ZnCh catalyst

Furthermore, inorganic polyphenylsiloxane has been prepared by hydrolysis of PhSiCls and subjected to further functionahzation [183-185]. After initial formation of a prepolymer, the polymer formed in the presence of KOH catalyst [183], is subjected to chloromethylalion with chloromethylmethylether and ZnCh catalyst, and phosphination by reaction with LiPRj (Scheme 1.12) [184,185] ... 

As catalysts Lewis acids such as AICI3, TiCU, SbFs, BF3, ZnCh or FeCl3 are used. Protic acids such as FI2SO4 or FIF are also used, especially for reaction with alkenes or alcohols. Recent developments include the use of acidic polymer resins, e.g. Nafion-Fl, as catalysts for Friedel-Crafts alkylations and the use of asymmetric catalysts. ... 

Figure 5 IR spectra of acylated PS films after thermotreatment at 75 C during 45 min (1) and at 150°C during 30 min (2), Catalysts (a) BF3-OEt2, (b) TiCU, (c) SnCU, (d) ZnCh, (e) FeCb-...

A homogeneous catalyst exists in the same phase as the reactants. Homogeneous catalysts most often catalyze gaseous and aqueous reactions. For example, aqueous zinc chloride, ZnCh, is used to catalyze the following reaction. 

The reaction takes place in aqueous solution, and the catalyst is soluble in water. Therefore, ZnCh is a homogeneous catalyst when it is used with this reaction. 

The original conditions, called the Gattermann Reaction / Formylation, were to add HCN, HC1 and ZnCh (known as Adam s Catalyst) directly. Use of Adam s catalyst avoids using gaseous HCN. 

Silica-supported Lewis acids are useful catalysts with microwave irradiation for conjugate additions. The silica-supported catalysts are obtained by treatment of silica with ZnCh [Si(Zn)], Et2AlCl [Si(Al)] or TiCl4 [Si(Ti)] [ 150-152], The Michael addition of methyl a-acetamidoacrylate (196) with indole (2) under Si(M) heterogeneous catalysis assisted by microwave irradiation afforded the alanine derivative 197 within 15 min and/or bis-indolyl 198, depending on the reaction conditions (Scheme 45) [153]. While the bis-indolyl product 198 is only formed when Si(Zn) was used as catalyst, the alanine derivative 197, as a single product is formed under thermal heating in a yield of 12%. The best yields were observed with Si(Al) (Table 5). The product 198 was obtained by elimination of acetamide followed by a-Michael addition between intermediate 199 with a second mole of indole. 

Heterosubstituted 1,3-dienes. Some alkenylmetals substituted by OR, SR, or SiR, can be coupled with alkenyl halides (or aryl halides) in the presence of this Pd(0) catalyst. Alkenylzinc reagents, prepared by reaction of alkenyllithiums with ZnCh, and alkenyl-alanes are the most useful. Yields from reactions with organoboronates are low. 

The order of reactivity of this Ru/silane combination to various functional groups differs greatly from that of its Pd/silane/ZnCh analog. While the latter is very useful for allylic reductions and essentially useless for unsaturated esters, the Ru-based system exhibits opposite reactivity. This complementary che-moselectivity is illustrated by the reduction of cinnamyl cinnamate (Scheme 59), a substrate containing both an allylic carboxylate and an a, -unsaturated ester.Each of these can be reduced separately by silicon hydride and the appropriate transition metal catalyst. 

Sometimes the addition of a small amount of ZnCh as a catalyst is recommended, e.g. succinic acid is transformed to the dichloride only in the presence of this catalyst. It has to be noticed, however, that under these conditions hydrogen chloride is evolved and, therefore, acid sensitive compounds cannot be taken. Af-acylamino acid chlorides and chlorides of fully acetylated sugar acids are also obtainable in yields of 70% or more. Whereas most reagents seem to be unsuited to the preparation of acid halides of a-keto acids, dichloromethyl methyl ether is successful in this reaction (equation 10). Pyruvyl chloride can now be made in ca. 50% yield. In some examples the corresponding a,a-dibromomethyl methyl ether has also been tested. ... 

Diselenoacetals are also useful for the protection or umpolung of the carbonyl group. - Bis(al-kylseleno)alkanes and arylseleno analogs are prepared by the reaction of carbonyl compounds with sele-nols and trimethylsilyl selenides in the presence of acid catalysts (cone. HCl, cone. H2SO4, Lewis acids such as BP3 Et20, TiCU, ZnCh or AICI3) or with tris(methylseleno)borane in the presence of LAH and BF3Et20.274... 

Allyl halides have been used to effect selective haloalkylations catalyzed by proton acids. For example, 3-chloropropylene reacts with benzene and toluene in the presence of H2SO4 to give haloalkyl-ated products almost exclusively (equation 87). The same result is obtained when ZnCh is used as Ae catalyst. ... 

Selective haloalkylation of aromatic compounds has also been achieved by employing alkyl haloalkyl ethers as the haloalkylating agents, as seen from equations (88) and (89). However, the use of chlo-romethyl ethers in chloromethylation (equation 89) is discouraged due to the carcinogenic nature of the ether substrate. As an alternative, chloromethylation reactions can also be carried out using paraformal-dehyde-HCl in the presence of ZnCh or SnCU as catalyst. ... 

