Synthesis of - and Z -Enolborinates from Saturated Ketones

The literature describes number of methods for the generation of enolborinates [1]. Enolborinates are valuable intermediates [2] and react with high stereospecificity in aldol condensation [3-8]. Brown and coworkers [9] have carried out a detailed study of the stereochemistry of ketone enolization with B-C1-9-BBN [10], B-OTf-9-BBN [3c], Chx BCl [10], and ChXjBOTf [9] to get ( )- or (Z)-enolborinates. It is found that stereochemical outcome of the reaction varies not only with the steric requirements of RjB and steric requirements of the amine [4, 5b, c], but also with the nature of the leaving group. Cl or OTf (Table 20.1) [9]. 

Tabis 20.1 Enolization of propiophenone and diethylketone with RjBX and amines [9] 

The steric requirement of the substituent R in EtCOR also plays a significant role in the control of enolate geometry. The smaller R groups favor the formation of Z-enolates, while bulkier R groups favor the formation of -enolate. Consequently, EtCOBu-f with all the RjBX (except RjBI) favor the formation of -enolborinate, either exclusively or predominantly (Table 20.5) [11]. 

The stereoselective enolboration of propiophenone (Table 20.6) [11] is also controlled by the nature of leaving groups of H-X-9-BBN and of Chx BX. Chx BI behaves unusually, favoring the formation ofZ-enolate. The selective generation of -enolborinate with Chx BCl/EtjN from propiophenone is one of the significant achievements. 

RjBOTf reagents are prepared [13] by the controlled addition of trifluoro-methane sulfonic acid (1 equiv) to R BH (1 equiv). RjBOMs reagents are obtained by the controlled addition of methanesulfonic acid (1 equiv) to RjBH (1 equiv) [11]. ChXjBBr is prepared by the direct hydroboration of cyclohexene with monobromoborane methylsulfide [11]. B-Br-9-BBN is synthesized by treating 9-BBN with HBr gas. 

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