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. 2013 Jul 31;135(30):11222-31.
doi: 10.1021/ja4041016. Epub 2013 Jul 18.

Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes

John R Coombs  1 , Fredrik Haeffner, Laura T Kliman, James P Morken
Affiliations

Scope and mechanism of the Pt-catalyzed enantioselective diboration of monosubstituted alkenes

John R Coombs et al. J Am Chem Soc. .

Abstract

The Pt-catalyzed enantioselective diboration of terminal alkenes can be accomplished in an enantioselective fashion in the presence of chiral phosphonite ligands. Optimal procedures and the substrate scope of this transformation are fully investigated. Reaction progress kinetic analysis and kinetic isotope effects suggest that the stereodefining step in the catalytic cycle is olefin migratory insertion into a Pt-B bond. Density functional theory analysis, combined with other experimental data, suggests that the insertion reaction positions platinum at the internal carbon of the substrate. A stereochemical model for this reaction is advanced that is in line both with these features and with the crystal structure of a Pt-ligand complex.

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Figures

Figure 1
Figure 1
Reaction rates with Pt(nbe)3, Pt2(dba)3, and Pt(dba)3. Reactions employed 1 mol% platinum, 1.2 mol% (R,R)-L4 and 1.2 equiv. B2(pin)2. Initial [tetradecene] = 1.0 M.
Figure 2
Figure 2
Unreactive Diboration Substrates.
Figure 3
Figure 3
Reaction progress kinetic analysis of the catalytic enantioselective diboration of 1-tetradecene. (a) Reaction rate versus substrate concentrations. [Alkene]= [B2(pin)2] for both 1.0 M (red) and 1.5 M (blue) reactions, with (R,R)-L4: Pt(dba)3 = 1.2: 1.0 (b) Absolute reaction rate versus [alkene], employing 1.2 mol% (R,R)-L4 and 1.0 mol% Pt(dba)3 (red), and 2.4 mol% (R,R)-L4 and 2.0 mol % Pt(dba)3 (blue). (c) Heat flow versus time of normal diboration reaction conditions (red) and with excess B2(pin)2 (blue). (d) Heat flow versus time of normal diboration reaction conditions (red) and with excess alkene (blue).
Figure 4
Figure 4
A. Relative energetics of migratory insertion involving bis(boryl)platinumpropene complexes. B. Energy profile for the migratory insertion of 45 and 48 to give 49 and 52. C. Calculated transition state structures for conversions to 45→49 and 48→52. All energies are in kcal/mol and are relative to 48.
Figure 5
Figure 5
A. ORTEP of trans-Cl2Pt[(R,R)-L2]2 with 50% probability ellipsoids. B. “Front” view of complex with substituents from one phosphorous ligand removed. C. Top view of complex with substituents from one phosphorous ligand and the dioxolane removed for clarity.
Figure 6
Figure 6
Hypothesis for the stereochemical outcome in enantioselective diborations.
Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3
Scheme 4
Scheme 4
Scheme 5
Scheme 5
Scheme 6
Scheme 6
Scheme 7
Scheme 7
See this image and copyright information in PMC

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