Borinic acids formation

Replacement of pyridine I-oxide by borinic acid in the above reaction leads to formation of vinyloxyborane intermediate 7 which further reacts with aldehydes to give the aldol products 8 and 9. 2-Deoxy-D-ribose is prepared efficiently by application of this reaction O). 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

Isolation of free boronic acids using an aqueous work up may lead to low yields, especially for small or polar ones, which tend to be water-soluble even at a low pH (Section 1.4). In such cases, it is often better to isolate the desired boronic acid as an ester. In an improved procedure that does not involve an aqueous work-up. Brown and Cole reported that the reaction of several types of organolithium intermediates with triisopropylborate was very effective for the synthesis of arylboronic esters [180]. To help minimize the possible formation of borinic acids and boranes by multiple displacements (i.e.. Equation 28 in Figure 1.19), the reaction protocol involves the slow addition of the organolithium to a solution of triisopropylborate in diethyl ether cooled to -78 °C. The use of smaller borate esters such as trimethylborate gave large proportions of multiple addition products (i.e., borinic acid and borane). With triiso-... 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

Although stereoselective formation of enolates from acyclic ketones with bases such as LDA is rather difficult, stereodefined boron enolates are more readily accessible. In the Mukaiyama method, an ethyl ketone is treated with a dialkylboron triflate and a tertiary amine, usually i-Pr2NEt. The resultant Z-(0) boron enolates (also known as enol borinates) are believed to be formed under kinetic control by deprotonation of the Lewis acid-complexed substrate. Brown and co-workers have shown that E- 0) boron enolates may be prepared by treatment of ethyl ketones with dicyclohexylboron chloride in the presence of Et3N. ... 

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