Dialkylketenes

Group transfer polymerization allows the synthesis of block copolymers of different methacrylate or acrylate monomers, such as methyl methacrylate and allyl methacrylate [Hertler, 1996 Webster and Sogah, 1989]. The synthesis of mixed methacrylate-acrylate block copolymers requires that the less reactive monomer (methacrylate) be polymerized first. The silyl dialkylketene acetal propagating center from methacrylate polymerization is more reactive for initiation of acrylate polymerization than the silyl monoalkylketene acetal propagating center from acrylate polymerization is for initiation of methacrylate polymerization. Bifunctional initiators such as l,4-bis(methoxytri methyl si loxymethylene)cyclohexane (XXXIII) are useful for synthesizing ABA block copolymers where the middle block is methacrylate [Steinbrecht and Bandermann, 1989 Yu et al., 1988]. 

Carbonyl cyanide reacts readily with ketene and dialkylketenes to give the corresponding dicyano-/3-lactones (equation 109). This reaction seems entirely analogous to the addition of carbonyl cyanide with alkenes to give oxetanes, described in the preceding section (75MI51302). 

If, however, a more reactive ketene is used with excess of the less reactive ketene, cross-dimerization becomes more efficient. The haloketenes represent a more reactive class of ketenes, compared to dialkylketenes, and will give cross-dimerization products as seen in the following examples.15 16... 

Chiral CHdo-alcohols (Fig. 35D (R = Et), F and G) as proton source mediate the enantioselective protonation of Sm-enolates according to Scheme 31 [255]. The optimal molar ratio of DHPEX (Fig. 35G) and HMPA were about 0.7 and 0.6, respectively. Steric factors dominate the enantioselectivity of this reaction sequence when unsymmetrical dialkylketenes are used. High enantiomeric excesses were achieved when the difference between the bulkiness of the alkyl groups for a given substrate is large. The relationship between the enantioselectivity of the protonation and the E/Z selectivity of Sm-enolate formation which is dependent on type of alkyl substitiution was examined. 

SCHEME 42. Addition of butyllithium to isolated186 or in situ generated187 dialkylketenes... 

Cycloaddition to iminolactones (1). Ketene itself does not undergo this reaction, but dialkylketenes react to form spiro-/3-lactams (2). A one-pot procedure starting with maleic anhydride, an amine, and a dialkylketene can also be used. ... 

The anhydrides can be prepared by the action of acetic anhydride on the corresponding malonic acid in the presence of a small amount of sulfuric acid, followed by neutralization of the mineral acid with powdered barium carbonate and evaporation to dryness in a high vacuum. The residual malonic anhydride is then heated to the decomposition point at a low pressure, and the ketene is collected in a cold receiver. This procedure has been applied to the synthesis of low-molecular-weight dialkylketenes (R is methyl, ethyl, f2-propyl, or isopropyl) in 50-80% yields. ... 

Two syntheses involving cycloaddition reactions were the self-condensation of N-aryltrichloroacetimido chlorides, in the presence of antimony pentachloride, which gave 45-63% yields of 3-aryl-2,4-6wtrichloromethyl-3,4-dihydroquinazolines, and the dimerization of A-aryl dialkylketen-imines (71). The imines 71, in the latter reaction, required a temperature of 125°C for several days to give the 4-methylene-3,4-dihydroquinazolines (72) in ca. 50% yields. Quinazolines could not be formed in this reaction when both the ortho positions of the A-aryl group were substituted. 

Dialkylmalonic anhydrides can be converted into dialkylketenes by heating them under diminished pressure at 100-200° 24... 

Only the simpler dialkylketenes such as the dimethyl-, diethyl-, dipropyl-, and diisopropyl-ketene can, however, be obtained by this method. 

Dialkylketenes react with carbon dioxide, mediated by Ph3P, to aflbrd the six-membered ring [2 + 2 + 2] cycloadducts 170 (Scheme 58) (1971JOC2205). The C=C bond in ketenes also contribute in cycloaddition reactions with other cumulenes such as carbon disulfide to provide cycloadduct 171 (1971JOC2205). 

Nitrile oxides, formed in situ from chloro-oximes and triethylamine, react with the keten to give good yields of 1,3-dipolar cycloadducts which are unfortunately contaminated, in most cases, with 0-acylation products, e.g. Scheme 35. It is to be noted that these cycloadditions occur at the keten s carbonyl bond, also the site of its ring-forming reactions with electron-rich olefins and dialkylketens. The 1,3,4-dioxazoles so formed may also be obtained from perfluoroisobutene by treating it with the sodium... 

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... 

Dialkylketenes 39 react with carbon dioxide, catalyzed by PhsP, to give the six-membered ring [2+2+2] cycloadducts 40 . 

Cycloadducts and linear adducts derived from ketenes and ketene 0,0, OJ, NJ and S,S -acetals are also known. However, four-membered ring [2+2] cycloadducts are only obtained in the reaction of diphenylketene with dialkylketene acetals. ... 

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