysis of Li/I exchanges is the fast reaction between ArI and the ate complex, in which the latter COMMUNICATIONS 3436 , b ¼ 90.998(2), g ¼ 105.658(2)8, V ¼ 5628.9(8) ä 3 , Z ¼ 2, 1 calcd ¼ 1.312 Mg m À3 , m ¼ 3.039 mm À1 , F(000) ¼ 2266, 44 317 reflections collected, 16 076 independent reflections, GOF ¼ 1.021, R ¼ 0.0723, wR 2 ¼ 0.1947. 3: C 94 H 102 N 8 O 12 K 4 SiU 2 , M r ¼ 2196.34, monoclinic, space group P2 1 /m, a ¼ 16.322(1), b ¼ 17.691(2), c ¼ 19.772(2) ä, b ¼ 98.956(2)8, V ¼ 5643.4(9) ä 3 , Z ¼ 2, 1 calcd ¼ 1.293 Mg m À3 , m ¼ 3.076 mm À1 , F(000) ¼ 2184, 44 186 reflections collected, 6134 independent reflections, GOF ¼ 1.058, R ¼ 0.0529, wR 2 ¼ 0.1737. 4: C 96 H 156 N 10 O 12-K 5 U 2 , M r ¼ 2313.87, monoclinic, space group P2 1 /n, a ¼ 13.111(1), b ¼ 22.953 (3), c ¼ 17.976(2) ä, b ¼ 91.634(2)8, V ¼ 5407(1) ä 3 , Z ¼ 2, 1 calcd ¼ 1.421 Mg m À3 . m ¼ 3.241 mm À1 , F(000) ¼ 2354, 12 592 reflections collected, 6924 independent reflections, GOF ¼ 1.002, R ¼ 0.0383, wR 2 ¼ 0.0908.

ChemInform Abstract In order to determine the potential of transition-metal Lewis acids as catalysts, complexes such as (I)-(III) are investigated. These compounds are catalytically active with interesting differences in rates of cycloaddition and catalyst turnover. It is shown that trace impurities and/or decomposition products can be catalytically active and serve to mask the true organometallic catalysis.