Lewis acid-base system

Drago and co-workers have correlated a large body of enthalpies of adduct formation in Lewis acid-base systems, including some solvents as reactants, with this four-parameter equation ... 

Lewis acid-base systems lone-pair interaction in, 16 100 PES of, 16 94... 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

SF4 forms adducts with main group inorganic fluorides, and these have been variously described as simple Lewis acid-base systems (with SF4 behaving as the base) or as ionic systems such as [Sp3]+ [BF4] . The reactions of SF4 with organic molecules have been widely studied. The most important reaction is the conversion of a carbonyl gronp to a diflnoride (eqnation 51). 

Acetals are also isomerized by a similar Lewis acid-base system [63]. Interestingly, according to Rychnovsky s work, selective protection of secondary alcohols is feasible by isomerization of asymmetric cyclic acetal 46 then cyclopropanation (47) (Sch. 36) [64]. 

Just a few reviews on this quickly developping field a) D. H. Pauli, C. J. Abraham, M. T. Scerba, E. Alden-Danforth, T. Lectka, Bifunctional asymmetric catalysis cooperative lewis acid/base systems, Acc. Chem. Res., 2008,41, 655-663 b) M. Kanai, N. Katob, E. Ichikawab, M. Shibasaki, Power of cooperativity Lewis acid-Lewis base bifimctional asymmetric catalysis, Synlett, 2005, 1491-1508, c) M. Shibasaki, M. Kanai K. Funabashi, Recent progress in asymmetric two-center catalysis, Chem. Commun., 2002, 1989-1999. 

Use as a Dual Catalyst in Addition of Alkynylzinc Species to Aldehydes. A dual Lewis acid/base system was reported involving NMI, Ti(OiPr)4, and (/f)-BINOL in the enantioselective alkynylation of aldehydes with diethylzinc at room temperature. Other methods to access these products have been hindered by the need for higher temperatures to generate the alkynylzinc species. A variety of propargy lie alcohols could be prepared from aromatic aldehydes in high yields and with excellent enantioselectivities (eq42). ... 

Pauli DH, Abraham CJ, Scerba MT, Alden-Danfrath E, Lectka T (2008) Bifunctional asymmetric catalysis cooperative Lewis acid/base systems. Acc Chem Res 41(5) 655-663. doi 10.1021/ ar700261a... 

Kroeger, M.K. and Drago, R.S. (1981) Quantitative prediction and analysis of enthalpies for the interaction of gas-phase ion-ion, gas-phase ion-molecule, and molecule-molecule Lewis acid-base system. J. Am. Chem. Soc., 103, 3250-3262. 

Bases in combination with Lewis acids are powerful in promoting the HWE reaction with many substrates that are incompatible with strong bases. In 1984, Masamune and Roush et al. reported mild conditions using lithium chloride and DBU (or i-Pr2NEt) [172], and later Rathke et al. extended this Lewis acid/base system to lithium or magnesium halides with triethylamine [173]. Recently, Helquist et al. have employed Zn(OTf)2, TMEDA, and DBU to promote the HWE reaction of aldehydes with diethylphosphonoacetic acid to give a,p-unsaturated carboxylic acids with excellent selectivity [174]. 

CHART 5 Perfluoroarylated Lewis acid based systems that initiate polymerization protonically. 

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Moreover, these experiments reveal some unique properties of the chlorostan-nate ionic liquids. In contrast to other known ionic liquids, the chlorostannate system combine a certain Lewis acidity with high compatibility to functional groups. The first resulted, in the hydroformylation of 1-octene, in the activation of (PPli3)2PtCl2 by a Lewis acid-base reaction with the acidic ionic liquid medium. The high compatibility to functional groups was demonstrated by the catalytic reaction in the presence of CO and hydroformylation products. 

The literature3 contains some limited work on decaborane-based polymer systems. Typically, most utilize the Lewis acid/base reaction between decaborane (Lewis acid, B10H12) and amines and phosphines (Lewis bases, L) resulting in the formation of complexes (see scheme 2) having the general formulas B10H12L2. For example, the... 

Two such well studied systems are pyridine chemisorbed on alumina (15) and pyridine chemisorbed on silica-alumina (16). It had been previously shown that alumina contains only sites which adsorb pyridine in a Lewis acid-base fashion whereas silica-alumina has both Lewis and Bronsted acid sites. These two different kinds of sites are distinguishable by the characteristic vibrational bands of pyridine adducts at these sites (see Table I). Photoacoustic and transmission results are compared in Table II. Note that the PA signal strength depends on factors such as sample particle size and volumes of solid sample and transducing... 

The important role of thermodynamics in complex formation, ionic medium effects, hydration, solvation, Lewis acid-base interactions, and chelation has been presented in this chapter. Knowledge of these factors are of great value in understanding solvent extraction and designing new and better extraction systems. 

