Triplet state ketenes

Addition of ci5-butene-2 to the photolysis of ketene causes a reduction in the rate of formation of carbon monoxide. Ethylene production diminishes more rapidly and is approaching zero while the yield of carbon monoxide is still finite. The decline in production of ethylene is accompanied by a steady increase in isomerisation of ci5-butene-2 to the trans isomer. This behavior is ascribed to an energy transfer involving triplet-state ketene molecules . 

Oxo-2,5-cyclohexadienylidene [83] was generated in solid argon at 9 K by irradiation of diazo compound [84] with visible light (A>495 nm) (Sander et al., 1988 Bucher and Sander, 1992 Bucher et al., 1992). The IR, UV, and esr spectra of [83] were in accord with a structure having a triplet state with one delocalized electron. In the IR spectrum of the carbene [83] the r (CO) mode was found at 1496 cm which indicates a bond order of the C—O bond considerably less than 2. The low-temperature reaction of carbene [83] with CO generated the keto-ketene [85]. Irradiation (A = 543 10 nm) of [83] led to its transformation into a very labile species, presumed to be [86], which rearranged back to [83] not only under UV or... 

Enthalpies of formation for the singlet and triplet states of methylene were obtained from the photodissociation of ketene.131 The data for CH2 (3Bi) were recently confirmed by methods which do not rely on ketene.132,133 In a widely applicable procedure, threshold collision energies for the loss of halide ion from RR C-X- were combined with gas phase acidities of RR CH-Cl to give AHf (RR C ) (Eq. 11).134 Similarly, gas phase acidities of the radicals RR CH were combined with ionization energies of the radical anions RR C -, or electron affinities of the carbenes RR C (Eq. 12).135136... 

Herz and Iyer have proposed an a-cleavage mechanism with the intermediacy of a cyclopropyl ketene for formation of the polycyclic acetal 42 from photolysis of the rigid cyclopentenone 4339 The cleavage reaction is thought to occur via an n,7r triplet state of unusually high energy for a cyclopentenone (75 kcal/mole). 

The lower triplet state corresponds to the 3a (3a ) transition of the tt,x type. This band could underlie the 385-to 200-nm band since the Franck-Condon accessible region may be quite high in energy. Excited electronic states of ketene have been recently studied by electron impact spectroscopy (87). 

Depending on the mode of generation, a carbene may be initially formed in either the singlet or triplet state, irrespective of its stability. Common methods used for the generation of carbenes include photolytic, thermal, or metal catalyzed decomposition of diazocompounds, elimination of halogenfrom gem-dihalides, elimination of Hx from CHX3, decomposition of ketenes, thermolysis of a-halo-mercury compounds and cycloelimination of shelf stable substrates such as cyclopropanes, epoxides, aziridines and diazirines. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

Basch investigated the photodecomposition of keten and carried out both SCF and MCSF calculations with an augmented DZ basis set. It was concluded that the first excited triplet state of keten can form CH2( Ba) and CHaCMi) relatively rapidly and the first excited singlet state of keten can give CH2( R2) easily in a near-least-motion path. However, the formation of CHj(. 4i) from the first excited singlet state of keten by a near-least-motion path seemed to be very improbable. 

The photochemical behavior of cyclobutanone (IS) contrasts sharply with that of other ketones. Cyclobutanone undergoes a cleavage also from the (n, r ) state, with subsequent fragmentation to ketene and olefin, decarbonylation to cyclopropane or cyclization to oxacarbene (16), whose concerted formation has also been proposed on the basis of stereochemical observations (Stohrer et al., 1974). In contrast, cyclohexanone cleaves exclusively from the triplet state and undergoes disproportionation reactions. The photochemical activity of cyclobutanone persists even at low temperatures (77 K) where cyclohexanone is photostable. 

D. Dopp provides an example of the para cycloaddition which, apparently, is observed with dienes or as a secondary process of preformed ortho cycloadducts. Here the reactions involve the triplet state of a-acetylnaphthalene finally leading to formal ketene cycloadducts in a [4+2] mode. 

Both oxetanes and p-hydroxy esters are formed following irradiation of aromatic carbonyl compounds in the presence of silyl ketene acetals. The products arise either by SET processes or by direct Paterno-Biichi additions. Griesbeck and Bondock have reported the influence of substrate concentration on the dia-stereoselectivity of the photochemical addition of aldehydes to (Z)- and ( )-cyclooctene. Miranda and co-workers have published physical evidence for the quenching of the triplet state of 2,4,6-triphenylpyrylium salts by 2,3-diaryloxetanes. 

At short wavelengths, the methylene formed is almost entirely in its singlet state, whereas at long wavelengths the photolysis of ketene produces predominantly triplet methylene (methods for the estimation of singlet and triplet methylene fractions are discussed on p. 393). The intervention of a triplet state of ketene is indicated by quenching experiments with biacetyP and, more conclusively , ci5-butene-2. 

The Norrish Type I reaction usually leads to decarbonylation. This is the case with dicyclopropyl ketone on irradiation at 193 nm. Decarbonylation, however, is a second step and this is preceded by ring opening of the cyclopropyl moieties to diallyl ketone. Calculations have shown that decarbonylation of cyclobutanone occurs from the nji triplet state. The resultant triplet trimethylene biradical undergoes ISC to the ground state before formation of cyclopropane. On the other hand, the cycloelimination reaction to yield ketene and ethene arises from the singlet excited state.Irradiation of cyclopentanone in aqueous and frozen aqueous solutions has been examined and the influence of applied magnetic fields assessed. Photodecarbonylation in the crystalline phase of the ketone (3) at 310 nm takes place stereospecifically with the formation of the cyclopentane derivative (4). The latter can be readily transformed into racemic herbertenolide (5). ... 

From singlet state it has been found that the following reactions often compete with the 1,3-rearrangements cis-trans isomerization, decarbonyl ation, aldehyde formation, ketene formation, olefin reduction, Norrish t q)e-II cleavage with cydobutanol formation, [2-J-2] cycloaddition, and inter tem crossing with concomitant 1,2-acyl shift. From the triplet state a similar series of reactions have been reported including 1,3-acyl shift. 

No quenching and/or sensitization studies are available which establish with certainty ketene formation proceeding from the triplet state. The ketene formation from 24 and 726 most likely occurs from the triplet state, since in related systems with an ,p-enone as part of the chromo-phore, only reactions from the triplet are found. 3i,90)... 

Figure 7.13 Potential energy profile for the lowest three electronic states of ketene. Note that in addition to the low barrier for dissociation from the triplet state, Ti, there is a higher barrier for formation of a symmetric structure, oxirene, that can rearrange to a ketene molecule in which the two C atoms have exchanged. In the transition state for dissociation the reaction coordinate is a C—C stretch, as shown. The products are detected by laser-induced fluorescence of CO [adapted from A. P. Scott etal., JACS 116,10159 (1994) see also Moore and Smith (1996)].

Kaledin, A.L., J. Seong, and K. Morokuma (2001), Predominance of nonequilibrium dynamics in the photodissociation of ketene in the triplet state, J. Phys. Chem. A, 105, 2731-2737. 

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