Active hydrogen compounds base-catalyzed

The base-catalyzed Michael—type addition of active hydrogen compounds to activated double bonds is generally performed under homogeneous conditions and lays therefore at the borderline of the scope of this paper, even if it has been sometimes carried out under typical phase transfer conditions. However, considering that the catalysts promoting phase transfer reactions, i.e. ammonium salts, amines and crown ethers, are generally active in the Michael addition, the reported reactions will be discussed here in some details also for the reasons mentioned in the introduction. 

There are three principal ionic ring-opening polymerization reactions of epoxides acid-initiated, base-initiated, and coordinate-initiated polymerizations. The acid-initiated reaction involves addition of an active hydrogen compound, HY, such as ethanol, to an epoxide ring and is catalyzed by an acid, HX, such as perchloric acid. The reaction sequence involves formation of an oxonium complex, followed by ring opening by an 8 2 cleavage of an oxonium carbon bond. 

During the course of base-catalyzed exchange in O-deuterated alcohols, the vinylic hydrogen in the a position to the ketone is replaced by deuterium, in addition to the hydrogens activated by enolization. Thus, under these conditions the exchange of androst-l-en-3-one (16, R = H) gives a trideuterio derivative (18) instead of the expected 4,4-d2 analog (16, R = D). " (For other examples see compounds 13, 19, 21, 23, 26 and 27.) Incorporation of this deuterium is due to rapidly reversible alcohol addition (16 -+17) and elimination (17 18) which competes with the enolization step. " ... 

It is of interest that antibacterial activity can be retained even when the imine carbon-nitrogen bond is replaced by a carbon to carbon double bond. Base-catalyzed condensation of 5-nitrofurfuraldehyde (24) with 2,6-dimethylpyridine (38) affords olefin 39. Treatment of this compound with hydrogen peroxide gives the corresponding N-oxide (40). Heating of... 

Based on the extraordinary selectivity in hydrosilylation reactions when an alkyne competes with other groups for a silicon-bonded active hydrogen, further derivatisation can be carried out. The hydrosilylation of 2-methyl-3-butynol, which works very well with polymeric siloxanes, gives hydroxyal-kenylsilicon compounds - a l-silylalkenyl/2-silylalkenyl mixture from cis-addition across the triple bond. Elimination of water from the tert. alcohol produced, catalyzed by traces of a strong acid, results in isoprenylic siloxanes in more than 90 % overall yield (Eq. 8). 

The carbon-bond-forming reactions based on hydrogen transfer catalyzed by Cp Ir complex have been extended to the alkylation of active methylene compounds. Grigg et al. reported the alkylation of arylacetonitriles catalyzed by the... 

The system is not a homogeneous one, as the organometallic compounds and alkali metal hydrides are insoluble in benzene, the solvent used however, the rate of reaction was found not to be affected by the addition of platinum or palladium catalysts. The reaction is formally similar to the base-catalyzed activations of hydrogen studied by Wilmarth this becomes clear if Eq. (45) is written as... 

Important laboratory applications involve the addition of hydrogen cyanide to an olefinic linkage which is activated by another group such as carbonyl, carbalkoxyl, cyano, or nitro on the adjacent carbon /3-cyano compounds are formed. The reaction is related to the Michael condensation (method 301). For the most part, the additions are base-catalyzed and are carried out by treating the unsaturated compound with an alkali cyanide in aqueous or aqueous-alcoholic solution. 

TABLE 16.2. Base-Catalyzed Condensations Showing the Active-Hydrogen Components and the Carhonyl Compounds... 

While Baker and Gaunt postulate that the mechanism of the base-catalyzed reaction of isocyanates and alcohols involves the attack of the isocyanate-base complex by the alcohol, according to Iwakura and Okada the higher acidity of the active hydrogen containing compounds changes... 

Condensation of urea with carbonyl compounds A rapid and efficient MW-assisted synthesis of hydantoins and thiohydantoins was described by Muccioli et al. [117]. The most straightforward conditions for synthesis of phenytoin are the base-catalyzed condensation using benzil and urea, known as the Biltz synthesis (Eq. 33). MW activation of the Biltz synthesis of phenytoin improved both yield and reaction time. The first step consists in MW activation of the reaction of benzil with (thio)urea the second includes the conversion of the resulting 2-(thio)hydantoin to hydantoin using hydrogen peroxide. When reactions were performed at the same temperature under both reaction conditions, yields were by far better under the action of MW and emphasized the evident specific MW effects. These are perfectly expected when one considers the polar TS involved in the first step (nucleophilic addition of neutral NH2 group on carbonyl moiety). 

PO also reacts with active hydroxyl hydrogen derived from the ring opening of other compounds such as EO and tetrahydrofuran thus, a copolymer polyol is obtained. Typically, polyols are obtained from base-catalyzed reactions with aqueous ammonia, sodium or potassium hydroxide, or lower alkyl tertiary amines such as trimethyl- and triethylamine. The reaction of PO with tetrahydrofuran is catalyzed by boron trifluoride etherate. The molecular weights of polyols prepared according to the reactions described earlier range from 200 to 7000. 

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