Addition Lewis acid

A trend emerged in that the best conditions involved either the use of a Lewis acid additive (BF3 OMe2) or a more polar solvent-such as trifluoroethanol (TFE), although the beneficial effect of TFE did depend on the ligand employed [27]. The... 

Absorption by ladder polysilanes, 143-144, 148-149, 150-151 Acetylation of thiobetaines, 63-64 Acids see Lewis acids Addition elimination in butadiene cyclo-oligomerization, 170 Addition reactions... 

On the other hand, in the presence of Lewis acid, addition of dialkylzinc to ketones occurs (equation 14)45. A stoichiometric amount of Ti(OPr-i)4 and a catalytic amount of camphorsulfonamide 33 enable an enantioselective addition of dialkylzincs to ketones46. Later, bis(sulfonamide) ligand 34 was found to be a more enantioselective catalyst in this... 

Tricyclic molecules having 10 carbon atoms are converted to adamantane with Lewis acids. Additional carbon atoms become alkyl appendages ... 

The 3 + 2-cycloaddition of nitrile oxides to 2-crotyl-l,3-dithiane 1-oxides produces exclusively 5-acyldihydroisoxazoles.92 Lewis acid addition to 1,3-dipole cycloaddition reactions of mesityl nitrile oxide with a, /i-unsaturated 2-acyl-1,3-dithiane 1-oxides can reverse the sense of induced stereoselectivity.93 The 1,3-dipolar cycloaddition of 4-t-butylbenzonitrile oxide with 6A-acrylainido-6A-deoxy-/i-cyclodextrin (68) in aqueous solution favours the formation of the 4-substituted isoxazoline (69) rather than the 5-substituted regioisomer (Scheme 24).94 Tandem intramolecular cycloadditions of silyl nitronate, synthons of nitrile oxides, yield functionalized hydrofurans.95... 

For both modern reagents PIFA and MoCls an inner-sphere radical transfer is expected, as depicted in Scheme 7. The Lewis acidic additives involved in the PIFA-mediated transformation create an iodonium species that forms a Jt-com-plex 26 with the substrate this subsequently leads to an electron transfer. In contrast, the electrophilic molybdenum chloride most probably coordinates to the oxygen atoms of the donor functions which will then start the transformation. Smooth conversions are obtained if the substituent R" adjacent to the donor (27) is another methoxy group or a bulky moiety. 

For a-bromo ketones and related compounds, enhancement of the enantioselectivities has been achieved by the addition of simple Lewis acids (e.g., BF3, Cp2TiCl2, or magnesium salts, which presumably form Lewis acid-Lewis base complexes with the carbonyl functions of the substrates).2627 The stereochemical effect of some Lewis acid additives on the reduction of a racemic a-bromo ester, namely ethyl 2-bromo-2-phenylpropanoate, is summarized in Scheme 27.2. 

Moreover, as highlighted in Table 5.4, the use of Bronsted or Lewis acid additives increases the reaction rate, yield and selectivity, but these results are normally substrate-dependent. 

The first test of the chiral /V-acylhydrazones was in tin-mediated radical addition [47,48]. Addition of isopropyl iodide to propionaldehyde hydrazone 3a was chosen for initial screening (Scheme 2). Using the tin hydride method with triethylborane initiation [51, 52] (Bu3SnH, Et3B/02), with InCl3 and ZnCU as Lewis acid additives, desired adduct 13a was obtained with high diastereoselectivity. In contrast, 13a was produced with poor selectivity (diastereomer ratio, dr 2 1) in the absence of Lewis acid. 

