1.4- Cyclohexadiene, reactions with hydrogen

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

To our knowledge, the first published report of a photocatal-ytic reaction at elevated pressure was W. Strohmeyer1s hydrogenation of 1,3-cyclohexadiene under hydrogen at 10 atm /22/. On photolysis, the iridium complex 8 formed a very active catalyst, probably by dissociation of a phosphine ligand (Equation 17). At 70 C, with hydrogen at 10 atm, and a catalyst/substrate ratio of 1/100,000, the activity was 196 per minute and the turnover number was 96,000 mol of product/mol catalyst. 

Scheme 4.8 shows that the CH2 of cyclohexadiene moiety acts as the H donor with formation of cyclohexadienyl radical as the intermediate, which rapidly ejects the silyl radical upon re-aromatization. The silyl radical is able to propagate the chain by reaction with a starting halide. The hydrogen donation of silylated cyclohexadienes toward primary alkyl radicals is reported to be 1 X 10 M s at 70 °C [120], which is in accord with the reported range of 10 -10" s at room temperature for the reaction of primary and second-... 

A second example from the same group is the synthesis of an elaborate diethynyltriphenylene derivative (Scheme 7 Table 8,entries 12,13) [58].Zn/Pd-promoted homocoupling of a 4-iodo-l,2-dialkoxybenzene furnishes the desired tetraalkoxybiphenyl, an electron-rich aromatic system. Iron trichloride-catalyzed Friedel-Crafts arylation of the biphenyl derivative with dimethoxy-benzene furnishes an unsymmetrical triphenylene derivative. Deprotection, oxidation, and subsequent Diels-Alder reaction with cyclohexadiene is followed by catalytic hydrogenation and reoxidation. TMS-CC-Li attack on the quinone delivers the alkyne modules, treatment with SnCl2 aromatizes the six-mem-bered ring, while KOH in MeOH removes the TMS groups cleanly to give the elaborate monomer. 

Benzene is reduced in 95% current yield to a mixture of 23% cyclohexadiene, 10% cyclohexene and 67% cyclohexane. HMPTA as a solvent additive seems to play a dual role. Firstly it is selectively adsorbed at the cathode surface, thereby preventing hydrogen evolution from the protic solvent. Thus it permits the attainment of a potential sufficiently cathodic for the generation of the solvated electron. It secondly stabilizes the solvated electron, thus suppressing its reaction with protic solvents (eq. (130) ). With decreasing HMPTA concentration in the electrolyte the current efficiency for reduction decreases and hydrogen evolution dominates. In pure ethanol the current efficiency is less than 0,4%. 

Although O2 reacts with proton sources to form HOO- (which dispropor-tionates via a second 02), with limiting fluxes of protons to control the rate of HOO- formation from 02, the rate of decay of HOO- is enhanced by reaction with the allylic hydrogens of excess 1,4-cyclohexadiene (1,4-CHD).25 Because HOO- disproportionation is a second-order process, low concentrations favor hydrogen-atom abstraction from 1,4-CHD. This is especially so for MezSO, in which the rate of disproportionation for HOO- is the slowest (PhCl > MeCN > H20 > DMF > Me O).16... 

Michael BD, Hart EJ. The rate constants of hydrated electron, hydrogen atom, and hydroxyl radical reactions with benzene, 1,3-cyclohexadiene, 1,4-cyclo-hexadiene, and cyclohexene. J Phys Chem 1970 74 2878-2884. 

On the other hand, lower reaction temperature (0-30 °C) was indispensable to decrease the background reaction. Under these conditions, a wide range of other 0C, 3-unsaturated ketones and substituted 2-pyrones had been converted into bicyclic chiral compounds 17 in high yield, diastereomeric ratio, and enantiomeric excess (Table 10.8). Interestingly, theauthors noted that, incontrastto 2-pyrone, electron-rich dienes bearing neither a hydrogen-bond acceptor nor donor such as cyclopentadiene and cyclohexadiene were inactive for the Diels-Alder reaction with benzylideneace-tone catalyzed by lp and TFA. They propose that the activation of 2-pyrone by the multifunctional amine IP is also required for the D-A reaction to occur [30],... 

