Fulvenes oxidation

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

Suitable carbonyl compounds can thus be olefmated photochemicully with (diazobenzyl)diphenylphosphine oxide (7), the oxygen function being substituted by a diphenylmethylene group 18,20). Hence irradiation of 7 for a sufficient length of time in the presence of the corresponding unsaturated ketones affords the hepta-fulvene 27 23), the trans-1,3-butadiene 28 22 and the cross-conjugated hexatriene 2922> by direct olefination with the intermediate 9. 

The compounds in this section—fulvenes, isotoluenes, xylylenes—are characterized by trivial names. Our various reference citations document their nontrivial chemistry isomerization, polymerization and oxidation befall the unwary experimentalist who would study them. 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

Cyclopentadiene triphenylarsorane reacted withp-nitrobenzaldehyde in chloroform to give triphenylarsine (10% yield), triphenylarsine oxide (80% yield), and two other unidentified products (24). The corresponding 2,3,4-triphenyl- or tetraphenylcyclopentadienylide under similar conditions afforded triphenylarsine oxide and the corresponding substituted fulvene (25, 68). 

In connection with the above discussion, formation of 3,3-disubsti-tuted 2 (3 H)-oxepinones (73) in the dye-sensitized photooxygenation of 6,6-disubstituted fulvenes is of special interest. 57>58> The reaction may be pictured in terms of an allene oxide intermediate which, as shown in Scheme 11, isomerizes to a cyclopropanone, followed by intramolecular rearrangement. 

The Diels-Alder adduct of fulvene and di(2,2,2-trichloroethyl)azodiearboxylate after selective monohydrogenation of the endocyclic pi bond can lead to the bicyclic biscarbamate 107. The electrochemical removal of the TV-protecting carbamoyl groups in a DMF—LiC104—(Hg) system is followed by the oxidation with potassium ferricyanide to give the azo compound 108 which on thermal decomposition forms the linearly fused tricydopentanoid 109 in over 50 % yield (from 107, Scheme 3-41)88a). 

Methylenepyran Fischer-carbene complexes undergo self-dimerisation in the presence of a Pd(0) catalyst, giving electron-rich bispyrans. Oxidation leads to a methylenepyran fulvene (Scheme 1) <02TL3967>. 

A comparison between the photoreactions of 2-chlorophenol and 2-bromo-phenol in a low-temperature argon matrix was carried out by Akai et al. by means of IR spectroscopy [17,18]. The formation of fulvene 6-oxide was evidenced in both systems. Homolytic C - Br cleavage was found as an additional pathway in the case of 2-bromophenol. 

We have described the cycloadditions of a variety of dienes, ranging from cyclo-pentadiene to cyclopentadienones with alkyl and aryl fulvenes80-82. In these cases, only the [4 + 2] cycloadducts across the 2 and 3 positions are observed. Similarly, 1,3-dipoles such as nitrones and nitrile oxides add in this fashion, as well. We discovered the first authentic [6 + 4] cycloaddition of a fulvene in 197083. The cycloadditions of tropone to fulvenes, which we originally suggested involved [6-fulvene + 4-tropone] cycloadditions, now appear to be [6-tropone + 4-fulvene] cycloadditions8... 

The mode of reaction of fulvenes with o-quinones varies with structure. As illustrated below, 7,7-disubstituted fulvenes (91) react with o-benzoquinone and alkyl substituted o-quinones to give bridged diones 90,91) (92). The regiochemistry of these reactions was determined by photobisdecarbonylation to 93. Subsequent oxidation with chloranil provided a synthesis of benzofulvenes (94). 

A further illustration of the electronic influence of substituents on the course of reactions with 1,3-dipoles can be seen in the cycloaddition of an amino-substituted fulvene to benzonitrile oxide. Adduct (76) was obtained in 60% yield when the electron-rich dipolarophile 6-dimethylaminofulvene was exposed to benzonitrile oxide. The structure of the resultant product can be conveniently explained by an initial [6... 

Many phenoxy-Substituted fulvenes, e.g. 368, are oxidized to the corresponding highly coloured [3]radialenes. Reduction of the latter species can be effected with hydroquinone (equation 107). Oxidative cleavage of some triafulvenes to allenes is... 

An unusual example of the allene oxide-cyclopropanone isomerization is found in the results of singlet oxygen oxygenation of 6,6-disubstituted fulvenes (18) The production of 3,3-disubstituted 2(3//)-oxepinones (20) in this process can be rationalized by the sequence of transformations outlined in equation 8. The initially formed endoperoxide (19) undergoes a well-precedented rearrangement to the epoxy ketone (19a) which contains the allene oxide residue. Rearrangement to the cyclopropanone form is... 

Cp (C5Me4CH2CH2NMe2)TiCl2 (Cp = Cp, Cp ) are obtained by oxidation of the Ti(m) compounds Cp (C5Me4CH2CH2NMe2)TiCl with PbCl2. The molecular structure of the Cp derivative has been determined by X-ray diffraction, which reveals a non-coordinated NMe2 group. The dichloro compounds have been converted to a series of dialkyl, vinyl, titanacyclobutane, and fulvene complexes by the appropriate reactions (Scheme 531 Section 4.05.4.2.4).1093... 

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