Lewis bulky

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

The effect of ligands on the endo-exo selectivity of Lewis-acid catalysed Diels-Alder reactions has received little attention. Interestingly, Yamamoto et al." reported an aluminium catalyst that produces mainly exo Diels-Alder adduct. The endo-approach of the diene, which is normally preferred, is blocked by a bulky group in the ligand. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The inverse electron-demand Diels-Alder reaction is also accelerated by Lewis acids, but the successful application of chiral Lewis acids to this kind of Diels-Alder reaction is very rare. Marko and coworkers applied Kobayashi s catalyst system (Yb(OTf)3-BINOL-amine) to the Diels-Alder reaction of 3-methoxycarbonyl-2-py-rone with vinyl ether or sulfide [58] (Scheme 1.72, Table 1.29). A bulky ether or... 

As far as investigated77, most reactions of the allyllithium-sparteine complexes with electrophiles proceed antarafacially, either as SE2 or anti-SE2 reactions. As a working hypothesis it is assumed that the bulky ligand obliterates the Lewis acid properties of the lithium cation. 

Ethyl (Z)-2-bromomethyl-2-heptenoate and aldehydes condense on reaction with chromium(II) chloride to furnish cw-3,4-disubstituted dihydro-3-methylene-2(3 //)-( uranones exclusively16, indicating that a (Z)-allylchromium complex might serve as reactive intermediate in the. mv-selec-tive addition step due to the bulky 2-substitucnt. Alternatively, an acyclic transition state for the reaction of the ( )-diastereomer, mediated by the Lewis acid dichloroaluminum hydride, has been discussed16. 

Diene 265, substituted by a bulky silyl ether to prevent cycloaddition before the metathesis process, produced in the presence of catalyst C the undesired furanophane 266 with a (Z) double bond as the sole reaction product in high yield. The same compound was obtained with Schrock s molybdenum catalyst B, while first-generation catalyst A led even under very high dilution only to an isomeric mixture of dimerized products. The (Z)-configured furanophane 266 after desilylation did not, in accordance with earlier observations, produce any TADA product. On the other hand, dienone 267 furnished the desired macrocycle (E)-268, though as minor component in a 2 1 isomeric mixture with (Z)-268. Alcohol 269 derived from E-268 then underwent the projected TADA reaction selectively to produce cycloadduct 270 (70% conversion) in a reversible process after 3 days. The final Lewis acid-mediated conversion to 272 however did not occur, delivering anhydrochatancin 271 instead. 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

Sterically bulky 3,3 substituents reduced the enantioselectivity, while coordination between La and O atoms of ortho-substituents improved ees. 6,6 -diphenyl-BI-NOL gave the best results (69% ee for para-tolualdehyde) it was proposed that the phenyl substituents affected the Lewis acidity of the catalyst via electronic effects. With this catalyst, ee and yield depended strongly on solvent, THE being the most... 

Cu2+(aq) to 0.9 in [Cu(tetb)]2+. There is also a modest increase in softness due to changes in EA and CA parameters, which can be interpreted in terms of the symbiosis proposed by Jorgensen (33). The low Da parameter for [Cu(tetb)]2+ suggests that the main effect in transforming Cu(II) to a soft Lewis acid in [Cu(tetb)]2+ is the altered steric situation in coordination of bulky donor atoms to the metal ion. 

The intramolecular Lewis acid-base interaction of type B is of course always in competition with an intermolecular interaction, as indicated by formula C. Again, a bulky group in a-position to X can favor the formation of monomer B. 

