Cyclobutene photochemistry. Involvement of carbene intermediates in the photochemistry of alkylcyclobutenes

Self Cite

WILLIAM J. LEIGH. Can. J. Chem. 71, 147 (1993). Direct photolysis of alkyl-substituted cyclobutene derivatives in solution with monochromatic far-UV light sources results in competing ring opening to the corresponding substituted 1,3-butadiene derivatives and fragmentation to the corresponding alkene and alkyne via formal [2+2] cycloreversion. The excited states leading to the two types of products have been identified. Cycloreversion occurs stereospecifically and arises as a result of excitation of the n,R(3s) Rydberg state, which is of comparable energy to the n , n * state in alkylcyclobutenes. In spite of its stereospecificity, the reaction is nonconcerted; evidence is presented to suggest that it occurs by a mechanism involving [I ,2] migration (ring contraction) to yield a cyclopropylcarbene intermediate, which then fragments to yield the alkyne and alkene. The stereochemistry at C3 and C4 in the starting cyclobutene is retained in the alkene produced in the reaction. Ring opening proceeds nonstereoselectively (from the n , n * singlet state) in well over a dozen systems that have been investigated, and possible mechanisms to account for this are discussed. It is proposed that orbital symmetry plays a role in the reaction, but only in the early stages of ring opening. Bicyclic cyclobutene derivatives in which the cyclobutene double bond is located across the [O] bridge undergo ring opening with a high degree of disrotatory stereoselectivity. Possible reasons for this unusual behavior are discussed in light of recent a b irlitio theoretical results.WILLIAM J. LEIGH. Can. J. Chem. 71, 147 (1993). La photolyse directe de derives du cyclobutkne substituts par des groupes alkyles, realisee en solution, a I'aide de sources de lumikre UV-lointain monochromatique, conduit a une competition entre l'ouverture de cycle conduisant aux derives buta-1,3-dibnes substitues correspondants et une fragmentation conduisant h I'alc6ne et a l'alcyne correspondants par le biais d'une cyclortversion [2+2] formelle. Les Ctats excites conduisant a ces deux types de produits ont etC identifies. La cyclorCversion se produit stCrtospkcifiquement et elle est le rtsultat de I'excitation de 1'Ctat Rydberg 7r,R(3s) dont l'knergie est comparable celle de 1'Ctat n , a * des alkylcyclobutknes. Malgrt sa stCrtospCcificit~, la reaction n'est pas concertee; on presente des donnees suggerant qu'elle se produit par un mtcanisme impliquant une migration [1,2] (contraction de cycle) qui conduit 6 un intermediaire cyclopropkne qui se fragmente ensuite pour donner l'alcene et l'alcyne. La stereochimie en C3 et en C4 du cyclobutkne de depart est conservke dans l'alckne qui se forme au cours de la reaction. L'ouverture du cycle se produit d'une faqon qui n'est pas sttrCospCcifique (a partir d'un etat singulet n,n4') dans plus d'une douzaine de systkmes que nous avons examines; on discute des divers mecanismes possibles pour expliquer les rksultats. On propose que la symetrie des orbitales joue un r81e dans la reaction, mais seulement au debut de la rea...

supporting

confidence: 84%

“…[ l I]), has been investigated directly (24). Cyclopropylcarbenes are known to undergo ring contraction to yield cyclobutenes in addition to fragmenting to an alkene and acetylene.…”

Section: Scope and Rnechanism Of The Cycloreversion Reactionmentioning

confidence: 99%

1992 Merck–Frosst Award Lecture Orbital symmetry and the photochemistry of cyclobutene

Leigh¹

1993

Self Cite

“…There is a similar substituent effect on the relative energies of the n , n * and n,R(3s) states in alkenes, as determined by gas-phase UV absorption (12) and fluorescence emission spectroscopy (13), and the n,R(3s) state is the lowest energy state in tri-and tetrasubstituted alkenes (12,14). We have recently reported evidence for carbene involvement in cyclobutene photochemistry (15). Although we were unable to quantify the extent of carbene involvement in those cases, the results suggest that the n,R(3s) state does contribute to the photochemistry of alkylcyclobutenes, as it does in the case of simpler alkenes.…”

