Dehydrogenations of hydroaromatic compounds

DDQ was first introduced for the dehydrogenation of hydroaromatic compounds, such as tetralin and bibenzyl, which yield naphthalene and stilbene, respectively. A benzene ring or an olefinic bond provides sufficient activation, although it is sometimes difficult to force the reaction to completion. Phis high-potential quinone has since found wide a )plication, jiarticularly... 

Dehydrogenation of hydroaromatic compounds. Used in the last step of the synthesis of 3,4-benzpyrene from l-oxo-l,2,3,4,5,6,7,8-octahydroanthracenel ... 

Aromatization. The high-potential quinone was introduced for use in the dehydrogenation of hydroaromatic compounds for example, in boiling benzene it converts tetralin into naphthalene and acenaphthene into acenaphthylene. The hydroaromatic compound with a blocking group (I) undergoes aromatization with a 1 2-shift of a methyl group. 

Scheme 2.30. Mechanistic aiternatives of quinone dehydrogenations of hydroaromatic compounds. (1) Hydrogen atom transfer, (2) direct hydride transfer, (3) singie electron transfer, and (4) pericyclic hydrogen transfer.

Dehydrogenation. The reagent is comparable to trityl perchlorate and trityl tetrafluoroborate (1, 1287) for dehydrogenation of hydroaromatic compounds to aromatic compounds. In a typical experiment the hydroaromatic compound is refluxed with triphenylmethanol in TFA for 1-20 hr. Yields, in general, are high, and the method is sometimes superior to dehydrogenation with n-butyl-lithium-TMEDA (5, 86). 

Palladium, Pd, and platinum, Pt, usually on activated carbon or asbestos, are catalysts for the dehydrogenation of hydroaromatic and some heterocyclic compounds to aromatic compounds. The reaction takes place at high temperatures (300-350 °C), and consequently, side reactions such as rearrangements often take place [495, 496, 497, 945, 946, 947, 948], The catalytic dehydrogenations played a very important role in the elucidation of terpene and alkaloid structures. Because spectroscopic methods, especially NMR spectroscopy, can help to determine structures much more reliably, catalytic dehydrogenation over palladium and platinum are rare nowadays. 

Dehydrogenation. The perchlorate is an eifective reagent for the aromatization of hydroaromatic compounds. It converts 9,10-dihydroanthracene into anthracene in quantitative yield. It provides the best means available for the conversion of perinaphthanones into perinaphthenones and for the dehydrogenation of chrom-anones to chromones. ... 

Dehydrogenation (the conversion of alicycllc or hydroaromatic compounds into their aromatic counterparts by removal of hydrogen and also, in some cases, of other atoms or groups) finds wide application in the determination of structure of natural products of complex hydroaromatic structure. Dehydrogenation is employed also for the s)mthesis of polycyclic hydrocarbons and their derivatives from the readily accessible synthetic hydroaromatic compounds. A very simple example is the formation of p-methylnaphthalene from a-tetra-lone (which is itself prepared from benzene—see Section IV, 143) ... 

Methylnaphthalene. This preparation illustrates the general procedure for catalytic dehydrogenation. The apparatus used is shown in Fig. 6.1. Heat a mixture of 3.2 g (0.02 mol) of the above hydroaromatic compound with 0.3 g of palladised charcoal (Section 4.2.54, p. 452) at 250-270 °C in a slow current of dry carbon dioxide in a Silicone oil or fusible metal bath for 3 hours (1). Cool, dissolve the residue in ether and filter off the catalyst. Wash the extract with dilute aqueous sodium hydroxide and dry it over anhydrous sodium sulphate. Remove the ether and distil the residual oil under reduced pressure use a small-scale distillation apparatus (cf. Fig. 2.111). Collect the 1-methylnaphthalene, b.p. 121-123 °C/20mmHg. The yield is 2.5g (89%). 

Potassium ferricyanide [potassium hexacyanoferrate(lll)], K3Fe-(CN), in the presence of a base, dehydrogenates hydroaromatic compounds to aromatic compounds [919] and can cause dehydrogenative cy-clizations [920]. The reagent is used for the conversion of acid hydrazides into aldehydes [921], of sterically hindered phenols into phenoxy radicals [922, 923], and of primary amines into nitriles [924], Tertiary amines are demethylated to secondary amines [925, 926]. 

In an elegant modification of dehydrogenation, hydrogen is transferred from the hydroaromatic compound to a compound that readily accepts hydrogen. For example ... 

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