Dehydrogenations with sulfur

Dibenzofuranols are available by Michael addition of l-morpholino-1-cyclohexenes (198) to 1,4-benzoquinones (197, Scheme 50). Treatment of the intermediate adducts 199 with acid affords 6,7,8,9-tetrahydro-2-dibenzofuranols (200), which on dehydrogenation with sulfur or palladized charcoal afford the dibenzofuranols 201 in good yield. 

A2-Pyrazolines are converted into pyrazoles by oxidation with bromine or Pb(OAc)4 and they can also be dehydrogenated with sulfur. 3,5-Diphenylpyrazoline (419) on heating with platinum disproportionates to the pyrazole (420) and the pyrazolidine (421) (66AHC(6)347). 

Dihydrobenzo[6]thiophene 426 and its 2-methyl derivative182 are readily dehydrogenated with sulfur to the corresponding benzo[ ]-thiophene. 2,3-Dihydrobenzo[6]thiophenes are oxidized by peracetic acid to the corresponding sulfones (Section VI, P, 2), and give crystalline complexes with mercuric chloride. 

Problem 30.5 Cadinene C15H24, is found in oil of cubebs. Dehydrogenation with sulfur converts cadinene into cadaiene, CisHig, which can be synthesized from carvone by the following sequence ... 

Hydrogenation of diarylamines yields dialicyclic amines, but hydrogenolysis of the carbon-nitrogen bond also occurs. Dehydrogenation of DPA yields catbazole and alkyl derivatives of DPA give acridines. Dehydrogenation with sulfur gives phenothiazines (12). 

Plattner P A 1948 Dehydrogenation with sulfur, selenium, and platinum metals. In Newer methods of preparative organic chemistry. Wiley-Interscience New York, 21-59... 

Most synthetic camphor (43) is produced from camphene (13) made from a-piuene. The conversion to isobomyl acetate followed by saponification produces isobomeol (42) ia good yield. Although chemical oxidations of isobomeol with sulfuric/nitric acid mixtures, chromic acid, and others have been developed, catalytic dehydrogenation methods are more suitable on an iadustrial scale. A copper chromite catalyst is usually used to dehydrogenate isobomeol to camphor (171). Dehydrogenation has also been performed over catalysts such as ziac, iadium, gallium, and thallium (172). 

Before dehydrogenation of ethane became the dominant method, ethylene was prepared by heating ethyl alcohol with sulfuric acid. 

Naphthalic anhydride has been prepared by the hydrolysis of the dinitrile of ,2-naphthaIic acid,1 by the oxidation of suitably substituted hydrocarbons or ketones,2 or by the dehydrogenation of the 3,4-dihydro compound with bromine 3 or with sulfur.4... 

Tetraphenylphthalic anhydride has been prepared by condensation of tetraphenylcyclopentadienone and maleic anhydride in nitrobenzene,1 followed by dehydrogenation of the tetraphenyl-dihydrophthalic anhydride with sulfur.2 3... 

The procedure given here is essentially that described previously by the submitters2 and is based on the early work of Knoevenagel.8 2-Phenylindazole has been prepared by reduction of N-(o-nitrobenzyl)aniline with tin and hydrochloric acid,4 by reduction of N-(o-nitrobenzyl) -N-nitrosoaniline with tin and hydrochloric acid,5 by dehydration of 2-(phenylazo)benzyl alcohol,6 by elimination of acetic acid from 2-(phenylazo)benzyl acetate,7 by dehydrogenation of 3,3a,4,5,6,7-hexahydro-2-phenyl-indazole with sulfur,8 and by thermal decomposition of o-azido-benzalaniline.9... 

An elegant synthesis of 1,3,7,9-tetramethyldibenzothiophene has been recorded as shown in Eq. (2). Cyclization of the diene (34) to the octahydrodibenzothiophene was accomplished with either aluminum chloride or a mixture of acetic acid and sulfuric acid (90%). Subsequent dehydrogenation with palladium on carbon gave the tetramethyl compound. The NMR spectrum of this compound has been discussed earlier... 

In contrast to linear unsaturated acids and anhydrides vide supra), 3,4,5,6-tetrahydrophthalic anhydride reacts with sulfur tetrafluoride to give mainly bicyclo products, lactone 31 and tetrafluoro ether 32, with the trifluoromethyl derivatives 33 and 34 as minor products. The ratio of products strongly depends on the reaction conditions at elevated temperature (> 150°C) cleavage of the C —O —C bond and dehydrogenation occur resulting in l,2-bis(tri-fluoromethyl)benzene (35) in high yield.252... 

The catalytic dehydrogenation of dicyclohexeno-4-methyl-4f/-thiopyrans 340 (R = H, Me) on palladium-charcoal or with sulfur led to aromatization of both carbocycles and only dibenzo derivatives 426 (R = H, Me) were obtained.95,102... 

In another set of experiments the effect of prior dehydrogenation on the yields of gases (especially that of methane) was studied. As shown earlier, dehydrogenation with either sulfur (17, 18) or iodine (22) leads to the complete inhibition of tar formation and fixation of the corresponding carbon (alicyclic) in char. It is thus possible to study the contribution, if any, of the hydroaromatic structure towards gas formation by partial or complete fixation of the hydroaromatic carbon in char. 

Klarman s nietliod779 of dehydrogenation of 2,3-dihydrobenzofurans is achieved with sulfur at 220° (45-85% yields)4,10 or with selenium.214 The method has led to methoxybenzofurans which are difficult to obtain otherwise.10 It has been applied to polycyclic compounds furoquinolines.223,780 naphtho[2,l-5]furan.499... 

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