Tag Archives: Catalysis

Uncovering alkenes: complex products from all-carbon substrates

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Molecules containing simple alkene functionality are attractive substrates in synthesis. Carbon-carbon double bonds are relatively stable groups, lacking polarisation, they can be carried through synthetic processes involving acid, base, mildly oxidative and reductive conditions. But, the availability of the π-electrons for reaction embodies remarkable potential for constructing substituted contiguous stereocentres and the possibility of introducing complex functionality to an all-carbon system.

In recent years, functionalisation of unactivated alkenes has blossomed with developments in robust metal-catalysed processes and new oxidating agents. In the last few weeks several reports of new alkene functionalisations have been published that cover many aspects of this broad area, and show the diverse utility of alkenes in synthesis.

Hayashi and co-workers have published an elegant cyclising difunctionalisation of dienes.[1] The process is an iridium-catalysed C-H activation of cyclic aryl N-sulfonyl ketimines and gives spirocyclic aminoindane scaffolds with high diastereoselectivity.

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We’ve already discussed an asymmetric cyclising aminofluorination reaction carried out by a chiral hypervalent iodine fluoride oxidant.[2] The reaction is endo-selective and prepares fluoropiperidine-based systems. Extension of the reaction to a racemic intermolecular process with styrene substrates was also demonstrated.

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In a similar example of intermolecular alkene difunctionalisation, Zhang reports aminocyanation and diamination of (predominantly) styrene substrates.[3] The principle reagent for functionalisation is NFSI (N-fluorobenzenesulfonimide), which aminates the terminal end of the double bond. Subsequent reaction with TMSCN gives the aminonitrile product. Alternatively, an alkylnitrile may be used to give the Ritter product of the second addition.

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The authors suggest a copper(I)-catalysed radical mechanism, generating a carbon-centered radical intermediate after amination by •N(SO2Ph)2. This reaction gives some idea of the kind of complex functionality that can be introduced to all-carbon alkene substrates by oxidative processes.

Hydrofunctionalisation of alkenes doesn’t provide the same high degree of complexity as oxidative difunctionalisation processes, but reactions in this category do offer a great deal of diversity in synthetic options while still providing regio- and stereocontrol of sp3 carbon centres. A report from Qing and co-workers describes a general and high yielding hydrotrifluoromethylation of unactivated alkenes.[4] Trifluoromethyl groups are useful moieties in pharmaceutical development of lead compounds. The group apply the method to a wide scope of unactivated alkene substrates showing tolerance to many other functionalities.

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Hartwig’s group in Berkeley have developed an asymmetric hydroheteroarylation of bicycloalkenes.[5] The reaction is catalysed by an iridium DTBM-Segphos complex and tolerates various heteroaromatic coupling partners.

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Alkenes offer chemists widely available, stable and highly tolerant substrates for synthesis. Careful application of modern reactions allows the uncovering of diverse and complex functionality from a carbon-carbon double bond, and new developments provide ever more effective methods for the preparation of desirable adducts.

References:
1. DOI: 10.1021/ja311968d
2. DOI: 10.1002/anie.201208471
3. DOI: 10.1002/anie.201209142
4. DOI: 10.1002/anie.201208971
5. DOI: 10.1021/ja312360c

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Catalytic Asymmetric C–N Bond Formation: Phosphine-Catalyzed Intra- and Intermolecular γ-Addition of Nitrogen Nucleophiles to Allenoates and Alkynoates

Rylan J. Lundgren, Ashraf Wilsily, Nicolas Marion, Cong Ma, Ying Kit Chung, and Gregory C. Fu
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201208957

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A carbonyl group provides a pivot point for the functionalisation of its proximal positions – α and β. This can be demonstrated by the reactivity of classical enolates as α-functionalisation nucleophiles and Michael acceptors as β-functionalisation electrophiles, as well as their corresponding reactivities in enamine and imminium organocatalysis.

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In a recent report Lundgren et al. have successfully demonstrated that γ-functionalisation of carbonyl compounds is also achievable, both intra- and inter- molecularly. Using a spirophospine ligand to induce stereoselectivity, the group has demonstrated the feasibility of asymmetric C-N bond formation with nitrogen nucleophiles and alkynoates or allenoates as electrophiles.

