Phenols, direct acylation metal phenolates

V. Snieckus and co-workers developed a new carbamoyl Baker-Venkataraman rearrangement, which allowed a general synthesis of substituted 4-hydroxycoumarins in moderate to good overall yields. The intermediate arylketones were efficiently prepared from arylcarbamates via directed ortho metallation and Negishi cross coupling. The overall sequence provided a regiospecific anionic Friedel-Crafts complement for the construction of ortho-acyl phenols and coumarins. 

Metal triflates are efficiently utilized as reusable catalysts in the direct acylation of phenols as well as in the Fries rearrangement. Scandium triflate is employed as catalyst (5% mol) in the direct acylation of phenols and naphfhols wifh acyl chlorides in toluene-nitromefhane mix-fures af 100°C for 6 h. Resulfs from Table 5.3 show fhaf fhe reacfion is very efficienf complefe regioselecfivify is observed wifh mefa-cresol and 1-naphfhol derivafives. 

Carboxylates Among various 0-substituted electrophiles, O-acylated phenols share a few distinct advantages over other pseudohahdes (i) they are readily available and affordable, (ii) they are relatively stable substrates to various reaction conditions, and (iii) they can serve as directing groups for aromatic substitution reactions such as directed ortho-metallations (DoMs) [58]. Moreover, these carboxylates are much more environmentally friendly, generating acetate salts as the only by-product in the Suzuki-Miyaura cross-coupling reaction [59,... 

Recently, Eberlin and coworkers130 proposed an EI-MS/MS approach for the direct structural assignment of ortho- and metal para-substituted acyl and amidyl anilines in general (as well as phenols). The approach is based on the assertion that most compounds of this class should produce upon El the respective aminoacylium ion (Scheme 39), and that these diagnostic ions (and hence the position of the ring substituents) could be distinguished via selective ion/molecule reactions. 

A further application of the metal-template effect in the ortho-regioselective acylation of phenols is represented by the direct synthesis of salicyloyl chloride and derivatives by the reaction of bromomagnesium phenolates 4 with phosgene. The reaction affords the unstable salicyloyl chloride-magnesium complexes 5 by a pathway similar to the mechanism depicted in Scheme 5.2. These intermediates can be in situ converted into the corresponding acids 6 (Scheme 5.3), esters, amides, or ketones by reaction with suitable reagents. 

Sartori, G., Casnati, G., Bigi, R, and Bonini, G. 1988. Metal template ortho acylation of phenols direct synthesis of salicylic acid chlorides and derivatives. Synthesis 763-766. 

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