Asymmetric Direct Aldol Reactions of Acetoacetals Catalyzed by a Simple Chiral Primary Amine

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Pushing the limits of aminocatalysis: enantioselective transformations of α-branched β-ketocarbonyls and vinyl ketones by chiral primary amines

Enantioselective α-functionalizations of carbonyl compounds are fundamental transformations for the asymmetric synthesis of organic compounds. One of the more recent developments along this line is in aminocatalysis, which leads to the direct α-functionalization of simple aldehydes and ketones. However, most of the advances have been achieved with linear aldehydes and ketones as substrates. Effective aminocatalysis with α-branched carbonyls, particularly α-branched ketones, has remained elusive. The primary difficulty arises from the space-demanding α-substituent, which impedes iminium/enamine formation. In 2005, synthetic organic chemists revived catalysis using primary amines, which brought new attention to these challenges, because of the conformational flexibility of primary amines. On the basis of early biomimetic studies by Hine, in 2007 we developed the bioinspired chiral primary amine catalysts featuring primary-tertiary diamines. This type of catalyst involves enamine/iminium catalysis, and we could apply this chemistry to all of the major types of ketones and aldehydes. In this Account, we present research from our laboratory that significantly expands aminocatalysis to include α-branched ketones such as β-ketocarbonyls and α-substituted vinyl ketones. Our primary amine catalysis methodology, when used alone or in conjunction with metal catalysts, provides convenient access to both enantiopure α-tertiary and quaternary ketones, structures that are not available via other approaches. Our mechanistic studies showed that acidic additives play the critical role in facilitating catalytic turnover, most likely by shuttling protons during the enamine/iminium tautomerizations. These additives are also critical to induce the desired stereochemistry via ammonium N-H hydrogen bonding. Proton transfer by shuttling is also stereoselective, resulting in enantioselective enamine protonation as observed in the reactions of α-substituted vinyl ketones. In addition, we have carried out density functional theory studies that help to delineate the origins of the stereoselectivity in these reactions.

Construction of an all-carbon quaternary stereocenter by organocatalytic enantioselective α-functionalization of α-substituted β-ketocarbonyls with electron deficient vinylarenes

An all-carbon quaternary stereocenter is efficiently constructed by a chiral amine catalyzed enantioselective α-functionalization of α-substituted β-ketocarbonyls with electro-deficient vinylarenes.

Asymmetric enamine catalysis with β-ketoesters by chiral primary amine: divergent stereocontrol modes

α-Branched ketones remain a challenging type of substrates in aminocatalysis due to their congested structures as well as the associated difficulties in controlling chemo- and stereoselectivity. In this work, we have explored asymmetric aminocatalysis with α-substituted β-ketoesters. A simple chiral primary amine catalyst was identified to enable unprecedentedly effective catalysis of β-ketoesters in α-hydrazination and Robinson annulation reaction with good yields and high enantioselectivities. Stoichiometric experiments with preformed enamine ester intermediates revealed their enamine-catalytic nature as well as the critical roles of acidic additives in facilitating catalytic turnovers and in tuning the chemo- and stereoselectivity. With the identical catalytic system, the two reactions demonstrated opposite chiral inductions in terms of the absolute configurations of the newly formed stereogenic centers. Investigations into this intriguing issue by DFT have revealed divergent stereocontrol modes. For α-hydrazination, H-bonding-directed facial attack determines the stereoselectivity, whereas a steric model is applied to the Robinson annulation where dual activations of both β-ketoester and vinyl ketone/aldehyde are involved.

Origins of the enantio- and N/O selectivity in the primary-amine-catalyzed hydroxyamination of 1,3-dicarbonyl compounds with in-situ-formed nitrosocarbonyl compounds: a theoretical study

Chemoselective control over N/O selectivity is an intriguing issue in nitroso chemistry. Recently, we reported an unprecedented asymmetric α-amination reaction of β-ketocarbonyl compounds that proceeded through the catalytic coupling of enamine carbonyl groups with in-situ-generated carbonyl nitroso moieties. This process was facilitated by a simple chiral primary and tertiary diamine that was derived from tert-leucine. This reaction featured high chemoselectivity and excellent enantioselectivity for a broad range of substrates. Herein, a computational study was performed to elucidate the origins of the enantioselectivity and N/O regioselectivity. We found that a bidentate hydrogen-bonding interaction between the tertiary N(+)-H and nitrosocarbonyl groups accounted for the high N selectivity, whilst the enantioselectivity was determined by Si-facial attack on the (E)- and (Z)-enamines in a Curtin-Hammett-type manner. The bidentate hydrogen-bonding interaction with the nitrosocarbonyl moieties reinforced the facial selectivity in this process.

Asymmetric syn-selective direct aldol reaction of protected hydroxyacetone catalyzed by primary amino acid derived bifunctional organocatalyst in the presence of water

A new series of water compatible primary-tertiary diamine catalysts derived from natural primary amino acids bearing a hydrophobic side chain have been synthesized. These new primary-tertiary diamine-Brønsted acid conjugates bifunctional organocatalysts efficiently catalyzes the asymmetric direct syn selective cross-aldol reaction of different protected hydroxyacetone with various aldehydes in high yield (94%) and high enantioselectivity (up to 97% ee of syn) and dr of 91 : 9 (syn/anti) under mild reaction conditions.

Effective enantiomeric separations of racemic primary amines by the isopropyl carbamate-cyclofructan6 chiral stationary phase

A new chiral stationary phase (CSP) was developed by bonding isopropyl-carbamate functionalized cyclofructan6 (IP-CF6) to the silica gel. It was evaluated by injecting 119 racemic primary amine-containing compounds. This CSP showed pronounced enantioselectivity toward all types of primary amines, separating 93% of all tested compounds. Baseline separation was achieved even for some simple aliphatic racemic amines that contained no other functionality. The polar organic mode was shown to be the effective mobile phase owing to higher efficiency. This new chiral stationary phase showed great potential for preparative-scale separations. It is also interesting that the chiral selector, R-naphthylethyl-carbamate functionalized CF6 (RN-CF6), was found to provide complementary selectivity for the relatively few amine analytes that did not separate on IP-CF6. Thus between the two CSPs, 98% of attempted amine compounds were separated.