Organocatalysis

In organic chemistry, the term Organocatalysis (a concatenation of the terms "organic" and "catalyst") refers to a form of catalysis, whereby the rate of a chemical reaction is increased by an organic catalyst referred to as an "organocatalyst" consisting of carbon, hydrogen, sulfur and other nonmetal elements found in organic compounds ^{[3] [4] [5] [6] [7]}. Because of their similarity in composition and description, they are often mistaken as a misnomer for enzymes due to their comparable effects on reaction rates and forms of catalysis involved.

The term "organocatalysis" was created by David MacMillan in 2000 from the old and well known concept of "organic catalysis" introduced by the German chemist Wolfgang Langenbeck; "organocatalysis" is nothing more than a new name for an old methodology, but thus gives fresh impulses for intensive research in the following years.

Organocatalysts which display secondary amine functionality can be described as performing either enamine catalysis (by forming catalytic quantities of an active enamine nucleophile) or iminium catalysis (by forming catalytic quantities of an activated iminium electrophile). This mechanism is typical for covalent organocatalysis. Covalent binding of substrate normally requires high catalyst loading (for proline-catalysis typically 20-30 mol%). Noncovalent interactions such as hydrogen-bonding facilitates low catalyst loadings (down to 0.001 mol%).

Two main advantages of organocatalysis are:

• there is no need for metal-based catalysis thus making a contribution to green chemistry
• when the organocatalyst is chiral an avenue is opened to asymmetric catalysis, for example the use of proline in aldol reactions,

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Introduction

Regular achiral organocatalysts are based on nitrogen such as pyridine used in the Doebner modification of the Aldol condensation, DMAP used in esterfications and DABCO used in the Baylis-Hillman reaction. Thiazolium salts are employed in the Stetter reaction. These catalysts and reactions have a long history but current interest in organocatalysis is focused on asymmetric catalysis with chiral catalysts and this particular branch is called asymmetric organocatalysis or enantioselective organocatalysis . A pioneering reaction developed in the 1970s by teams of Hoffmann-La Roche and Schering AG that sums it all up is the Hajos-Parrish-Eder-Sauer-Wiechert reaction:

In this reaction, naturally occurring chiral proline is the chiral catalyst in an Aldol reaction. The starting material is an achiral triketone and it requires just 3% of proline to obtain the reaction product, a ketol in 93% enantiomeric excess. The asymmetric synthesis of the Wieland-Miescher ketone (1985) is also based on proline and another early application was one of the transformations in the total synthesis of Erythromycin by Robert B. Woodward (1981) ^[8].

Many chiral organocatalysts are an adaptation of chiral ligands (which together with a metal center also catalyze asymmetric reactions) and both concepts overlap to some degree.

Organocatalyst classes

Organocatalysts for asymmetric synthesis can be grouped in several classes:

• Biomolecules: notably proline, phenylalanine, the cinchona alkaloids, certain oligopeptides.
• Synthetic catalysts derived from biomolecules. Examples of proline derivatives are McMillan Imidazolidinones and the CBS catalyst
• TADDOLS
• Derivatives of BINOL such as NOBIN
• Triazolium salts as next-generation Stetter reaction catalysts
• Organocatalysts based on thioureas

Examples of asymmetric reactions involving organocatalysts are:

• Asymmetric Diels-Alder reactions
• Asymmetric Michael reactions
• Asymmetric Mannich reactions
• Shi epoxidation
• Organocatalytic transfer hydrogenation

Imidazolidinone organocatalysis

This catalyst works by forming a iminium ion with carbonyl groups of α,β-unsaturated aldehydes (enals) and enones in a rapid chemical equilibrium. This iminium activation is similar to activation of carbonyl groups by a Lewis acid and both catalysts lower the substrates LUMO ^[10]:

The transient iminium intermediate is chiral which is transferred to the reaction product via chiral induction. The catalysts have been used in Diels-Alder reactions, Michael additions, Friedel-Crafts alkylations, transfer hydrogenations and epoxidations.

One example is the asymmetric synthesis of the drug warfarin (in equilibrium with the hemiketal) in a Michael addition of 4-hydroxycoumarin and benzylideneacetone ^[11]:

A recent exploit is the vinyl alkylation of crotonaldehyde with an organotrifluoroborate salt ^[12]:

For other examples of its use: see organocatalytic transfer hydrogenation and asymmetric DA reactions.

Thiourea organocatalysis

In nature noncovalent interactions such as hydrogen bonding ("partial protonation") play a crucial role in enzyme catalysis that is characterized by selective substrate recognition (molecular recognition), substrate activation, and enormous acceleration of organic transformations. Based on the pioneering exmaninations by Kelly, Etter, Jorgensen, Hine, Curran, Göbel, and De Mendoza (see review articles cited below) on hydrogen bonding interactions of small, metal-free compounds with electron-rich binding sites Schreiner and co-workers performed series of theoretical and experimental systematic investigations towards the hydrogen-bonding ability of various thiourea derivatives ^{[13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]}. This purely organic compounds revealed effective acceleration of simple Diels-Alder reaction, act like weak Lewis acid catalyst, but act through explicit double hydrogen bonding instead of covalent binding known from traditional metal-ion mediated catalysis. Schreiner and co-workers identified and indroduced electron-poor thiourea derivatives as hydrogen-bonding organocatalysts. N,N'-bis[[3,5-bis(trifluormethyl)phenyl thiourea is to date the most effective achiral thiourea derivative and combines all typical structural features for double H-bonding mediated organocatalysis:

• electron-poor
• rigid structure
• non-coordinating, electron withdrawing substituents in 3,4, and/or 5 position of a phenyl ring
• the trifluoromethyl-group is the preferred substituent

Advantages of thiourea derivatives:

• no product inhibition due to weak enthalpic binding, but specific binding-“recognition“
• low catalyst-loading (down to 0.001 mol%)^{[citation needed]}
• high TOF values (up to 2,000 h^–1)^{[citation needed]}
• simple and inexpensive synthesis
• easily to modulate and to handle, no inert atmosphere necessary
• immobilization on solid phase (polymer-bound organocatalysts), catalyst recovery and reusability
• catalysis under almost neutral conditions (pk_a thiourea 21.0), acid-sensitive substrates are tolerated
• metal-free, not toxic (compare traditional metal-containing Lewis-acid catalysts
• water-tolerant, even catalytically effective in water or aqueous media
• environmentally benign ("Green Chemistry")

To date various organic transformations are organocatalyzed through hydrogen-bonding N,N'-bis[[3,5-bis(trifluormethyl)phenyl thiourea at low catalyst loadings and in good to excellent product yields. This electron-poor thiourea derivative has proven to be the benchmark for noncovalent organocatalysis utilizing explicit hydrogen-bonding as well as to be the basis for development of a wide range of catalytically active derivatives.

Since 2001 research groups world-wide (e.g., Berkessel, Connon, Jacobsen, Nagaswa, Takemoto) have realized the potential of thiourea derivatives and developed various achiral/chiral mono- and bifunctional derivatives incorporating the electron-poor 3,5-bis(trifluoromethyl)phenyl substrate-"anchor" functionality. Meanwhile a broad spectrum of organic transformations are performed through hydrogen-bonding organocatalysis and the research ist still in the focus of interest.

References