Chemistry of cycloheptadienones.  VI.  Structure and thermal isomerization reactions of protonated cyclohepta-3,5-dienones in superacids.  Convenient preparation of cyclohepta-2,4-dienone

. Protonated 2,4-hexadienal (IH) and 2,4,6-octatrienal (2H), prepared by protonation of the analogous aldehydes in FS03H, isomerized to give cyclized products 3H and 4H at 30°C and -20°C respectively. The rate constants for the cyclization of 1H were measured in both FS03H and CF3S03H. It was found that the rate constant for isomerization decreased when CF3S03H was used as the reaction medium. It is suggested that the thermal cyclizations of 1H and 2H involve diprotonated species with protonation occurring on oxygen. 1H and 2H underwent photoisomerization at -78°C in FS03H to yield 5H and 7H respectively. It was found that 5H underwent a cyclization reaction at -40°C to give 6H, which subsequently rearranged to give 3H at -10°C. However, 7H was found to isomerize to 2H at -50°C. The mechanisms of the thermal isomerization are discussed. GEORGE R. ELIA, RONALD F. CHILDS et GARY S. SHAW. Can J . Chem 70, 2065Chem 70, (1992. L'hexa-2,4-diknan (IH) et l'octa-2,4,6-triknal (2H protonts, prCpares par protonation des aldehydes correspondants par le FS03H, s'isomCrisent pour donner les produits cycliques 3H et 4H, a 30°C et -20°C respectivement. On a mesurC les constantes de vitesse pour la cyclisation du produit l H , tant dans le FS03H que dans le CF3S03H. On a trouvC que la constante de vitesse pour 1'isomCrisation diminue lorsqu'on utilise le CF3S03H comme milieu rkactionnel. I1 est suggCrC que les cyclisations thermiques des composes 1H et 2H impliquent des espkces diprotonCes dans lesquelles la protonation se fait sur l'oxygkne. Dans le FS03H, 2 -78"C, les composCs 1H et 2H subissent des photo-isomkrisations qui conduisent respectivement aux composCs 5H et 7H. On a trouvC que le composC 5H subit une reaction de cyclisation i -40°C qui conduit au produit 6H qui se transpose subsCquemment pour donner le compose 3H a -10°C. Toutefois, on a trouvC que, a -50"C, le composC 7H s'isomkrise en composC 2H. On discute des mecanismes des isomCrisations thkrmiques.[Traduit par la redaction]The chemistry of polyenyl carbocations has been investigated extensively (1 -3). The characteristic reaction of this important class of carbenium ions is ring closure of a pentadienyl fragment to form a substituted cyclopentenyl cation. This cyclization reaction has been shown to proceed via a stereoselective 4-n electron electrocyclic ring closure (4, 5). These reactions are generally facile, occurring, for example, at very low temperatures when the ions are generated as long-lived species in strong acid media.There have been fewer studies reported on the chemistry on the hydroxy-or alkoxy-substituted polyenyl carbenium ions, despite the ease by which these ions can be prepared by the protonation of unsaturated carbonyl compounds and their derivatives (6). One important example is the cyclization of protonated cross-conjugated dienones to form protonated cyclopentenones, a reaction that forms the key step in what has become known as the Nazarov reaction (7). This cationic cyclization reaction parallels the cyclization reactions observed ...

Section: Resultssupporting

confidence: 72%

Thermal cyclizations of protonated poly-unsaturated aldehydes

Elia¹,

Childs²,

Shaw³

1992

“…Such a reaction, which may be compared to the photoenolization of the cis isomer of the unprotonated ketone (12) and other related enones (13), would produce the oxygen protonated dienol 8H. This species would be expected to be very short lived in FSOIH and be converted to 2H by protonation C3, proton loss from oxygen, and further protonation at C4 (14). If a "photoenolization" type process were operative, then deuterium incorporation at both C3 and C4 of 2H would be expected to occur when the reaction is carried out in FS03D.…”

Section: Tlme Hmentioning

confidence: 99%

Photoisomerizations of protonated 5-methylhex-3-en-2-one and 4-methylpent-2-enoic acid. Evidence for charge localization during photoisomerization

Childs¹,

DiClemente²,

Lund-Lucas³

et al. 1983

.J. Chcm. 61, 856 (1983).The photoisomcrizations of protonated 5-mcthylhcx-3-en-2-one and 4-methylpent-2-cnoic acid have been exalnined. In each case a relatively rapid c~i,s/trrrtl.s ison~crization about the carbon-carbon partial doublc bond and a slower conversion to protonated dihydrofuran 2H or prvtonatcd lactonc 5H, respcctivcly, were observed. The transformation of the acyclic cations to the five-membered ring cations was shown to occur by a photo-initiated. intramolecular hydride shift from the y to P carbon atom. This shift is fully in accord with an excited state twisting/chasge localization model for the excited states of these systems. Control experiments and the thermal chemistry of these ions is described.

