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Stereodivergent Hydroboration of Allenes
Yoshiyuki Nagashima1, Keiji Sasaki1, Takahiro Suto1
1Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1, Hiyoshi, Kohoku-ku, Yokohama, 223-8522, Japan.
This study introduces a stereodivergent hydroboration of allenes, yielding either (E)- or (Z)-allylic alcohols. This method enables selective synthesis of challenging trisubstituted olefins and valuable natural product motifs.
Area of Science:
- Organic Chemistry
- Synthetic Chemistry
Background:
- Stereoselective synthesis of trisubstituted olefins is challenging.
- Allenes are versatile synthetic precursors.
- Controlling stereochemistry in allylic alcohol synthesis is crucial for natural product synthesis.
Purpose of the Study:
- To develop a stereodivergent hydroboration of allenes.
- To achieve selective synthesis of (E)- and (Z)-allylic alcohols.
- To apply the method for synthesizing complex organic molecules.
Main Methods:
- Hydroboration of allenes using 9-BBN and disiamylborane (HB(Sia)2).
- Oxidative work-up to yield allylic alcohols.
- Stereochemical analysis of products.
Main Results:
- Hydroboration with 9-BBN yielded thermodynamically stable (E)-allylic alcohols.
- Hydroboration with HB(Sia)2 yielded kinetically controlled (Z)-allylic alcohols.
- High stereoselectivity was achieved in the synthesis of trisubstituted olefins.
Conclusions:
- A novel stereodivergent hydroboration of allenes was established.
- The method provides access to both (E)- and (Z)-allylic alcohols.
- The developed conditions are applicable to synthesizing motifs found in biologically active natural products.
Related Concept Videos
03:08Hydroboration-Oxidation of Alkenes
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 stereochemistry.
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.