Dihydrobenzoic Acid

Dihydrobenzoic acid has a very penetrating, repulsive odor, and care should be taken to avoid contamination of hands or clothing. 

Samples of the 1,4-dihydrobenzoic acid, after both the first and the second distillations, are transparent in the ultraviolet region between 220 mu and 300 m/i, indicating the absence of benzoic acid or conjugated dihydrobenzoic acids. The refractive index cited in Reference 3 is in error. 

In the presence of air, 1,4-dihydrobenzoic acid slowly gives benzoic acid and hydrogen peroxide.2... 

Apparently, 1,4-dihydrobenzoic acid has been prepared only by the Birch reduction of benzoic acid, as illustrated by the present procedure.2 3... 

Benzil, 4,4 -bis(di-n-propylamino)-, 41,3 Benzoic acid, conversion to peroxy-benzoic acid, 43, 93 reduction to 1,4-dihydrobenzoic acid, 43, 22... 

A clever application of this reaction has recently been carried out to achieve a high yield synthesis of arene oxides and other dihydroaromatic, as well as aromatic, compounds. Fused-ring /3-lactones, such as 1-substituted 5-bromo-7-oxabicyclo[4.2.0]oct-2-en-8-ones (32) can be readily prepared by bromolactonization of 1,4-dihydrobenzoic acids (obtainable by Birch reduction of benzoic acids) (75JOC2843). After suitable transformation of substituents, mild heating of the lactone results in decarboxylation and formation of aromatic derivatives which would often be difficult to make otherwise. An example is the synthesis of the arene oxide (33) shown (78JA352, 78JA353). 

Bromolactonization of /3,y-unsaturated acids has proven to be a much more satisfactory method of synthesis of /3-lactones, giving good yields of stable crystalline -y-bromo-/3-lactones, except when the substitution at the -y-carbon atom can favor development of carbonium character there. Thus 1,4-dihydrobenzoic acid and 2-methyl-l,4-dihydrobenzoic acid form /3-lactones (equation 95), while 3-methyl-l,4-dihydrobenzoic acid forms the -y-lactone (75JOC2843). The reaction of the sodium salt of a-methylcinnamic acid with bromine in water or methanol also gives /3-lactone, but the yield is low (78JOC3131). 

As mentioned earlier in the discussion of cyclizations leading to (3-lactones, the (3-lactones formed from halolactonization of 1,4-dihydrobenzoic acids readily rearrange to produce bridged ring y-lac-tones.19 In some cases, the substitution pattern favors formation of the y-lactone even under conditions of kinetic control (equation 23).20 Synthesis of a variety of y-lactones by iodolactonization of dihydroben-zoic acid derivatives has been reported recently by Hart (equation 24).91 Attempted iodolactonization of the acid in the case where R = H resulted primarily in an oxidative decarboxylation however, iodolactonization was effected using the amide derivative. 

With substituted benzenes the regioselectivity and the rate of the reduction are crucially dependent on the electron-donating or electron-withdrawing characteristics of the substituent. Thus with anisole, the rate of reaction is decreased and the product is 1-methoxycyclohexa- 1,4-diene (2,5-dihydroanisole) (42) with benzoic acid the rate of reaction is increased and the product is cyclohexa-2,5-dienecarboxylic acid (1,4-dihydrobenzoic acid) (43). 

Cognate preparations. Cyclohexa-2,5-dienecarboxylic acid (1,4-dihydroben-zoic acid).17a A solution of benzoic acid (10g, 0.082 mol) in ethanol (100 ml) and liquid ammonia (600 ml) is stirred and sodium (6.2g, 0.27g-atom) added in small pieces, followed by ammonium chloride (14.6 g, 0.27 mol). The ammonia is evaporated and the residual material dissolved in ice-water (500 ml). After acidification with 10 per cent hydrochloric acid the solution is extracted with four 100 ml portions of ether, the ether washed once with saturated sodium chloride solution, dried over magnesium sulphate and concentrated in vacuo. The remaining pale yellow oil is distilled at 96-98 °C (0.01 mmHg) to give 9.0 g (89%) of 1,4-dihydrobenzoic acid this product shows no u.v. absorption above 220 nm. 

Stereoselective denomination. A key step in a notably efficient synthesis of shi kimic acid (3) from 1,4-dihydrobenzoic acid (see also, 8,84-86) is the stereoselective debromination of 1 with Bu3SnH to give 2 in 72% yield. When BujSnD is used for this reduction, 6/J-deuterioshikimic acid is obtained after alkaline hydrolysis with... 

The 1,4-dihydrobenzoic acids derived from reductive alkylation may undergo facile rearomatization with either loss of the carboxylic acid group or the alkyl group. The gibberellin synthesis intermediate (82), for example, was found to be especially labile, forming (83) simply on exposure to air." Oxidative decarboxylation may be deliberately achieved with lead tetraacetate or electrochemically." Loss of the 1-alkyl group is likely to be a problem when the alkyl moiety can form a reasonably stable free radical, since a chain reaction may then be sustained." ... 

Birch reduction of benzoic acids in the presence of an alcohol (proton donor) furnishes 1,4-dihydrobenzoic acids. The carboxylate salt (-CO2 M ) formed during reduction of benzoic acid derivatives is sufficiently electron rich that it is not reduced. 

DIHYDROBENZOIC ACID (2,5-C7clohexadiene-l-carboxylic acid) CO2H CO2H... 

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