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Investigating the scope of the reaction revealed that a good range of functional groups can be accommodated in either intramolecular reactions with alkynoates or intermolecular reactions with allenoates.

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The process provides access to novel reactivity and it may even prove complementary to the classical α- and β- asymmetric functionalisation of carbonyl compounds. It would be interesting to see if a γ-, β-, α-cascade addition would be possible.

Catalytic transformation of alcohols to carboxylic acid salts and hydrogen using water as the oxygen atom source

Ekambaram Balaraman, Eugene Khaskin, Gregory Leitus and David Milstein
Nature Chem.
DOI: 10.1038/nchem.1536

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We’ve discussed ruthenium-catalysed oxidative cross-coupling before, and the area appears to developing rapidly. Milstein and co-workers have reported dimerisation of alcohols (to esters)1 and coupling of alcohols with amines (to amides)2 previously, and now report a remarkable oxidation of alcohols to acids using water as the terminal oxidant.

The reported reaction is operationally straightforward, requiring only the alcohol substrate, catalyst, a slight excess of hydroxide and water at reflux. Alcohols are oxidised with high tolerance and high yield, though any present alkenes are subsequently hydrogenated.

The ruthenium catalyst operates with a bipyridyl ligand that can oxidise the metal centre through dearomatising loss of a proton. The proposed mechanism of the reaction cycles this dearomatising/aromatising process with a bound alochol and water to give the acid and two equivalents of hydrogen.

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The authors comment on the important role hydroxide plays in the mechanism. Without it, only trace product is observed. They conclude that hydroxide is necessary to scavenge the acid (as the salt) in order to regenerate the active catalyst.

Direct oxidations from alcohols to acids are uncommon, yet, obviously, very useful. Not only does this report address a general synthetic problem, it does so with an incredibly mild and accessible method.

1. J. Am. Chem. Soc. 2005, 127, 10840
2. Science 2007, 317, 790

Highly Diastereoselective Multicomponent Cascade Reactions: Efficient Synthesis of Functionalized 1-Indanols

Jun Jiang, Xiaoyu Guan, Shunying Liu, Baiyan Ren, Xiaochu Ma, Xin Guo, Fengping Lv, Xiang Wu, and Wenhao Hu
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201208391

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Wenhao Hu has been reporting remarkable reactions of transition metal-generated oxonium ylides for some time now. Oxonium ylides formed from alcohol insertion into carbenoids tend to undergo rapid 1,2-proton shift to give ethers (path A). However, Hu and coworkers report reactions where this process is delayed long enough for the ylide to react as a carbon nucleophile with electrophilic coupling partners (path B).

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Recently, these oxonium ylides have been shown to undergo a cascade reaction with a dielectrophilic compound containing an unsaturated ester and an aldehyde. The ylide adds selectively to the Michael acceptor, which is followed by intramolecular aldol-type addition to the aldehyde to generate a complex indanyl adduct. The product is formed as a single diastereoisomer.

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The group also report an asymmetric variant of the reaction using a chrial auxiliary on the diazo substrate.

Subsequent studies into the reaction attempted an intermolecular multicomponent cascade. Unfortunately, the product of 1,2-proton shift was preferred and no 4-component products were observed. More interestingly, the product of addition of the ylide into the unsaturated ester was only observed in 13% yield. This suggests that trapping of the oxonium ylide by the unsaturated ester in the dielectrophilic compound above is synergistically promoted by the presence of the aldehyde and subsequent aldol cyclisation.

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Palladium-Catalyzed Aerobic Dehydrogenative Aromatization of Cyclohexanone Imines to Arylamines

Alakananda Hajra, Ye Wei, and Naohiko Yoshikai
Org. Lett.
DOI:10.1021/ol302568b

Substituted anilines are common core structures for a wide range of molecules. They are usually synthesised by SNAr chemistry, transition metal-catalysed C-N bond forming couplings or the reduction of nitroarene precursors. The common feature of these methodologies is that the aryl ring is already present in the starting material.

A recent report outlines a more indirect way of preparing such molecules employing a Pd-catalysed dehydrogenative aromatisation of cycloheximines. Rather than sourcing the aryl ring from the starting material, this methodology forms the substituted arene by a dual dehydrogenative catalytic cycle that leads to aromatisation of the cycloheximine substrate.

