Ketenes, matrix isolation

The results of low-temperature matrix-isolation studies with 6 [41a] are quite consistent with the photochemical formation of cyclo-Cif, via 1,2-diketene intermediates [59] and subsequent loss of six CO molecules. When 6 was irradiated at A > 338 nm in a glass of 1,2-dichloroethane at 15 K, the strong cyclobut-3-ene-1,2-dione C=0 band at 1792 cm in the FT-IR spectrum disappeared quickly and a strong new band at 2115 cm appeared, which was assigned to 1,2-diketene substructures. Irradiation at A > 280 nm led to a gradual decrease in the intensity of the ketene absorption at 2115 cm and to the appearance of a weak new band at 2138 cm which was assigned to the CO molecules extruded photo-chemically from the 1,2-diketene intermediates. Attempts to isolate cyclo-Cig preparatively by flash vacuum pyrolysis of 6 or low-temperature photolysis of 6 in 2-methyltetrahydrofuran in NMR tubes at liquid-nitrogen temperature have not been successful. 

An interesting carbene, 1-oxobutatrienylidene [25], having cumulated double bonds, has been found by IR spectroscopy in the photolysis (A>230nm) products of matrix-isolated l,2,3,4-pentatetraene-l,5-dione [26] (Maier et al., 1988) (in its turn the unstable dione [26] was generated by thermo- or photo-destruction of compound [27]). The second product was carbon monoxide. The linear structure of the carbene [25] has been suggested on the basis of two intense IR bands at 2222 cm and 1923 cm indicating respectively ketene and allene fragments. 

But there is also good news about the members of the C2H2O family and that is the matrix isolation of oxiranylidene (78). Besides ketene 74 and ethynol (77)104 carbene 78 should be a minimum on the C2H2O potential surface101 with a considerable barrier to isomerization. Indeed, oxiranylidene 78 is observable under matrix conditions.105... 

The matrix photochemistry of 2v proved to be fairly complicated.108 The primary product of the photolysis of 2v is carbene lv, which was identified by ESR spectroscopy. Under the conditions of matrix isolation the carbene showed the expected reactivity towards molecular oxygen (formation of carbonyl oxide 7v) and carbon monoxide (formation of ketene lOv) (Scheme 22). In contrast to the oxocyclohexadienylidenes (la and derivatives) carbene lv slowly reacted with CO2 to give an a-lactone with the characteristic C=0 stretching vibration at 1896 cm-1. The latter reaction indicates that lv is — as expected — more nucleophilic than la. 

Silaacrylate 17 is another example of a matrix-isolated silene36. It is formed simultaneously with ketene 18 on irradiation of methyl (pentamethyldisilanyl)diazoacetate... 

During the irradiation of 2,5-diiodothiophene 50 under matrix-isolation conditions <2006JOC9602> ethynylthio-ketene 52, formed via the bis-alkyne 51, could be identified by IR spectroscopy (Scheme 3). 

Carboxylate esters readily undergo photodecomposition with loss of carbon dioxide. Not surprisingly, lactones and related oxygen heterocycles undergo related transformations. A wide variety of lactones behave in this fashion" for example, the cyclic dilactone (505) is converted on irradiation to the [2.2]paracyclophane (506). Of particular interest is the use of the jS-lactone (507) as a precursor of matrix-isolated cyclobutadiene (508)." Phthaloyl peroxide (509) has similarly been employed as a precursor of benzyne (510). A careful examination in an argon matrix at 8 K has revealed that the / -lactone (511) and the keto ketene (512) are intermediates... 

Under matrix isolation, liberation of N2 from 3,4-dihydro-l,2,3-benzotriazin-4-ones is not the only process that may be observed. Irradiation of 3-methoxy-3,4-dihydro-l,2,3-benzotriazin-4-one 19g (Ar matrix, 10 K, >300nm) resulted in photoreversible formation of ( /Z)-benzazetinones 85 (photo-interconvertible), while there was no firm indication of a ketene intermediate. Thus, a 1,3-sigmatropic shift of the C=0 group from N-3 to N-1 was proposed (Equation 12). A similar reaction was observed for the 3-butoxy- but not for the 3-methylbenzotriazinone 19f <1992CL361>. 

A very good example of the enormous benefits that DFT computations of IR spectra have brought to matrix-isolation studies giving the technique a veritable new lease of life in the study of organic reactive intermediates - has been provided by a study of the photolysis of a-pyrone (32) and its 4,6-dimethyl derivative (Breda et al.. Chapter 6). The photochemistry of a-pyrone was the subject of some of the earliest matrix-isolation studies of organic species, but the use of DFT computations has now allowed a virtually complete identification of the individual rotamers of the ring opened aldehyde-ketene (33)-(36). 

A new statistical quantum mechanical approach - the statistical adiabatic product distribution method has been applied to the photodissociation of ketene in parallel theoretical and experimental studies. The main focus of the work, however, was H-atom production via C-H cleavage rather than the elimination of CO. The photodecomposition of formohydroxamic acid (HCONHOH) has been investigated in matrix-isolation FTIR and DFT stud-ies. " Irradiation of the acid in Ar or Xe matrices with the full output of a xenon arc lamp generated H-bonded HNCO- H2O and NH20H- CO complexes. In the latter case, the IR spectra also suggest the existence of a structure with the NH2 group interacting with the carbon atom. 

Flash vacuum thermolysis with Ar-matrix isolation or matrix photolysis of 2-phenylbenzo-l,3-dioxan-4-one as well as other precursors gave the ketene 6-carbonylcyclohexa-2,4-dienone 112, as detected by the IR absorption at 2134 cm Flash vacuum thermolysis above 600 °C or matrix photolysis resulted in formation of the fulvenone 114, proposed to arise by decarbonylation of 112 and Wolff rearrangement. The use of C-labeled precursor gave 114 with scrambling revealing the intermediacy of bisketene intermediates 113 in the reaction (Scheme 4.21). 

The chemical reactivity of the cumulenes under discussion ranges from highly reactive species to almost inert compounds. While some cumulenes can only be generated in a matrix at low temperatures, others are indefinitely stable at room temperature. For example, sulfines and sulfenes are only generated in situ, but some cumulenes with bulky substituents are sometimes isolated at room temperature for example, C=C=S was detected in interstellar space by microwave spectroscopy, and its spectrum was later verified by matrix isolation spectroscopy. In contrast, some cumulenes, such as carbon dioxide and carbon disulfide, are often used as solvents in organic reactions or in the extraction of natural products. The reactivity of some center carbon heterocumulenes in nucleophilic reactions is as follows isocyanates > ketenes > carbodiimides > isothiocyanates. However these reactivities do not relate to the reactivities in cycloaddition reactions. Often reactive cumulenes are isolated as their cyclodimers. Aromatic diisocyanates are more reactive than aliphatic diisocyanates in nucleophilic as well as cycloaddition reactions. 

Irradiation of diazoketones (reactions 8.7 and 8.8) [17-19], diazoesters (reaction 8.9) [20], and various other diazoalkanes (reactions 8.10 and 8.11) [21,22] as well as diazirines (reactions 8.12 and 8.13) [23,24] under matrix isolation conditions also led to the detection of the corresponding carbene intermediate, which could be trapped with carbon monoxide in the form of ketenes. For example. 

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