ORGN 372 |
| The palladium-catalyzed diboration of prochiral allenes (1) with bis(pinacolato)diboron results in the formation of 1,2-bis(boronate)esters (2) possessing allylic and vinylic functionality. High levels of enantioselection (up to 98% ee) are achieved when employing TADDOL-derived phosphoramidite (R,R)-3. Catalyst optimization and substrate scope for this transformation will be discussed. The mechanism for allene diboration was probed and is summarized in Scheme 2. Kinetic studies support oxidative addition of bis(pinacolato)diboron to palladium (I) as the rate-determing step. An inverse dependence on allene concentration, possibly due to the formation of a LPd(0)•allene complex, was also observed. Isotope labeling studies illustrate that the insertion of the terminal bond of the allene into I occurs preferentially, forming intermediate II. DFT calculations support the formation of π-allyl intermediate II followed by reductive elimination to afford diboron product III. These mechanistic studies as well as other experiments, culminating in a better understanding of this transformation, will be discussed.
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Asymmetric Reactions and Syntheses
1:00 PM-5:00 PM, Tuesday, March 27, 2007 McCormick Place East -- Room E351, Level 3, Oral
Division of Organic Chemistry |
