Ketene synthesis

SCHMIDLIN KETENE SYNTHESIS. Formation of kelene by thermal decomposition of acetone over electrically heated wire at 500-750 degrees by a reaction involving radical formation with generation of methane and carbon monoxide. 

Other industrially important uses of P4O10 include the reactions with ethers, an example of which is the formation of triethyl phosphate via reaction with diethyl ether followed by pyrolysis. The product (which has a worldwide production of many thousands of tons per annum) finds use as ketene synthesis, a flame retardant, and a plasticizer within the plastics industry a less conventional use is as a simulant for the sarin when modeling situations involving the latter nerve agent. 

Monoalkylketenes. Methylketene and ethylketene can be obtained in 60-65% yield by reaction of 2-bromopropionyl bromide and 2-bromobutyryl bromide in THF with zinc activated by treatment with hydrochloric acid (1, 1276) under reduced pressure (100 mm). The ketenes codistill with THF and are free from starting materials and zinc salts. The present method is a modification of Standinger s ketene synthesis, which gave only low yields of these ketenes. ... 

It is assumed that acetone at such high temperatures decomposes into methyl and acetyl radicals, and the latter further degrades into methyl radical and carbon monoxide. Then methyl radical abstracts one hydrogen atom from acetone to give methane and an acetyl-methyl radical that decomposes into ketene and another methyl radical. Displayed here is the reaction mechanism for the Schmidlin ketene synthesis. 

Other references related to the Schmidlin ketene synthesis are cited in the literature. ... 

Carbalkoxymethylstannanes from alkoxystannanes and ketene Synthesis of stannanes from diorganomercury compounds... 

The reaction can be applied to the synthesis of q, /3-unsaturated esters and lactones by treatment of the ketene silyl acetal 551 with an allyl carbonate in boiling MeCN[356]. The preparation of the q,, 3-unsaturated lactone 552 by this method has been used in the total synthesis of lauthisan[357]. 

Ketone Synthesis. In the Friedel-Crafts ketone synthesis, an acyl group is iatroduced iato the aromatic nucleus by an acylating agent such as an acyl haUde, acid anhydride, ester, or the acid itself. Ketenes, amides, and nittiles also may be used aluminum chloride and boron ttitiuotide are the most common catalysts (see Ketones). 

P-Hydroxy acids lose water, especially in the presence of an acid catalyst, to give a,P-unsaturated acids, and frequendy P,y-unsaturated acids. P-Hydroxy acids do not form lactones readily because of the difficulty of four-membered ring formation. The simplest P-lactone, P-propiolactone, can be made from ketene and formaldehyde in the presence of methyl borate but not from P-hydroxypropionic acid. P-Propiolactone [57-57-8] is a usehil intermediate for organic synthesis but caution should be exercised when handling this lactone because it is a known carcinogen. 

The first synthetic route for isocyanates was reported in 1848 (10,11)- Subsequent efforts by Hofmann, Curtius, and Hentschel pioneered alternative synthetic approaches (12). These efforts highlighted the phosgene—amine approach. Staudinger presented the stmctural similarities between isocyanates and ketenes and stimulated interest in this class of compounds (13). However, it was not until 1945, when the world was pressed for an alternative to natural mbber, that synthetic routes to isocyanates became an area of great importance. Several excellent review articles covering the synthesis and chemistry of isocyanates have been presented (1 9). 

Ketenes and related compounds have been reviewed extensively (1 9). For the synthesis and synthetic uses of conjugated ketenes see Reference 10. Ketenes with three or more cumulated double bonds have been prepared (11,12). The best known is carbon suboxide [504-64-3] 3 2 preparative uses and has been reviewed (13—16). Thioketenes (17,18), ketenimines (19—21), and their dimers show interesting reactivity, but they have not achieved iadustrial importance to date. 

Cyclo ddltion. Ketenes are ideal components ia [2 + 2] cycloadditions for additions to the opposite sides of a TT-system as shown ia the cyclobutane product (2) ia Figure 1. Electron-rich double bonds react readily with ketenes, even at room temperature and without catalysts. In conjugated systems, ketenes add ia a [2 + 2] fashion. This is illustrated ia the reaction foUowiag, where the preferential orientation of L (large substituent) and S (small substituent) is seen (40). This reaction has been used ia the synthesis of tropolone [533-75-5]. 

Ketene has also been used on a large scale for C-acetylation in the synthesis of the carbapenem antibiotic thienamycin [59995-64-1] (86,87). 

Ketene trimer can be recovered from the tarry residue of diketene distillation and converted into valuable building blocks like 1,3-cyclobutanedione and squaric acid [2892-51-5] (140,141), an important intermediate in the synthesis of pharmaceuticals and squaryHum dyes used in photostatic reproduction (142,143). 

The first synthesis of sorbic acid was from crotonaldehyde [4170-30-3] and malonic acid [141-82-2] in pyridine in 32% yield (2,17,18)- The yield can be improved with the use of malonic acid salts (19). One of the first commercial methods involved the reaction of ketene and crotonaldehyde in the presence of boron trifluoride in ether at 0°C (20,21). A P-lactone (4) forms and then reacts with acid, giving a 70% yield. 

Group-Transfer Polymerization. Living polymerization of acrylic monomers has been carried out using ketene silyl acetals as initiators. This chemistry can be used to make random, block, or graft copolymers of polar monomers. The following scheme demonstrates the synthesis of a methyl methacrylate—lauryl methacrylate (MMA—LMA) AB block copolymer (38). LMA is CH2=C(CH2)COO(CH2) CH2. 

Bielectrophiles have found appreciable applications in the synthesis of ring-fused systems, especially those involving [5,6] fused systems. The following serve to illustrate these applications. Reaction of pyrazole with (chlorocarbonyl)phenyl ketene (214) (Type 1, Scheme 6) readily formed the zwitterionic pyrazolo[l,2-a]pyrazole derivative (215) (80JA3971). With l-methylimidazole-2-thione (216), anhydro-2-hydroxy-8-methyl-4-oxo-3-phenyl-4//-imidazo[2,l-6][l,3]thiazinium hydroxide (217) was obtained (80JOC2474). 

Flash thermolysis of compounds of the type (120), derivatives of Meldrum s acid , is a fairly general synthesis of ketenes (Scheme 103). Brown and coworkers (77AJC179) found that the spirooxirane (121) gave ketene, possibly via the expected carbonyloxirane (122) and probably by isomerization of carbenaoxirane (Scheme 104). 

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. 

Ketene, CH2=C=0, is an extremely reactive, toxic gas that sees little use in the laboratory, but is very important in the eommereial synthesis of aeetic anhydride. 

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