Solvent effects ether formation

What effect does the solvent have on the structure, charges and reactivity of Grignards Compare geometries, atomic charges and electrostatic potential maps of the diethyl ether complex to that of methylmagnesium chloride itself. How does solvent-magnesium bond formation affect the reactivity of the methyl group Explain. 

Recently, use of LiCl/DMAc and LiCl/l,3-dimethyl-2-imidazolidinone as solvent systems for acetylation of cellulose by acetic anhydride/pyridine has been compared. A DS of 1.4 was obtained the substituent distribution in the products synthesized in both solvents was found to be the same, with reactivity order Ce > C2 > C3. Therefore, the latter solvent system does not appear to be better than the much less expensive LiCl/DMAc, at least for this reaction. It appears, however, to be especially efficient for etherification reactions [178]. It is possible, however, that the effect of cellulose aggregation is more important for its reaction with the (less reactive) halides than with acid anhydrides this being the reason for the better performance of the latter solvent system in ether formation, since it is more efficient in cellulose dissolution. 

For complex III, the Na+ Is probably as accessible to solva-tlon by solvent molecules as is the Na In the tight Fl-,Na+ Ion pair. Hence, no externally bound solvent molecules need to be removed. This may be different In other systems. For example, the formation constant of a loose Ion pair complex between FI", Na+ and tetraglyme (tetraethylene glycol dimethyl ether) Is nearly four times lower In dloxane than In THF (10). This may be caused by specific solvent effects rather than by the difference In solvent dielectric constant. The flexible glyme ligand wraps Itself around the Na+ Ion, and this may make It more difficult for solvent molecules to remain bound to Na+ In the glyme-separated Ion pair. 

Table 2 shows the maxima of the K-bands of phenolic substances and their methyl ethers in hexane (or benzene) and in ethanol, as well as the displacements of the K-bands of the phenols as compared with the corresponding methyl ethers (D, Me II) in both solvents and the displacements attributable to hydrogen bond formation (D, H-bond) obtained by subtracting the solvent effect observed for the methyl etier from that observed for the corresponding phenol. 

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. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Furthermore, such a C3 -> C4 ring expansion could even be induced by lithium chloride. Thus, the cyclopropylcarbinol 228, prepared by addition of acetylenic Grignard reagents to the cyclopropanecarboxaldehyde 171a in 80-90% yield1101, was transformed into the tosylate 229 upon successive treatment with one equivalent of methyllithium in ether at 0 °C and with one equivalent of tosyl chloride at —40 °C, lithium chloride being formed as by-product. The formation of tosylate 229 appeared, however, to be strongly dependent upon the nature of the solvent effectively, the same... 

Coupling attempts conducted with (R,S)-214 led to lower enantioselection upon C-C bond formation, an observation that points to the significant role played by the relative configuration (R,R) of the binaphthyl and ethylenediamine units in promoting asymmetric induction. This complex was found to be the most efficient among several different structural variations. Solvent effects on this transformation were also studied (Table 35), with toluene and chlorobenzene giving the best results. Low solubility of the catalyst (diethyl ether and diiso-... 

Macroscopic solvent effects can be described by the dielectric constant of a medium, whereas the effects of polarization, induced dipoles, and specific solvation are examples of microscopic solvent effects. Carbenium ions are very strong electrophiles that interact reversibly with several components of the reaction mixture in addition to undergoing initiation, propagation, transfer, and termination. These interactions may be relatively weak as in dispersive interactions, which last less than it takes for a bond vibration (<10 14 sec), and are thus considered to involve "sticky collisions. Stronger interactions lead to long-lived intermediates and/or complex formation, often with a change of hybridization. For example, onium ions are formed with -donors. Even stable trityl ions react very rapidly with amines to form ammonium ions [41], and with water, alcohol, ethers, and esters to form oxonium ions. Onium ion formation is reversible, with the equilibrium constant depending on the nucleophile, cation, solvent, and temperature (cf., Section IV.C.3). 

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