Ketene, commercial preparations

The production of ketene by this method has no significant environmental impact. The off-gases from the ketene furnace are either circulated to the furnace and burned to save energy or led to a flare system. The reaction can also be carried out at 350—550°C in the presence of alkaH-exchanged zeoHte catalysts (54). Small quantities of ketene are prepared by pyrolysis of acetone [67-64-1] at 500—700°C in a commercially available ketene lamp (55,56). 

Ketene itself is commercially prepared in this manner. Carboxylic acids have also been converted to ketenes by treatment with certain reagents, among them TsCl, ... 

This new technology offers considerable promise for commercial preparations of living polymers of methyl methacrylate without resorting to low-temperature anionic polymerizations. Although the mechanism or polymerization is not completely explained, the propagation is generally believed to be covalent in character. A silyl ketene acetal is the initiator. It forms from an ester enolate ... 

Various processes involve acetic acid or hydrocarbons as solvents for either acetylation or washing. Normal operation involves the recovery or recycle of acetic acid, any solvent, and the mother Hquor. Other methods of preparing aspirin, which are not of commercial significance, involve acetyl chloride and saHcyHc acid, saHcyHc acid and acetic anhydride with sulfuric acid as the catalyst, reaction of saHcyHc acid and ketene, and the reaction of sodium saHcylate with acetyl chloride or acetic anhydride. 

Low DS starch acetates ate manufactured by treatment of native starch with acetic acid or acetic anhydride, either alone or in pyridine or aqueous alkaline solution. Dimethyl sulfoxide may be used as a cosolvent with acetic anhydride to make low DS starch acetates ketene or vinyl acetate have also been employed. Commercially, acetic anhydride-aqueous alkaU is employed at pH 7—11 and room temperature to give a DS of 0.5. High DS starch acetates ate prepared by the methods previously detailed for low DS acetates, but with longer reaction time. 

Dimethylpropylcyanobetene can be prepared in an analogous fashion starting from the commercially available 2,5-bis(l,l-dimethyl-propyl)-l,4-benzenediol. This ketene seems to be very similar in stability and reactivity to its [Pg.38]

A. Preparation of ketene. The arrangement of the apparatus is shown in Fig. 2 (Note 1). The graduated separatory funnel, shown in the diagram, filled with 125 cc. of commercial acetone, leads into a 500-cc. round-bottom flask which, in turn, is connected by gas-tight joints (Note 2) to a glass combustion tube filled with broken porcelain, a spiral or bulb condenser, a two-way stopcock, and a reaction flask. In the reaction flask is placed the material with which the ketene is to react (Note 3). A second reaction flask may be placed in series, if desired, to ascertain if any ketene escaped reaction in the first flask. 

The catalytic aldol addition process has been extended to include the addition reactions of dienolsilane 49 to a broad range of aldehydes (Eq. (8.12)) [26]. The addition reactions of 49 are conducted at 23 "C utilizing 5 mol% of catalyst, giving adducts in up to 94% ee. This dienolsilane is easily prepared by enolization of the commercially available acetone-ketene adduct followed by quenching with chlorotrimethyl silane. The resulting dienolsilane is isolated typically in 78% yield as a clear colorless liquid that can be conveniently purified by distillation. 

Preparative Methods most often prepared (eq 1) by pyrolysis of ethoxy(trimethylsilyl)acetylene at 120 °C (100 mmol scale, 65% yield). Recently, pyrolysis of t-butoxy(trimethylsilyl) acetylene has been shown to be a convenient alternative for the preparation of trimethylsilylketene (1). Thermal decomposition of t-butoxy(trimethylsilyl)acetylene causes elimination of 2-methylpropene slowly at temperatures as low as 50 °C and instantaneously at 100-110 °C (30 mmol scale, 63% yield). The main advantage of this method is that it is possible to generate trimethylsilylketene in the presence of nucleophiles, leading to in situ trimethylsilylacetylation (eq 2). Increased shielding of the triple bond prevents problems such as polymerization and nucleophilic attack that occur when the ketene is generated in situ from (trimethylsilyl)ethoxyacetylene. Trimethylsilylketene can also be prepared (eq 3) via the dehydration of commercially available trimethylsilylacetic acid with 1,3-dicyclohexylcarbodiimide (DCC) in the presence of a catalytic amount of triethylamine (100 mmol scale, 63%). Other typical methods used for ketene generation such as dehy-drohalogenation of the acyl chloride and pyrolysis of the... 

Included among the many types of vinyl monomers that have been subjected to photoinitiated cationic polymerization are styrene," substituted styrenes, a-methylstyrenes, N-vinylcarbazole, alkyl vinyl ethers, prop-l-en-l-yl ethers, ketene acetals, and alkoxyallenes. Most useful in the crosslinking photopolymerizations employed for UV curing applications are multifunctional vinyl ethers and multifunctional prop-l-en-l-yl ethers. A number of multifunctional vinyl ether monomers are available from commercial sources, while multifunctional prop-l-en-l-yl ethers can be readily prepared by catalytic isomerization from their corresponding allyl ether precursors. The photoinitiated cationic... 

Carbon-carbon bond formation can also be used to assemble enantiomeiically-pure secondary alcohols. Herfried Griengl of Graz University of Technology has foimd (Adv. Synth. Cat. 2007, 349,1445) that a commercial nitrile lyase effects addition of nitromethane to an aldehyde such as 24 to give the nitro alcohol 25 in high ee. Markus Kalesse of Leibniz Universitat Hannover has constructed a catalyst (Organic Lett. 2007, 9, 5637) for the enantioselective addition of the ketene silyl acetal 27 to aldehydes. Hajime Ito and Masaya Sawamura of Hokkaido University (J. Am. Chem. Soc. 2007,129, 14856) (depicted), and Dennis G. Hall of the University of Alberta (Angew. Chem. Int. Ed. 2007, 46, 5913) have reported complementary enantioselective preparations of allyl boronates such as 31. 

---