Ketenes with alcohols

Exercise 17-38 Write reasonable mechanisms for the reaction of ketene with alcohols and amines. Would you expect these reactions to be facilitated by acids, or by bases ... 

The methoxyketene 297, coordinated to Cr carbonyl, is formed from methoxy-carbene easily by insertion of CO under irradiation [90]. An ester is formed by the reaction of ketene with alcohol. The aminocarbene complex 298 was prepared from benzamide and converted to phenylalanine ester 300 under irradiation of sunlight in alcohol via ketene 299 [91]. The eight-membered lactone 304 was prepared in high yield by the reaction of the alkyne 301 having the OH group in a tether with Cr carbene without irradiation. The vinylcarbene 302 is formed at first and converted to the vinylketene intermediate 303 as expected. The keto lactone 304 is formed from 303 by intramolecular reaction with the OH group and hydrolysis [92],... 

The reaction of ketene with alcohols to produce esters is attractive, since all the ketene goes into the product arid there are no by-products. Ketene CHj=CO + CjHsOH CHiCOOCjH,... 

The facile reaction of ketenes with alcohols and amines led to investigations of the possible utihty of ketenes in polymer chemistry more than 75 years ago. There is renewed interest in the utilization of ketenes in polymer chemistry, and this work has been reviewed, including many examples of ketenes derived from Meldrum s acid derivatives. 

In the presence of strong acid, such as boron trifluoride [7637-07-2] appropriately substituted acyl chlorides (7, R = CCl ) add to ketene to form the corresponding acetoacetyl chlorides, which can further react with alcohols to form the corresponding acetoacetates. 

Petoxycatboxyhc acids have been obtained from the hydrolysis of stable o2onides with catboxyhc acids, pethydtolysis of acyhinida2ohdes, reaction of ketenes with hydrogen peroxide, electrochemical oxidation of alcohols and catboxyhc acids, and oxidation of catboxyhc acids with oxygen in the presence of o2one (181). 

Ketene, like acid anhydrides, reacts with alcohols to form (acetate) esters ... 

Photodriven reactions of Fischer carbenes with alcohols produces esters, the expected product from nucleophilic addition to ketenes. Hydroxycarbene complexes, generated in situ by protonation of the corresponding ate complex, produced a-hydroxyesters in modest yield (Table 15) [103]. Ketals,presumably formed by thermal decomposition of the carbenes, were major by-products. The discovery that amides were readily converted to aminocarbene complexes [104] resulted in an efficient approach to a-amino acids by photodriven reaction of these aminocarbenes with alcohols (Table 16) [105,106]. a-Alkylation of the (methyl)(dibenzylamino)carbene complex followed by photolysis produced a range of racemic alanine derivatives (Eq. 26). With chiral oxazolidine carbene complexes optically active amino acid derivatives were available (Eq. 27). Since both enantiomers of the optically active chromium aminocarbene are equally available, both the natural S and unnatural R amino acid derivatives are equally... 

As with alcohols, ketenes add thiols to give thiol esters (R2C=C=0- - RSH-R2CHCOSR). 

Conversion of Free or Silylated Carboxylic Acids into Esters, Thioesters, Lactones, or Ketenes. Transesterification of Esters with Alcohols... 

The thermolysis of isopropylidene 2-azacycloalkylidenemalonates (468, R2 = R4 = H, n = 0-2) gave unstable ketenes (1641), which were reacted with alcohols, thiols, and protic amines to afford Z-enamino ester derivatives (1642) in 63-98% yields (81TL963). 

These reactions may first form the ketene acetals which react with alcohols to give the ortho esters. The generality of these reactions is still unknown thus further research on these methods is required. 

The addition of alcohols to ketene acetals allows the synthesis of mixed ortho esters [96, 120a-c, 121a, b, 124, 125a, b]. a-Haloaldehydes may be converted to ortho esters by the following process (a) acetal formation, (b) de-hydrohalogenation, and (c) reaction with alcohols via addition reaction (33). In general, the method above, using ketene acetals, is not practical since ketene acetals are not readily available and are difficult to prepare. However, the method is useful because it allows the synthesis of mixed ortho esters and other ortho esters more difficult to synthesize [122-127]. Recently a simple one-step synthesis of ketene acetals and ortho esters has been reported (see p. 56). 

