Catalysts, acidic electron-withdrawal

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. ... 

Nitrones are a rather polarized 1,3-dipoles so that the transition structure of their cydoaddition reactions to alkenes activated by an electron-withdrawing substituent would involve some asynchronous nature with respect to the newly forming bonds, especially so in the Lewis acid-catalyzed reactions. Therefore, the transition structures for the catalyzed nitrone cydoaddition reactions were estimated on the basis of ab-initio calculations using the 3-21G basis set. A model reaction indudes the interaction between CH2=NH(0) and acrolein in the presence or absence of BH3 as an acid catalyst (Scheme 7.30). Both the catalyzed and uncatalyzed reactions have only one transition state in each case, indicating that the reactions are both concerted. However, the synchronous nature between the newly forming 01-C5 and C3-C4 bonds in the transition structure TS-J of the catalyzed reaction is rather different from that in the uncatalyzed reaction TS-K. For example, the bond lengths and bond orders in the uncatalyzed reaction are 1.93 A and 0.37 for the 01-C5 bond and 2.47 A and 0.19 for the C3-C4 bond, while those in... 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

Although there are several reports in the literature on boron-mediated amide formations, the boron reagents had to be used in stoichiometric amounts.1-4-5-6-7-8-9 Recently, Yamamoto et al. presented the first truly catalytic method allowing for a direct amide formation from free carboxylic acids and amines as the reaction partners.10-1112 Best results were obtained by using phenylboronic acids bearing electron withdrawing substituents in the meta- and/or para-positions such as 3,4,5-trifluorophenylboronic acid or 3,5-bis(trifluoromethyl)boronic acid as the catalysts. 

In the absence of bromide ion the p-xylene undergoes rapid autoxidation to p-toluic acid but oxidation of the second methyl group is difficult, due to deactivation by the electron-withdrawing carboxyl group, and proceeds only in low yield at elevated temperatures. Although bromide-free processes were subsequently developed (ref. 5) they require the use of much higher amounts of cobalt catalyst and have not achieved the same importance as the Amoco-MC process. Indeed, the... 

The rate of acid-induced demetalation depends only slightly on the nature of the head substituents X (Table I). In contrast, the tail-R groups dramatically affect k and, for the most part, k3, suggesting that tail amide O-atoms are sites of peripheral protonation. Thus, the acid tolerant Fem-TAML catalysts with tail electron-withdrawing groups should be more acid resistant and replacement of R = Me with R = F results in a remarkable stabilization. The rate constants (Table I) show that under weakly acidic conditions (pH 2-3), when the k pathway dominates over k3, fluorinated lk is 105-fold more H +-tolerant than la. 

Simple reaction occurs with aryl halides only when the ring is sufficiently substituted with electron-withdrawing functions to allow attack by the nucleophilic phosphorus.53-56 Generally, reaction with aryl halides requires the presence of a Lewis acid catalyst or some other means of reaction initiation. These reactions will be considered in detail in Chapter 6 of this work. Interestingly, while reactions involving vinylic halides seem to correlate with those of aromatic halides (see Chapter 6), acetylenic halides undergo facile reaction with these phosphorus reagents.57 58... 

A bulkyl silsesquioxane has been used to prepare metathetically active Mo complexes [91]. A silsesquioxane mimics the manner in which a metal might be attached to silica in a heterogeneous metathesis system. The conjugate acid of the silsesquioxane appears to have a relatively high pKa, a hint that the silsesquioxane itself may be relatively electron withdrawing. Silsesquioxane complexes were found to be highly active catalysts in metathesis reactions. 

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