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Photolysis of ‘naphtho-o-carborane’ under oxygen leads to the quinone, 5,8-diketonaphthocarborane. Photolysis in the presence of hydrogen donors such as acetonitrile or 1,4-cyclohexadiene leads both to the quinone and 5-ketodihydronaphthocarborane. By contrast, photolysis of ‘benzocarborane’ under similar conditions leads only to a highly stereo- and regiospecific dimerization.

Photolysis of ‘naphtho-o-carborane’ in the presence of excellent hydrogen donors such as 1,4-cyclohexadiene leads to quantitative double hydrogen abstraction. Other molecules, including supercoiled DNA, similarly act as hydrogen atom donors towards photolyzed naphtho-o-carborane. A diradical intermediate is proposed.

Cyclohept-1-en-3-yne rearranges thermally to toluene via cycloheptatriene, and to a mixture of 1- and 2-vinylcyclopentadiene an intermediate allene, 1-ethenylidenecyclo-2-pentene.

Almost thirty years after the first (and only) description of 1,4-addition of a carbene to cyclooctatetraene, we report that direct photolysis of ethyl diazomalonate in cyclooctatetraene leads to 13% of the product of 1,4-addition. When the decomposition of the diazo compound is photosensitized by benzophenone, 1,4-addition becomes virtually the sole reaction pathway.

Generation of benzocyclohex-1-en-3-yne in two ways leads to naphthalene and methyleneindene (benzofulvene). Labeling studies favor a multistep mechanism involving an initial electrocyclic ring opening, and carbene−alkene interconversions.

Photolysis of naphtho[b]cyclopropene in hydrocarbons leads to products derived from hydrogen abstraction by a diradical. Photolysis in a cryogenic matrix allows ESR detection of this diradical, which has zero field spliting parameters D/hc = 0.057, E/hc −1 .

The details of the nitroxylation of 2-nitroxy- endo -tetrahydrodicyclopentadiene with nitric acid were studied. A bridged carbocation is suggested as the intermediate responsible for the parallel and irreversible formation of 2,6-dinitroxy- endo -tetrahydrodicyclopentadiene and 2,7-dinitroxy-1,7-trimethylenenorbornane.

In contrast to photolysis or thermal decomposition in solution, which is dominated by ionic reactions, flash vacuum pyrolysis of alkylchlorodiazirines in the gas phase generates radicals. The cyclopropylcarbinyl system is re-examined and the 1-norbornylcarbinyl system is studied for the first time.

9-Carboranylcarbene is formed by reaction of carbon atoms with o-carborane. The triplet state reacts with alkenes by hydrogen abstraction, not the more usual addition reaction.

The phonylcarbene rearrangement is used to produce carbenes that are compared to the intermediates formed on photolysis and pyrolysis of diazo compounds.

The factors influencing the carbon-hydrogen insertion reaction of dichlorocarbene into tertiary carbon-hydrogen bonds are discussed.

A combination of chemical trapping and laser flash photolysis experiments is used to demonstrate that homocub-9-ylidene and homocub-1(9)-ene are in equilibrium. The equilibrium constant can be shown to be close to unity at +20°C (0.23≤K≤4)

PREFACE. CONTRIBUTORS. PART 1 REACTIVE INTERMEDIATES. 1. Tetrahedral Intermediates Derived from Carbonyl Compounds, Pentacoordinate Intermediates Derived from Phosphoryl and Sulfonyl Compounds, and Concerted Paths Which Avoid Them (J.P. Guthrie). 2. Silicon-, Germanium-, and Tin-Centered Cations, Radicals, and Anions (V.Y. Lee and A. Sekiguchi). PART 2 METHODS AND APPLICATIONS. 3. An Introduction to Time-Resolved Resonance Raman Spectroscopy and Its Application to Reactive Intermediates (D.L. Phillips, W.M. Kwok, and C. Ma). 4. Time-Resolved Infrared (TRIR) Studies of Organic Reactive Intermediates (J.P. Toscano). 5. Studies of the Thermochemical Properties of Reactive Intermediates by Mass Spectrometric Methods (P.G. Wenthold). 6. Reactive Intermediates in Combustion (J.K. Merle and C.M. Hadad). 7. Reactive Intermediates in Crystals: Form and Function (L.M. Campos and M.A. Garcia-Garibay). 8. The Chemical Reactions of DNA Damage and Degradation (K.S. Gates). 9. Conical Intersection Species as Reactive Intermediates (M.J. Bearpark and M.A. Robb). 10. Quantum Mechanical Tunneling in Organic Reactive Intermediates (R.S. Sheridan). INDEX.

