Reactivity of Carbenes and Ketenes in Low-Temperature Matrices. Carbene CO Trapping, Wolff Rearrangement, and Ketene−Pyridine Ylide (Zwitterion) Observation

Abstract

Dicarbalkoxyketenes 8a and 8b are obtained by photolysis of diazo esters 7 in cryogenic matrices or by FVT of 7 followed by matrix isolation. The photochemical Wolff rearrangement of diazomalonates 1 in Ar matrix at temperatures as low as 6.5 K produces alkoxy(alkoxycarbonyl)ketenes 3, also obtained by FVT of 1. Photolysis of 1 in CO matrix at ≥6.5 K produces the dicarbalkoxyketenes 8. Ketenes 3, 8, and 12 react with pyridine above 40 K to produce ketene−pyridine ylides (zwitterions) 9, 10, and 13, respectively.

Synthetic and Theoretical Aspects of New Dimroth Rearrangement of6‐Aminopyran‐2‐ones to 6‐Hydroxypyridin‐2‐ones via Carbamoyl Ketenes

“…On the other hand the formation of 1,2,3-thiadiazole-4-carboxamides in the reaction of pyranone 1a with piperidine and primary amines [28] can be rationalised by an initial nucleophilic attack at C-4 prior to ring opening (Scheme 3), which is in line with the accepted mechanism of the Dimroth rearrangement. [30,31] It was demonstrated earlier [32,33] that reactions between ketenes and nucleophiles, pyridine in particular, take place at extremely low temperatures (15-40 K), to generate ketene-pyridine zwitterions. The kinetic monitoring of such reactions revealed that they have extremely low activation enthalpies (approx.…”

100 Years of the Wolff Rearrangement

“…In CO-doped argon matrices, the Wolff rearrangement of 157 was suppressed to an extent depending on the concentration of CO, and 166 (R 1 ϭ OMe, R 2 ϭ CO 2 Me) was formed. [153,154] The Wolff rearrangement of 157 was also suppressed by oxygen, but less efficiently than by CO. [153] The trapping of carbonyl carbenes by O 2 produces carbonyl oxides 164 whose secondary reactions are variable. A ketone was eventually obtained from 157 [153] whereas an acid anhydride resulted from 158.…”

“…[150] The generation of carbonyl carbenes in pyridine-doped matrices gave rise to ketene-derived zwitterions 163 rather than to carbene-derived ylides 165. [154,155] In contrast, matrix photolysis of 2-diazoacetylpyridine (167) yielded ylide 169 in addition to ketene 170. [156] Continued irradiation of 169 led to ring opening with formation of 170.…”

Theoretical Studies on the Addition Reactions of Ketene with NH3in the Gas Phase and in Non-Aqueous Solutions

“…For example, the for the rate-limiting step of the reaction path through the C=O bond addition with NH 3 is lower by 2. 16 . This indicates that the solvent effects on the are larger for reactions with (NH 3 ) 2 than those with NH 3 .…”

“…Zwitterions, formed from the reaction of ketene with tertiary amines, 13 have in fact been identified experimentally by UV and IR by use of time-resolved laser flash photolyses 14 and theoretically in AM1 15 or BLYP calculations. 16 The addition of NH 3 or a primary amine to ketene can proceed by two different pathways, stepwise via a ketenenucleophile zwitterion, (I), or concerted via a neutral fourcenter cyclic transition state as shown in Scheme 1. Experimentally, evidence for the zwitterion (I) has been reported by UV/VIS spectroscopic 17 However, it is difficult to determine unambiguously whether (I) or (II) is the initial intermediate, because the characteristic IR absorption of the zwitterionic ylide (I) is similar to the IR absorption for C=C bond stretching of the enol amide (II).…”