Ether solvates

The crystal structures of many organolithium compounds have been determined.44 Phenyllithium has been crystallized as an ether solvate. The structure is tetrameric with lithium and carbon atoms at alternating corners of a highly distorted cube. The lithium atoms form a tetrahedron and the carbons are associated with the faces of the tetrahedron. Each carbon is 2.33 A from the three neighboring lithium atoms and an ether molecule is coordinated to each lithium atom. Figures 7.2a and b show, respectively, the Li-C cluster and the complete array of atoms, except for hydrogen 45 Section 6.2 of Part A provides additional information on the structure of organolithium compounds. 

The crystal structures of many organolithium compounds have been determined.35 Phenylhthium has been crystallized as an ether solvate. The structure is tetrametic, with lithium and carbon atoms at alternating comers of a strongly distorted cube. Each carbon is... 

In order to determine how many ether molecules are favored by the Schlenk dimer, MeMgCliMgMe in Scheme 10, the geometry of 9b (four ethers) is compared to that of 9a (two ethers). In 9a, two Mg-O distances (2.103 A and 2.109 A) are close to that (2.104 A) of the MgO ionic crysral. In 9b, they are 2.265 A, 2.261 A, 2.272 A and 2.272 A and are larger than those in 9a. In spite of the large Mg to O affinity, two Mg atoms do not favor the coordination of four ether molecules. Thus, 9a is a saturated complex, although there seems to be room on the two Mg atoms for further nucleophilic coordination. Mg atoms seem to persist in tetra-coordination. Ether solvation of the Schlenk equilibrium species does not block reaction channels completely. 

FIGURE 1. Transition state for the addition of (M)-l,2-butadienylzinc fluoride, as a dimethyl ether solvate, to acetaldehyde calculated at the B3LPY/6-31G level of theory... 

All the absolute values of k reported in the literature are collected in Table 1. In all cases the figures should refer to the reaction of the ion-pair with monomer. The results are too fragmentary and in some cases of uncertain accuracy for a detailed discussion of the effect of environment on reactivity. A few points are clear. The reactivity of the relatively unsolvated ion-pair in hydrocarbon solvents is relatively large and may even be comparable with that of the solvated ion-pair in tetrahydrofuran despite the large difference in dielectric constant. The reactivity of the ether-solvated ion-pair in solvents of lower dielectric constant is lower than either. The first effect of etherate formation is to decrease the reactivity of the ion-pair which can be increased again by an increase of the dielectric constant of the solvent. 

In this section, we review our first examinations of tryptophan probing sensitivity and water dynamics in a series of important model systems from simple to complex, which range from a tripeptide [70], to a prototype membrane protein melittin [70], to a common drug transporter human serum albumin [71], and to lipid interface of a nanochannel [86]. At the end, we also give a special case that using indole moiety of tryptophan probes supramolecule crown ether solvation, and we observed solvent-induced supramolecule folding [87]. The obtained solvation dynamics in these systems are linked to properties or functions of these biological-relevant macromolecules. 

Ethers solvate cations. An ionic substance such as lithium iodide (Lil) is moderately soluble in ethers because the small lithium cation is strongly solvated by the ether s lone pairs of electrons. Unlike alcohols, ethers cannot serve as hydrogen bond donors, so they do not solvate anions well. 

Crown Ether Complexes In Chapter 6, we encountered the use of crown ethers, large cyclic polyethers that specifically solvate metal cations by complexing the metal in the center of the ring. Different crown ethers solvate different cations, depending on the relative sizes of the crown ether and the cation and the number of binding sites around the cation. The EPM of 18-crown-6 shows that the cavity in the center of the molecule is surrounded by electron-rich oxygen atoms that complex with the guest potassium cation. 

Among unsolvated organolithium compounds only the alkyllithiums are soluble in noncoordinating solvents such as alkanes and arenes. Their states of aggregation depend on the structure close to lithium. Thus primary, tertiary and secondary alkyllithiums, all unsolvated, assemble into respectively hexamers, tetramers and equilibrium mixtures of hexamers and tetramers. Most organolithium compounds dissolve in and coordinate with donor compounds such as ethers and tertiary amines. The actual structures depend critically on the nature of the donor. Thus, diethyl ether solvates tend to be mainly cubic tetramers (with some dimers) while THF favors mixtures of monomers and dimers. Tertiary vicinal diamines such as TMEDA and 1,2-di-Af-piperidinoethane, DPE, favor bidentated coordinated dimers. Finally, in the presence of triamines such as pentamethyl-triethylenediamine PMDTA and l,4,7-trimethyl-l,4,7-triazacyclononane TMTAN, many organolithium compounds form tridentately complexed monomers. 

As noted, the sample of neopentyllithium in diethyl ether-dio, described above, contained neopentyllithium dimer solvated by diethyl ether-dio, 0.125 M, in addition to the 14 PMDTA monomer 0.34 M. Averaging of the 6Li NMR for these two species indicated a fast mutual exchange of lithiums between PMDTA coordinated monomer and ether solvated dimer. NMR line shape analysis of the 6Li resonance gave AH = 12 kcalmol-1 and AS = +10 eu for this exchange process. It is interesting that at 230 K the pseudo-first-order rate constants for inversion in 14-PMDTA and exchange between the latter monomer and dimeric etherate are, respectively, 5.06 s 1 and 2.57 s-1. This implies that the two processes may be mechanistically linked and that nitrogen inversion in 14 PMDTA alone must be a much slower process. 

Cp I and Lil in ether. The corresponding chloro derivative was prepared by reacting YbCl3 with Cp Li. This method has been used to prepare a series of chloro and iodo complexes of different lanthanides as ether solvates. 

Abbotto, Streitwieser and Schleyer performed an exhaustive study, using the B3LYP /6-31-bG //PM3 calculation level, on the effect of dimethyl ether solvation on aggregated forms of the lithium enolate of acetaldehyde (CH2=CH0Li) (Me20) c. 

Three-membered ring carbenoids have been structurally characterized for species where X = OR or NR2. The crystal structure of the diethyl ether-solvated a-(dimethylamino)-benzyllithium dimer 2 shows crystallographically equivalent three-membered Li-C-N rings, with (Lij-C) = 2.475(6) A, and . As predicted by theory, the anionic carbon sits closer to the lithium in the other three-membered ring (,7(LiiA-C) = 2.230(7) A) than to Lij. This seems to present a means for the anionic carbon to delocalize its charge. 

In contrast, diisopropyl ether-solvated 2-lithio-3-bromobenzofuran dimer 4 contains two crystallographically identical Li-C-O cycles, a difference ascribed to monodentate ether versus bidentate TMEDA in 3. Again the anionic carbon lies ca. 0.3 A closer to the other lithium LijA than to Lij. Bond distance t/(Li-0) = 1.954(6) A is much smaller than that for unsubstituted 3, while d(C-0) = 1.470(4) A is almost 0.1 A longer than typical for benzofuran C-O bonds. 

The basicity of the solvent influences the course of the reactions of dialkylmagnes-ium solvents more basic than diethyl ether solvate the reagent strongly... 

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