Azide-alkyne Huisgen cycloaddition
Azide-alkyne Huisgen cycloaddition | |
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Named after | Rolf Huisgen |
Reaction type | Ring forming reaction |
Identifiers | |
Organic Chemistry Portal | huisgen-1,3-dipolar-cycloaddition |
RSC ontology ID | RXNO:0000269 |
The azide-alkyne Huisgen cycloaddition is a
In the reaction above[4] azide 2 reacts neatly with alkyne 1 to afford the product triazole as a mixture of 1,4-adduct (3a) and 1,5-adduct (3b) at 98 °C in 18 hours.
The standard 1,3-cycloaddition between an azide 1,3-dipole and an alkene as dipolarophile has largely been ignored due to lack of reactivity as a result of electron-poor olefins and elimination side reactions. Some success has been found with non-metal-catalyzed cycloadditions, such as the reactions using dipolarophiles that are electron-poor olefins[5] or alkynes.
Although azides are not the most reactive 1,3-dipole available for reaction, they are preferred for their relative lack of side reactions and stability in typical synthetic conditions.
Copper catalysis
A notable variant of the Huisgen 1,3-dipolar cycloaddition is the copper(I) catalyzed variant, no longer a true concerted cycloaddition, in which organic azides and terminal alkynes are united to afford 1,4-regioisomers of 1,2,3-triazoles as sole products (substitution at positions 1' and 4' as shown above). The copper(I)-catalyzed variant was first reported in 2002 in independent publications by
While the reaction can be performed using commercial sources of copper(I) such as cuprous bromide or iodide, the reaction works much better using a mixture of copper(II) (e.g. copper(II) sulfate) and a reducing agent (e.g. sodium ascorbate) to produce Cu(I) in situ. As Cu(I) is unstable in aqueous solvents, stabilizing ligands are effective for improving the reaction outcome, especially if tris(benzyltriazolylmethyl)amine (TBTA) is used. The reaction can be run in a variety of solvents, and mixtures of water and a variety of (partially) miscible organic solvents including alcohols, DMSO, DMF, tBuOH and acetone. Owing to the powerful coordinating ability of nitriles towards Cu(I), it is best to avoid acetonitrile as the solvent. The starting reagents need not be completely soluble for the reaction to be successful. In many cases, the product can simply be filtered from the solution as the only purification step required.
NH-1,2,3-triazoles are also prepared from alkynes in a sequence called the Banert cascade.
The utility of the Cu(I)-catalyzed click reaction has also been demonstrated in the
The copper-mediated azide-alkyne cycloaddition is receiving widespread use in material and surface sciences.
The CuAAC click reaction also effectively couples
Copper catalysts
The use of a Cu catalyst in water was an improvement over the same reaction first popularized by Rolf Huisgen in the 1970s, which he ran at elevated temperatures.[11] The traditional reaction is slow and thus requires high temperatures. However, the azides and alkynes are both kinetically stable.
As mentioned above, copper-catalysed click reactions work essentially on terminal alkynes. The Cu species undergo metal insertion reaction into the terminal alkynes. The Cu(I) species may either be introduced as preformed complexes, or are otherwise generated in the reaction pot itself by one of the following ways:
- A Cu2+ compound is added to the reaction in presence of a reducing agent (e.g. sodium ascorbate) which reduces the Cu from the (+2) to the (+1) oxidation state. The advantage of generating the Cu(I) species in this manner is it eliminates the need of a base in the reaction. Also the presence of reducing agent makes up for any oxygen which may have gotten into the system. Oxygen oxidises the Cu(I) to Cu(II) which impedes the reaction and results in low yields. One of the more commonly used Cu compounds is CuSO4.
- Oxidation of Cu(0) metal
- Halides of copper may be used where solubility is an issue. However, the iodide and bromide Cu salts require either the presence of amines or higher temperatures.
Commonly used solvents are polar aprotic solvents such as THF, DMSO, acetonitrile, DMF as well as in non-polar aprotic solvents such as toluene. Neat solvents or a mixture of solvents may be used.
Mechanism
A mechanism for the reaction has been suggested based on
The reaction is assisted by the copper, which, when coordinated with the acetylide lowers the pKa of the alkyne C-H by up to 9.8 units. Thus under certain conditions, the reaction may be carried out even in the absence of a base.
In the uncatalysed reaction the alkyne remains a poor electrophile. Thus high energy barriers lead to slow reaction rates.[20]
Ligand assistance
The ligands employed are usually labile i.e. they can be displaced easily. Though the ligand plays no direct role in the reaction the presence of a ligand has its advantages. The ligand protects the Cu ion from interactions leading to degradation and formation of side products and also prevents the oxidation of the Cu(I) species to the Cu(II). Furthermore, the ligand functions as a proton acceptor thus eliminating the need of a base.[21]
Ruthenium catalysis
The ruthenium-catalysed 1,3-dipolar azide-alkyne cycloaddition (RuAAC) gives the 1,5-triazole. Unlike CuAAC in which only terminal alkynes reacted, in RuAAC both terminal and internal alkynes can participate in the reaction. This suggests that ruthenium acetylides are not involved in the catalytic cycle.
The proposed mechanism suggests that in the first step, the
Cp*RuCl(PPh3)2, Cp*Ru(COD) and Cp*[RuCl4] are commonly used ruthenium catalysts. Catalysts containing cyclopentadienyl (Cp) group are also used. However, better results are observed with the pentamethylcyclopentadienyl(Cp*) version. This may be due to the sterically demanding Cp* group which facilitates the displacement of the spectator ligands.[22][23]
Silver catalysis
Recently, the discovery of a general Ag(I)-catalyzed azide–alkyne cycloaddition reaction (Ag-AAC) leading to 1,4-triazoles is reported. Mechanistic features are similar to the generally accepted mechanism of the copper(I)-catalyzed process. Silver(I)-salts alone are not sufficient to promote the cycloaddition. However the ligated Ag(I) source has proven to be exceptional for AgAAC reaction.[24][25] Curiously, pre-formed silver acetylides do not react with azides; however, silver acetylides do react with azides under catalysis with copper(I).[26]
References
- .
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- ^ Development and Applications of Click Chemistry Gregory C. Patton November 8, 2004 http://www.scs.uiuc.edu Online[permanent dead link]
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- ^
Vsevolod V. Rostovtsev; Luke G. Green; Valery V. Fokin; K. Barry Sharpless (2002). "A Stepwise Huisgen Cycloaddition Process: Copper(I)-Catalyzed Regioselective Ligation of Azides and Terminal Alkynes". PMID 12203546.
- PMID 16113739.
- doi:10.1071/CH06457.
- PMID 16100593.
- ^ 1,3-Dipolar Cycloaddition Chemistry, published by Wiley and updated in 2002
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