Stereoselective synthesis of alcohols, XLI. Chirality transfer to generate quaternary stereogenic centers by an allylboration reaction

One out of Four: Kinetic Resolution of Stereoisomeric Mixtures of Secondary Alcohols with a Quaternary Carbon Atom in the β-Position by Cu-H-Catalyzed Enantioselective Silylation

An enantio- and diastereoselective Cu-H-catalyzed silylation of acyclic secondary alcohols with a vicinal quaternary stereocenter is reported. The reaction kinetically selects one out of four stereoisomers, affording the fastest-reacting stereoisomer as the silyl ether in enantio- and diastereomerically enriched form. The obtained motif with a quaternary carbon atom in the β-position of the hydroxy group is otherwise not easy to access.

Nonracemic allylic boronates through enantiotopic-group-selective cross-coupling of geminal bis(boronates) and vinyl halides

Under the influence of a chiral palladium catalyst, 1,1-bis(pinacolboronate) esters undergo asymmetric cross-coupling with bromoalkenes to generate nonracemic allyl boronates with high levels of enantioselectivity. The so-formed allyl boronates may be oxidized with hydrogen peroxide to provide secondary allylic alcohols or with nitrosobenzene to furnish nonracemic tertiary allylic alcohols. Mechanistic experiments suggest the operation of a pathway involving outer-sphere stereoinvertive transmetalation.

Catalytic asymmetric allylation of carbonyl compounds and imines with allylic boronates

Enantioselective allylation is a highly used organic reaction to prepare chiral homoallylic alcohols and amines, which serve as important building blocks in the synthesis of a variety of natural products and pharmaceuticals.

Axial preferences in allylation reactions via the Zimmerman-Traxler transition state

The reaction of a substituted allylmetal with a prostereogenic carbonyl compound can give rise to up to two racemic diastereomers (syn and anti). Classically, in such reactions, when pure E-isomers have afforded anti-selectivity and the Z-isomers exhibit syn-selectivity, researchers have used the empirical Zimmerman-Traxler model. In this model, chair-like transition states dominate over boat-like arrangements. The incoming aldehyde alkyl (aryl) residue occupies a pseudoequatorial rather than a pseudoaxial position to avoid potential 1,3-diaxial steric interactions. However, the reaction of γ,γ-disubstituted allylzinc species with carbonyl compounds generates two gauche interactions, which may result in a completely different stereochemical outcome. With these two gauche interactions, would a transition state in which the aldehyde substituent occupies a pseudoequatorial position or a pseudoaxial position be preferred? In this Account, we show that reaction of γ,γ-disubstituted allylzinc species with carbonyl compounds proceeds through a chair-like transition state and the substituent of the incoming aldehyde residue prefers to occupy a pseudoaxial position to avoid these two gauche interactions. Theoretical calculations on model systems support our experimental results. We have extended this new stereochemical outcome to describe the formation of α-alkoxyallylation of aldehydes through the formation of the rather uncommon (E)-γ,γ-disubstituted alkoxyallylzinc species. We also used this method to transform aromatic ketones and α-alkoxyaldehydes and ketones into functionalized adducts. In a one-pot reaction and using simple alkynes, three new carbon-carbon bonds and two to three stereogenic centers, including an all-carbon quaternary stereocenter could be created in acyclic systems. Because 1,3-diaxial interactions are now produced with the axial substituent, an increase in the substituent size on the zinc atom decreases the diastereoselectivity.

Catalytic enantioselective 1,2-diboration of 1,3-dienes: versatile reagents for stereoselective allylation

More with boron: The development of catalytic enantioselective 1,2-diboration of 1,3-dienes enables a new strategy for enantioselective carbonyl allylation reactions (see scheme). These reactions occur with outstanding levels of stereoselection and can be applied to both monosubstituted and 1,1-disubstituted dienes. The carbonyl allylation reactions provide enantiomerically enriched functionalized homoallylic alcohol products.

Highly (E)-selective BF(3).Et(2)O-promoted allylboration of chiral nonracemic alpha-substituted allylboronates and analysis of the origin of stereocontrol

Delta-methyl-substituted homoallylic alcohols 2 were prepared in 71-88% yield, E:Z >30:1 and 93% to >95% ee via BF(3).Et(2)O-promoted allylboration with alpha-Me-allylboronate 1. The origin of high (E)-selectivity is proposed.

Creation of quaternary stereocenters in carbonyl allylation reactions

Despite the advances in stereoselective carbonyl allylation reactions, the creation of quaternary stereocenters in the addition of 3,3'-disubstituted allylmetals to aldehydes is still a challenging issue. This feature article describes the most powerful approaches that have been devised to address this problem.

New Multicomponent Approach for the Creation of Chiral Quaternary Centers in the Carbonyl Allylation Reactions

The combined regio- and stereo-controlled carbometalation reaction of alkynyl sulfoxide followed by a zinc homologation and finally an allylation reaction led, in a single-pot operation, to chiral homoallylic alcohols in excellent yields and diastereoselectivities for the creation of chiral quaternary and tertiary centers. The key features in all of the reactions that are described in this article are the high degree of stereocontrol, the level of predictability, and "the ease of execution" which ensures success in the application of such methods. The chiral sulfinyl moiety can be finally removed by simple treatment with alkyllithiums, which allows further functionalizations of the carbon skeleton. By using one of these methods, the creation of chiral quaternary centers with the smallest possible difference, namely CD3 versus CH3 and 13CH3 versus CH3, was easily performed.