Solvents, Lewis basic

As an aside, benzotrifluoride is a slightly polar, non-Lewis basic solvent that has favorable properties for many kinds of organic reactions. A. Ogawa, D. P. Curran, Benzotrifluoride A useful alternative solvent for organic reactions currently conducted in dichloro-methane and related solvents , J. Org Chem. 1997, 62, 450. 

An investigation of keto-enol tautomerism for perfluorinated keto-enol systems was undertaken. N-methylpyrrolidone (NMP) catalyzes equilibration of the keto and enol forms, but if used in more than trace amounts, it drives the equilibrium strongly toward enol because of hydrogen bonding to the amide. The enol is much more thermodynamically stable than its ketone, and it was found that in mildly Lewis basic solvents, such as ether, THE, acetonitrile, and NMP, the enohzation equilibrium lies too far right to allow detection of ketone (Correa et al., 1994). 

A mixed metal siloxide complex YCu(OSiPh3)4(PMe2Ph) was obtained by a simple addition reaction (Eq. 3) [48], The geometry around the yttrium atom is distorted tetrahedral. The molecule is cleaved in solution by Lewis basic solvents like THF. 

Fig. 10.2. The structures of alkyllithium compounds in solution I Deaggregation of the alkyllithium structures, as preferred in the solid state, usually occurs when Lewis bases are added or alkyllithium compounds are dissolved in Lewis-basic solvents. By the way, this process is accompanied by a decrease of entropy, i.e., occuring more at low than at higher temperatures.

Neutral nickel bis(dithiolene) complexes undergo ligand substitution reactions with a variety of amines to form mixed-ligand dithiolene complexes (52, 60). Dance and Miller (60) pointed out that reduction of electron-poor bis(dithiolene) complexes ( i/2 > 0 V vs. SCE) by halides or pseudo-halides, unhindered amines, or common weakly Lewis basic solvents (such as acetonitrile)... 

Enantioselective Aziridination of Alkenes. Copper complexes with neutral methylenebis(oxazoline) ligands (1) and (2) have also been employed as enantioselective catalysts for the reaction of alkenes with (Al-tosylimino)phenyliodinane, leading to A-tosylaziridines. The best results have been reported for cinna-mate esters as substrates, using 5 mol % of catalyst prepared from CuOTf and the phenyl-substituted ligand (2) (eq 6). The highest enantiomeric excesses are obtained in benzene, whereas in more polar and Lewis basic solvents, such as acetonitrile, the selectiv-ities are markedly lower. The chemical yield can be substantially improved by addition of 4X molecular sieves. Both Cu - and Cu"-bisoxazoline complexes, prepared from Cu or Cu triflate, respectively, are active catalysts, giving similar results. In contrast to the Cu-catalyzed cyclopropanation reactions discussed above, in which only Cu complexes are catalytically active, here Cu complexes are postulated as the actual catalysts. ... 

Oxazaborolidine catalyzed reductions are generally performed in an aprotic solvent, such as dichloromethane, THF, or toluene. When the reactions are run in a Lewis basic solvent, such as THF, the solvent competes with the oxazaborolidine to complex with the borane, which can have an effect on the enantioselectivity and/or rate of the reaction. The solubility of the oxazaborolidine-borane complex can be the limiting factor for reactions run in toluene, although this problem has been circumvented by using oxazaborolidines with more lipophilic... 

Complex 446 is a doubly chloro-bridged dimer in the solid state but is essentially monomeric in dichloromethane (or chloroform) solution (Scheme 101). Thus, coordination of an additional Lewis-basic solvent (S) molecule such as acetone, THF, and acetonitrile is feasible in a position tram to the Cp ligand to form a solvate, but no such stable solvates could be isolated. Methanol or water (X) displaces the ether side chain to form the bis(solvate) adducts however, the chelate is re-formed on removal of the solvent under vacuum to give stable methanol or water monoadducts. The THF-solvate of the water adduct has been structurally characterized. 

Many organolithium compounds are soluble in hydrocarbons exceptions are methyllithium and phenyllithium w hich are associated in these solvents. Butyllithium is mostly hexameric and /c/Y-butyllithium is tetrameric in cyclohexane. A Lewis basic solvent can interact with an organolithium oligomer, thereby decreasing the degree of association. Thus, methyllithium. which is tetrameric in the solid phase, becomes a solvated tetramer in ether, and BuLi. hexameric in hydrocarbons, becomes tetrameric in ether. In the more basic THE BuLi has a degree of association between dimeric and tetrameric at -108 °C, and phenyllithium is between monomeric and dimeric [3]. 

Arene tricarbonyl molybdenum complexes are yellow, often crystalhne compounds. They are weakly air-sensitive in the sohd state and have to be stored under inert atmosphere and out of hght. They are best purified by crystaUization. In solution, they are unstable to air. The trait that has most hampered development of the use of (arene)Mo(CO)3 complexes in organic synthesis, however, is the lability of the arene metal bond. Lewis basic solvents such as THF, DMF, DM-SO, acetone and acetonitrile rapidly displace benzene in (benzene)Mo(CO)3. This lability of the arene-Mo bond, while making handling difficult, holds promise for the catalytic use of this class of compounds. 

In most Lewis basic solvents tiriodid ion prevails. Triiodide ion is easily formed from 1 and I2, and the higher the basicity of the solvent, the easier the triiodide ion is determined by UV-spectra. However, in DMF and PC triiodide ion is unstable. 

Finally, the di-tert-butyl or DBP derivatives are all found to be monomeric, adopting 3-5 coordination environments. These derivatives are the few that can be isolated witiiout solvents however, tiiey can be easily solvated by simple dissolution in a Lewis basic solvent. The monomeric nature is not solely due to the 2,6-r-butyl substituents, since the mesityloxide (OMes) species have been found to form both monomeric and dinuclear complexes. Figure 6 shows... 

The formulas RLi and RMgX oversimplify the true structures of these reagents. Thus, as written, the metal ions are highly electron deficient. To make up the desired electron octet, they function as Lewis acids (Section 2-3) and attach themselves to the Lewis basic solvent molecules. For example, alkylmagnesium halides are stabilized by bonding to two ether molecules. The solvent is said to be coordinated to the metal. This coordination is rarely shown in equations, but it is ancial for the formation of the Grignard species. 

Webb JA, Klijn JE, Hill PA et al (2004) Experimental studies of the C NMR of iodoalkynes in Lewis-basic solvents. J Org Chem 69 660-664... 

This trend persists in solvents (e.g., toluene, dioxane, and dichloromethane) where macrocyclization can take place, albeit with lower yields of 18a and higher yields of 16a (entries 7-9). This AlMes-mediated cyclohexamerization reaction likely proceeds via an intermediate aluminum amide formed by reaction of AlMes with RNH2 with loss of methane, followed by coordination of the A1 center to the carbonyl to activate the ester and deliver the amide nucleophile to form amide bonds. In light of this mechanism, such reactions are expected to be prone to inhibition by Lewis basic solvents and additives. The use of Lewis basic solvents such as DMF, DMSO, CH3CN, acetone, and ethyl acetate indeed completely halts the macrocyclization reaction, resulting in no generation of 16a and 18a. Similarly, in the presence of Lewis basic additives such as HMPA, TMEDA, and PMDTA, circular products 16a and 18a remain undetectable either. 

---