Undeniable Confirmation of the syn‐Addition Mechanism for Metal‐Free Diboration by Using the Crystalline Sponge Method

Abstract

The stereochemical outcome of the recently developed metal-free 1,2-diboration of aliphatic alkenes has, until now, only been elucidated by indirect means (e.g. derivatization). This is because classical conformational analysis of the resulting 1,2-diboranes is not viable; in the (1)H NMR spectrum the relevant (1)H resonances are broadened by (11)B, and the occurrence of the products as oily compounds precludes X-ray crystallographic analysis. Herein, the crystalline sponge method is used to display the crystal structures of the diboronic esters formed from internal E and Z olefins, evidencing the stereospecific syn addition mechanism of the reaction, which is fully consistent with the prediction from DFT calculations.

Inter‐Element Boration Reactions of Carbon‐Carbon Multiple Bonds via Lewis‐Basic Activation of Boron Reagents

“…The high stereospecificity observed in the reaction using internal alkenes is consistent with this mechanism. The stereochemistry of the products formed from internal alkenes was unambiguously assigned by X‐ray analysis with the aid of the crystalline sponge method, [17] confirming the stereospecificity of the process. Non‐racemic products were subsequently obtained by employing two equivalents of ( S )‐1‐phenylpropanol instead of methanol, albeit with only moderate selectivity [18] .…”

Stereospecific Reactions Leading to Allylboronic Esters Within Acyclic Systems Bearing Distant Stereocenters

“…The presence of the quaternary stereocenter on the three‐membered ring can also be on the carbon C 2 (Scheme 2, R 1 =Ph, 4 f ). The relative stereochemistry was established by X‐Ray diffraction analysis of 4 d (from an E ‐olefin) and 4 f (from a Z ‐olefin), respectively confirming the syn ‐diboration reaction [14, 15] . Configurations of all other products were determined by analogy.…”

“…The relative stereochemistry was established by X-Ray diffraction analysis of 4 d (from an E-olefin) and 4 f (from a Z-olefin), respectively confirming the syn-diboration reaction. [14,15] Configurations of all other products were determined by analogy. It should be noted that the diastereoselectivity of the diboration reaction decreases with the distance between the olefin and the cyclopropyl methanol core (i.e.…”

Combined Analysis Based on a Crystalline Sponge Method

“…Advance Publication by J-STAGE Received August 28, 2020; Accepted October 22, 2020; Published online on October 30, 2020 DOI: 10.2116/analsci.20SAR07 determined by the CS method and its reaction selectivity for the diboration was evaluated. 20 Using the crystalline nano-space, the CS method is capable of such potential applications as trace biomaterial characterization and unknown sample characterization, and will be applicable to other fields as a sample characterization technique.…”