In the past few decades, it has become clear that asymmetric catalysis is one of the most powerful methods for the construction of carbon-carbon as well as carbon-heteroatom bonds in a stereoselective manner. However, when structural complexity increases (i.e., all-carbon quaternary stereogenic center), the difficulty in reaching the desired adducts through asymmetric catalytic reactions leads to a single carbon-carbon bond-forming event per chemical step between two components. Issues of efficiency and convergence should therefore be addressed to avoid extraneous chemical steps. In this Perspective, we present approaches that tackle the stimulating problem of efficiency while answering interesting synthetic challenges. Ideally, if one could create all-carbon quaternary stereogenic centers via the creation of several new carbon-carbon bonds in an acyclic system and in a single-pot operation from simple precursors, it would certainly open new horizons toward solving the synthetic problems. Even more important for any further design, the presence of polyreactive intermediates in synthesis (bismetalated, carbenoid, and oxenoids species) becomes now an indispensable tool, as it creates consecutively the same number of carbon-carbon bonds as in a multi-step process, but in a single-pot operation.
The formation of all-carbon quaternary stereocentres in acyclic systems is one of the most difficult contemporary challenges in modern synthetic organic chemistry. Particularly challenging is the preparation of all-carbon quaternary stereocentres in aldol adducts; this difficulty is problematic because the aldol reaction represents one of the most valuable chemical transformations in organic synthesis. The main problem that limits the formation of these stereocentres is the absence of an efficient method of preparing stereodefined trisubstituted enolates in acyclic systems. Here we describe a different approach that involves the formation of two new stereogenic centres--including the all-carbon quaternary one--via a combined carbometalation-oxidation reaction of an organocuprate to give a stereodefined trisubstituted enolate. We use this method to generate a series of aldol and Mannich products from ynamides with excellent diastereomeric and enantiomeric ratios and moderate yields.
The applicability of computational descriptors extracted from metal pyridine-oxazoline complexes to relate both site and enantioselectivity to structural diversity was investigated. A group of computationally derived features (e.g., metal NBO charges, steric descriptors, torsion angles) were acquired for a library of pyridine-oxazoline ligands. Correlation studies were employed to examine steric/electronic features described by each descriptor, followed by application of the said descriptors in modeling the results of two reaction types, the site-selective redox-relay Heck reaction and the enantioselective Carroll rearrangement, affording simple, well-validated models. Through experimental validation and extrapolation, parameters derived from ground state metal complexes were found to be advantageous over those from the free ligand.
Reactions that involve metal enolate species are amongst the most versatile carbon-carbon bond forming processes available to synthetic chemists. Enolate species are involved in a multitude of powerful applications in asymmetric organic synthesis, but the generation of fully substituted enolates in a geometrically defined form is not easily achieved especially in acyclic systems. In this Feature Article we focus on the most prominent examples reported in the literature describing the formation of highly diastereo- and enantiomerically enriched quaternary stereocentres in acyclic molecules derived from stereodefined non-cyclic trisubstituted metal enolates.
The development of efficient catalysts and processes for synthesizing functionalized (olefinic and/or chiral) primary alcohols and fluoral hemiacetals is currently needed. These are valuable building blocks for pharmaceuticals, agrochemicals, perfumes, and so forth. From an economic standpoint, bench-stable Takasago Int. Corp.’s Ru-PNP, more commonly known as Ru-MACHO, and Gusev’s Ru-SNS complexes are arguably the most appealing molecular catalysts to access primary alcohols from esters and H2 (Waser, M. et al. Org. Proc. Res. Dev. 2018, 22, 862). This work introduces economically competitive Ru-SNP(O) z complexes (z = 0, 1), which combine key structural elements of both of these catalysts. In particular, the incorporation of SNP heteroatoms into the ligand skeleton was found to be crucial for the design of a more product-selective catalyst in the synthesis of fluoral hemiacetals under kinetically controlled conditions. Based on experimental observations and computational analysis, this paper further extends the current state-of-the-art understanding of the accelerative role of KO-t-C4H9 in ester hydrogenation. It attempts to explain why a maximum turnover is seen to occur starting at ∼25 mol % base, in contrast to only ∼10 mol % with ketones as substrates.
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