Irradiation cyclobutadiene

THE cvcLOBUTADENE-TETRAHEDRANE SYSTEM. A related reaction is the photoisomerization of cyclobutadiene (CBD). It was found that unsubstituted CBD does not react in an argon matrix upon irradiation, while the tri-butyl substituted derivative forms the corresponding tetrahedrane [86,87]. These results may be understood on the basis of a conical intersection enclosed by the loop shown in Figure 37. The analogy with the butadiene loop (Fig. 13) is obvious. The two CBDs and the biradical shown in the figure are the three anchors in this system. With small substituents, the two lobes containing the lone electrons can be far... 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Irradiation of tricarbonyl(r(4-cyclobutadienc)iron(0) with ethyl 1//-azepine-l-carboxylate results in the formation of the novel sandwich compound (t 4-cyclobutadiene)[l-(ethoxycar-bonyl)-1 //-azepine]iron(0) (27).223... 

A final, related example is the formation of tetrahedranes (84), from irradiation of tert-butyl substituted cyclobutadienes such as 83 (equation 31), a reaction which has been... 

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

Irradiation of 1 -ethoxycarbonyl-1 //-azepine with tricarbonyl(cyclobutadiene)iron produces the novel metallocene (166) as a stable orange crystalline solid, the structure of which was confirmed by X-ray crystallography (71JA262). 

Irradiation of pyridine itself gives Dewar pyridine, observable spectroscopically, which in water is hydrolytically ring-opened to form H2N(CH = CH)2CHO, but in a matrix fragments to cyclobutadiene and HCN. 

Further irradiation results in decarboxylation and the formation of cyclobutadiene... 

When irradiated, fluonnated isomers of Dewar benzene yield pnsmane denvatives that rearrange thermally to benzene Photolysis of hexakis(tnfluoromethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [147]... 

Irradiation of diphenyl acetylene (Formula 430) in hexane solution gives 1,2,3-triphenylazulene (Formula 431), 1,2,3-triphenylnaphthalene (Formula 432), hexaphenylbenzene (Formula 433), and octaphenyl-cubane, (Formula 434) (180a, 190). The products presumably arise by isomerization, addition, or dimerization of an intermediate tetraphenyl-cyclobutadiene (180a,190). 

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... 

Cyclopentadienyl dicarbonyl ruthenium dimer 132 reacts with silver tetrafluoroborate and diphenylacetylene to afford the cyclobutadiene ruthenium complex 133 (Scheme 12). Irradiation of 133 in dichloro-methane in the presence of several alkynes leads to the arene cyclopentadienyl ruthenium complexes 125 in high yield. This reaction appears to be a general route to sterically crowded ruthenium arene cations (55). 

Cyclobutadieneiron tricarbonyl may also be produced through reaction of 3,4-dichlorocyclobutene with Na2Fe(CO)4,5 and by irradiation of a-pyrone followed by treatment with Fe2(CO)9 . The method outlined here is the most convenient procedure especially when considerable quantities (10 g. or more) of cyclobutadieneiron tricarbonyl are required. The analogous reaction of derivatives of 3,4-dihalocyclobutenes with Fe2(CO)9 affords the corresponding cyclobutadieneiron tricarbonyl complexes. Cyclobutadieneiron tricarbonyl can be oxidized to generate cyclobutadiene in situ.7... 

However, on irradiation (i.e. photochemically) an electron is promoted from i 3 to vj 4 in hexatriene. Now all the orbitals of hexatriene (in the excited state) correlate with the orbitals of cyclobutadiene. 

The irradiation of a-pyrone in the presence of Fe(CO)5 results in formation of (cyclobutadiene)tricarbonyliron and the yellow (a-pyrone)-tricarbonyliron, m.p. 140°-141°C (141). This latter complex can also be... 

Matrix irradiation (240nm) of tricyclopentanone (111) yields cyclobutadiene, CO, cyclopentadienone, and acetylene.No evidence for the formation of tetrahedrane (112a) was obtained, in contrast with the previously reported behaviour of the tetra-t-butyl derivative of (111), which gave the tetrahedrane... 

We found also that (77-C5H,)V(CO)3PPh3 as well as its niobium analog, when irradiated, underwent substitution of one PPh3 and one CO by tolan. Hence, niobium (and tantalum) complexes of the type are able to attach a third molecule of tolan and transform this to hexaphenylbenzene, unlike the vanadium analog, which transforms just two tolans to tetraphenyl-cyclobutadiene or, if irradiated, combines them with carbon monoxide and produces tetraphenylcyclone. 

Interest in the photochemistry of the phthalimide systems has continued. The phthalimide derivatives (316) are phot ochemically reactive and on irradiation in acetone yields the cyclized products (317). The reaction involves hydrogen abstraction to yield the biradical (318) which subsequently bonds to afford the observed products. A recent study has examined the behaviour of the anion (319) in an attempt to reduce electron transfer processes. In t-butanol irradiation affords the solvent addition product (320) as the principal product presumably by a free radical path. Minor products (321) and (322) are also formed but are probably artefacts of the work-up procedure. Irradiation of (319) in methanol with added cyclohexene follows a different reaction path. In this system the reaction with methanol is minor while the dominant reaction is addition of the alkene to afford the adduct (323) in 20 % yield. The Dewar benzene derivative (324) is photocheraically unstable and irradiation affords t etramet hyl cyclobutadiene. ... 

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