In search for control of absolute stereochemistry, the reaction of thio-chalcones was investigated with unsaturated amides bearing an Evans chiral oxazolidinone [223] and dimenthyl fumarate [224, 225]. For the first time with thiocarbonyl compounds, the efficiency of Lewis acid addition was demonstrated, and reactions could be conducted at room temperature. With EtAlCl2 (Table 4, entry 2) or A1C13 (entry 3), levels of induction up to 92% were attained for the endo isomer. Yb(OTf)3 in DMSO also caused the acceleration of the reaction with chiral acrylamides with p-facial selectivity [226]. This group has also reported [227] an intramolecular hetero Diels-Alder reaction with divinyl thioketones and the double bond of an allyloxy group (Table 4, entry 4). 

In previous sections we discussed the typical methodologies and practical aspects of the controlled/living carbocationic polymerizations of vinyl ethers, isobutene, styrene, and other monomers. It is possible to select optimal conditions such as the structure and concentrations of initiators, Lewis acids, additives (nucleophiles and salts), solvent, and temperature for each class of monomers in order to control and to prepare well-defined polymers. However, further progress requires a better understanding of the mechanisms of the involved reactions. 

A communication by Yamamoto and co-workers details the use of a bulky Lewis acid additive which serves to direct the addition in a 1,4-fashion rather than the typical 1.2-fashion, e.g. formation of 3. 

Clusters, particularly those described in Section 2.2 as orbitally rich, can act as Lewis acids. Addition of a Lewis base formally adds two electrons to the cluster bonding system. For some clusters, this causes an opening of the cluster, for example, closo to nido interconversion of B3HgMn(CO)3 and B3Fl8Mn(CO)4 by the addition and loss of CO (equation 7). If the cluster is formally unsaturated with respect to the electron counting mles, no change in the core structure takes place, for example, the interconversion of Fe4(CO)n(PPh)2 and Fe4(CO)i2(PPh)2 (equation 8). The base pair may be supplied by a conventional Lewis base (equation 7 or 8), but also can come from bond pairs, for example, H2 (equation 9). The latter is equivalent to oxidative addition in mononuclear... 

Jang et al. reported a highly diastereoselective tandem radical reaction to prepare -polysubstituted homoallylic alcohols (Scheme 64) [175]. This new process relies on the initial addition of benzoyl radicals onto an olefin. The intermediate radicals such as 201 underwent a stereoselective vinylation (two elementary steps) to form the desired Bz-protected homoallylic alcohols in good yields. The stereochemical outcome of the reaction is strongly dependent on polar factors such as solvent polarity of Lewis acid additives. More sophisticated domino processes including cyclizations can be devised, as is the case for the formation of 203. 

Radical reactions can often be rationalized on the basis of frontier orbital considerations for intermediate radical species, the reactivity and stereochemistry of which can certainly be regulated with Lewis acid additives [21-23]. The first appearance of Lewis acids in radical reactions was in polymerization reactions resulting in alternation of copolymers different from that obtained without Lewis acids [24-26]. This concept, Lewis acid-directed radical reactions, has been applied to reductions and alkylations of organic halides or olefins, and has resulted in highly stereospecific processes. 

The 1/BQ system alone or with Lewis acid additives was also employed for asymmetric catalysis in the synthesis of /3-amino acids235. During an elaboration of the tandem catalytic asymmetric chlorination/esterification process, Lectka and coworkers found that proton sponge 1 competes with ketenes in the reaction with halogenating agents, such... 

The pioneering discovery by Mukaiyama in 1974 of the Lewis acid mediated aldol addition reaction of enol silanes and aldehydes paved the way for subsequent explosive development of this innovative method for C-C bond formation. One of the central features of the Mukaiyama aldol process is that the typical enol silane is un-reactive at ambient temperatures with typical aldehydes. This reactivity profile allows exquisite control of the reaction stereoselectivity by various Lewis acids additionally, it has led to the advances in catalytic, enantioselective aldol methodology. Recent observations involving novel enol silanes, such as enoxy silacyclobutanes and O-si-lyl M(9-ketene acetals have expanded the scope of this process and provided additional insight into the mechanistic manifolds available to this versatile reaction. 

The ratio in parenthesis is for reactions without Lewis acid additive. 

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