Irradiation conjugated diene complexes

Less complex non-conjugated diene systems also lead to cubane-like derivatives as in the diene 175. Here the outcome of the reaction is dependent upon the excited state. Thus, direct irradiation brings about fragmentation with the formation of 1,4-difluorobenzene and excited-state naphthalene while triplet-sensitized irradiation follows a different path with the formation of the cage compound 17682. 

Very stable diene complexes can be prepared by the reaction of conjugated dienes with Fe(CO)5, Fe2(CO)9 or Fe3(CO)12 by heating or under irradiation. For synthetic purposes, the complexes of acyclic 1,3-dienes, 1,3-cyclohexadienes and 1,3-cycloheptadienes are useful. 

Upon UV irradiation in hydrocarbon solution, the hexacarbonyls of chromium, molybdenum, and tungsten react differently with conjugated dienes like 1,3-butadiene (la), ( )-l,3-pentadiene (lb), 2-methyl-1,3-butadiene (lc), ( , )-2,4-hexadiene (Id), ( )-2-methyl-l,3-pentadiene (le), 2-ethyl-1,3-butadiene (If), or 1,3-cyclohexadiene (Ig). Chromium hexacarbonyl (2) yields, with the acyclic dienes la-lf, tetracarbonyl-r/2-dienechromium(0) complexes (3a-3f) in a smooth reaction (8-10). With 1,3-cyclohexadiene, in addition to 3g, dicarbonylbis(>/4-l,3-cyclohexadiene)chromium(0) (4g) is obtained [Eqs. (7) and (8)j. During chromatography on silica gel, the 1,3-cyclohexadiene complex 3g dismutates readily to [Cr(CO)6] and 4g [Eq. (9)]. Under the same conditions with 2 1,3-cyclopentadiene (lh) yields, in a hydrogen-transfer reaction, the stable dicarbonyl- / 5-cyclopentadienyl-r/ 3-cyclopent-enylchromium (5) (11-13) [Eq. (10)]. 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

When the electron acceptor monomer is a relatively weak acceptor, e.g. acrylonitrile, the reaction with a conjugated diene such as butadiene, to yield the Diels-Alder adduct, proceeds slowly. However, in the presence of aluminum chloride, the acrylonitrile is converted to a stronger electron acceptor and the formation of the ground state complex and the adduct therefrom proceeds more rapidly. When the reaction is carried out under UV irradiation, the yield of adduct is greatly reduced and the equi-... 

The naphthalene-photosensitized irradiation of a-phellandrene (Scheme 10), which has a peppery odor, gives the dimmers (Baldwin and Nelson, 1966). The sensitized irradiation is more complex than direct irradiation since the dimer can be converted photo-chemically into the other isomers. Dimerization reaction is readily achieved when the conjugation is increased in diene system which results in more accessible energy levels (Horspool, 1976). 

The photochemical reaction of Group VI metal hexacarbonyls with mono-enes was studied several years ago, but only recently has the reaction of one of the hexacarbonyls, W(CO)s in fact, with conjugated dienes been investigated. The W(CO)6 proves to be an effective catalyst for cis trans isomerization of such dienes compounds of the type W(CO)6(diene), which must act as intermediates in such isomerizations, have been isolated and their reactions studied. Under normal conditions iron pentacarbonyl reacts with acetylene to produce a complicated mixture of products, but under irradiation in an argon matrix at — 256 °C the product is but-l-en-3-yne, complexed to the iron through the carbon-carbon triple bond only. This iron-alkyne-carbonyl compound is presumably an intermediate in the reaction under normal thermal conditions. In an argon matrix at — 256 C, iron pentacarbonyl reacts with ethylene to give Fe(CO)4(QH4). ... 

The reaction of enantiomerically pure sulfinyl dienes with Fe(GO)s yields ( 7" -(lZ)-sulfinyl diene)iron(O) tricarbonyl complexes.Planar and axial chirality are introduced upon complexation of similar ligands with (dba)Fe(GO)3 (dba = dibenzylideneacetone). Gomplexation of sorbic acid with Fe(GO)s in acetone upon irradiation with UV light produced the ( 7" -2,4-hexadienoic acid)Fe(GO)3 complex in good yields.Stobart efal. reported the reaction of 1,4,5,8,9,10-hexahydroanthracene with Fe(GO)s under UV irradiation to form a complex in which two tricarbonyliron moieties are bound in an 77 -fashion to the ends of the isolated conjugated diene units (Equation (8)). 

Dienes are formed regioselectively when acetonitrile solutions of tj -allylpalladium complexes are irradiated at 366 nm (e.g. Scheme 97) unsym-metrical complexes furnish isomeric mixtures of 1,5-dienes arising from coupling at either end of the allyl moiety.Acetate or even conjugated enone functions do not interfere with the dimerization. 

---