Pyridyl ketenes

Laser flash photolysis of 3-pyridyl diazomethyl ketone in acetonitrile containing n-butylamine generates 3-pyridyl ketene as a reactive intermediate, as shown in Scheme 1.5. Subsequent sequential IR measurements at 2125 cm-1 on the microsecond time scale allowed determination of the pseudo-first-order rate constant for its capture by the amine. From the rate law for the capture of the ketene,... 

Scheme 1.5 3-Pyridyl ketene generated by laser flash photolysis in acetonitrile, and trapping by n-butylamine investigated by time-resolved IR measurements [14].

The synthesis of indolizines by 1,5-dipolar cyclization, the reaction of picolinium salts with ketene dithioacetals, the reaction of 2-pyridyl ketene dithioacetals with halogencarbonyls, and a reaction of A7-imines or S-imines with ketene dithioacetals have been reviewed <85YGK669>. 

In the fiash vacuum thermolysis (FVT) or photolysis of 3-acyltriazolo[1.5a]pyridines 60, the corresponding phenyl(pyridyl)ketenes 61 are generated at low temperatures, and they undergo rapid dimerization to give the quinolizine-2,4-diones 62. The dimers, upon... 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Scheme 6.8 gives some examples of ketene-alkene cycloadditions. In Entry 1, dimethylketene was generated by pyrolysis of the dimer, 2,2,4,4-tetramethylcyclobutane-l,3-dione and passed into a solution of the alkene maintained at 70° C. Entries 2 and 3 involve generation of chloromethylketene by dehydrohalo-genation of a-chloropropanoyl chloride. Entry 4 involves formation of dichloroketene. Entry 5 is an intramolecular addition, with the ketene being generated from a 2-pyridyl ester. Entries 6, 7, and 8 are other examples of intramolecular ketene additions. 

Benzene addition to Ir(P Pr3)2Cl is an exothermic reaction (22kcal mol-1), while addition to Ir(P Pr3)2(CO)Cl is endothermic (—5kcal mol-1).501 The reaction enthalpies of substitution reactions to complexes containing the Ir(P1Pr3)2Cl fragment are supplied. Reaction of Ir(P1Pr3)2Cl with 2-pyridyl esters gives a p2 (C,0)-bound ketene, (307), where R2 = R1 = aryl or R2 = aryl,... 

The ketene valence isomers of mesoionic pyrimidinylium olates have been shown to undergo three different reaction pathways (Scheme 1) either retro-ene-type fragmentation to give C3O2 and 2-aminopyridine (path A), electrocycli-zation to a naphthyridine (path B), or cycloreversion to 2-pyridyl isocyanate and a ketene (path C) <2000J(P2)1841>. The exact nature of the product varied with the substituents present. 

In the Reaction of Ketenes with N-(2-Pyridyl)formimidates and Schiff... 

Katagiri et al. investigated the reactions of ketenes with ethyl N-(2-pyridyl)formimidate and Schiff bases derived from 2-aminopyridines (83CPB2899, 83H597 84JHC407, 84MI3 85CPB2671). 

Acetyl-4//-pyrido[ 1,2-a]pyrimidin-4-one 186 was formed in the reaction of ethyl iV-(2-pyridyl)formimidate 187 (R = H) and ketene in acetone at ambient temperature for 2 days in 12% yield (83H597). It was assumed that diketene formed first from ketene, then it reacted with the formimidate 187 (R = H). When excess ketene gas was passed over formimidates 187 (R = H and Me) at 75°C for 0.5-1.5 hours without solvent, 4//-pyrido[ 1,2-a]pyrimidin-4-ones 188 and 7V-(2-pyridyl)formamides were obtained in 25-85% and 0-45% yields, respectively. From the 5-methyl derivative of the formimidate 187 (R = 5-Me) only pyridopyrimidinone 188 (R = 7-Me) was obtained. It was proposed that both products were formed by 1,4-... 

The cycloaddition of ketene diethyl acetal to benzo[6]quinolizinium salt 247, followed by hydrolysis and thermolysis, gave over-crowded 1-pyridyl-8-rerf-butylnaphthalene 248. The naphthalene 248 was oxidized electro-chemically or with anhydrous CuCl2 to afford azoniafluoranthene salt 249,... 

Cyclopropylation. The complex formed from bis(allylpalladium chloride) by treatment with AgBp4 and 2-(2-pyridyl)imidazole mediates cyclopropylation of ketene silyl acetals with allyl acetates. 

Preparation of Carboxylic Acid Chlorides (and Anhydrides). Oxalyl chloride has found general application for the preparation of carboxylic acid chlorides since the reagent was introduced by Adams and Ulich. Acid chlorides produced by this means have subsequently featured in the synthesis of acyl azides, bromoalkenes, carboxamides, cinnolines, diazo ketones, (thio)esters, lactones, ketenes for cycloaddition reactions, intramolecular Friedel-Crafts acylation reactions, and the synthesis of pyridyl thioethers. ... 

Overcrowded 4,5-bis(2-pyridyl)phenanthrenes 28a and 28b were prepared from the Diels-Alder reactions between 4a,5a-diazoniapentaphene diperchlorate (26) and ketene acetals 27a and 27b, respectively (Scheme 5) [16]. The X-ray structure... 

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