The zinc halides have a mixed reputation in the field of epoxide chemistry. Znh is recommended as a catalyst for epoxide opening, without rearrangement, in the addition of thiosilanes and selenosi-lanes. Conversely, rearrangements catalyzed by zinc salts have been clearly documented for several epoxides. Sutherland s comparison suggests that ZnCh should be rated on the weaker end of the strong acid scale. 

A special case of a Negishi coupling was performed with 1-hydroxy-imidazole (92), which could be etherified with Merrifield and Wang resin (91) [210]. Upon treatment with Bull, the H-2 is removed giving a stabilized carbanion, which can react with various electrophiles such as aryl iodides (2). Both ZnCh and Pd(PPh3)4 were required for the reaction. The Z.riCL was used in excess and displaces the Li from the imidazole. The Pd catalyst was added in small quantities (0.1 equiv.) and performs an insertion into the aryl-iodine bond creating the electrophile to be trapped by the zinc imidazolide. Several 2-arylated 1-hydroxyimidazoles (95) were obtained by this method in almost quantitative yields (Scheme 21). 

Lewis acid catalysts increase the reactivity of dienophiles in Diels-Alder reactions by complexing to basic sites on the dienophile. ° The Lewis acid lowers the LUMO of the adjacent ir-system, which strengthens the overlap between the LUMO of the dienophile and the HOMO of the diene. In 1979 Scheeren reported that ZnCh catalyzes the cyclocondensation reaction of unactivated aldehydes with l-methoxy-3-(trimethylsilyloxy)-l,3-butadiene. Experimental details of this reaction, however, were not fully documented. In 1982 Scheeren also reported the use of aluminum alkoxydichlorides as catalysts... 

The major development in the Lewis acid catalyzed reaction came in 1981 when Danishefsky and coworkers reported that ZnCh and BF3-OEt2 catalyze the cyclocondensation of silyloxy dienes with unactivated aldehydes. The use of these Lewis acid catalysts permits a variety of simple aldehydes to be used in the reaction (Table 2). Danishefsky also critically evaluated the mechanism and stereochemistry of the cyclocondensation reaction with a series of dienes, aldehydes and catalysts. These studies resulted in conditions to control the relative and absolute stereochemistry of the reaction and allowed the use of this chemistry in natural product synthesis. 

In further studies by Larson and Danishefsky it was shown that boron trifluoride etherate (BF3-OEt2) also catalyzes the reaction but with a dramatically different stereochemical outcome. When ZnCh is used as a catalyst the reaction between benzaldehyde and diene (14) gives predominately the cis dihydro-pyrone (16). Under BF3-OEt2 catalysis, however, the trans dihydropyrone (17) is the major product (Table 4). 

Diene (14) reacted with a series of aldehydes under BFs-OEtz catalysis in CH2CI2 to give predominantly trans products (Table lO). " Aldol-type products, such as p-hydroxy enones, are isolated (along with dihydropyrones) from the reaction mixtures. Using TFA as a catalyst, the p-hydroxy enones are, as previously described, converted into dihydropyrones. The stereoselectivity of these reactions is consistent with a Mukaiyama-aldol reaction rather than a Diels-Aider cycloaddition. The stereochemistry of the P-hydroxy enones is also consistent with the observation that the (Z)-alkoxysilane reacts with the aldehyde in an extended transition state to give anti (threo) aldol products (Scheme 16). In the cases using ZnCh or lanthanide ions as catalysts aldol products have not been detected. 

Lactone (170) is reduced to aldehyde (171) and used as a heterodieneophile in a Diels-Alder reaction with diene (14), with ZnCh in THF as a catalyst, to give (172 Scheme 48). The major product of this cycloaddition is the syn pyrone derived from a Cram-Felkin attack (syn-CF) 27% of the syn-ACF product is also produced. The jyn-CF pyrone (172) is ozonized and subjected to an oxidative work-up fol-... 

These results stimulated us to synthesize tetra-O-methyl-P-o-glucopyranosylamine (126)." The 4CC products of (126) undergo acidolytic N—C cleavage under milder conditions than (124)." We are now studying the stereoselective model 4CC of (126), as well as of (127) and (128), in the presence of chelating catalysts like ZnCh and Ti(OPr )4. The latter enhances the stereoselectivity of 4CC with (127) considerably. " - ... 

Since the investigations which used ZnCh and BF3-OEt2 as catalysts, other Lewis acids have been shown to catalyze the cyclocondensation reaction. MgBr2, for example, was used by Pearson and Danishefsky40 to add the power of chelation control to the reaction. When chiral a-alkoxy aldehydes are condensed with dienes under the influence of MgBr2, products derived from an ACF transition state predominate. The chelation of the metal by the a-alkoxy group of the aldehyde forces the aldehyde side chain to occupy the same side of the aldehyde as the metal. The diene, therefore, can attack the aldehyde from the least hindered exo face giving rise to the trans-ACF product (Scheme 5). When an aldehyde is used that cannot form a chelate to the metal (such as benzaldehyde) syn (endo) selectivity is observed. 

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