Alkyl groups using bulky base

When X = NMe. The elimination reaction forms predominantly the least highly substituted alkene in a Hofmann elimination. The H—C bond is broken before the C— NMes bond and the transition state resembles a carbanion product. As a result, the base removes to form a more stable carbanion (i.e. the - is situated on a carbon bearing the least number of electron-donating alkyl groups). The Hofmann elimination is also normally expected when using bulky bases (e.g. BuO ). This is because is more accessible (i.e. less hindered and more easily attacked) than H . 

Potassium Alkoxides. The most widely used potassium bases are potassium tert-hu. oAde [865-47-4] (KTB) and potassium / i -amylate [41233-93-6] (KTA). These strong alkoxide bases offer such advantages as base strength (pX = 18), solubiUty (Table 5), regio/stereoselectivity because of bulky alkyl groups, and stabiUty because of the lack of a-protons. On storage, KTB and KTA have long shelf Hves under inert atmosphere (see... 

Lithium triethylcarboxide (1) is the base of choice for these conversions. The use of less hindered alkoxide or amide bases results in poorer yields. In this procedure 1.5-2.0 equivalents of base are required, although with more bulky alkyl groups attached to boron only 1 equivalent is necessary. The use of the more hindered 2,6-diisopropylphenol to form the borinic ester gives a 96% yield of the bicyclic ketone with only 1 equivalent of base however, in the work-up procedure this phenol is more difficult to separate from the ketone. 

Use of a Bulky Base If the substrate is prone to substitution, a bulky base can minimize the amount of substitution. Large alkyl groups on a bulky base hinder its approach to attack a carbon atom (substitution), yet it can easily abstract a proton (elimination). Some of the bulky strong bases commonly used for elimination are ferf-butoxide ion, diisopropylamine, triethylamine, and 2,6-dimethylpyridine. 

Among N-Mannich bases, derivatives of polyacrylamide (517) are investigated particularly for their flocculant properties in water treatment. The corresponding quaternary ammonium salts are also used. Benzimidazolic and bcnzotriazolic Mannich bases 518, usually with the bulky alkyl group linked to the amine moiety, are used effectively against corrosion in a manner similar to alkylphenols 513, which have the apolar group linked to the substrate moiety. 

Remember that halo-substituted alkyl groups can also undergo E2 and El reactions (Section 11.8). Notice that a bulky base (terr-BuO ) is used to encourage elimination over substitution. 

Based on the high efficiency of the (dialkylamino)alkyl side chain on the fer-rocenylphosphines, a series of /3-(diaIkylamino)alkylphosphines 10 was prepared and used for Ni-catalyzed cross-coupling. Those substituted with a sterically bulky alkyl group at the chiral carbon center are more effective than the ferrocenylphosphine ligands. Valphos (10c) and Meuphos (lOd), which were prepared starting with valine and tert-leucine, respectively, gave the product 3 with over 81% Use of polymer-... 

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