Dicyclohexylboron chloride

Entry 2 is an example of the polar (3-oxy directing effect. Entries 3 and 4 involve formation of E-enolates using dicyclohexylboron chloride. The stereoselectivity is consistent with a cyclic TS in which a polar effect orients the benzyloxy group away from the enolate oxygen. 

Boron-mediated ketone-ketone aldol reactions have been described, using boron enolates formed with dicyclohexylboron chloride and triethylamine.124 Following addition of the acceptor ketone to form a boron aldolate, oxidation with peroxide yields the aldol product. 

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. ... 

Since the reactions of lithium (El-iOl-enolates are not stereoselective (see Table 6.3b), formation of 2,3-anti aldol products is conveniently accomplished by ketone enolization using dicyclohexylboron chloride. Reaction of the resultant boron... 

Solid-phase-bound (2R,4R,5R) -1 - (henzyloocy) -5,S-dihydroxy-2,4-dimethyloctan-3-one (613) (Scheme 128) [415] To a solution of dicyclohexylboron chloride... 

It is all very well knowing what enolates give rise to what aldol products, but this is not much use unless we can choose to have cis or trans enolates. Clearly with cyclopentanone there is no option but to form a trans enolate. But with an acyclic ketone we will need a little more care. Boron enol ethers provide an answer. Two common boron reagents used are 9-BBN chloride 46 (derived from 9-BBN 45) and another is dicyclohexylboron chloride 47. Other reagents include the corresponding triflates. There are also the chiral boron reagents that we shall meet later chapters. 

Aldol reactions. In conjunction with an amine, dicyclohexylboron chloride is used to generate kinetic enolates from ketones. Some interesting and useful information concerns the stereoselectivity of the aldol reactions. ... 

To 10 mL CH2CI2 was added 0.28 mL triethylamine (2.00 mmol), and the resulting solution was cooled to 0°C. To this solution, was added 2.0 mL 0.5 M dicyclohexylboron chloride in hexane (1.00 mmol), and the solution was stirred for 5 min. Then 0.13 g diox-anone (1.00 mmol) was added and after being stirred for 15 min, 0.1 mL benzaldehyde (1.00 mmol) was added. After stirring for another 15 minutes, 0.28 mL of triethylamine (2.00 mmol) was added, followed by 2.0 mL of 0.5 M dicyclohexylboron chloride in hexane... 

In a mechanistic study, the reaction of 4-bromobenzaldehyde with dicyclohexylboron chloride was monitored by NMR spectroscopy. The NMR data suggest that the reaction proceeds via formation of a borinate ester followed by migration of the alkyl group (Scheme 23.59). 

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