Phenols from boronic acids

SCHEME 29 6 Photocatalytic access to phenol from boronic acid. 

Coumarone—Indene Kesins. These should be called polyindene resins (17) (see Hydrocarbon resins). They are derived from a close-cut fraction of a coke-oven naphtha free of tar acids and bases. This feedstock, distilling between 178 and 190°C and containing a minimum of 30% indene, is warmed to 35°C and polymeri2ed by a dding 0.7—0.8% of the phenol or acetic acid complex of boron trifluoride as catalyst. With the phenol complex, tar acids need not be completely removed and the yield is better. The reaction is exothermic and the temperature is kept below 120°C. When the reaction is complete, the catalyst is decomposed by using a hot concentrated solution of sodium carbonate. Unreacted naphtha is removed, first with Hve steam and then by vacuum distillation to leave an amber-colored resin. It is poured into trays, allowed to cool, and broken up for sale. 

Finally, the remarkably simple solution came from Evans et al. [21a] and researchers of DuPont [21b] simultaneously. Their method allows the coupling of structurally and electronically diverse phenols and aryl boronic acids in the presence of copper]11) acetate, trie-thylamine or pyridine, and molecular sieves at ambient temperature (Scheme 5). Even phenolic amino acid derivatives react smoothly without racemization. The only limitation has been observed when using orfho-heteroatom substituted boronic acids which resulted in lower product yields. The initial step in the proposed pathway (Scheme 6) is the trans-metallation of the boronic acid residue with the copper salt. 

A long-standing reaction is the oxidation of aryl boronic acids to phenols by alkaline peroxide, usually in the work-up of a borate-organolithium reaction, without isolation of the boronic acid, i.e. an efficient ArBr ArOH conversion. A variant under milder conditions uses sodium perborate (for the conversion of 5-bromopyrimidines), and, using oxone, oxindoles can be prepared from 1-Boc indoles via direct... 

Unlike the synthesis of phenols from non-aromatic sources that of thiophenol by similar methodology is virtually unknown. The synthesis shown of the diacetate of a tricyclic compound containing an o-substituted thiophenol system would appear to be applicable to the simpler central portion and to bicydic analogues. Ethyl 4-(1-benzyl-2-thioxo-pyrrolidin-3-yl)-5-(diazoacetyl)-pyrrole -2-carboxylate in dichloromethane was treated with boron trifluoride etherate in the same solvent and after 15 mins, the intermediate shown resulted which in a sealed tube (under vacuum) with acetic acid/acetic anhydride (3 1) at 132°C during 90mins. produced by a Pummerer rearrangement, ethyl... 

Boron, Phosphorus, and Selenium Compounds. Oxone has been used to oxidize carbon-boron bonds during the work-up of hydroboration reactions to obtain high yields of the resultant alcohols (eq 73). Aqueous Oxone/acetone oxidizes electron-poor and electron-rich aromatic and aliphatic boronic acids and esters to the corresponding alcohols rapidly and efficiently (eq 74). A one-pot procedure for the synthesis of iweta-substituted phenols from benzenes has been developed, and a similar strategy has been devised for the synthesis of Boc-oxindoles from Boc-indoles. i3i... 

Non-metallic stractures of boron, silicon, phosphorus, and bromine can be bonded to phenolics. In most cases, these resins show an improved thermal stability in comparison to the nonmodified ones [1], Boron modified phenolics are prepared from aryl borates that are synthesized from phenol and boric acid (Scheme 26). The aryl borates react with methanal or 1,3,5-trioxane as shown in Scheme 26 to give boron modified prepolymers [167,168],... 

During these catalytic or stoichiometric processes, possible side reactions do occur which are shown in Scheme 4.3. These explain why the use of more than lequiv. of boronic acid is necessary. Furthermore, during the oxidation of copper(ll) to copper(lll), hydrogen peroxide is produced, which can decrease the yields of the reaction as a result of its reaction with the aryl boronic add. This may also explain why more than 1 equiv. of aryl boronic acid provides enhanced yields [10]. Moreover, the aryl boronic acid can form triaryl boroxines [11], and in doing so forms water, which can be removed from the reaction by molecular sieves. Evans and coworkers postulated that phenolic products would be formed as a result of the competitive arylation of water formed during the reaction process. 

The convergent total synthesis of (S,S)-isodityrosine has been accomplished from natural a-amino acids, l-tyrosine, and L-phenylalanine (Scheme 20.49). In this synthesis, boronic acid is used as the aryl donor, and tyrosine derivatives are used as the phenolic partner [148]. 

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