Halogeno-hydrocarbons, reactions

Aryl halides do not usually give a positive test for halogen when wanned with alcoholic silver nitrate solution, but certain activating groups may modify the normal reaction, e.g. see nitro-halogeno-hydrocarbons, (page 92). 

Since the nitrogen in pyridine is electron attracting it seemed reasonable to predict that the trihalopyridynes would also show the increased electrophilic character necessary to form adducts with aromatic hydrocarbons under similar conditions to those employed with the tetra-halogeno-benzynes. The availability of pentachloropyridine suggested to us and others that the reaction with w-butyl-lithium should lead to the formation of tetrachloro-4-pyridyl-lithium 82 84>. This has been achieved and adducts obtained, although this system is complicated by the ease with which pentachloropyridine undergoes nucleophilic substitution by tetrachloro-4-pyridyl lithium. Adducts of the type (45) have been isolated in modest yield both in the trichloro- and tribromo- 58) series. 

Optical resolution of some hydrocarbonds and halogeno compounds by inclusion complexation with the chiral host (9a) has been accomplished.11,12 Preparation of optically active hydrocarbons is not easy and only a few example of the preparation of optically active hydrocarbons have been reported. For example, optically active 3-phenylcyclohexene has been derived from tartaric acid through eight synthetic steps.11 Although one-step synthesis of optically active 3-methylcyclohexene from 2-cyclo- hexanol by the Grignard reaction using chiral nickel complex as a catalyst has been reported, the enantiomeric purity of the product is low, 15.9%.11 In this section, much more fruitful results by our inclusion method are shown. 

The intramolecular esterification of simple m-hydroxy-acids (for example, those containing only a saturated hydrocarbon chain) or of its co-halogeno derivative can be realized easily (Fig. 5), but the harsh reaction conditions are not compatible with the numerous sensitive functions present in the seco co-hydroxyacids of natural origin. The necessity to perform the lactonization in a smoother way has been an efficient motor for progress. The same considerations apply for the macrolactamization or the formation of macrocarbocycles of natural origins. 

The Diels-Alder reaction of 9,10-dichloro-anthracenes with 1,1-dichloro-ethene gives a mixture of (360) and (361), and an interesting high-temperature reaction of hydrogen sulphide with the 9-halogeno-derivatives of polynuclear aromatic hydrocarbons led to the corresponding sulphide R2S, e.g. (362). 

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