Lewis complexed

Probably the solubility of C02 and CS2 can be explained by the formation of a Lewis complex with pyridine, so that the titration equation must be written as an exchange reaction with OH-. 

A species which accepts protons in a reaction (Lowry-Bronsted) or donates electrons (Lewis). Complex... 

Purine nucleosides of type 1.4 (1177) were prepared by the reductive cleavage at the anomeric position of the ribofuranosyl moiety of 1176 with diisobutylaluminium hydride (DIBALH). The reductive ring opening was explained by the initial formation of a Lewis complex (93TL4835). 

Many catalysts do not use metals in their pure reduced metallic forms. Anchored organometallic complexes are often analogs of homogenous catalysts fixed on a solid support. In particular, titanate complexes both in solution and in supported form have been found to be especially active in transesterifications of simple esters.It was proposed that titanates catalyze the transesterification reaction through a Lewis acid mechanism where the reactant ester and metal form a Lewis complex activating the carbonyl groups for a nucleophilic attack by the reactant alcohol. The tetrahedral intermediate that is formed breaks down into the product alcohol and an ester-metal Lewis... 

X-ray crystallography revealed that the arsolidinium cation (50) exists as a dimeric dication. The dimmer is formed via intermolecular As- -S interactions (2.422(6) A). Addition of another equivalent of [(CH2CH2S2)AsCl] to (50) affords a bicyclic monocation, which can be viewed as the Lewis complex (52) of the corresponding chloroarsolidine [(CH2CH2S2)AsC1], and the arsolidine cation (50). ... 

With silyl-substituted oxiranes, dibal-H favors the primary alcohol and Bu 3A1H favors the secondary alcohol. These observations have been interpreted in terms of the timing of the hydride transfer to one of the oxirane carbons. dibal-H, which exists as a Lewis complex in donor media (R3N-A1H(Bu )2, or R20-A1H(Bu )2) acts as a nucleophilic hydride source, which preferentially attacks the least-hindered carbon. With Bu 3A1, complexation with the oxirane oxygen precedes isobutene elimination and the generation of the Al—H bond. A considerable carbocation character is acquired in the transition state, hence formation of the primary alcohol is favored. It is worthy of note that trialkylstannyl-substituted oxiranes are reduced with Red-Al invariably at the oxirane... 

The formation of such Lewis complexes may find increasing application in stereochemical studies. Two cases in point might be cited. First, etherate formation allows the isolation of the optically active tri-2-methylbutyl-aluminum etherate (97). This compound is stable to racemization up to 100°. Second, diphenylpalladium (II) has recently been prepared as its bis-triethylphosphine complex in both a cis- and a irans-planar complex form (20). The latter seems to represent the first example of geometrical isomerism among organometallic compounds (Section II.D). 

A final point concerning Lewis complex possibilities should be mentioned. The adducts formed between Group III organometallics and alkali metals are somewhat remindful of Lewis salt formation except only one electron is involved (compare oxidation of R—M, Section IV.C.l) ... 

Free boroles are highly reactive four t-electron antiaromatic systems, and only a few sterically protected derivatives have been isolated up to 1995. The only known isolable monomeric boroles without annulated aromatic groups are derivatives of 1,2,3,4,5-pentaphenylborole (1), first described in 1961. The antiaromaticity is evident by the rapidity of reactions that remove the empty 2p -orbital on boron from conjugation (Lewis complexation) or disrupt the borole ring system altogether (e.g., Diels-Alder addition). An earlier review in the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) has only dealt with the borole ring <84CHEC-I(1)629>. 

The empty 2/j,-orbital on boron interacting with the ji-electrons of the butadienylidene system in borole (5) is proved by the pronounced bathochromic shift in the visible spectrum compared with that of its pyridine adduct. This Ti-electron delocalization in boroles is destabilizing and produces a 471-electron antiaromatic system. The antiaromaticity is further evidenced by the rapidity of reactions that remove the 2p ,-orbital on boron from conjugation. These reactions include Lewis complexation, transition metal complexation, or borole ring opening reactions such as Diels-Alder addition, oxidation, and protodeboration. This removal of the antiaromaticity from borole results in transforming the compound to a typical, low reacting, sterically hindered arylborane. 

MnAPO-44 in clusters (Mn—0—P—0-Mn) IR Adsorption of CD3CN results in strong Lewis complex with Mn + (143)... 

Among nonmetal chiral complexes, chiral oxazaborolidines have been found effective in many D-A reactions. The adduct obtained from the D-A reactions of 5-benzyloxymethyl-l,3-cyclopentadiene 98a with a-bromoacrolein in the presence of catalyst 98 (S-tryptophan-derived oxazaboroUdine) is an important intermediate in the synthesis of prostaglandins. The aldehyde group of the dienophile is bound to the catalyst by coordination with boron by Lewis interaction and the Lewis complex is stabilized by H-bonding. The upper face of the aldehyde is shielded by indole moiety of the catalyst. The benzyloxymethyl substituent of the cyclopentadiene produces a steric differentiation on the two faces of cyclopentadiene ring resulting the approach of the diene preferably from one face. 

The reaction is believed to proceed through a para-protonated Lewis complex. The presence of excess AlBrs prevents the cyclo-reversion of the product. 

The formation of surface Lewis complexes through a plain cr-coordination being fully reversible upon evacuation of pco, this process entirely fulfilled the requirements imposed by the method. Equation 1.26 was employed ... 

---