Cyclobutadiene matrix isolation

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

During all these studies on cyclobutadienes and tetrahedranes formed via carbenes as transient species we wondered whether matrix isolation IR spectroscopy might be a good tool for the direct observation of cyclopropenylidene (2) and trimethylenemethane (3). This is indeed the case. 

Our interest in ketocarbenes originated from the aim to matrix-isolate oxirene (76) (Scheme 1), the oxygen-containing hetero analog of cyclobutadiene (1)... 

Species identified by matrix isolation include cyclobutadiene and benzyne, where products derived from these molecules are formed at higher temperatures. 

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 fashion443 for example, the cyclic dilactone (505) is converted on irradiation to the [2.2]paracyclophane (506).444 Of particular interest is the use of the /(-lactone (507) as a precursor of matrix-isolated cyclobutadiene (508).445... 

Matrix-isolated diene (96) undergoes photochemical transformation into the isomeric compounds (97) and (98) as the primary photoproducts. Maier and his coworkers have observed the photoisomerization of the cyclobutadiene dimer (99) into the pentacyclic compound (100). The u.v. irradiation of the Dewar benzene (101) a.R ords the paracyclophane (102, 13%). The prismane (103) can be produced by the irradiation of the Dewar benzene (104). The intermediacy of the benzene (105) in the transformation is likely. ... 

Although several derivatives of cyclobutadiene are known and are discussed shortly, cyclobutadiene itself has been observed only as a matrix-isolated species, that is trapped at very low temperature in a frozen inert gas. The first successful synthesis of cyclobutadiene was achieved by release from a stable iron complex." " Various trapping agents react with cyclobutadiene to give Diels-Alder adducts, indicating that it is reactive as both a diene and a dienophile." Dehalogenation of trans-3, 4-dibromocyclobutene gave the same reaction products." ... 

Antiaromatic compounds are especially unstable compared with their acyclic analogs. Cyclobutadiene is isolable only in an inert matrix at very low temperatures. Cyclopentadienone is extremely unstable because the C—O resonance structure is antiaromatic. Cycloctatetraene avoids being antiaromatic by bending into a tub shape so that its p orbitals don t overlap continuously. 

Later, the experimental evidence for square cyclobutadiene was called into question. Krantz reported the photolysis of bicyclopyranone in which the carbon atom eliminated as CO2 was labeled with C. One important infrared band that had been assigned to a vibration of square planar cyclobutadiene in earlier studies was altered by the isotopic change, suggesting that this band was due to CO2 trapped with the cyclobutadiene in the rigid rare gas matrix. Thus, the experimental data did not answer the question of the structure of cyclobutadiene. Later work on the theoretical determination of the infrared spectrum of cyclobutadiene ° and further matrix isolation spectroscopy experiments, including the use of polarized IR spectroscopy... 

The anti-aromaticity of cyclobutadiene (46) has made this molecule a topic of choice in research by matrix isolation, where it occupies an important chapter. It was initially obtained by matrix photodecomposition of a-pyrone by a prolonged irradiation at 20.4 K in argon matrix via a series of intermediates (Scheme 6.21). The peaks marked in the unexpectedly simple spectrum obtained were attributed to such species (Fig. 6.30). To finally establish the structure of this molecule, rectangular or square, many further experiments were required, in particular the generation from a different precursor and testing the effect of specific deuteration in the spectrum [8]. 

Lin AY, Kranz A (1972) Matrix isolation of cyclobutadiene. J Chem Soc Chem Commun 1111-1112... 

Many photochemical reactions are carried out at low temperatures as low as 4 K to slow down the reaction rate for the study of the lifetimes of the reactive intermediates. The most useful matrix materials are solid argon, solid neon and solid nitrogen. The initial photoproduct is trapped within a rigid matrix that inhibits the decay of the reactive species in diffusion process. For example, 5-hydroxy-a, P, y, 5-unsaturated valerolactone 1 on photochemical decomposition gives cyclobutadiene 2 and carbon dioxide. The intermediate and the products of this reaction are characterized in low-temperature matrix isolation process. 

The story of the generation, detection, and determination of the structure of cyclobutadiene isolated in low temperature matrices is one of the most celebrated and long running in the history of matrix isolation, and it illustrates both the strengths and pitfalls of the technique. 

Fraga, S., Possible interactions of cyclobutadiene with by-products in inert matrix isolation studies. Tetrahedron Lett., 22, 3343,1981. 

Arnold, B.R., Radziszewski, J.G., Campion, A., Perry, S.S., and Michl, J., Raman spectrum of matrix-isolated cyclobutadiene. Evidence for environmental hindrance to heavy-atom tunnehng /. Am. Chem. Soc., 113, 692, 1991. 

Cyclobutadiene (118) has been the subject of numerous matrix isolation investigations over a long period. It was first generated in matrices from a-pyrone (116), which gave j3-lactone 117 on photolysis, from which CO2 was subsequently eliminated to yield cyclobutadiene. " At first, an insufficient number of IR bands of 118 were observed to settle the question of whether the molecule has a square or rectangular geometry, but eventually a fuller spectrum was obtained, which contained too many bands to be consistent... 

The Dewar isomer, 123, of 3-methyl-4(3H)-pyrimidinone (122) was generated in argon matrices by 308 nm irradiation of the parent molecule and identified by comparison of the experimental matrix IR spectrum with spectra computed for 123 and other possible products. " For several 4(3H)-pyrimidinones not methylated at N3, two other types of matrix photoreactions were observed in addition to the isomerization to the Dewar isomer phototautomerism and ring opening. Somewhat later, matrix isolated Dewar pyridine (124) was produced by UV-irradiation of pyridine in solid argon and was identified with the aid of DFT computations. On further photolysis, 124 produced cyclobutadiene (118) and HCN. 

Irradiation of pyridine 109 matrix isolated in argon at 8K gave rise to hydrogen cyanide and cyclobutadiene 119 as secondary photoproducts." " It was assumed that these products arise from photofragmentation of the initially formed Dewar-pyridine 112 (Scheme 33). 

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