Ketene generation, photochemical

A stereoselective synthesis of aminoalkyl-substituted P-lactams (41) has been developed, the key step of which is [2+2] cycloaddition of imines to ketenes generated photochemically from diazoketones (42), which are derived from protected a-amino acids. A number of steroidal diazoketones related to progesterone have been synthesized as potential photoaffinity labelling reagents for the mineralocorticoid receptor. ... 

Schultz and Kulkarni have explored the possibilities of using this process in asymmetric synthesis by trapping the ketene generated from these photochemical ring-opening... 

The orientation of addition of unsaturated ketones to allenes and ketens dififers. The reactions of the structurally similar allene (129) and keten (130) have been studied. Photochemically, the allene gives the methylenecyclobutane (131), which was converted into the ketone (132). The keten, generated by photolysis of the diazoketone (133), gave the isomeric tricyclic ketone (134). If the additions occur by the Corey mechanism, then the different ground-state charge distributions of the allene and the keten could be responsible for the different orientations of addition. 

The comparison with ketenes is made because, for example, the photochemical reaction to give j9-lactams via chromium carbene complexes gives product distributions that closely parallel those obtained using ketenes generated using acid chloride. ... 

There are several reactions that are conceptually related to carbene reactions but do not involve carbene, or even carbenoid, intermediates. Usually, these are reactions in which the generation of a carbene is circumvented by a concerted rearrangement process. Important examples of this type are the thermal and photochemical reactions of a-diazo ketones. When a-diazo ketones are decomposed thermally or photochemically, they usually rearrange to ketenes, in a reaction known as the Wolff rearrangement.232... 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

Scheme 16.17. Photochemical generation of 50 and 51 from anhydrides 61 and 62. Compound 50 is photocarhonylated to cyclopentadienylideneketene (63). The analogous photoreaction of 57 to ketene 64 was not observed. ...

Intramolecular photoaddition of (terminal) allenes or ketenes to a cyclohex-2-enone C —C double bond have been investigated. The former reaction occurs regiospecifically in high yield,2 the latter, wherein the ketene moiety is photochemically generated, proceeds in moderate yield.3... 

The diverse chemistry of carbenes is beyond the scope of this account, but a few typical reactions are shown here to illustrate the usefulness of the photochemical generation of these reactive species. A carbene can insert into a C—H bond, and this finds application in the reaction of an a-diazoamide to produce a P-lactam (5.29). Carbenes derived from o-diazoketones can rearrange to ketenes, and thus a route is opened up to ring-contraction for making more highly strained systems <5.301. Carbenes also react with alkenes, often by cycloaddition to yield cyclopropanes in a process that can be very efficient (5.31) and highly stereoselective (5.321. 

CARBENE. The name quite generally used for the methylene radical, CH,. It is formed during a number of reactions. Thus the flash photochemical decomposition of ketene (CH2=C=0) has been shown to proceed in two stages. The first yields carbon monoxide and CHj. the latter then reacting with more ketene to form ethylene and carbon monoxide. Carbcne reacts by insertion into a C- H bond to form a C-CH, bond. Thus carbene generated from ketene reacts with propane to form, i-butane and isobutane. Carbene generated by pyrolysis uf diazomethane reacts with diethyl ether to form ethylpropyl ether and ethylisopropyl ether. 

The photochemical generation of metal-bound ketenes from carbene-chromium complexes and the subsequent coupling with imines to give azetidin-2-ones is treated separately (Section 2.01.3.10.5). 

Single pulse, shock tube decomposition of acetic acid in argon inv olves the same pair of homogeneous, molecular first-order reactions as thermolysis (19). Platinum on grapliite catalyzes the decomposition at 500—800 K at low pressures (20). Ketene, methane, carbon oxides, and a variety of minor products are obtained. Photochemical decomposition yields methane and carbon dioxide and a number of free radicals, wliich have complicated pathways (21). Electron impact and gamma rays appear to generate these same products (22). Electron cyclotron resonance plasma made from acetic acid deposits a diamond [7782-40-3] film on suitable surfaces (23). The film, having a polycrystalline stmcture, is a useful electrical insulator (24) and widespread industrial exploitation of diamond films appears to be on the horizon (25). 

In contrast to the carbene and carbenoid chemistry of simple diazoacetic esters, that of a-silyl-a-diazoacetic esters has not yet been developed systematically [1]. Irradiation of ethyl diazo(trimethylsilyl)acetate in an alcohol affords products derived from 0-H insertion of the carbene intermediate, Wolff rearrangement, and carbene- silene rearrangement [2]. In contrast, photolysis of ethyl diazo(pentamethyldisilanyl)acetate in an inert solvent yields exclusively a ketene derived from a carbene->silene->ketene rearrangement [3], Photochemically generated ethoxycarbonyltrimethyl-silylcarbene cyclopropanates alkenes and undergoes insertion into aliphatic C-H bonds [4]. Copper-catalyzed and photochemically induced cyclopropenation of an alkyne with methyl diazo(trimethylsilyl)acetate has also been reported [5]. 

In 1968, Qriffiths and Hatt established that although dienylketene (282) (generated as the only photochemical product from cyclohexadienone 281) cyclizes thermally to (281), other thermal cyclizations compete, such as the foimaticxi of the bicyclo[3.1.0]hexenone (283) or solvent (amines or alcohols) addition product (284).The thermal formadm of bicyclo[3.1.0]hexenones (283) from ketenes (282) can be (diotochemically reversed. Thus 2,4-cyclohexadienones (287) can be obtained from 2,S-cyclohexa-dienones (285).The stereospecificity of the thermal ring opening of the bicyclo[3.1.0]hexenones to dienylketenes has been established. 

The competition of nucleophilic attack by solvent at the ketene sp center has been shown to be useful for obtaining aza analogues of dienylketenes, namely dienylketenimines (297) fixHn ketene intermediates generated from (295). Upon dehydration, the addition product (296) afforded directly the Af-alkylated cy-clohexadienimines (298) in go( yield. The expected dienylketenimines (297) were not isolated. Regeneration of spectroscopically detectable (297) can be achieved by photochemical irradiation of (298), its stability above 0 C being solvent depen nt. ... 

R.L. Danheiser and co-workers generated a key vinylketene intermediate via tandem Wolff rearrangement-ketene-alkyne cycloaddition to utilize it in a photochemical aromatic annulation reaction Danheiser benzannulation) for the total synthesis of the phenalenone diterpene salvilenone. ... 

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