The intermediate formation of ketocarbenes and the conformational control in the Wolff rearrangement of α-diazoketones

Abstract

This prrper is dedictrteil to Prqf'e~sor Hnrry E. Gut~tlitzg 011 the occ.n.siotz of' his 65th b i r t h~l q MANUEL TORRES, JUAN RIBO, ALLAN CLEMENT, and O R 0 P. STRAUSZ. Can. J. Chem. 61, 996 (1983). The conformational control of the Wolff rearrangement (WR) has bcen studied by Incans of a comparative study of the low temperature photolysis of argon-matrix isolated a-diazo ketones, 1, and of vinylcne thioxocarbonates, 2. Under these conditions thc s-E, s-Z conformational equilibrium of 1 will be hindered whereas 2 has a fixed s-Z conformation. The results indicate that the conformational control observed in the WR of a-diazo ketones is not per se a proof for a concerted mechanism and that a mechanism involving a ketocarbene intermediate is the most reasonable.MANUEL TORRES, JUAN RIBO, ALLAN CLEMENT ct ORO P. STRAUSZ. Can. J. Clicm. 61, 996 (1983). On a CtudiC le contrble conformationnel de la transposition de Wolff (TW) au moyen d'une Ctude comparative de la photolyse i basse tempkrature des a-diazo cCtones 1 et des thioxocarbonates de vinylhe 2, isol6s dans une matrice d'argon. Dans ces conditions, I'Cquilibre conformationnel s-E, s-Z du composC 1 sera emp&chC tandis que le composC 2 a une conformation s-Z fixe. Les rtsultats indiquent que le contrble conformationnel observe dans la TW des a-diazo cCtones n'est pas en soi une preuve de I'existence d'un mecanisme concert6 et qu'un mCcanisme impliquant un citocarbkne comme intermidiaire est plus raisonnable.[Traduit par le journal]The Wolff rearrangement (WR) of a-diazo ketones and a-diazo esters to ketenes can be brought about either photolytically, thermally, or catalytically and ketocarbenes are generally thought to be implicated as short-lived reactive intermediates in the photochemical and thermal reactions (1 -12).Unambiguous evidence for the intermediacy of ketocarbenes and carbalkoxycarbenes came from the detection of oxirenes as reactive intermediates in the WR (4, 10, 11) kinetic studies (7) as well as from the effects of the substituent upon nitrogen elimination (6). Moreover, molecular orbital computations predict a prohibitively high energy requirement for concerted nitrogen elimination (12). Also, the low temperature stabilization and direct esr.detection of a series of triplet state ketocarbenes from the photolysis of a-diazo ketones has been reported (1 3 -18). These observations are in agreement with molecular orbital predictions of a triplet ground state for these species, with a small singlet-triplet energy separation (19).A concerted mechanism for the WR would be in clear contradiction to the now well established transient formation of oxirene in a large number of photochemical WR (for a recent review, see ref. 20). Recently, however, Kaplan and Mitchell (21) have restated an earlier suggestion (22) of an alternative concerted mechanism for the decomposition WR sequence of those a-diazo ketones in which the intermediacy of oxirene was not detected. The a-diazo ketones may exist as two rotational isomers of the type s-E and s-Z and the propose...

On the infrared spectrum and dipole moment of dichloroketene

“…I(b)). These are the same as the wavenumber values observed for dichloroketene by Torres et al (5); indeed, the strong band at 2160 cm-', attributable to the CO stretching frequency, provides a unique fingerprint for ketene-like molecules (7), and in similar experiments we have observed such a band for monochloroketene. When the reaction products were isolated in the cell these bands disappeared over the space of about 30 min, leaving bands attributable to CO and C 0 2 (Fig.…”

Oxidation of Difluorovinylidene

“…The frequency of n as (C-CO) is close to that of ketene 4 (in argon: 2142 cm À1 ) [26] and dichloroketene 5 (in argon: 2155 cm À1 ) [27] , while n s (CCO) is blue-shifted by 46 cm À1 relative to 4 and 137 cm À1 relative to 5.…”

100 Years of the Wolff Rearrangement

“…[158] When butane-2,3-dione (175) was deoxygenated by cocondensation with carbon vapor at 77 K, 3-buten-2-one was the predominant product. [159] Both dimethylketene and 3-buten-2-one were formed in the photolysis of 3-diazobutan-2-one [158] and in the reaction of 2-butyne with O( 3 P) atoms (triggered by photolysis of Ar/ ozone/2-butyne matrices). [160a] The products point to the intervention of 3-oxo-2-butylidene (176), but how to explain the divergent product ratios?…”