Lewis bivalent

First, the use of water limits the choice of Lewis-acid catalysts. The most active Lewis acids such as BFj, TiQ4 and AlClj react violently with water and cannot be used However, bivalent transition metal ions and trivalent lanthanide ions have proven to be active catalysts in aqueous solution for other organic reactions and are anticipated to be good candidates for the catalysis of aqueous Diels-Alder reactions. 

DeFrees, S., Kosch, W., Way, W., Paulson, J., Sabesan, S., Halcomb, R., etal. (1995) Ligand recognition by E-selectin Synthesis, inhibitory activity and conformational analysis of bivalent sialyl Lewis x analogs. J. Am. Chem. Soc., 117, 66-79. 

Similarly, relevant electrophiles (Lewis acids) including A-type metal cations (hard), bivalent transition metal ions (borderline), and B-type metal ions (soft) can be categorized (see Stumm and Morgan 1996). Note that in organic molecules, the atom where a nucleophile attacks (i.e., the electrophilic site) may possess harder (e.g., C=0, P=0) or softer (e.g., CH3-X) character. 

Figure 4.2. Mechanism proposed for vegetable oils alcoholysis promoted by Lewis acid based on tin and others bivalent pirone complexes.

Bivalent tin, in its compounds, is rarely found with a spherical symmetry. This is a consequence, in the condensed phases, of a ready distortion of the nonbonding pair of electrons (which are 5s in the free gaseous ion) due, for example, to the adoption of some p character. The lone pair therefore frequently has stereochemical activity. In addition, the nonbonding pair of electrons in tin(II) species can be used in coordination with acceptors or Lewis acids furthermore, the empty 5p or d orbitals can be used to interact with donors or Lewis bases. 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

Hatheway, C.H., Snyder, J.D., Seals, J.E., Edell, T.A., Lewis, G.E. (1984). Antitoxin levels in botulism patients treated with bivalent equine botulism antitoxin to toxin types A, B, and E. J. Infect. Dis. 150 407-12. 

The stereochemical complexity of the reaction can be further increased when an ( )- or (Z)-2-butenylsilane reacts with a chiral aldehyde. Herein both diastereo-selection processes are operative, relative (between the reacting faces) and internal with respect to the original stereogenic center in the aldehyde. Thus, the reaction of jff-benzyloxy aldehyde 32 and silane ( )-31 with bivalent Lewis acids (SnCU, TiCU) was examined in the presence of an additive, e.g. MgBr2, ZrCp2Cl2, TiCp2Cl2 (Scheme 10-12) [32]. The reactions all afford mixtures of the four possible diastereomeric products, favoring the syn homoallylic alcohol. When the com-... 

The reaction of //-methyl-2-butenylsilanes 36 and stannanes with chiral a-al-koxyaldehydes has also been reported [33]. Surprisingly, the anti homoallylic alcohols were predominantly observed (94/6, ant i/syn) when a bivalent Lewis acid such as SnCl4 was used (Scheme 10-13). A synclinal transition structure is proposed to account for the observed selectivity. In the chelation-controlled reactions the synclinal transition structure is favored over the corresponding antiperiplanar transition structure because there exists an open space wherein the complexed Lewis acid can reside. The monovalent Lewis acid BF3-OEt2 provides the expected syn homoallylic alcohol, presumably through the antiperiplanar transition structure shown (66% of the product was the syn alcohol 37). 

The Lewis acid-promoted reaction of chiral y -methyl ( )-2-butenylsilane (5)-43 with a-(benzyloxy)propanal affords homoallylic alcohols favoring the anti or syn diastereomers depending on the Lewis acid employed (Scheme 10-17) [36]. The BF3 0Et2-promoted reaction provided a 6.5/1 (44/45) ratio of the syn to anti diastereomers presumably through the antiperiplanar transition structure shown below. When MgBr2 was employed, the anti homoallylic alcohol was favored by a 12/1 (45/44) ratio. A synclinal transition structure accounts for the reversal in selectivity observed with the bivalent Lewis acid. 

The BF3-OEt2-promoted reaction of the enantiomerically enriched MOM-pro-tected alkoxystannane (S)-150 with the aldehyde (5)-26 (mismatched series) affords a mixture of syn and anti homoallylic alcohols 152 and 153 (Scheme 10-65) [103]. The matched series gives much higher selectivity for the syn homoallylic alcohol. Reaction of the TBS-protected alkoxystannane (Sl-lSl with the aldehyde (S)-26 provided the syn homoallylic alcohol 154 and a cyclopropane derivative (not shown). When the bivalent Lewis acid MgBr2 was employed, the stereochemical outcome of the reaction was significantly altered. The anti homoallylic alcohol 156 was favored with the MOM-protected alkoxystannane, while the syn homoallylic alcohol 159 was the only product observed with the TBS-protected alkoxystannane. Antiperiplanar transition structures are proposed to account for the observed selectivity in these reactions. 

Timpe classified the onium salts as bivalent initiators , that is, capable of initiating both radical and cationic cures simultaneously, hence capable of so-called dual cures of mixtures of radical and cationically polymerizable monomers [18]. This theme has been developed in the more recent review by Rabek [19]. More recently, use of photogenerated Bronsted or Lewis acids to effect decomposition of acid-labile polymers has found utility in photoimaging and been reviewed [20],... 

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