In a manner similar to OsH(OH)(CO)(P Pr3)2, the hydride-metallothiol complex OsH(SH)(CO)(P Pr3)2 adds Lewis bases that are not bulky such as CO and P(OMe)3 to give the corresponding six-coordinate hydride-metallothiol derivatives OsH(SH)(CO)L(P Pr3)2 (L = CO, P(OMe)3). OsH(OH)(CO)(PiPr3)2 and OsH(SH)(CO)(P Pr3 also show a similar behavior toward dimethyl acetylenedi-carboxylate. Treatment of OsH(SH)(CO)(P Pr3)2 with this alkyne affords 6sH SC(C02Me)CHC(OMe)6 (CO)P Pr3)2, which is the result of the tram addition of the S—H bond to the carbon-carbon triple bond of the alkyne. Phenyl-acetylene, in contrast to dimethyl acetylenedicarboxylate, reacts with OsH(SH) (CO)(P Pr3)2 by insertion of the carbon-carbon triple bond into the Os—H bond to give the unsaturated alkenyl-metallothiol derivative Os ( )-CH=CHPh (SH) (CO)(P Pr3 )2, the inorganic counterpart of the organic a, (3-unsaturated mercaptans (Scheme 46).92... 

The controlled polymerization of (meth)acrylates was achieved by anionic polymerization. However, special bulky initiators and very low temperatures (- 78 °C) must be employed in order to avoid side reactions. An alternative procedure for achieving the same results by conducting the polymerization at room temperature was proposed by Webster and Sogah [84], The technique, called group transfer polymerization, involves a catalyzed silicon-mediated sequential Michael addition of a, /f-unsaluralcd esters using silyl ketene acetals as initiators. Nucleophilic (anionic) or Lewis acid catalysts are necessary for the polymerization. Nucleophilic catalysts activate the initiator and are usually employed for the polymerization of methacrylates, whereas Lewis acids activate the monomer and are more suitable for the polymerization of acrylates [85,86]. 

The rate of polymerization may be dramatically accelerated upon addition of a bulky Lewis acid. For example, addition of (184) to a sample of living PMMA generated by irradiation of (181)/MMA causes an increase in polymerization rate by a factor of >45,000.444 The dualcomponent systems (181)/(184), and (181)/(185), have been used to prepare monodisperse, ultra-high-molecular-weight samples of PMMA (Mn > 106, Mw/Mn= 1.2).445... 

The scheme below depicts the novel use of a carbonyl ene cyclization (A, Lewis acid-catalyzed) and a closely related Prins cyclization (B, Brpnsted acid-catalyzed) to generate predominantly trans (cyclization condition A) or cis (cyclization condition B), di and tri substituted piperidines 160 and 161 <06JOC2460 06OBC51>. Of note, in the formation of di-substituted derivatives, R1 = H and R2 = Ph, no reaction occurs under cyclization condition B and the cis isomer 160 is obtained exclusively under cyclization condition A. In the case of tri-substituted derivatives, when bulky substituents at the 2-position (R1 = f-Bu or Ph) are present the trans diastereomer 161 is obtained almost exclusively under cyclization condition A, while no diastereoselectivity is seen under cyclization condition B. 

Dove et al. designed and synthesized unsymmetrically substituted /3-diketimininate ligands 97, bearing bulky diisopropylphenyl and Lewis-basic anisyl groups for the purpose of lowering the Lewis acidity of their zinc complexes.155 In both solution phase and the solid state, however, the oxygen atom does not seem to interact sufficiently with the zinc atom to have a noticeable effect on the Lewis acidity of the metal center. 

Optically active selenoxides are known to be unstable toward racemization. An optically active selenoxide having a steroidal frame was obtained for the first time by Jones and co-workers in 1970.7 Enantiomeric selenoxides were prepared by Davis et al. in 1983,8 and an enantiomerically pure selenoxide was isolated for the first time by us in 1989.9 Many optically active selenoxides, which are kinetically stabilized by bulky substituents, were synthesized over the last two decades, and their stereochemistry and stability toward racemization were studied.3,5,10 Recently, some optically active selenoxides, which were thermodynamically stabilized by the intramolecular coordination of a Lewis base to the selenium atom, have been isolated. Optically active selenoxides 1 and 2 were obtained by optical resolution on chiral columns, and their stereochemistry and stability toward racemization under various conditions were clarified (Scheme 1).11,12... 

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