The exploration of synthetic methodologies toward heavy alkaline-earth chalcogenolates resulted in the preparation and structural characterization of a family of calcium thiolates, including [Ca(SC(6)F(5))(2)(py)(4)], 1 (py = pyridine), the separated ion-triple [Ca(18-crown-6)(NH(3))(3))][SMes](2).2THF, 2 (Mes = 2,4,6-tBu(3)C(6)H(2)), and the contact triple [Ca(18-crown-6)(SMes)(2)].THF, 3. Compound 1 was prepared by treating [Ca(N(SiMe(3))(2))(2)](2) with 4 equiv of HSC(6)F(5) under addition of pyridine. The thiolates 2 and 3 were synthesized by treatment of calcium metal dissolved in dry, liquid NH(3) under addition of 2 equiv of HSMes and crown ether or, alternatively, by the reduction of MesSSMes with calcium metal in dry, liquid ammonia. We also report two reaction products isolated during attempted calcium thiolate syntheses: [CaBr(4)(THF)(2)(&mgr;(2)-Li)(2)(THF)(4)], 4, isolated as the product of a salt elimination reaction between CaBr(2) and 2 equiv of [Li(THF)(n)()S-2,4,6-(i)()Pr(3)C(6)H(2)](m)(). [(NH(4))(py)(SC(6)F(5))], 5, was obtained as the sole product in the reaction of metallic calcium with HSC(6)F(5) in liquid ammonia under addition of pyridine. All compounds were characterized by single-crystal X-ray crystallography in addition to IR and NMR spectroscopy.
Rate studies of the lithiation of benzene and related alkoxy-substituted aromatics by n-BuLi/TMEDA
mixtures implicate similar mechanisms in which the proton transfers are rate limiting with transition structures
of stoichiometry [(n-BuLi)2(TMEDA)2(Ar−H)]⧧ (Ar−H = benzene, C6H5OCH3, m-C6H4(OCH3)2, C6H5OCH2OCH3, and C6H5OCH2CH2N(CH3)2). Cooperative substituent effects and an apparent importance of inductive
effects suggest a mechanism in which alkoxy−lithium interactions are minor or nonexistent in the rate-limiting
transition structures. Supported by ab initio calculations, transition structures based upon triple ions of general
structure [(n-Bu)2Li]-//+Li(TMEDA)2 are discussed.
The metathesis reaction of butyllithium or sodium or potassium hydride with pyridine-2-thiol is effected in ethereal or hydrocarbon media, where the addition of crown ether (which is essential in the case of sodium hydride) facilitates the metalation step. The reaction is driven forward irreversibly by the evolution of a volatile component (C 4 C NMR (solubility permitting), IR spectroscopy, and melting point determination.
n-Butyllithium/N,N,N',N'-tetramethylethylenediamine-mediated ortholithiations of aryloxazolines are described. Methyl substituents on the aryloxazoline and substituents at the meta position of the arenes (methoxy, oxazolinyl, and fluoro) influence the rates and the mechanisms. Monomer- and dimer-based reactions are implicated. Density functional calculations probe details of the mechanism and suggest the origins of cooperative effects in meta-substituted aryl oxazolines.
The synthesis and structural identification of a family of
crown-ether-coordinated and
crown-ether-free potassium thiolates are described.
[Li(15-crown-5)(SCPh3)] (1),
[K(18-crown-6)(SCPh3)(C6H6)0.5]
(2),
[K(18-crown-6)(SCPh3)(thf)0.5]
(3),
[K(dibenzo-18-crown-6)(SCPh3)(hmpa)0.5] (hmpa = hexamethylphosphoramide;
4), and
[K(dibenzo-18-crown-6)(SCPh3)(tol)]
(tol = toluene; 5) are available via metathesis
of nBuLi or KH with triphenylmethanethiol
in toluene, added crown ether, and respective Lewis donors. In the
absence of crown ether,
however, metathesis with NaH or KH and triphenylsilanethiol or
triphenylmethanethiol in
toluene leads to the hexameric aggregates
[(NaSSiPh3)6(tol)2]
(6),
[(KSCPh3)6(hmpa)2]·2
tol
(7), and
[(KSCPh3)6(tol)2]
(8). It is shown that the strongly coordinating hmpa
can ligate
the hexameric framework in 7; employment of stoichiometric
amounts of pyridine, 1,4-dioxane, or chelating pmdta
(N,N,N‘,N‘,N‘‘-pentamethyldiethylenetriamine)
results in the
isolation of toluene-coordinated 8. Compounds
1−8 were unequivocally identified by
X-ray
crystallography, 1H NMR, 13C NMR (depending on
solubility), IR spectroscopy, and melting
point. The monomeric formulations of 1−5
are clearly related to the ability of the appropriate
crown ether to sequester the alkali-metal cation, thereby satiating
much of its coordination
environment. In contrast, aggregation is observed in the absence
of crown ether (compounds
6−8), where the M6S6
hexameric core is stabilized through numerous cation−π
interactions
with several of the 18 phenyl groups comprising the organic periphery
and two donor
molecules.
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