Lewis acid mechanism

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... 

The regio- and enantioselective pentadienylation of aldehydes is also catalyzed by the BINAP/silver complex.312 The reaction of dienylstannanes 351 with various aldehydes takes place in the 7-position exclusively to afford the products 352 with good to high enantioselectivities (Scheme 100). The reaction was proposed to occur either via a Lewis acid mechanism (transition state 353) or by transmetallation (transition state 354). 

Diazo compounds react with aldehydes and ketones to give homologated carbonyl compounds and epoxides. Lewis acids accelerate these processes, and in some cases direct the reactions to single products. frara-Epoxides result from aldehydes and ethyldiazoacetates in the presence of MeReOs (although this can occur via a metal carbene pathway rather than a Lewis acid mechanism) [139], whereas... 

Chiral tridentate chromium(III) complexes catalyze highly enantio- and diastereo-selective hetero-Diels-Alder reaction between various aldehydes and dienes with one oxygen substituent (Sch. 60) [206]. Complex formation between catalysts and aldehydes could not be detected by IR and the successful use of acetone as a solvent casts doubt on a simple Lewis acid mechanism. 

The Lewis acid mechanism of catalysis is shown in Figure 6.2.8. In this mechanism, the tin compound functions as a classic Lewis acid, coordinating to the isocyanate either through the carbonyl oxygen or nitrogen in the initial step to polarize the carbonyl. This increases the electrophilicity of the isocyanate carbon toward nucleophilic attack by alcohol. It has been shown by cryoscopy that there is a strong association of isocyanate with di-n-butyltin dicarboxylates. ... 

Figure 6.2.8 Lewis acid mechanism for tin catalyzed urethane formation...

Key Words Ethylene oxide, Propylene oxide. Epoxybutene, Market, Isoamylene oxide. Cyclohexene oxide. Styrene oxide, Norbornene oxide. Epichlorohydrin, Epoxy resins, Carbamazepine, Terpenes, Limonene, a-Pinene, Fatty acid epoxides, Allyl epoxides, Sharpless epoxidation. Turnover frequency, Space time yield. Hydrogen peroxide, Polyoxometallates, Phase-transfer reagents, Methyltrioxorhenium (MTO), Fluorinated acetone, Alkylmetaborate esters. Alumina, Iminium salts, Porphyrins, Jacobsen-Katsuki oxidation, Salen, Peroxoacetic acid, P450 BM-3, Escherichia coli, lodosylbenzene, Oxometallacycle, DFT, Lewis acid mechanism, Metalladioxolane, Mimoun complex, Sheldon complex, Michaelis-Menten, Schiff bases. Redox mechanism. Oxygen-rebound mechanism, Spiro structure. 2008 Elsevier B.V. 

HOMOGENEOUS EPOXIDATION BY EARLY TRANSITION METALS (LEWIS ACID MECHANISM)... 

The group of Busch also presented evidence that the Mn(n) complex of a cross-bridged cyclam ligand, 4,ll-dimethyl-l,4,8,ll-tetraazabicyclo[6.6.2]hexadecanc, denoted as Mn"(Me2EBC)CL, forms a Mn(IV) adduct with lodosylbenzene which is a new active intermediate in epoxidation reactions [600,609,610]. These examples with Mn, and in the previous section with Fe, are instances of late transition metals catalyzing epoxidation reactions by both the redox and the Lewis acid mechanisms (see Chapter 3). 

Mid- to Late Transition Metal Catalysts also Perform Epoxidation Reactions by the Lewis Acid Mechanism... 

The Lewis acid mechanism occurs with early transition metals (e.g.. Mo, V, Re) which generally utilize organic peroxides as oxidizing agents. The mechanism involves the transfer of the oxygen atom next to metal center, the proximal oxygen, in a concerted step that leaves the oxidation state of the metal unchanged. 

Most of the catalytic methods reported so far are the chiral metal compound-catalyzed allylation reactions using allylic silanes or allylic stannanes. Two types of catalytic mechanism, Lewis acid mechanism and transmetallation mechanism, can be considered for these reactions as illustrated in Scheme 2. If the chiral metal catalyst acts as a Lewis acid in the asymmetric allylation, an al-... 

Cationic Ni(II)-pincer complexes have also been exploited for the hydroamination of acrylonitrile with anihne [258]. Stoichiometric reactions in this case suggest that a Lewis acid mechanism promoting the anti-Markovnikov hydroamination is active (Figure 15.7) [258]. 

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