“…The plnr spectrum of the rearranged product, 4H, was identical in all respects to that obtained by protonation of authentic 4 (5). Cyclohepta-2,4-dienone, 4, was obtained on careful neutralization of the acid solution and this was identical in all respects to an authentic sample (5,9). Other acids, such as FS03H or H2SO4, could be used to effect this isomerization; however, with H2S04 some charring occurred on the initial dissolution.…”

Section: Resultsmentioning

confidence: 91%

“…The Cl and C5 methyl signals were sharp singlets whereas the C6 methyl signal was a doublet, J = 7.2 Hz. A11 the resonances of 6H occurred in the regions of the spectrum expected for a protonated cyclohepta-2,4-dienoiie (5).…”

Section: Resultsmentioning

confidence: 93%

“…It might well be expected that a dication such as 17 which could adopt a conformatioil whereby interaction between the two positive centres is minimized, would be preferred over a more rigid one derived by protonation of 16. The alternative deprotonation-protonation sequence is of course also possible with this system; however, with related ketones it has been found that, in FS03H, proton removal from carbon is a slow process and that diprotonated species are frequently involved (5).…”

Section: Discussionmentioning

confidence: 95%

See 1 more Smart Citation

The formation of cycloheptenone and cycloheptadienones from bicyclo[3.2.0]heptanone derivatives. An acid-catalyzed, two carbon ring expansion reaction

Hine¹,

Childs²

1976

In strong acids, such as FSO3H and 96% HzS04, bicyclo[3.2.0]heptan-6-one and bicyclo-[3.2.0]hept-2-en-7-one undergo a clean isomerization to form protonated cyclohept-2-enone and cyclohepta-2,4dienone, respectively. Substituted derivatives undergo comparable ring expansions when dissolved in these strong acids. The seven-membered ring ketones can be recovered on quenching the acid solutions with a NaHCO3-ether mixture. In contrast, bicyclo-[3.2.0]hept-2-en-6-one when dissolved in FSO3H rearranged to give protonated l-acetylcyclopentadiene. The mechanism and synthetic utility of these reactions is discussed. KENNETH E. HINE et RONALD F. CHILDS. Can. J. Chem. 54, 12 (1976).Dans les acicies forts comme FSO3H et HlS04 96y0 la bicyclo[3.2.0]heptanone-6 et la bicyclo[3.2.0]heptkne-2 one-7 subissent des isornerisatio~~s propres conduisant respectivement aux formes protonCes de la cycloheptkne-2 one et de la cycloheptadikne-2,4 one. Des derives portant des substituants subissent des extensions de cycle comparables lorsque dissous dans ces acides forts. On peut rCcupCrer les cCtones cycliques a sept chainons en traitant les solutions acides par un melange NaHCOs-Cther. Par ailleurs le traitement de la bicycIo[3.2.O]heptkne-2 one-6 par FSO3H conduit k la forme protonee de l'acetyl-1 cyclopentadikne. On discute du mecanisme et de l'utilite en synthiese de ces reactions.[Traduit par le journal]There are numerous examples of fused bicyclic ring systems in which one of the rings is a cyclobutanone moiety. Indeed, one common and facile synthetic entry into this type of ring system involves the cycloaddition of a ketene to a cycloalkene (I). If a means could be found of inducing cleavage of the cyclobutanone bond that is conunon to both rings, then combined with this ketene addition, we would have a route t o functionalized monocyclic systems possessing a ring two carbons larger than that of the alkene used in the original cycloaddition. A viable two carbon ring expansion reaction would be of value in general syntheses of medium size rings (2). The well known interconversion of cyclobutyl, cyclopropylcarbinyl, and open chain cationic systems (3) suggested t o us that one method of inducing the desired cleavage of the cyclobutanone bond would be to react this carbonyl function with an electrophile. There are several reports of the isomerization of cyclobutanones upon protonation and it would appear that in nlany of these rearrangements the carbon-carbon bond / 3 to the carbonyl group is cleaved (4). In view of our interest in the chemistry of the monocyclic, unsaturated, seven-membered ring ketones (5), we have examined the effect of protonation on several bicyclo[3.2.0]heptan-6-ones and bicyclo[3.2.0Jheptenones and report here the results of this study. ResultsCareful solution of bicyclo[3.2.0]heptan-6-one, 1, in FS03H at -78 "C yielded a clear yellow solution whose pmr spectrum at +20 "C (