Palladium/chiral diphosphine-catalyzed umpolung cyclization of allylic acetate-aldehyde using formate as a terminal reductant affords cis-disubstituted pyrrolidine, tetrahydrofuran, and spiro carbocycle in high enantioselectivity. The formate does not cause allylpalladium reduction under the catalysis. The highly stereoselective cyclization would proceed through a cationic η(1)-allylpalladium ligated by diphosphine.

Palladium‐catalyzed umpolung type‐II cyclization of allylic carbonate‐aldehydes leading to 3‐methylenecycloalkanol derivatives was developed. The formate reductant was effective for the cyclization without causing a reduction of the η3‐allylpalladium intermediate. One‐pot decarboxylative allylation of aldehyde‐containing malonate with 2‐[(acetyloxy)methyl]‐2‐propenyl methyl carbonate followed by the cyclization of the allyl acetate‐aldehyde formed in situ was also achieved. The high diastereoselectivities observed in the cyclization of branched substrates indicates that a chair‐chair transition state should be involved. Based on the presumed transition state, we could predict the enantioselectivity of the cyclization using SEGPHOS as a chiral diphosphine ligand and obtain optically active alcohols in up to 95:5 er.magnified image

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Two palladium/chiral diphosphine-catalyzed umpolung cyclizations of aldehyde-containing allylic acetates and allenes with arylboronic acid are fully investigated to establish key factors in their high stereoselectivities. Both cyclization reactions afford cis-disubstituted pyrrolidine and tetrahydrofuran. These occur in high diastereo- and enantioselectivities through a common cationic (Z)-η1-allylpalladium, toward which a ring strain generated in the cyclization step leading to trans-isomers biases the equilibrium through η3-η1-η3-complex in the former cyclization. Varied diastereoselectivities were observed in the formation of five-membered carbocycles and six-membered heterocycles. These reflect release of a ring strain generated in the cyclization step leading to trans-isomers and a different distribution of the (Z)- and the (E)-η1-allylpalladium intermediates generated by the oxidative addition of allylic acetates to Pd(0) or carbopalladation of allenes, respectively. A sterically demanding substituent at the center of the allyl moiety is necessary for high diastereo- and enantioselectivity. The enantioselectivity of the former cyclization was lowered by the presence of organometallic reductants or reagents, possibly causing the formation of neutral η1-allylpalladium species. We used a chiral allylic acetate containing (E)-deuterium-labeled alkene to demonstrate that the electrophilic attack of the aldehyde to the allyl ligand occurred on the side where the palladium existed, consistent with the Zimmerman-Traxler transition state.

Asymmetric Palladium-Catalyzed Umpolung Cyclization of Allylic Acetate-Aldehyde Using Formate as a Reductant. -The first example of an enantioselective umpolung cyclization of allylic acetates or aldehydes is described. The highly enantioselective cyclization is suitable for the synthesis of various heterocycles and spiro--carbocycles otherwise hardly accessible. -(TSUKAMOTO*, H.; KAWASE, A.; DOI, T.; Chem. Commun. (Cambridge) 51 (2015) 38, 8027-8030, http://dx.doi.org/10.1039/C5CC02176F ; Grad. Sch. Pharm. Sci., Tohoku Univ., Aoba, Sendai 980, Japan; Eng.) -R. Staver 36-037