A study of the vacuum pyrolysis of 4-diazoisothiochroman-3-one with Hel ultraviolet photoelectron spectroscopy

Abstract

A newly developed ultraviolet photoelectron spectrometer apparatus that utilizes a tunable 50 W CW CO:, laser as a directed heat source is used to study the vacuum pyrolysis of 4-diazoisothiochroman-3-one (la). Analysis of the pyrolysate with ultraviolet photoelectron spectroscopy shows that l a undergoes a facile pyrolysis at a laser power level of less than 26 W, yielding two new compounds: thiaketene 3a, the product of a Wolff rearrangement, and benzocyclobutenthione (6a), which can be derived from thiacarbene 4a, the decarbonylation product of 3a. Activation enthalpieslenergies calculated at the AM 1 and ab initio levels of theory indicate that, unlike the case of 4-diazoisochroman-3-one (lb), the Wolff rearrangement of the incipient carbene may be concerted with loss of nitrogen from l a . The activation enthalpylenergy calculated for the decarbonylation of 3a is significantly higher (AM1,20.5 kcallmol; RHFl6-3 lG(d), 11.7 kcallmol; MP2(full)llRHFl6-31G(d), 14.3 kcallmol) than the activation enthalpylenergy for the decarbonylation of 3b. This result is in keeping with the fact that we detect 3a, but 3b is not found in detectable amounts in the pyrolysate of lb.Key words: vacuum pyrolysis, 4-diazoisothiochroman-3-one, He1 ultraviolet photoelectron spectroscopy, AM1 and ab initio calculations.Resume : On a utilist un spectromktre de photoCmission de photoklectrons dans l'ultraviolet rkcemment mis au point qui utilise un laser ajustable au CO1 de 50 W CW comme source dirigCe de chaleur, pour Ctudier la pyrolyse sous vide de la 4-diazoisothiochroman-3-one (la). L'analyse du pyrolysat par la spectroscopie de photoCmission de photoClectrons dans l'ultraviolet montre que le composC l a subit facilement la pyrolyse i un niveau de puissance du laser infkrieure i 26 W pour donner deux nouveaux composCs : la thiacCt2ne 3a, le produit d'un rearrangement de Wolff, et la benzocyclobutenthione (6a) qui peut provenir du thiacarbkne 4a, le produit de dCcarbonylation du produit 3a. Les Cnergieslenthalpies d'activation calculCes aux niveaux thkoriques AM1 et ab initio indiquent que, contrairement au cas de la 4-diazoisochroman-3-one (lb), la transposition de Wolff du carbkne naissant peut-&tre concertCe avec la perte d'azote i partir de la. L'tnergielenthalpie d'activation calculte pour la dCcarbonylation de 3a est de f a~o n significative plus ClevCe (AM1,20,5 kcallmol; RHFl6-31G(d), 11,7 kcallmol; MP2(complet)llRHFl6-3 lG(d), 14,3 kcallmol) que 1'Cnergielenthalpie d'activation pour la dkcarbonylation de 3b. Ce rCsultat est en accord avec le fait que nous avons detect6 le composC 3a mais le compost 3b n'est pas present en quantitks detectables dans le pyrolysat de lb.Mots clPs : pyrolyse sous vide, 4-diazoisothiochroman-3-one, He1 spectroscopie de photoCmission de photoClectrons dans l'ultraviolet, calculs AM1 et ab initio.[Traduit par la rkdaction] I

A study of the vacuum pyrolysis of para-substituted diazoacetophenones with He(I) ultraviolet photoelectron spectroscopy

“…3(c)) and no CO (it exhibits a sharp band at 14.01 eV (39)) was formed when the laser power was increased from 26 W to 42 W (~850 ± 50°C) indicates that the loss of CO from 2a is subject to a high activation energy. 4 This observation is in keeping with the results of our previous calculational study (5). The spectrum of 2a obtained in this study differs to some degree from the published spectrum of phenylketene that was prepared by the zinc-induced dechlorination of 2chloro-2-phenylacetyl chloride (13).…”

“…While the calculated first IP for pchlorophenyloxirene is close to the first IP in the pyrolysis spectrum, at this point we cannot say with certainty whether it actually is a pyrolysis product of 1d. 5 The low volatility of 1e thwarted our attempts to obtain its spectrum and study its pyrolysis. Even heating a relatively large sample (40 mg) of 1e at 94°C near its melting point of 116°C in a 4 mm outer diameter tube inserted into the probe did not yield an acceptable spectrum.…”

“…To widen our research in this field, we undertook a study of the vacuum pyrolysis of diazoacetophenone (1a), p-methyldiazoacetophenone (1b), p-methoxydiazoacetophenone (1c), p-chlorodiazoacetophenone (1d), and pnitrodiazoacetophenone (1e) that are potential precursors of ketocarbenes and the corresponding para-substituted phenylketenes, which are the Wolff rearrangement products (7). Only a limited number of ketenes which cannot be isolated in the condensed phase at ambient temperature have been studied in the gas phase with PE spectroscopy (5,(8)(9)(10)(11)(12)(13), and there is comparatively little experimental data available on the vacuum pyrolysis of α-diazocarbonyl compounds (see, for example, ref. 14) in contrast to the large body of work that has been accumulated on the photochemistry of this class of compounds (14)(15)(16)(17)(18)(19)(20)(21).…”

100 Years of the Wolff Rearrangement

“…When 4-diazoisothiochroman-3-one (390, X ϭ S) was thermolyzed, both 391 and 392, X ϭ S, were detected by PE spectroscopy. [300] Scheme 49. Wolff rearrangement followed by ketene decarbonylation Broad-band irradiation of 2-diazo-1,3-diketones 26 in Ar matrices at 12 K produced ketoketenes 393 in less than 10 min.…”

Thermolysis of 2,2-dimethoxy-5,5-dimethyl- Δ3- 1,3,4- oxadiazoline studied with photoelectron spectroscopy. He(I) photoelectron spectrum of dimethoxycarbene

“…Our unique ultraviolet PE spectrometer -CW CO 2 laser apparatus (3) allows us to use the laser as a directed heat source to produce a hot zone as short as 1 mm in length on the tip of a quartz tube positioned at about 1 cm from the point of intersection with the ionization beam. We used this setup successfully to obtain PE spectra of short-lived reactive species such as benzazete and o-benzyne (4), 1,2-cyclohexadiene (5), 1-(2thiaindane)ketene and benzocyclobutenethione (6), as well as p-substituted phenylketenes (7) through pyrolyses of suitable precursors (Scheme 1).…”