Inversion of Enantioselectivity in Allene Gas versus Allyl Acetate Reductive Aldehyde Allylation Guided by Metal-Centered Stereogenicity: An Experimental and Computational Study
Seung Wook Kim
Cole C Meyer
Binh Khanh Mai
Peng Liu
Michael J Krische
Author Contribution.
S. K. and C. C. M. contributed equally.
Corresponding Authors.:pengliu@pitt.edu,mkrische@mail.utexas.edu
Issue date 2019 Oct 4.
Abstract
The use of gaseous allene as an allyl pronucleophile in enantioselectivealdehyde reductive coupling is described. Notably, using the same antipode ofchiral ligand, (S)-tol-BINAP, an inversion ofenantioselectivity is observed for allene vs allyl acetate pronucleophiles.Experimental and computational studies corroborate intervention ofdiastereomericπ-allyliridium-C,O-benzoate complexes,which arise via allene hydrometalation (from a pentacoordinate iridium hydride)vs allyl acetate ionization (from a square planar iridiumspecies).
Keywords: iridium, allylation, enantioselective, transfer hydrogenation, reductive coupling
Graphical Abstract
In connection with longstanding efforts to use abundant chemical feedstocks aspronucleophiles in metal-catalyzed carbonyl reductive coupling,1f a diverse suite of enantioselective C-Ccouplings was developed in our laboratory.1,2 A distinguishing featureof these processes resides in the use of inexpensive terminal reductants (e.g.H2, 2-PrOH) or, more ideally, dual use of alcohols as carbonylproelectrophiles and reductants in carbonyl additionvia hydrogenauto-transfer.1 Given the highoccurrence of allenes as constituents in the C3, C4 and C5 petroleum cracking fractions(Figure 1), these patterns of reactivity wereapplied to the first (2007) allene-carbonyl reductive couplings to furnish homoallylicalcohols (Figure 2).3 Using allene, methylallene and dimethylallene aspronucleophiles, products of carbonyl allylation, crotylation andtert-prenylation were obtained in reactions conducted from either thealcohol or aldehyde oxidation level.3Shortly thereafter (2009), we reported the first enantioselective reactions of this typeusing dimethyl allene.4a Following thisand other catalytic enantioselective allene-carbonyl additions developed in ourlaboratory,4,5 Buchwald reported an allene-ketone reductivecoupling mediated by (MeO)2MeSiH.6
Figure 1.
Allene feedstocks formed in petroleum cracking.
Figure 2.
Milestones in metal-catalyzed allene-carbonyl reductive coupling andrelated hydrogen auto-transfer processes.
In this account, we report the first enantioselective allene-aldehyde reductivecouplings.7 Additionally, wedemonstrate that using thesame antipode of chiral ligand,(S)-tol-BINAP, an inversion of enantioselectivity is observed forcarbonyl allylations that employ gaseous allene vs allyl acetate as pronucleophilesolely in response to stereogenicity at iridium.8,9 Experimental andcomputational studies corroborate intervention of diastereomericπ-allyliridium-C,O-benzoate complexes,which arise via allene hydrometalation (from a pentacoordinate iridium hydride) vsionization of allyl acetate (from a square planar iridium species).
An initial series of experiments were conducted in which a sealed reaction vesselback-filled with gaseous allene1a and charged with aldehyde2a (100 mol%), 2-propanol (200 mol%), THF (0.4 M) and an iridiumcatalyst (5 mol%) were heated to 60 °C (Table1) for 24 hours. It was determined that the(S)-tol-BINAP-modified iridium catalyst, Ir-V, deliveredthe desired product3a with the highest levels of enantioselectivity as the(R)-enantiomer, which is the opposite enantiomer observed incorresponding carbonyl allylations mediated by allyl acetate1b.10 This surprising result compelled us toevaluate the diastereomeric composition of the iridium catalyst Ir-V, whichis easily accomplished via LCMS due to the chromatographic stability ofπ-allyliridium-C,O-benzoates (Figure 3). The catalyst prepared from allyl acetateis enriched in diastereomerD, which, as indicated by experiment andcomputation, is the thermodynamically most stable isomer. In contrast, the catalystrecovered from the reaction mixture using allene or prepared from allene itself isenriched in diastereomerC. It was posited that use of Ir-Vderived from allene would improve enantioselectivity in the allene-mediated allylationof aldehyde2a. Indeed, an increase from 86% to 92% ee in the formation ofhomoallylic alcohol3a was observed (Table1).
