Silyl catalysts

In Table 2 and Fig. 2 the results of cyclohexene epoxidation over hydrophilic TM-1 and silylated catalysts (TM-2 and TM-3) are presented. Apparently, the silylation applied to Ti-MCM-41 improves the activity of cyclohexene epoxidation, enhances the yield of epoxide and reduces the formation of l-ol and 1-one. In contrast to the Ti-beta, the selectivity of diol remained almost unchanged. In accordance with the characterization results, the MSTFA silylated catalyst, TM-3, gives lower selectivities to l-ol and 1-one than TM-2 does. This indicates that the more hydrophobic the catalyst is, the less by-products are produced, while the higher selectivity of epoxide is obtained. In addition, we observe that the sum of selectivities to 1 -ol and to 1 -one remains unchanged during reaction for epoxidation over both Ti-beta and Ti-MCM-41. This implies that l-ol is the primary product and can be further oxidized to 1-one. According to these observations, we can further conclude that the hydrophilic nature of catalysts leads to the formation of l-ol. 

Silylated catalysts prepared from methoxytrimethylsilane [71,85] contacted with amorphous, disordered mesoporous titanosUicate (2 mol%) followed by deposition of Au via DP resulted in an increase in PO production rate from 52 to 67 gpo kg f h over the unsilylated catalyst at 150 °C. There was no significant effect on H2 efficiency (an increase from 33.3% to 35.3%). Catalyst deactivation rate appears to have been decreased slightly, 58% retention of activity over the first 4 h relative to 44% for the unsilylated catalyst. SUylation in combination with Ba promotion and the high Ti content of the disordered mesoporous materials has resulted in some of the most active catalysts thus reported with a PO rate of 92 gpo kg j h at 150 °C. 

TMCS was one of the first silylation reagents prepared [61]. Although continuing to find wide use in synthetic chemisty, TMCS is rarely used alone in analytical applications. The common analytical use of TMCS is in mixtures, e.g. HMDS/TMCS/pyridine or BSA/TMSIM/ TMCS, which function as very powerful broad-spectrum silylation reagents (see below). The role of TMCS as a silylation catalyst is well recognized, as already mentioned. The facile reaction of TMCS with F ions has found application to the determination of fluorides in air, exhaust gases, bone ash and water [62]. The use of TMBS and TMIS as silylation catalysts has been discussed briefly above their analytical use is discussed below in the appropriate sections. 

The data in Table 2.10 indicate that Catalysts 1 and 3 that were treated with the silylation reagent to convert Si-OH groups to Si-O-SiMe groups provided much more active catalysts after been treated with DBM and TiCl than the non-silylated Catalysts 2 and 4. Catalyst 5 in which the silica was treated with the silylation reagent did not react with TiCl and, therefore, had no polymerization activity, while Catalyst 6, which did contain Si-OH groups reacted with TiCl and was not treated with DBM, provided a catalyst with very low activity. Consequently, Catalysts 1-4 treated with DBM were about 7 to 60 times more active than Catalyst 6, which was not treated with DBM. 

Fig. 2 Possible interaction between the A1 atom and the OTf of silyl derivative for generation of reactive silyl catalyst...

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

A trialkylsilyl group can be introduced into aryl or alkenyl groups using hexaalkyidisilanes. The Si—Si bond is cleaved with a Pd catalyst, and trans-metallation and reductive elimination afford the silylated products. In this way, 1,2-bis-silylethylene 761 is prepared from 1,2-dichloroethylene (760)[625,626], The facile reaction of (Me3Si)2 to give 762 proceeds at room temperature in the presence of fluoride anion[627]. Alkenyl- and arylsilanes are prepared by the reaction of (Me3Si)3Al (763)[628],... 