The outstanding features of metal clusters prepared in block copolymer micelles [81] are their high catalytic activity combined with high stability. Such micellar catalyst systems can be recovered after reaction by precipitation or ultrafiltration. In many cases high selectivity and stability have been observed. Cyclohexadiene, for instance, is selectively hydrogenated by Pd colloids just to cyclo-octene [69]. High activity and stability of such catalyst particles have been reported for the Heck-reaction with unusually high turnover numbers of... 

Antkracyclinones. The use of latent quinone reagents such as 2-lithio-3,3,6,6-tetramethoxy-l,4-cyclohexadiene has been extended to a synthesis of an anthra-cyclinone. The starting material 1 was converted into the quinone bisketal 2 by anodic oxidation. The corresponding lithio compound was then condensed with dimethyl 3-methoxyphthalate (3). The reaction fortunately was stereoselective and resulted in 4 in satisfactory yield. The conversion of 4 to the anthracyclinone 5 was conducted in three steps without isolation of intermediates reductive hydrolysis to the hydroquinone, saponification, and finally cyclization with hydrogen fluoride. The overall yield of 5 from 3-bromo-2,5-dimethoxybenzalde-hyde, the precursor of 1, was 8%. ... 

Matthews and Strange [42] reported a similar reaction of isoprene with sulfur dioxide in the presence of hydrogen chloride. Seyer and King [Id] reported that 1,3-cyclohexadiene reacted with sulfur dioxide to give a white amorphous compound, as earlier reported by Hofmann and Damm [43]. 2-Methyl- and 4-methyl-1,3-pentadiene were reported by Morris and Van Winkle [44] to react with sulfur dioxide to yield a cyclic sulfone and some hydrocarbon polymer. Starkweather [45] in 1945 reported that chloroprene (2-chloro-l,3-butadiene) reacted with sulfur dioxide in an emulsion system to give a copolymer. Poly-sulfone is the major product when radical initiators are used. Cyclic products predominate when radical inhibitors (hydroquinone) or temperatures in excess of the ceiling temperature are used. For example ... 

The substrate scope and mechanism of Rh2(cap)4-catalysed TBHP oxidation of phenol and aniline was discussed. The rate of oxidation of para-substituted phenols to 4-(f-butyldioxy)cyclohexadien ones increased significantly in aromatic hydrocarbon solvents. Comparative results with RuCl2(PPh3) and Cul were provided. The results were consistent with hydrogen atom abstraction by the f-butyl peroxy radical followed by combination of the phenoxy and the f-butylperoxy radicals. Under similar reaction conditions,para-substituted anilines were oxidized to the corresponding nitroarenes, and primary amines were oxidized to carbonyl compounds in moderate to good yields. ... 

James and Troughton ( ) obtained ethylene and 1,3,5-hexatriene as the primary products in their study on the reaction of diallyl with the ethyl radical at 134- 175 C. Furthermore, they obtained 1,3-cyclohexadiene as a successive product. Recently Orchard and Thrush (19) reported the thermal isomerization of 1,3,5-hexatriene to 1,3-cyclohexadiene at ca. 400 C and the consecutive formation of benzene at ca. 550 C. In the present work, 1,3-cyclohexadiene (reaction 17) and benzene (reaction 18) were obtained as the secondary products. The hydrogen atom produced in reactions 12,... 

Grieco et al. have also probed an intramolecular variant of the iminium reaction with cyclohexadiene [19]. The intramolecular cyclization of cyclo-hexadienyl aldehyde 45 provides the precursor to dihydrocannivonine 46. Initial attempts to execute the intramolecular iminium-ion Diels-Alder reaction under standard conditions gave rise to poor yields (<10%), probably due to the inherent instability of dienyl aldehyde 45. Yields are dramatically improved by slow addition of a solution of 45 in ethanol over 20-30 h to monomethylamine hydrochloride in aqueous ethanol at 70°C. This modified protocol results in a 66% yield of tricyclic amine 46. Subsequent hydrogenation of 46 affords racemic dihydrocannivonine 47 ... 

Brennecke, Chateauneuf, and coworkers used laser flash photolysis to investigate the excited triplet-state reactions of benzophenone, including triplet-triplet annihilation and hydrogen abstraction reactions with a variety of hydrogen donors in supercritical fluids (184-191). For example, when 2-propanol and 1,4-cyclohexadiene were used as hydrogen donors, the hydrogen abstraction reactions of the triplet benzophenone in supercritical CO2 were found to be... 

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