Ketene is known to be a very active acetylating agent. This reagent reacted with the complex, [(CH3)4N] [Cr(HO-A)2], in acetonitrile solution to produce the diester. However, ketene, which normally reacts with alcohols very quickly, reacted slowly in this case (Table I). In all ketene reactions a large excess of ketene was passed through the solution the critical factor determining yield appears to be the total time of refluxing. 

Cleavage is often associated with elimination of small, stable, neutral molecules, such as carbon monoxide, olefins, water, ammonia, hydrogen sulfide, hydrogen cyanide, mercaptans, ketene, or alcohols, often with rearrangement (Section 2.8). 

New synthesis of/11 CJpropyl ketene (201) and its reactivity with alcohols... 

If the alcoholate or the alkylating reagent contains a carboxylic acid ester, acylation of the alcoholate can compete with alkylation. This potential side reaction does not cause trouble in the examples sketched in Scheme 6.14 (first and third reactions), because these esters are sterically hindered and devoid of a hydrogen (no ketene formation can occur) but, as illustrated in Scheme 6.15, less hindered esters can readily undergo transesterification with alcoholates. 

The cycloaddition of chiral (-)menthylcarbodiimide with prochiral ketenes affords chi-rally selective cycloadducts In the reaction of an optically active alcohol with dicy-clohexylcarbodiimide complete inversion of the configuration occurs after hydrolysis. " Treatment of arenesulfenic acids with alcohols, thiols or secondary amines in the presence of optically active carbodiimides affords the corresponding optically active arenesulfenic acid derivatives. DCC is used to convert an optically active selenoxide into the corresponding optically active selenimide with TsNHa. ... 

The formation of ketene intermediates facilitates the carbodiimide mediated esterification reaction with alcohols and phenols when carboxylic acids with an electron withdrawing group in the a-position are used as substrates. ... 

Ketene acetals, preparation, 267, 269 Ketene dimers, depolymerization, 407 reaction with alcohols, 483... 

Chelation with copper(ll) Lewis acids has been used to change the reactivity of bis ketenes toward alcohols. The alcoholysis of 52 in the presence of copper acetylaceto-nate derivatives afforded the cyclic lactones 54 (Sch. 14) [35]. The addition of alcohols to 52 without the Lewis acid led to the formation of ketene esters 53 which would not undergo cyclization upon treatment with the copper(ll) complexes. It is suggested that chelation of the bis-ketene in a jr-fashion accounts for the lactone products. Although chiral Lewis acids were used, the products were obtained as racemates probably because of very facile epimerization. 

A significant advance in the study of cyclopropanones resulted from studies by the Turro and the de Boer groups on the formation of cyclopropanones in solution by the reaction of ketenes with diazoalkanes. These investigators found that it is possible to store the parent ketone only for short periods at low temperature because of its unusual reactivity and propensity for polymerization. Subsequent work has shown that cyclopropanone seems to show chemical behavior similar to that of ketene. Thus, it is attacked by nucleophilic species such as water, alcohol and amines and reacts rapidly with itself to form... 

Further reports have appeared on the reaction of trivalent phosphorus compounds with acetylene dicarboxylates. In the first, alkyl diphenylphosphinites (e.g.57) are shown to react with dialkylacetylene dicarboxylates (e.g.58) in the presence of carbon dioxide to form 1,2-oxaphosphol-3-enes (e.g.59) which in the presence of excess phosphinite decompose via (60) to give di-ylids (e.g.61). On the other hand, the phosphoranes (62) from phosphonites and phosphites react with a further phosphorus component to give the ylids (63) which are readily converted by treatment with alcohol into phosphonates (65) apparently via ketene intermediates (64) as evidenced by and isotopic tracer studies. ... 

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