The Selective Silylation of d‐Mannitol Assisted by Phenylboronic Acid and the Solid State and Solution Structures of the Intermediate 1,6‐bis(silyl) bis(phenylboronates)

Abstract

The influence of phenylboronic acid on the silylation of d‐mannitol by TBDMSCl and TBDPSCl has been investigated. Terminal silyation was found to dominate, with PBA significantly enhancing the yield of 1,6 di‐TBDMS d‐mannitol compared to the control. The intermediate 1,6‐disilyl bis(phenylboronates) showed unusually high stability, such that oxidative conditions were required to cleave the boronate esters. X‐ray crystal structures of both bis(phenylboronates) were determined, revealing both diboronate esters to exist as two fused six‐membered rings. The results of ¹¹B NMR experiments suggest that these structures also predominate in solution, whereas the bis(phenylboronate) ester of d‐mannitol itself exists as a mixture of five‐membered and six‐membered cyclic boronates.

Keywords:

• Silylation
• Mannitol
• Boronate
• Phenylboronic acid
• 1,3,2‐Dioxaborolane

Acknowledgments

This work was funded by Monash University, Biota Holdings Ltd and the Australian Research Council. E. M. T. is a recipient of an Australian Postgraduate Award.

Notes

^aThe crystal structure of the bis(phenylboronate) of d‐mannitol‐1,6‐dibenzoate, which had been prepared from d‐mannitol by direct benzoylation followed by condensation with phenylboronic acid has recently been reported.[4]

^bFor a preliminary discussion of the structure of the intermediate phenylboronate esters of d‐mannitol see footnote 12 in Ref. [3].

^cRelative peak intensities can give an indication of relative abundance of components in a mixture if the components give similar responses in the spectrometer and, as was the case here, there are few or no fragment ions.

^d 5 was synthesized by a similar method to that used by Kuivila et al.[17] and recrystallized from methanol.

^eThis compound was synthesized by the azeotropic removal of water from a suspension of 3/2 equiv PBA anhydride and 1 equiv d‐mannitol in toluene, followed by removal of the solvent.

^fTables of atomic coordinates, bond lengths, and bond angles have been deposited with the Cambridge Crystallographic Data Centre. These tables may be obtained, on request, from the Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK. Reference numbers; 1: CCDC 201656; 2: CCDC 201657.