The use of expensive or wasteful stoichiometric by-products is avoided by using an oxygen atmosphere to achieve successive oxidations. The authors demonstrate the applicability of their method by preparing a variety of substituted anilines.

This method provides an expedient preparation of anilines that complements traditional methods by bypassing possible problems embodied in aromatic substitutions.

Catalytic Functionalisation of Unactivated sp3 C−H Bonds via exo-Directing Groups: Synthesis of Chemically Differentiated 1,2-Diols

Zhi Ren, Fanyang Mo, and Guangbin Dong
J. Am. Chem. Soc.
DOI: 10.1021/ja3082186

Selective C-H oxidation of unactivated alkyl groups is a remarkably powerful way of installing new functionality in simple substrates. We’ve featured some developments in this area previously.

C-H oxidation at the β-carbon of an unactivated alcohol has, so far, been inaccessible due to the deactivating inductive effect of the proximal oxygen. Dong and co-workers have overcome this problem in a method that allows the selective preparation of orthogonally protected 1,2-diols from simple alcohol precursors.

Their method employs a neat oxime derivative of the alcohol to direct palladium C-H insertion β to oxygen. Oximes are known to direct palladation of C-H bonds on the ‘other side’ of the oxime moiety; by removing appropriate C-H bonds from the oxime directing group, palladation in this case occurs on the desired oxygen ‘side’.

The authors note that reaction concentration is critical to the success of the oxidation process. The optimised process uses a concentration of 0.2 M. At lower concentration (0.1 M) the reaction does not go to completion; at higher concentration (0.4 M) decomposition products are observed. Whether or not this narrow range of optimal conditions is variable by substrate is not discussed, but the scope of the reaction is exemplified in a number of substrates with consistently good conversion.

Selective preparation of 1,2 diol derivatives is critical to any application of this methodology in synthesis. The oxime directing group can be removed as a protecting group in the presence of the adjacent acetate. Similarly, the oxime is stable under conditions for acetate cleavage, allowing selective differentiation between the two masked hydroxyl groups.

Interestingly, the reaction gives better yields on larger scale. A test example showed an improvement from 61% yield on 0.1 mmol scale to 80% yield from a 5 mmol reaction.

In one example, a substrate derived from menthol was subjected to the reaction conditions. Instead of observing the expected 1,2 oxidation product, a postulated intermediate undergoes an interesting skeletal rearrangement to give an unusual substituted cyclopentane scaffold.

“Black Swan Events” in Organic Synthesis

William Nugent
Angew. Chem. Int. Ed.
DOI: 10.1002/anie.201202348

The moral of this article can be summarised in three words of folk wisdom: never say never. This article provides a refreshing insight into the misplaced preconceptions of chemists working in previous decades, and how these preconceptions were overturned.

The author lays out a series of dogmatic statements which over the past two decades have been proven incorrect. These statements are classed into two categories depending whether they were brought down by a single publication – “Change through revolution”, or a series of reports – “Change through evolution”.

A selected few are:

“Gold compounds are simply to unreactive to be useful as homogeneous catalysts”

A report by Teles et al. in 1998 demonstrated that a cationic Au(I) complex catalyses the addition of oxygen nucleophiles to acetylenes:

“Olefin metathesis is an ill-defined reaction of olefinic hydrocarbons and unlikely to find any use in organic synthesis”

For students studying organic synthesis today, it’s hard to imagine a time before ubiquitous olefin metathesis, more so that it was considered so far out of reach. R.H. Grubbs ultimately shared the Nobel Prize in Chemistry in 2005 for his research on ruthenium catalysed olefin metathesis.

“Efficient enantioselective catalysis requires the use of a metal complex”

And then came one of the most elegant and sophisticated aspects of catalysis in organic synthesis: organocatalysis, with the work of MacMillan, List, Barbas and others.

As the author points out, a common characteristic of the above overruled statements is that often, the scientific literature contained hints that the “common belief” was incorrect. However, these hints were not picked up on until a report was published proving that the original discounted phenomenon was in fact achievable.

The discovery of something completely new and revolutionary is often preceded by flashes of insights or moments of wisdom that will most probably be ignored until three or four decades later when someone has the curiosity to move it forwards.

So what assumptions about the behavior of molecules do we make today that will populate a surrogate of this article in 50 years?