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. 2018 Mar 14;140(10):3663-3673.
doi: 10.1021/jacs.7b12316. Epub 2018 Mar 2.

Carbohydrate/DBU Cocatalyzed Alkene Diboration: Mechanistic Insight Provides Enhanced Catalytic Efficiency and Substrate Scope

Lu Yan  1 , Yan Meng  1 , Fredrik Haeffner  1 , Robert M Leon  1 , Michael P Crockett  1 , James P Morken  1
Affiliations

Carbohydrate/DBU Cocatalyzed Alkene Diboration: Mechanistic Insight Provides Enhanced Catalytic Efficiency and Substrate Scope

Lu Yan et al. J Am Chem Soc. .

Abstract

A mechanistic investigation of the carbohydrate/DBU cocatalyzed enantioselective diboration of alkenes is presented. These studies provide an understanding of the origin of stereoselectivity and also reveal a strategy for enhancing reactivity and broadening the substrate scope.

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Conflict of interest statement

Notes

The authors declare no competing financial interest.

Figures

Figure 1.
Figure 1.
Preparation of [B2(rac-trans-cyclohexanediol)2] and crystal structure analysis of B6(TCD)5(ent-TCD) revealing the 1,2-bonding mode for the trans-cyclohexanediol ligand.
Figure 2.
Figure 2.
DOSY NMR analysis of B2(TBS-DHG)2in comparison to naphthalene as a molecular weight reference. Analysis was performed in CDCl3 by 600 MHz 1H NMR.
Figure 3.
Figure 3.
Kinetic analysis of TBS-DHG catalyzed diboration of 4-phenyl-1-butene with B2(neo)2 as stoichiometric diboron reagent. Effect of (a) catayst concentration, (b) alkene concentration, and (c) diboron concentration.
Figure 4.
Figure 4.
Kinetic analysis of TBS-DHG catalyzed diboration of 4-phenyl-1-butene with B2(pro)2. Effect of (a) diboron concentration, (b) alkene concentration, (c) DBU concentration.
Figure 5.
Figure 5.
Kinetic analysis of diboration of 4-phenyl-1-butene with B2(pro)2 catalyzed by TBS-DHG.
Figure 6.
Figure 6.
Calculated reaction mechanism for alkene diboration with B2(TCD)2.
Figure 7.
Figure 7.
Comparison of relative transition state energies for cycloboration employing TCD, ethylene glycol and 1,3-propanediol ligands in both the 1,1 and 1,2 bonding modes. Tomake meaningful comparison of transition states, the total energy for each ensemble was compared relative to the ensemble for TS-1A (ensemble energy includes the calculated transition state energyand the ground state energy for non-participating diols; see Supporting Information for additional discussion). Calculations performed with M06-2x/6-31+G*.
Figure 8.
Figure 8.
General comparison of 1,1- and 1,2-bonded transition states in metal-free diboration.
Figure 9.
Figure 9.
Calculated transition states for reaction of 1,2-B2(TCD)2•OMe with propene leading to enantiomeric reaction products. The inset structures depict and alternate perspective of the lowest energy transition state along with an electrostatic potential surface that suggests a possible origin of stereocontrol in diboration reactions.
Scheme 1.
Scheme 1.
Glycol-Catalyzed Alkene Diboration
Scheme 2.
Scheme 2.
Mechanism of Cyclic Diol-Catalyzed Alkene Diboration
Scheme 3.
Scheme 3.
Preparation of Carbohydrate-Derived Diboron Reagents
Scheme 4.
Scheme 4.
Equilibrium Between Neopentyl Glycol and trans-Cyclohexanediol Diboron Reagents
Scheme 5.
Scheme 5.
Proposed Exchange Process for Reaction of Cyclic Diol Ligands and B2(neo)2
Scheme 6.
Scheme 6.
Single Vessel Catalytic Enantioselective Diboration with B2(OH)4 as Reagent
See this image and copyright information in PMC

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