Dienes photolytic reactions

Photolytic reactions of dienes frequently give complex mixtures of rearranged products. Described here, however, is a photolytic isomerization of 1,5-cyclooctadiene (present in solution, in part, as a complex with cuprous chloride) that affords a good yield of one product. 

A systematic restudy of the thermal and photolytic reactions of perlluorodiene 12 demonstrated that ring closure to the bicyclie isomer 13 is reversible at 250 C and that at higher temperatures crosswise cycloaddition takes place to give perfluorobicyclo[2.1.1 (hexane (14).40 Mercury-sensitized photolysis at 254 nm of diene 12 gives bicyclie isomers 13 and 14 in a ratio of 1 4.4,1... 

Like the alkenes, dienes have a very limited chemistry with Re. Photolytic reactions of (1) in the presence of dienes give complex mixtures of products, with -allyl and bridging alkenyl structures, in a similar manner to the alkene dimers (8). Reaction of ReBr(CO)3(THF)2 with the cyclooctateraene dianion [CsHs] " produces a dimer (CO)3Re(/u-C8H8)Re(CO)3 (81). Each Re atom is coordinated to one half of the fluxional tetraene ligand. While the fragment [Re(CO)5]+ reacts with MeC=CMe to give the complex in equation (4), the fragment... 

Reaction of (402) with 2,5-dimethyl-3,4-diphenylcyclopentadienone gave the adduct (421). Photolytic decomposition at -45 °C yielded l,4-dimethyl-2,3-diphenylbenzene (422), a diene and another photoproduct identified as benzo[c]furan. The adduct (423) was obtained when the photolysis was carried out in the presence of dimethyl fumarate (Scheme... 

The molecular geometry of the lepidopterene skeleton remains a feature of unique importance for the observed photochemical reaction. The photolytic An + 27i cycloreversion generates an electronically excited product in which the diene and dienophile moieties are bound to face each other in an arrangement which, subsequent to deactivation to the Franck-Condon ground state, is an ideal one for bond formation. 

Photolytically generated 1-silabuta-l,3-dienes undergo a thermal reverse reaction to 2-silacyclobutenes. Thus 2-phenylsilacyclobut-2-ene 360 is easily opened to the 2-phenylsilabuta-1,3-diene 361 by irradiation in 3-methylpentane matrix at 77 K or by flash photolysis at ambient temperature (equation 97)183. The rate for the thermal reverse reaction was measured at room temperature and the activation energy for the 1 -siladiene ring closure was estimated to be 9.4 kcalmol-1183. 

Further cycloaddition reactions of silylenes generated by the photolysis of cyclotrisilanes have been published since Weidenbruch and coworkers summarized these reactions in an excellent review. Different siliranes were prepared by [2+1]-cycloaddition of di-t-butylsilylene to various alkenes and dienes (Scheme 6)46. Quite interesting results are obtained from the photolysis of hexa-i-butylcyclotrisilane in the presence of unsaturated five-membered ring compounds47 (Scheme 7). With cyclopentadiene and furane, [4 + 2]-cycloaddition of the photolytically generated disilene occurs only as a side reaction. Furthermore, [2 + 1]-cycloaddition of the intermediately formed silylene is highly favored and siliranes are primarily obtained. A totally different course is observed for the reaction in the presence of thiophene. The disilene abstracts the sulfur atom with the formation of the 1,2-disilathiirane as the major product with an extremely short Si—Si distance of 230.49 pm. 

The chemistry of pyrrol-1-ylbenzylidene pentacarbonyl chromium, molybdenum and tungsten complexes was investigated. Reaction with electrophilic alkenes gives l-(phenylcyclo-propyl)pyrroles Under photolytic decarbonylation conditions 2 + 2 cycloaddition products were obtained with nucleophilic alkenes, cyclic dienes and imines. <950M2522>... 

DIBAL-H/n-butyllithium, in cyclic and acyclic systems with iron pentacarbonyl, in cyclodecanes with lithium dihydrodimethoxyaluminate(III)/copper(I) iodide, and in cyclohexane and cyclopentane systems with NaH/sodium r-butylpentyl/Ni(OAc)2. ° The monoreduction of 1,3-diketones can be carried out under similar conditions, as illustrated by the reaction of a substituted cyclohexane-1,3-dione with oxalyl chloride to give the corresponding 1-chlorocyclohexenone, which was subsequently reduced to the enone with zinc-silver couple (Scheme 45). Kropp et al. have reported the photolytic reduction of vinyl iodides in acyclic systems however, when an a-hydrogen is present, formation of the diene product is a limiting side reaction (Scheme 46). For a more extensive discussion of vinyl halide reductions, see the preceding chapter in this volume. 

Tetrakis(triisopropylphenyl)distannene (21-33) is obtained as an intense red solid by the photolysis or thermolysis of hexakis(triisopropylphenyl)tristannane, by way of the intermediate stannylene which can be trapped with 1,3-dienes or 1,2-diones (see Section 21.1.2) the photolytic conversion is quantitative at -78 °C. In solution at 0 °C, the stannylene slowly reverts back to the the cyclotristannane. The X-ray crystal structure has not yet been determined, but the molecule is believed to be twisted with little pyra-midalisation.68 The distannene reacts with phenylacetylene to give the distannacyclobu-tene, with rSnSn 284.0(1) pm. The distannenes 21-35 and 21-36 are formed in the disproportionation reaction shown in equation 21-10. 

Cyclic exponents of the same elimination type are of particular interest. Thus, numerous cyclopentanones photolytically decarbonylate to give 1,4-dienes (p. 876 ff. in Ref. 108)). With thujone (194) this [l,2,(3)4]-elimination of carbon monoxide proceeds quantitatively to give 195 109). With silver nitrate the norcaradiene 196 yields 197 (95%)110) apparently regioselectively, and the [l,2,(3)4]-elimination of methanethiol from 198 to give 199 was realized thermally (16%), acid-catalyzed (59%), and photochemiciiily (ca. 5%)105). For the acid-catalyzed reaction (acetic acid, 100 °C) a non-stereospecific process has been proved 105). 

Cyclohexenes, " cycloheptene and cyclooctene" gave mixtures of endo- and exoisomers their ratio was strongly dependent on the reaction conditions. Cycloocta-1,5-diene by the photolytic method afforded the monoadduct 1 (major component), two isomeric bis-adducts 2 and 3 and rearranged product 4. ... 

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