The Henry reaction was first reported by Belgian chemist Louis Henry in 1895. The Henry reaction involves the addition of a nitronate salt to a carbonyl compound, usually an aldehyde. The reaction is also referred to as a Nitroaldol reaction. It is C-C bond-forming reaction used in organic synthesis.
Table of Page Contents
The Henry Reaction Mechanism can present in two steps
1) Nitronate preparation
Nitronate is usually prepared in situ by treating the nitroalkane with a strong base, like hydroxide, alkoxide, or even an amine base.
2) Addition of Nitronate to aldehyde
Next, an aldol reaction (C-alkylation of the nitroalkane) takes place with the carbonyl compound to form diastereomeric β-nitro alkoxides. Finally the β-nitro alkoxides are protonated to give the expected β-nitro alcohols.
Double Henry Reaction:
In rare cases, the Henry reaction is even too favorable and can occur twice because the product is nitroalkane. This overreaction is encountered almost exclusively with formaldehyde; in these cases, a 2-nitro-1,3-diol is formed, which can sometimes have synthetic value.
Henry Reaction Examples, Application
- The nitro group is quickly reduced to the amine; therefore, this is a route to 1,2-amino alcohols.
- The Henry product can also be easily dehydrated to a nitro-olefin, which is a Michael acceptor and can be reached at this position with several nucleophiles.
- Especially synthesis of alpha-nitro ketone, is a very versatile tool and is used extensively by organic chemists.
- 1,2-Nitroalcohols are essential intermediates in organic synthesis.
- An example of a tandem Henry plus Nef-reaction is the one-carbon homologation of pentoses to hexoses, meaning that carbon is added from a 5-carbon sugar to a 6-carbon sugar.
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Stereoselective Henry Reaction :
Henry’s reaction has been a prime target for diastereoselective and enantioselective synthesis. BINOL-metal catalysts, best if derivatives of lanthanides, create a chelated, rigid transition state for the reaction and therefore allow for excellent stereocontrol.
- Henry’s reactions with ketones can be unsatisfactory or even impossible because the equilibrium lies in favor of the nitronate and the ketone.
- Using aldehydes as reactants also brings the danger of base-promoted reactions like aldol condensations and Cannizzaro reactions of the aldehyde. These are the limitation of Henry’s reactions.
Related Reactions :
- Aza-Henry Reaction
- Nef Reaction
- Henry, Louis (1895). Synthetic formation of nitrated alcohols
- ScienceDirect Henry-reaction
- Professor Dave Explains _ Henry reactions
- Henry Reaction review
My name is Pradip Sanjay W. I’m an organic chemist originally from Maharashtra, India. I have qualified UGC NET-JRF, GATE in chemical sciences and MH-SET exam for assistant professor. I’m currently pursuing my Ph.D. in organic chemistry at the Indian Institute of Technology Hyderabad, India.