These qualitative explanations, whether they be hard-soft or ionic-covalent or Class A-Class B, all suffer from the arbitrary way in which they can be employed. All Lewis acid-base type interactions are composed of some electrostatic and some covalent properties, i.e., hardness and softness are not mutually exclusive properties. Predictions are straightforward when dealing with the extremes, but with other more ambiguous systems, one can be very arbitrary in explaining results and the predictive value is impaired. What is needed is a quantitative assessment of the essential factors which can contribute to donor strength and acceptor strength. Proper combination of these parameters should produce the enthalpy of adduct formation. Until this can be accomplished, one could even question the often made assumption that the strength of the donor-acceptor interaction is a function of the individual properties of a donor or acceptor. 

At present, the correlation contains one transition metal complex, Cu(Hfacac)2. The results on this complex are very interesting and somewhat unusual for a transition metal system in that enthalpies have been obtained in a poorly solvating solvent with nonionic donors (52), instead of the t5 ical stability constant study on a metal cation in some highly polar solvent. Data from this latter type of investigation have many practical uses, but are impossible to interpret and understand. The transition metal ion complex we have studied can be incorporated into the E and C scheme using the same base parameters that are used to correlate the enthalpies of formation of all the other Lewis acid-base adducts in the scheme. 

It was also of interest to generate the related symmetric primary benzylic trication. However, the ionization of the 2,4,6-tris(chloromethyl)mesitylene (112) in excess SbF5/S02ClF at -78°C gave apparently only the dienylic allylic dication Lewis acid-base complex (113). The unionized chloromethyl carbon displayed a relatively deshielded NMR absorption, 35.3, indicative of a weak Lewis acid-base interaction. The terminal methylene carbons of the dienylic system showed a 8 C of 197.7 and the terminal carbons of the allylic system displayed a 8 C of 194.8, quite similar to that of the previously described dienylic allylic dications, 107 and 108. 

The participation of the germanium dimers in nucleophilic/electrophilic or Lewis acid/base reactions has been the subject of several investigations on the Ge(100)-2x1 surface [16,49,255,288,294,313-318]. As for the case of silicon, adsorption of amines has provided an excellent system for probing such reactions. Amines contain nitrogen lone pair electrons that can interact with the electrophilic down atom of a tilted Ge dimer to form a dative bond via a Lewis acid/base interaction (illustrated for trimethylamine at the Si(100)-2 x 1 surface in Ligure 5.17). In the dative bond, the lone pair electrons on nitrogen donate charge to the Ge down atom [49]. 

The hypothesized delocalization of lone pair electrons in the above silicon compounds is supported by the lowered basicity of the silyl compounds as compared to the corresponding carbon compounds. This reduced basicity is contrary to that expected on the basis of electronegativity effects operating through the a system since silicon is less electronegative than carbon. It is consistent with an internal Lewis acid-base interaction between the nitrogen and oxygen lone pairs and empty acceptor d orbitals on the silicon. Experimentally this reduced basicity is shown by the absence of disiioxane adducts with BF3 and BO ... 

In 1997, the first truly catalytic enantioselective Mannich reactions of imines with silicon enolates using a novel zirconium catalyst was reported [9, 10]. To solve the above problems, various metal salts were first screened in achiral reactions of imines with silylated nucleophiles, and then, a chiral Lewis acid based on Zr(IV) was designed. On the other hand, as for the problem of the conformation of the imine-Lewis acid complex, utilization of a bidentate chelation was planned imines prepared from 2-aminophenol were used [(Eq. (1)]. This moiety was readily removed after reactions under oxidative conditions. Imines derived from heterocyclic aldehydes worked well in this reaction, and good to high yields and enantiomeric excesses were attained. As for aliphatic aldehydes, similarly high levels of enantiomeric excesses were also obtained by using the imines prepared from the aldehydes and 2-amino-3-methylphenol. The present Mannich reactions were applied to the synthesis of chiral (3-amino alcohols from a-alkoxy enolates and imines [11], and anti-cc-methyl-p-amino acid derivatives from propionate enolates and imines [12] via diastereo- and enantioselective processes [(Eq. (2)]. Moreover, this catalyst system can be utilized in Mannich reactions using hydrazone derivatives [13] [(Eq. (3)] as well as the aza-Diels-Alder reaction [14-16], Strecker reaction [17-19], allylation of imines [20], etc. 

Homopolymerization, epoxides aluminate-Lewis acid catalyst system, 11, 602 via aluminum-porphyrin-Lewis acid catalysts, 11, 599 aluminum-tetradentate ligand catalyst system, 11, 601 anionic polymerization, 11, 598 cationic aluminum catalyst system, 11, 603 cationic polymerization, 11, 598 characteristics, 11, 597 zinc-based catalyst system, 11, 605 Homopolymers, cyclic olefins, 11, 716... 

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