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Copper-Catalyzed Regioselective Dihydroboration of Alkynes to Construct gem-Diborylalkanes
Wenke Dong1, Zheming Liu2, Minghao Zhang2
1College of Chemistry and Chemical Engineering, Zhoukou Normal University, Zhoukou, Henan 466001, P. R. China.
Abstract:
We develop herein an efficient copper-catalyzed dihydroboration of alkynes with HBpin to synthesize gem-diborylalkanes. This catalytic process features a broad substrate scope, good functional group tolerance, and excellent regioselectivity. Notably, the gram-scale reaction and further product derivatizations demonstrate the practicality of this protocol. Preliminary mechanistic studies indicate that the process involves the hydroboration of the alkyne and the vinylboronate intermediate.
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02:47Alkynes to Aldehydes and Ketones: Hydroboration-Oxidation
One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
02:36Regioselectivity and Stereochemistry of Hydroboration
Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn...
02:24Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation
Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
02:38Electrophilic Addition to Alkynes: Halogenation
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a ÃÂÃÂ bond. In alkynes, the presence of two ÃÂÃÂ bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
02:34Preparation of Alkynes: Dehydrohalogenation
Alkynes can be prepared by dehydrohalogenation of vicinal or geminal dihalides in the presence of a strong base like sodium amide in liquid ammonia. The reaction proceeds with the loss of two equivalents of hydrogen halide (HX) via two successive E2 elimination reactions.
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