Section: Introductionmentioning

confidence: 63%

“…Cis-and trans-1,3,4-trimethylcyclobutene (cis-and trans-3) and cis and trans-1,2,3,4-tetramethylcyclobutene (cis-and trat~s-4) were prepared by routes similar to those published previously for related methylated cyclobutenes (1,15), starting with the acetophenonesensitized photocycloadditions of citraconic anhydride or dimethylmaleic anhydride (prepared by the method of Baumann and Bossard (26)) with 2-butene (17), followed by hydrolysis of the bicyclic anhydrides and bisdecarboxylation of the resulting dicarboxylic acids with lead tetraacetate (27). In both syntheses, the two cyclobutene isomers were obtained in similar yields.…”

Section: Preparation Of Compoundsmentioning

confidence: 99%

Cyclobutene photochemistry. Substituent and wavelength effects on the photochemical ring opening of monocyclic alkylcyclobutenes

Leigh¹,

Zheng²,

Clark³

1990

Self Cite

. Can. J. Chem. 68, 1988Chem. 68, (1990. The photochemical ring opening of cis-and trans-3,4-dimethyl-, 1,3,4-trimethyl-, and 1,2,3,4-tetramethylcyclobutene (1,3, and 4, respectively) has been investigated in hydrocarbon solution with 193 nm and 214 nm light sources. Ring opening is non-stereospecific in all cases at both wavelengths. The ratio of dienes formed by the formally allowed to formally forbidden pathways in the photolysis of these compounds is highest (ca. 2) for the trimethylcyclobutenes, and approximately 1 for both cis and trans isomers of the di-and tetramethylcyclobutenes with 193 nm excitation. The diene distributions from photolysis of all compounds but cis-3 show slight wavelength dependence. Gas-and solution-phase UV absorption spectra are reported for 3 and 4, and indicate that there are at least three singlet excited states accessible in the 185-230 nm region in these molecules. The .rr,R(3s) state is the lowest energy state in the gas phase in 3 and 4. The results verify that orbital symmetry factors do not play a role (or a consistent one, at least) in controlling the stereochemistry of the reaction, but they do not allow a firm assignment of the excited state(s) responsible for ring opening. Direct photolysis of these compounds also results in fragmentation to yield . .Z-2-butene (from cis-3 and 4) or E-2-butene (from trans-3 and 4) in addition to propyne or 2-butyne. The 2-butenes are formed A part le cas du produit cis-3, la distribution des dibnes est insensible i la longueur d'onde. On a determink les spectres UV des produits 3 et 4 tant en solution qu'en phase gazeuse; ils indiquent qu'il existe au moins trois ttats singulets excitCs accessibles dans la rCgion de 185 i 230 nm de ces moltcules. Pour les composts 3 et 4, en phase gazeuse, 1'Ctat .n,R(3s) possbde 1'Cnergie la plus basse. Les rtsultats confirment que les facteurs de symCtrie orbitalaire ne jouent pas un r61e (au moins important) dans le contr6le de la stCrtochimie de la reaction; ils ne permettent toutefois pas dkterminer avec certitude quel(s) est (sont) le(s) Ctat(s) excitC(s) de l'ouverture du cycle. La photolyse directe de ces composts conduit aussi i une fragmentation conduisant au Z-2-butbne (a partir des composts cis-3 et 4) ou au E-2-butbne ( i partir des trans-3 et 4) en plus du propyne ou du 2-butyne. Dans tous les cas, les 2-butbnes se forment avec une sttrtospCcificitt superieure a 90%. Les structures des quatre 3-mtthyl-2,4-hexadibnes isombres qui se forment par photolyse du produit 3 ont Ct C attribukes sur la base de leurs spectres RMN du 'H et des rtsultats de la thermolyse des deux cyclobutbnes isombres.

