Ketene forming reactions

The most studied of the mesoionic compounds are the mtinchnones (90), which are 3-substituted-5(4i/)-oxazolones with at most one substituent on the 4 position. They react in such a 1,3-dipolar form or in a ring-opened ketene form. Reactions of 3-unsubstituted-5(4/f)-oxazolones may also proceed through their mesoionic (91) or ketene (92) tautomers (Scheme 29). 

Ziegler and coworkers have reported several DFT studies on organometallic systems. Representative examples of this work include the study of (i) C-H bond activation by [(cp)M(L)] (M = Rh, Ir L = CO, PH3) and M(CO)4 (M = Ru, Os) [106], (ii) ketene forming reactions involving bromine abstraction from 2-bromoacetyl chloride by [Mn(CO)s]- [107], (iii) intermediates and reaction steps in the HCo(CO)4 catalysed hydroformylation reaction [108] as shown in Fig. 6, (iv) the role of the carbene L2Mo(X)CH2 and the molybdacyclobutane... 

A wide variety of /3-lactams are available by these routes because of the range of substituents possible in either the ketene or its equivalent substituted acetic acid derivative. Considerable diversity in imine structure is also possible. In addition to simple Schiff bases, imino esters and thioethers, amidines, cyclic imines and conjugated imines such as cinnamy-lidineaniline have found wide application in the synthesis of functionalized /3-lactams. A-Acylhydrazones can be used, but phenylhydrazones and O-alkyloximes do not give /3-lactams. These /3-lactam forming reactions give both cis and /raMS-azetidin-2-ones some control over stereochemistry can, however, be exercised by choice of reactants and conditions. 

The mechanism of the classic ketene-imine reaction to form /J-lactams [17,18] is thought to involve perpendicular attack of the imine nitrogen on the ketene carbonyl carbon from the side of the sterically smaller of the two groups, followed by conrotatory closure of the zwitterionic intermediate (Eq. 6). This... 

FIGURE 6.37 Synthesis of 3-(5-tocopheryl)-propionic acid (50) by trapping the intermediate ortho-QM 3 with a ketene acetal. Reaction products of 50 are formed in complete analogy to a-tocopherol (1). 

Unfortunately, some of the ketene formed decomposes further to form unwanted ethylene and carbon monoxide via the reaction ... 

Laboratory studies of these reactions indicate that the ketene selectivity S (kmol ketene formed per kmol acetone converted)... 

Denmark utilized chiral base promoted hypervalent silicon Lewis acids for several highly enantioselective carbon-carbon bond forming reactions [92-98]. In these reactions, a stoichiometric quantity of silicon tetrachloride as achiral weak Lewis acid component and only catalytic amount of chiral Lewis base were used. The chiral Lewis acid species desired for the transformations was generated in situ. The phosphoramide 35 catalyzed the cross aldolization of aromatic aldehydes as well as aliphatic aldehydes with a silyl ketene acetal (Scheme 26) [93] with good yield and high enantioselectivity and diastereoselectivity. 

We here further demonstrate the merits of Fe -exchanged montmorillonite over the conventional homogeneous acid catalysts, applying it to other types of liquid-phase carbon-carbon bond-forming reactions between carbonyl compounds and useful nucleophilic carbanion reagents such as silyl ketene acetal (an ester... 

Ketene. The reaction of CH2 with CH2CO has been studied in connection with the photoiysis of ketene. The major products are CO and ethylene,96 formed in the ratio of about 2.2.73 102 129 Ethane, methane,... 

The effects of reactant structures on the ketene-forming elimination reactions of aryl esters of substituted phenylacetic acids (9) and (10) with secondary amines in acetonitrile (Scheme 2) have been studied in anticipation that the transition state might have E cB- kc E2 character.3 The reactions are second order for R2NH-MeCN and... 

IOB alone epoxidizes some electron-deficient alkenes, in a reaction where its oxygen attacks the substrate nucleophilically. An example of this type was with tetracyanoethylene which was converted into its epoxide (74%) similarly, ketenes formed unstable a-lactones which polymerized [7],... 

Forms unstable explosive products in reaction with acetaldehyde + desiccants (forms polyethyUdine peroxide) acetic acid (forms peracetic acid) acetic + 3-thietanol acetic anhydride acetone (forms explosive peroxides) alcohols (products are shock-and heat-sensitive) carboxylic acids (e.g., formic acid, acetic acid, tartaric acid), diethyl ether, ethyl acetate, formic acid -f- metaboric acid, ketene (forms diacetyl peroxide) mercur f(II) oxide + nitric acid (forms mercur f(II) peroxide) thiourea -f- nitric acid polyacetoxyacryUc acid lactone + poly(2-hydroxyacrylic acid) + sodium hydroxide. 

Having developed an efficient catalytic asymmetric nitroaldol reaction, we next applied our attention to a direct catalytic asymmetric aldol reaction. The aldol reaction is generally regarded as one of the most powerful carbon-carbon bond-forming reactions. The development of a range of catalytic asymmetric aldol-type reactions has proven to be a valuable contribution to asymmetric synthesis. In all these catalytic asymmetric aldol-type reactions, however, preconversion of the ketone moiety to a more reactive speeies such as an enol silyl ether, enol methyl ether or ketene silyl... 

The main reaction is followed by a slow pressure increase , which is due to the thermal decomposition of the ketene formed. It was proved that ketene is an intermediate . 

The pressure increase approaches 100 % at 368 °C and 5-10 torr initial pressure, but is less at higher pressures (probably due to the secondary reactions of the ketene formed). Surface effects are not significant, although the secondary reactions of the products are, to some extent, heterogeneous. 

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