Table 1.
Selected optimization experiments in the enantioselective reductivecoupling of allene1 with aldehyde2a and divergentenantioselectivity observed upon use of allyl acetate vs allenepronucleophiles.a
 |
Yields are of material isolated by silica gel chromatography.Enantioselectivities were determined by chiral stationary phase HPLCanalysis.
PhMe (0.4 M).
PhMe (0.1 M).
(S)-Ir-V derived from allene. SeeSupportingInformation for experimental details.
Figure 3.
Diastereomeric composition of the (S)-Ir-Vand calculated thermodynamic stabilities.
Using catalyst (S)-Ir-V derived from allene, thescope of the allene-mediated reductive aldehyde allylation mediated by 2-propanol wasexplored (Table 2). Diverse aryl aldehydes2a-2i and heteroaryl aldehydes2j-2p were converted to the corresponding homoallylicalcohols3a-3p in good yield with uniformly high levels ofenantioselectivity. As illustrated by the formation of adduct3d, due tothe mild reaction conditions, sensitive functional groups such as pinacol boronates aretolerated. The formation of3l, which incorporates an unprotected indolenitrogen, also is notable. The α,β-unsaturated aldehyde2q,as well as linear and branched aliphatic aldehydes2r and2salso participate in highly enantioselective allylation. Allene-mediated allylation of2a mediated byd8-2-propanol deliversdeuterio-3a (eq. 1).11 The pattern ofdeuterium incorporation corroborates reversible allene hydrometalation with incompleteregiocontrol. The relatively low levels of deuterium incorporation are attributed toreversibility of the hydrometalation event and H/D-exchange with adventitiouswater.12
Table 2.
Enantioselective iridium-catalyzed reductive coupling of gaseous allene1a with aldehydes2a-2s mediated by2-propanol.a
 |
Yields are of material isolated by silica gel chromatography. Thecatalyst (S)-Ir-V prepared from allene wasused. Enantioselectivities were determined by chiral stationary phase HPLCanalysis. SeeSupportingInformation for experimental details.
 | (eq. 1) |
The experimental data suggest that the observed divergence in enantioselectivityin reactions of allene vs allyl acetate is due to intervention of diastereomericπ-allyliridium-C,O-benzoate complexesC andD, which arise via allene hydrometalation (from anpentacoordinate iridium hydride) vs ionization of allyl acetate (from a square planariridium species), respectively (Figure4).13,14 To challenge the veracity of our hypothesis, wethen turned our efforts to density functional theory (DFT) calculations.15 The reaction pathways to form theπ-allyliridium complexes under the different experimental conditions were firstcomputed. When allyl acetate pronucleophile is used, the π-allyliridium is formedvia coordination of allyl acetate to theC,O-benzoatecomplex4 followed by ionization (TS1,Figure 5A). Although the common intermediate4can potentially lead to all four diastereomeric π-allyl complexes, formation ofD is kinetically and thermodynamically favored (seeFigure S2 for less favorable pathways toA andB). The relatively low barrier ofTS1Dsuggests a reversible ionization process. The thermodynamically more stable complexD is operative in the subsequent aldehyde addition step (videinfra). On the other hand, π-allyliridium complexes derived fromallene are formed via hydrometalation from a square pyramidal iridium hydride(6C or6D,Figure5B). DFT calculations suggest6C is thermodynamically more stablethan6D, and subsequent allene coordination to form7C andhydrometalation viaTS3C are both very facile. Therefore, π-allylcomplexC is formed preferentially from allene hydrometalation.