“…From this it has been concluded that ring opening is a .rr,.rr* singlet state process, while cycloreversion arises from an excited singlet state with substantial (.rr,R(3s)) Rydberg character. The observation of skeletal rearrangements in the photochemistry of asymmetrically substituted, monocyclic alkylcyclobutenes suggests that cycloreversion may proceed via cyclopropyl carbene intermediates formed by ring contraction (7)(8)(9).…”

Section: Introductionmentioning

confidence: 99%

Cyclobutene photochemistry. Substituent effects on the photochemistry of 1-phenylcyclobutene

Leigh¹,

Postigo²

1995

Self Cite

The photochemistry and photophysics of I-phenylcyclobutene and five aryl-substituted derivatives have been studied in various solvents at room temperature. All six compounds fluoresce with quantum yields in the 0.2-0.3 range in cyclohexane and acetonitrile solution. I-Phenylcyclobutene undergoes [2+2]-cycloreversion ( 4 = 0.09) to yield phenylacetylene upon photolysis in either hydrocarbon or acetonitrile solution, and undergoes (Markovnikov) solvent addition upon irradiation in methanol solution ( 4 = 0.08) in addition to cycloreversion. Triplet sensitization and quenching experiments indicate that cycloreversion and methanol addition are both excited singlet state processes. None of the six compounds studied undergo ring opening to the corresponding 2-aryl-1.3-butadiene in detectable yield. Quantum yields for cycloreversion in cyclohexane, acetonitrile, and methanol solution and methanol addition have been determined for the six compounds, along with excited singlet state lifetimes. The quantum yields and rate constants for cycloreversion and methanol addition are both enhanced by substitution with electron-donating groups. ' The variation in the rate constant for [2+2]-cycloreversion with substituent indicates that there is substantial dipolar character developed in the cyclobutenyl a-bond framework during the reaction, in almost exact correspondence with that developed in the .rr system during photoprotonation. No deuterium scrambling is observed in I-phenylcyclobutene-2,4,4-4 after photolysis in pentane solution to ca. 80% conversion, indicating that skeletal rearrangements leading to cyclopropyl carbenes do not occur in the direct photolysis of arylcyclobutene derivatives. A pericyclic mechanism for the photocycloreversion reaction is suggested. Triplet-triplet absorption spectra and triplet lifetimes of I-phenyl-, 1-(porn-methylpheny1)-, and I-fj1arc2-trifluoromethyIphenyl)cyclobutene in hydrocarbon solution are also reported.Key words: photochemistry, cyclobutene, fluorescence, [2+2]-cycloreversion, substituent effects, nanosecond laser flash photolysis, lifetime, triplet state, styrene, photoaddition Resume : On a CtudiC, dans differents solvants et 6 diffkrentes temperatures, la photochimie et la photophysique du I-phenylcyclobutkne et de cinq derives aryles substitues. Tous ces six composes entrent en fluorescence avec des rende~nents quantiques de l'ordre de 0.2 6 0,3 en solution dans le cyclohexane et dans I'acCtonitrile. Le I -phCnylcyclobutkne subit une cycloCIimination [2+2] ( 4 = 0,09) pour conduire au phCnylacCtylkne par photolyse en solution dans un hydrocarbure ou dans I'ac6tonitrile. avec en plus une addition de solvant (de type Markovnikov) par irradiation dans une solution de methanol ( 4 = 0,08). Les experiences de sensibilisation du triplet et de dksactivation indiquent que la cycloClimination et I'addition de methanol sont toutes deux des Ctats singulets excites. Aucun des six composCs CtudiCs ne conduit par ouverture du cycle au 2-aryl-1,3-butadikne en quantite dCcelable. On ...