- Synthesis of α-Chiral Ketones and Chiral Alkanes Using Radical Polar Crossover Reactions of Vinyl Boron Ate Complexes
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Vinyl boron ate complexes of enantioenriched secondary alkyl pinacolboronic esters undergo stereospecific radical-induced 1,2-migration in radical polar crossover reactions. In this three-component process various commercially available alkyl iodides act as radical precursors and light is used for chain initiation. Subsequent oxidation and protodeborylation leads to valuable α-chiral ketones and chiral alkanes, respectively, with excellent enantiopurity.
- Gerleve, Carolin,Kischkewitz, Marvin,Studer, Armido
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supporting information
p. 2441 - 2444
(2018/01/27)
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- Development of Enantiospecific Coupling of Secondary and Tertiary Boronic Esters with Aromatic Compounds
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The stereospecific cross-coupling of secondary boronic esters with sp2 electrophiles (Suzuki-Miyaura reaction) is a long-standing problem in synthesis, but progress has been achieved in specific cases using palladium catalysis. However, related couplings with tertiary boronic esters are not currently achievable. To address this general problem, we have focused on an alternative method exploiting the reactivity of a boronate complex formed between an aryl lithium and a boronic ester. We reasoned that subsequent addition of an oxidant or an electrophile would remove an electron from the aromatic ring or react in a Friedel-Crafts-type manner, respectively, generating a cationic species, which would trigger 1,2-migration of the boron substituent, creating the new C-C bond. Elimination (preceded by further oxidation in the former case) would result in rearomatization giving the coupled product stereospecifically. Initial work was examined with 2-furyllithium. Although the oxidants tested were unsuccessful, electrophiles, particularly NBS, enabled the coupling reaction to occur in good yield with a broad range of secondary and tertiary boronic esters, bearing different steric demands and functional groups (esters, azides, nitriles, alcohols, and ethers). The reaction also worked well with other electron-rich heteroaromatics and 6-membered ring aromatics provided they had donor groups in the meta position. Conditions were also found under which the B(pin)- moiety could be retained in the product, ortho to the boron substituent. This protocol, which created a new C(sp2)-C(sp3) and an adjacent C-B bond, was again applicable to a range of secondary and tertiary boronic esters. In all cases, the coupling reaction occurred with complete stereospecificity. Computational studies verified the competing processes involved and were in close agreement with the experimental observations.
- Odachowski, Marcin,Bonet, Amadeu,Essafi, Stephanie,Conti-Ramsden, Philip,Harvey, Jeremy N.,Leonori, Daniele,Aggarwal, Varinder K.
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supporting information
p. 9521 - 9532
(2016/08/12)
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- Synthesis of enantioenriched alkylfluorides by the fluorination of boronate complexes
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The enantiospecific conversion of chiral secondary boronic esters into alkylfluorides is reported. Boronate complexes derived from boronic esters and PhLi were used as nucleophiles, with Selectfluor II as the electrophilic fluorinating agent, to afford alkylfluorides in short reaction times. The addition of styrene as a radical trap was found to enhance enantiospecificity. A broad range of alkyl boronic esters were converted into alkylfluorides with almost complete enantiospecificity by this method.
- Sandford, Christopher,Rasappan, Ramesh,Aggarwal, Varinder K.
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supporting information
p. 10100 - 10103
(2015/09/01)
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