N-Heterocyclic Carbene Adducts of Alkynyl Functionalized 1,3,2-Dithioborolanes.

Richard Böser,René Frank,Lars Denker

doi:10.3390/molecules24091690

Richard Böser, René Frank + Show 1 more

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https://doi.org/10.3390/molecules24091690

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Abstract

Alkynyl functionalized boron compounds are versatile intermediates in the areas of medicinal chemistry, materials science, and optical materials. In particular, alkynyl boronate esters [R1−C≡C−B(OR2)2] are of interest since they provide reactivity at both the alkyne entity, with retention of the B−C bond or alkyne transfer to electrophilic substrates with scission of the latter. The boron atom is commonly well stabilized due to (i) the extraordinary strength of two B−O bonds, and (ii) the chelate effect exerted by a bifunctional alcohol. We reasoned that the replacement of a B−O for a B−S bond would lead to higher reactivity and post-functionalization in the resulting alkynyl boronate thioesters [R1−C≡C−B(S2X)]. Access to this poorly investigated class of compounds starts form chloro dithioborolane cyclo-Cl−B(S2C2H4) as a representative example. Whereas syntheses of three coordinate alkynyl boronate thioesters [R1−C≡C−B(S2X)] proved to be ineffective, the reactions of NHC-adducts (NHC = N-heterocyclic carbene) of cyclo-Cl-B(S2C2H4) afforded the alkyne substituted thioboronate esters in good yield. The products NHC−B(S2C2H4)(C≡C-R1) are remarkably stable towards water and air, which suggests their use as boron-based building blocks for applications akin to oxygen-based boronate esters.

Highlights

Alkynyl boronates [R1 −C≡C−B(OR2 )2 ] and alkynyl borates [R1 −C≡C−B(OR2 )3 ]− (R1, R2 =hydrocarbyl moieties) can be viewed as ester of the parent alkynyl boronic acids R1 -C≡C-B(OH)2 and are versatile building blocks, which are widely employed as intermediates for various applications in the areas of medicinal chemistry, materials science, and optical materials [1,2,3]
The bivalent alcohols introduce a bridging backbone leading to improved stability, and the scope of 1,2-diols employed for alkynyl boronate esters includes pinacol (1) [12,13,14,15,16,17,18,19], catechol, (2) [20,21,22], and ethylene glycol (3) [23,24], Scheme 1
As indicated by the trend of bonds dissociation energies (BDE) for B−S (577 kJ/mol) vs. B−O bonds (810 kJ/mol) [25],the respective alkynyl boronate thioester derivatives are expected to be more reactive in terms of B−S bond scission, which offers options for post-functionalization at the boron center [26]

Summary

Introduction

Alkynyl boronates [R1 −C≡C−B(OR2 )2 ] and alkynyl borates [R1 −C≡C−B(OR2 )3 ]− (R1 , R2 =hydrocarbyl moieties) can be viewed as ester of the parent alkynyl boronic acids R1 -C≡C-B(OH) and are versatile building blocks, which are widely employed as intermediates for various applications in the areas of medicinal chemistry, materials science, and optical materials [1,2,3]. The bivalent alcohols introduce a bridging backbone leading to improved stability, and the scope of 1,2-diols employed for alkynyl boronate esters includes pinacol (1) [12,13,14,15,16,17,18,19], catechol, (2) [20,21,22], and ethylene glycol (3) [23,24], Scheme 1 These compounds 1–3 may be considered as boron substituted heterocycles and can be viewed as derivatives of the parent 1,3,2-dioxaborolane. As indicated by the trend of bonds dissociation energies (BDE) for B−S (577 kJ/mol) vs. B−O bonds (810 kJ/mol) [25],the respective alkynyl boronate thioester (or 1,3,2-dithiaborolane) derivatives are expected to be more reactive in terms of B−S bond scission, which offers options for post-functionalization at the boron center [26]

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