Transition-Metal-Free Borylation of Allylic and Propargylic Alcohols

Abstract

The base-catalyzed allylic borylation of tertiary allylic alcohols allows the synthesis of 1,1-disubstituted allyl boronates, in moderate to high yield. The unexpected tandem performance of the Lewis acid-base adduct, [Hbase](+) [MeO-B2 pin2 ](-) favored the formation of 1,2,3-triborylated species from the tertiary allylic alcohols and 1-propargylic cyclohexanol at 90 °C.

Keywords: allyl boronates; allylic alcohols; allylic borylation; metal-free; polyboronates.

Scheme 1

Optimized reaction conditions for the metal‐free allylic borylation of 1, with B₂pin₂, B₂nep₂, and B₂hex₂. % NMR yields [% Isolated yields].

Scheme 2

a) Scope of transition‐metal‐free allylic borylation of substituted cyclohexyl allylic alcohols (0.3 mmol), B₂pin₂ (2 equiv), Cs₂CO₃ (15 mol %), THF (2.0 mL), MeOH (10 equiv), at 70 °C for 16 h. IY: [% Isolated yields].

Scheme 3

Transition‐metal‐free allylic borylation of polyfunctional substituted cyclohexyl allylic alcohols (0.3 mmol), B₂pin₂ (2 equiv), Cs₂CO₃ (15 mol %), THF ((0.5 mL), MeOH (10 equiv), at 70 °C for 16 h. [% Isolated yields].

Scheme 4

Suggested mechanism for the metal‐free allylic borylation.

Scheme 5

Transition‐metal‐free allylic borylation of 1‐propargyl cyclohexanol (35, 0.3 mmol), B₂pin₂, Cs₂CO₃ (15 mol %), THF (0.5 mL), MeOH (10 equiv), % NMR yields, [% Isolated yields].

Scheme 6

Transition‐metal‐free tandem allylic borylation/diboration (0.3 mmol), B₂pin₂, Cs₂CO₃ (15 mol %), THF (0.5 mL), MeOH (10 equiv), % NMR yields, [% Isolated yields]. X‐Ray diffraction structure of 44.