Another preparative method for the enone 554 is the reaction of the enol acetate 553 with allyl methyl carbonate using a bimetallic catalyst of Pd and Tin methoxide[354,358]. The enone formation is competitive with the allylation reaction (see Section 2.4.1). MeCN as a solvent and a low Pd to ligand ratio favor enone formation. Two regioisomeric steroidal dienones, 558 and 559, are prepared regioselectively from the respective dienol acetates 556 and 557 formed from the steroidal a, /3-unsaturated ketone 555. Enone formation from both silyl enol ethers and enol acetates proceeds via 7r-allylpalladium enolates as common intermediates. 

Unstrained difluorotetramethyldisilane (84) gives the 1 1 adduct 85 as the main product and the 1 2 adduct 86 as a minor product[78,79]. On the other hand, the dimerization and double silylation of conjugated dienes with (Me3Si)2 catalyzed by PdCl2(PhCN)2 take place at 90" C[80]. Pd(dba)2 without phosphine is an active catalyst for the reaction, which proceeds in DMF to give 87 at room temperature[81], A five-membered ring is formed by the application of the reaction to the di-(2,4-pentadienyl)malonate (69)[82]. 

Silylation of alcohols, amines and carboxylic acids with hydrosilanes is catalyzed by Pd catalysts[l 19], Based on this reaction, silyl protection of alcohols, amines, and carboxylic acids can be carried out with /-butyldimethylsilane using Pd on carbon as a catalyst. This method is simpler and more convenient than the silylation with /-butyldimethylsilyl chloride, which is used commonly for the protection. Protection of P-hydroxymethyl-(3-lactam (125) is an example 120]. 

Numerous modifications of chromium-based catalysts have been made through the introduction of various additives, the most effective of which are titanium alkoxides (38,39). These additives apparentiy reduce surface silyl chromate moieties to chromium titanates, which are then oxidized to titanyl chromates. These catalysts offer a better control of the resin molecular weight (39). 

Dimethylaminopyridine [1122-58-3] (DMAP) (24) has emerged as the preferred catalyst for a variety of synthetic transformations under mild conditions, particularly acylations, alkylations, silylations, esterifications, polymeri2ations, and rearrangements (100). POLYDMAP resin [1122-58-3], a polymeric version of DMAP, is available, and is as effective as DMAP as a catalyst for acylation reactions. Furthermore, it can be recycled without regeneration more than 20 times with very Htde loss in activity. POLYDMAP is a trademark of Reilly Industries, Inc. 

Me3Si)2NH, Me3SiCl, Pyr, 20°, 5 min, 100% yield. ROH is a carbohydrate. Hexamethyldisilazane (HMDS) is one of the most common sily-lating agents and readily silylates alcohols, acids, amines, thiols, phenols, hydroxamic acids, amides, thioamides, sulfonamides, phosphoric amides, phosphites, hydrazines, and enolizable ketones. It works best in the presence of a catalyst such as X-NH-Y, where at least one of the group X or Y is electron-withdrawing. ... 

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Isopropenyloxytrimethylsilane. In the presence of an acid catalyst the reagent silylates alcohols and phenols. It also silylates carboxylic acids without added catalyst. 

Methyl 3-trimethylsiloxy-2-butenoate. This reagent silylates primary, secondary, and tertiary alcohols at room temperature without added catalyst. 

Et3SiCl, Pyr. Triethylsilyl chloride is by far the most common reagent for the introduction of the TES group. Silylation also occurs with imidazole and DMF arid with dimethylaminopyridine as a catalyst. Phenols, carboxylic acids, and amines have also been silylated with TESCl. 

TBDMSCl, DMAP, Et3N, DMF, 25°, 12 h. " These conditions were used to silylate selectively a primaiy over a secondary alcohol. Besides DMAP, other catalysts such as 1,1,3,3-tetramethylguanidine, 1,8-diazabicy-clo[5.4.0]undec-7-ene(83-99%), l,5-diazabicyclo[4.3.0]non-5-ene, and ethyldiisopropylamine have been used. ... 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

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