Elimination reactions ketenes from

The reaction involves an attack by the tellurium anion at the halogen atom followed by a rapid elimination of ketene from the resulting enolate. 

The elimination of alcohol from P-alkoxypropionates can also be carried out by passing the alkyl P-alkoxypropionate at 200—400°C over metal phosphates, sihcates, metal oxide catalysts (99), or base-treated zeoHtes (98). In addition to the route via oxidative carbonylation of ethylene, alkyl P-alkoxypropionates can be prepared by reaction of dialkoxy methane and ketene (100). 

Ketene acetal synthesis by /1-elimination of haloacids from halogenated acetals under well controlled conditions using thermal activation (A), ultrasound (US) or micro-wave irradiation [92] (MW) has been described. From a mechanistic point of view, as the TS is more charge delocalized than the GS and the polarity is enhanced during the course of the reaction, a favorable microwave effect can therefore be observed (Eqs. (37) and (38) and Scheme 3.13). 

Cyclic ketene acetals, which have utility as co-polymers with functional groups capable of cross-linking, etc., have been prepared by the elimination of HX from 2-halomethyl-l,3-dioxolanes. Milder conditions are used under phase-transfer conditions, compared with traditional procedures, which require a strong base and high temperatures. Solid liquid elimination reactions frequently use potassium f-butoxide [27], but acceptable yields have been achieved with potassium hydroxide and without loss of any chiral centres. The added dimension of sonication reduces reaction times and improves the yields [28, 29]. Microwave irradiation has also been used in the synthesis of methyleneacetals and dithioacetals [30] and yields are superior to those obtained with sonofication. 

Ketenes are normally prepared by the base-catalysed elimination of HC1 from an acid chloride 9 or by elimination of chlorine from a chloroalkyl acid chloride with zinc dust, often assisted by ultrasound. For reactions with nucleophiles, the solution would already contain the nucleophile before the ketene 6 was generated. 

A study of ketene A, O-acetals has shown that such compounds derived from lactam acetals may be used in the preparation of larger heterocyclic systems via reaction with a 1,3-dipolar species, the initial cycloadducts stabilizing their structures by aromatization. Pyrrolo[2,3-i/]-l,2,3-triazole derivatives, however, do not undergo such elimination reactions and stable cycloadducts may be obtained. Thus, the lactam acetals (133) give the ketene A,0-acetals (134), which on reaction with p-nitrophenyl azide yield substituted pyrrolo[2,3-d]-l,2,3-triazole derivatives (135) <86CB359l>. 

Carbenes are formed in a number of other similar reactions—for example, loss of carbon monoxide from ketenes or elimination of nitrogen from azirines—but these are rarely used as a way of deliberately making carbenes. 

The ketene-alkene cycloaddition gives cyclobutanones in a thermal reaction. Most ketenes are not kinetically stable, so they are usually generated in situ, either by E2 elimination of HC1 from an acyl chloride or by a Wolff rearrangement of an a-diazoketone (see Chapter 2). 

The ratio of spirohexan-4-one/spirohexan-5-one was close to 1 1 which can be rationalized by assuming a diradical mechanism as proposed for the cycloaddition of dimethylketene to methylenecyclopropane. Dichloroketene also underwent nonregioselective addition to methylenecyclopropane (49% yield) however, 4,4-dichlorospiro[2.3]hexan-5-one was formed as the main product. When methylenecyclopropane was reacted with chloro(2,2,2-tri-chloroethyl)ketene, generated by elimination of chlorine from 2,2,4,4,4-pentachlorobutanoyl chloride with zinc and phosphoryl chloride, 4-chloro-4-(2,2,2-trichloroethyl)spiro[2.3]hexan-5-one (10) was isolated in 31% yield. The isolation of a single isomer instead of mixtures as described above is possibly due to the different reaction conditions which do not guarantee an uncatalyzed cycloaddition. 

Acyl halides bearing a-hydrogen atoms, and acetyl chloride in particular, have limited stability in the presence of strong Lewis acids. Elimination of proton from the acylium ion leads to the ketene. This intermediate undergoes ready Friedel-Crafts acylation, acetyl chloride eventually forming the diacetylace-tylium ion, ° which is a poor acylating agent. For this reason, excessive reaction temperatures and times should be avoided in Friedel-Crafts acetylations. 

Methyl-l,2,4-triazines react with ketene OW-acetals to afford 5-[2-(dimethylamino)-prop-l-enyl]-l,2,4-triazines 10 (see Houben-Weyl, Vol. El5, p 1898). The reactions start with deprotonation of the methyl group by the ketene 0,Ar-acetal followed by combination of the two ions formed. Elimination of alcohol from the adduct 9 yields the isolated product 10.343... 

The first step in the reaction scheme involves the elimination of nitrogen from the diazoquinone (VI), forming a carbene (VII). The carbene undergoes a Wolf rearrangement, involving ring contraction, and leading to the formation of ketene (VIII). In the presence of small amounts of water in the novoiac binder," hydration of the ketene leads to the formation of indene carboxylic acid (IX). The main feature of the process is the transformation of a fairly hydrophobic... 

Enolates. The /f-H elimination from enolates involves formal elimination of ketene, and has been recognized in the decomposition of [Ru(Me) OC(CH2)H (PMe3)4] to [Ru(Me)(H)(PMe3)4], upon warming it in solution to 65°C. When this thermolysis was run in the presence of tert-butyl alcohol as a trap, tert-butyl acetate was formed in 10-15% yield, consistent with the formation of ketene during the course of the reaction [106]. 

Monoalkylketenes are also prone to dimerization, but dialkylketenes have longer lifetimes. The remarkably crowded and unreactive di-teri-butylketene 5 bears strong steric protection and was first prepared in 1960 from the acyl chloride using a strong base (Eqn (4.4)), and identified by the characteristic ketenyl IR absorption. The dehydrochlorination reaction has also been carried out with triethylamine as the base using ultrasound in 86% yield or by reaction with neat tri- -butylamine at 80 °C, also in 86% yield. The use of the aldehyde i-Bu2CClCH=0 as an alternative precursor to 5 by an elimination reaction has also recendy been reported. This ketene is stable indefinitely as a neat liquid and reacts slowly with and there is no... 

The most common method of formation of ketenes from carboxyUc acid derivatives is by ieh-amine catalyzed elimination from acyl haHdes, as used in the preparation ofdiphenylketene (Eqn (4.3)) and in many other examples, but under these conditions reactions of the ketenes with the terijaty-amines ate not ordinarily observed. The reactions of ketenes with tertiary-zmines shown above occur at high amine concentrations and in the absence of other reactants to consume the ketenes, therefore products from such reactions using stoichiometric amounts of amines are not ordinarily observed. ... 

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