From aryl Grignard reagents

Hi) From aryl Grignard reagents. Treatment of anhydrous chromium(III) halides with some aryl Grignard reagents gives bis-arene chromium complexes. The reaction is discussed in Chapter 7. 

This phenol synthesis complements the analogous reaction (see below) from arylthallium ditrifluoroacetates (147). Although yields are only moderate, the procedure represents a viable conversion of aryl Grignard reagents to phenols. It is a practical method, however, only when the diarylthallium trifluoroacetate precursor is formed via the Grignard route the alternative synthesis via symmetrization of arylthallium ditrifluoroacetates is obviously circuitous, since the latter compounds may be converted directly to phenols. 

More typical examples are the amino ethers (3) used as anti-histamine or anti-Parkinson drugs according to the substituents. These are obviously derived from the alcohols (4) which are made from an aryl Grignard reagent and a benzaldehyde. Either starting material may bear the substituent X the choice can be made according to availability and so that side reactions are avoided,... 

The order of reactivity of halides with magnesium is RI > RBr > RC1 (very few organomagnesium fluorides have been prepared), i) Aryl Grignard reagents are more easily prepared from aryl bromides and aryl iodides than from aryl chlorides, which react very sluggishly. 

Biaryl synthesisThis reagent promotes coupling of aryl Grignard reagents to symmetrical biaryls in 70-95% yield with formation of allene and 3-aryl-2-chloropropene as co-products. The reaction is retarded by galvinoxyl and evidently involves an electron-transfer from the Grignard reagent to the dichloropropene. 

The perfluorohalogenomethylraercapto compound is formed in about 50% yield, the by-products being aryl halides from the Grignard reagent in 5-15% yield. It is assumed that the reaction follows an Sn2 mechanism with a cyclic intermediate state in analogy with the reaction of Grignard reagents with alkyl halides 98, 155), e.g.,... 

The behavior of BnMgBr (5d) is similar to that observed for compounds with higher alkyl groups, i.e. only the coupling product was detected and the earlier report on the additional formation of benzyl alcohol was not confirmed. On the other hand, reactions of Ar" radicals formed in the anodic oxidation of aryl Grignard reagents are different from those established for Aik, as is evident from the percent distribution of parent radicals in major products given in Table 7. 

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