Figure 4.
Hypothesis for enantiodivergence in aldehyde allylations mediated bygaseous allenevs allyl acetate.
Figure 5.
Computed energy profiles of the kinetic pathways leading todiastereomersD andC and, therefrom,(S)-and (R)-product enantiomers,respectively.
The calculated transition states for allylation of aldehyde2a bythe diastereomeric complexesC andD provide further insightinto the origins of enantiodivergence. ComplexesC andD aresupported by the same antipode of the (S)-tol-BINAP ligand, but theirstereogenicity at iridium is opposite. Therefore, examination of the transition statesfor carbonyl addition will reveal whether enantioselectivity is influenced more by thechirality of the metal center16 or thebisphosphine ligand. The aldehyde addition withC andD occursby way of Zimmerman-Traxler-type transition states that place the Ar group of thealdehyde in a pseudo-equatorial position (Figure6).17 In the reactionwithD, addition to the (Si)-face of the aldehyde(TS2D-S) to form (S)-3a is 2.2 kcal/molmore favorable than the (Re)-face addition (TS2D-R) toform (R)-3a.TS2D-R is destabilized by the1,3-diaxial interactions between the aldehyde hydrogen and the benzene ring of thebenzoate, which is co-planar with the aldehyde. InTS2D-S, the axialaldehyde hydrogen and the benzoate are on opposite faces of the chair, which relievessteric repulsion. In the allylation transition states with complexC (Figure 6B), because the stereogenicity at iridium isinverted, the (S)-selective transition state (TS2C-S) isnow destabilized by the 1,3-diaxial interactions between the aldehyde hydrogen and thebenzoate. As such, the most favorable transition state fromC isTS2C-R, which eventually leads to the (R)-enantiomerof the homoallylic alcohol product3a.
Figure 6.
Enantioselectivity-determining aldehyde allylation transition stateswith π-allyliridium complexesD andC.Activation free energies are with respect toD andC,respectively. The (S)-tol-BINAP ligands are omitted forclarity
In summary, we report the first catalytic enantioselective aldehyde allylationsmediated by gaseous allene. These processes exploit a feedstock pronucleophile (allene)in combination with a feedstock reductant (2-propanol) under non-cryogenic conditionswith acetone as the sole stoichiometric byproduct. Remarkably, use of allenevs allyl acetate as pronucleophile results in an inversion ofenantioselectivity using the same antipode of chiral ligand,(S)-tol-BINAP. The collective experimental and computational datacorroborate intervention of diastereomericπ-allyliridium-C,O-benzoate complexes,which arise via allene hydrometalation (from a pentacoordinate iridium hydride)vs allyl acetate ionization (from a square planar iridium species).These data should facilitate the design of related chiral-at-metal complexes forenantioselective catalysis by providing insight into the structural and interactionsfeatures of the catalyst that influence enantioselectivity. More broadly, these studiesand other work from our laboratory demonstrate how reactions that traditionally haveemployed organometallic reagents may now be conducted catalytically in the absence ofpremetalated reagents using abundant feedstocks.1f,18
Supplementary Material
Acknowledgments.
The Robert A. Welch Foundation (F-0038), the NIH-NIGMS (RO1-GM069445,R35-GM128779). Ms. Leyah Schwartz is acknowledged for key preliminary experiments.Mr. Brian Lanigan is thanked for skillful technical assistance. P.L. acknowledgessupercomputer resources provided by the Center for Research Computing at theUniversity of Pittsburgh and the Extreme Science and Engineering DiscoveryEnvironment (XSEDE) supported by the NSF.
Footnotes
Supporting Information Available: Experimental proceduresand spectral data. Single crystal X-ray diffraction data for(S)-Ir-V derived from allyl acetate. Thismaterial is available free of chargevia the internet athttp://pubs.acs.org.
The authors declare no competing financial interest.
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