Pyrolysis of 1,6-diphenylhexa-1,5-diyne-cis-3-ene at 800–1000°C leads to a mixture of 1- and 2-phenylbiphenylene, along with triphenylene. Formation of the two biphenylenes is taken as strong evidence of the rearrangement of a p-benzyne into a m-benzyne through a shift of one of the phenyl groups. Copyright © 2004 John Wiley & Sons, Ltd.

Laser flash photolysis of diadamantyldiazomethane produces diadamantylcarbene which reacts with oxygen to form a carbonyl oxide with λ max=307 nm

9-Carboranylcarbene is formed by reaction of carbon atoms with o-carborane. The triplet state reacts with alkenes by hydrogen abstraction, not the more usual addition reaction.

Chlorodiadamantylmethane is shown to react with tert -butyllithium to give diadamantylmethane through a mechanism involving single electron transfer.

In the reactions of 4,4-dimethylcyclohexadienylidene no evidence for cyclopropene formation has been found. By contrast, 4,4-dimethyl-4-silacyclohexadienylidene yields a product on reaction with butadiene that is best rationalized by intramolecular cyclopropene formation.

Bicyclo[2.1.0]pentane reacts with phenylcarbene and dicarbomethoxycarbene by simple insertion at the cyclobutane methylene position. By contrast, difluorocarbene reacts by two bond cleavage to give 1,1-difluoro-1,5-hexadiene.

The reaction of quadricyclane with difluorocarbene to give endo -6-(2,2-difluoro-vinyl)bicyclo[3.1.0]hex-2-ene (4) along with previously reported minor products is described and compared with earlier work.

Development of a new source of alkylcarbenes allows the direct formation of homocuban-9-ylidene. Deuteriomethanol traps not only the carbene, but the product of its rearrangement, the bridgehead alkene homocub-1(9)-ene. Together with the earlier observation that the bridgehead alkene rearranged to the carbene this demonstrates the first example of an equilibrating carbene-bridgehead alkene pair.

When 1,1-diiodoneopentane is passed through a hot tube containing methyllithium-coated Pyrex chips, 1,1-dimethylcyclopropane and 2-methyl-2-butene are produced in near quantitative yield. The ratio of products indicates that the intermediate carbene is the same as is produced from thermal or photosensitized decomposition of tert-butyldiazomethane but different from that formed by direct irradiation of the diazo compound.

Photolysis of triene 3a or 3b yields dibromo- and dichlorocarbene which can be transferred to olefins in good yield. A warning is offered regarding the data recently interpreted as being “only consistent” with the formation of an intermediate in the addition of dibromocarbene to trans -cyclooctene.

Dicarbomethoxycarbene reacts with quadricyclane to give mainly 3,3-dicarbomethoxy-exo-tricyclo[3.2.1.02,4]oct-6-ene. Although this is the same major product as formed by reaction of the carbene with norbornadiene, it is shown that reaction with quadricyclane is direct and does not involve prior isomerization to the diene.

There is no evidence for hydride shift in the reaction of alcohols 1 and 2 with dichlorocarbene. Accordingly, the intermediate cannot be a free carbocation. It is suggested that the products are formed through a combination of S N i and direct displacement reactions in which tight ion pairs are involved.

Norcaradiene 2 undergoes two kinds of photochemical reaction. 4,4-Dimethylcyclohexadienylidene is extruded on irradiation through quartz, but not when 2 is shielded by Pyrex. The carbene can be trapped with various olefins and a comparison of the properties of the carbene formed from 2 and on photolysis of 4,4-dimethyldiazocyclohexadiene reveals them to be identical. In either quartz or Pyrex an isomer of 2 is formed which is thought to possess structure 3d. Heating of 3d at 100° produces still another isomer, 4a, which can be reconverted to 3d on irradiation. Structures for 3d and 4a are proposed and mechanisms for their formation suggested.

Les dibromocarbenes generes thermiquement ou photochimiquement s'additionnent a divers dimethylene-1,2 cyclanes pour donner des produits d'addition-1,2 et -1,4

Obtention et analyse aux RX de l'hexafluoro-2,3,4,5,6,7 spiro [cycloheptatriene-2,4,6-1,9'-fluorene] par reaction photochimique du perfluorobenzene avec le diazo-9 fluorene