Fluoroboration of phenols

The reaction of 2-carboxyphenyldiazonium fluoroborate with phenol constitutes a good route to xanthone (B-68MI22400). 

Diazo ketone cyclizathn. Some years ago Mander and his group1 demonstrated that the protonated diazomethylcarbonyl group can initiate cyclizations in unsaturatcd systems. In the case of phenolic diazo ketones, formation of spirodienones can predominate over competing side reactions (dienone-phenol rearrangement). Tetra-fluoroboric acid or boron trifluoride etherate can be used, but trifluoroacetic acid is usually the acid of choice. 

Phenol-formaldehyde reactions catalyzed by zinc acetate as opposed to strong acids have been investigated, but this results in lower yields and requires longer reaction times. The reported ortho-ortho content yield was as high as 97%. Several divalent metal species such as Ca, Ba, Sr, Mg, Zn, Co, and Pb combined with an organic acid (such as sulfonic and/or fluoroboric acid) improved the reaction efficiencies.14 The importance of an acid catalyst was attributed to facilitated decomposition of any dibenzyl ether groups formed in the process. It was also found that reaction rates could be accelerated with continuous azeotropic removal of water. 

Phenolic antioxidants in rubber extracts were determined indirectly photometrically after reaction with Fe(III) salts which form a red Fe(II)-dipyridyl compound. The method was applicable to Vulkanox BKF and Vulkanox KB [52]. Similarly, aromatic amines (Vulkanox PBN, 4020, DDA, 4010 NA) were determined photometrically after coupling with Echtrotsalz GG (4-nitrobenzdiazonium fluoroborate). For qualitative analysis of vulcanisation accelerators in extracts of rubbers and elastomers colour reactions with dithio-carbamates (for Vulkacit P, ZP, L, LDA, LDB, WL), thiuram derivatives (for Vulkacit I), zinc 2-mercaptobenzthiazol (for Vulkacit ZM, DM, F, AZ, CZ, MOZ, DZ) and hexamethylene tetramine (for Vulkacit H30), were mentioned as well as PC and TLC analyses (according to DIN 53622) followed by IR identification [52]. 8-Hydroquinoline extraction of interference ions and alizarin-La3+ complexation were utilised for the spectrophotometric determination of fluorine in silica used as an antistatic agent in PE [74], Also Polygard (trisnonylphenylphosphite) in styrene-butadienes has been determined by colorimetric methods [75,76], Most procedures are fairly dated for more detailed descriptions see references [25,42,44],... 

Fluoroboric acid supported on silica (HBF4-silica) has recently been found to be a highly efficient catalyst in the protection of various functional groups. Structurally diverse alcohols, phenols, thiophenols, and anilines can be acylated under solvent-free conditions at room temperature.669 Even acid-sensitive tertiary alcohols (1-alkylcyclo-hexanols) and sterically hindered compounds, such as endo-borneol, give the acylated products in high yields. A triflic acid-silica catalyst also shows high activity in the (9-acetylation with Ac20 of alcohols and phenols.359... 

A modification of this reaction involves conversion of the diazonium salt to the diazonium fluoroboride with fluoroboric acid. Treatment with glacial acetic acid then gives the aryl acetate, from which the free phenol is obtained by hydrolysis. ... 

The acyl phosphate (3) also N-methylates pyridine to form the salt (7). Treatment of (7) with trimethyloxonium fluoroborate regenerates (3). Treatment of (7) with alcohols or phenols gives the salt (8), which is rapidly hydrolyzed to the phosphate ester (9). 

Ethylation of alcohols, phenols, acids Triethyloxonium fluoroborate. 

Brockmann and Dorlars561 converted diazonium halides into phenols in the presence of phosphoric acid. Treating diazonium fluoroborates with glacial acetic acid gives phenol esters, which can be deacylated in the usual way.562 Diazonium trifluoroacetates are relatively stable and can be converted by warming into phenols in good yield.563... 

The classical diazonium salt route to phenolic ethers has been re-examined. The slow addition of methoxytrimethylsilane (4 moles) to benzenediazonium fluoroborate in ice-cold freshly distilled Freon-113 under nitrogen, warming of the mixture to 55°C with ultrasonic agitation (W-385 processor) and finally refluxing with continued sonication at the same temperature during 16 hours, afforded a 51% yield of anisole (ref.71). 

Although outside the scope of the present chapter, another transformation of interest is the conversion of the fully hydrogenated product from phenol, namely cyclohexanol, to cyclohexanone in 100% yield by addition of a dichloromethane solution to bis(quinuclidine)bromine fluoroborate and silver fluoroborate in dichloromethane followed by reaction for 30 mins.at ambient temperature (ref.65). 

Biogenetic-type syntheses of ( )-oxocrinine (21a) and ( )-oxomaritidine (21b) by an anodic oxidation procedure have been achieved. Thus oxidation of the trifluoroacetyl derivative (22a) in acetonitrile solution using platinum electrodes and fluoroboric acid electrolyte gave the spirodienone (23a) in 62% yield. Compound (23a) was readily converted into ( )-oxomaritidine (21b) by known procedures. This represents the highest observed yield of a phenol oxidative... 

Alcohols, Phenols, and Ethers. — Attempts to obtain evidence that l-(pentafluoro-phenyl)ethanoI or pentafluorobenzaldehyde ethyl acetal can act as a hydride ion donor towards triphenylmethyl fluoroborate have met with very limited success. The main products (82) and (83) of these reactions and the most likely routes to them are illustrated in Schemes 21 and 22, and although the traces of pentafluoro-... 

The widely used method for the determination of this stabiliser is based on the determination of phosphorus. This involves a tedious preliminary digestion with nitric and perchloric acids. Kellum [84] has described a colorimetric method for determining Polygard based on hydrolysis to nonyl phenol, followed by coupfing with p-nitro benzene-diazonium fluoroborate and colorimetric estimation at 550 nm ... 

High orthonovolac resins have been reported to be prepared by reacting phenol and formaldehyde in the presence of a divalent electropositive metal oxide, hydroxide, or organic salt based on sulfonates, fluoroborates, or acids selected from sulfonic or fluoroboric acid or mixtures [43]. This is in contrast to the conventional novolac resins prepared with strong acids (sulfuric acid, hydrochloric acid, or oxalic acid) and have a preponderance of para/para and ortho/para methylene bridges. The high ortho novolacs have mostly ortho/ortho methylene bridges such as ... 

The thermal decomposition of aryldiazonium salts also gives phenols, and in a few cases the photochemical processes offer superior yields. " Preparation and photodecomposition of diazonium salts in 50% fluoroboric acid has proved useful for the replacement by fluorine in cases where the diazonium salt is exceptionally stable and the usual Schiemann reaction gives poor yields." However the practical problems of avoiding overheating and the eventual screening of light by the reaction product limit the usefulness of these photochemical processes. 

---