Alternate Name: clayfen.
Solubility: clay support: insol water, organic solvents. Fe(NO3)3: sol water, acetone, ethanol.
Form Supplied in: rusty yellow, free-flowing floury powder.
Preparative Method: controlled solvent elimination from K10 clay (30 g) suspended in a solution of Fe(NO3)3.9H2O (22.5 g) in acetone (375 mL).1
Handling, Storage, and Precautions: most applications require freshly prepared (1-2 h) reagent. Clayfen is unstable, prone to vigorous or slow exothermic decomposition, together with evolution of NO2 gas. Careful observance of the preparation procedure1 is critical (in particular, T <50 °C and a crust on the glassware at the flaking-off stage). Never store in gas-tight containers, nor in contact with strongly reducing material such as aluminum. Nitrates are dangerous, and are to be used only with the appropriate precautions and protections. In particular, scaling-up should be attempted only after appropriate safety tests. This reagent should be handled in a fume hood.
Clayfen is an efficient and selective oxidation reagent4,5 for the synthesis of ketones from secondary alcohols, aromatic aldehydes from benzylic alcohols (eq 1), and benzils from benzoins (eq 2). The reaction with primary aliphatic alcohols leads to complex product mixtures.
Nitrites formed with retention of configuration2 are intermediates, indicating that in these reactions clayfen acts as an NO+ source.
As a source of nitrosonium ions, clayfen couples thiols to disulfides6 through transient thionitrites. In a manner similar to claycop (Copper(II) Nitrate-K10 Bentonite Clay), its reaction with Thiophenol (eq 3) is a good reactivity test.1
Clayfen regenerates carbonyl groups from a variety of masking functionalities. For such applications, advantages of this reagent include its low cost, its efficiency under mild conditions, and the easy workup. The following are examples of this use of clayfen.
Treatment of various hydrazines with clayfen gives the corresponding azides in fair yield, by the nitrosation route (eq 7).13 This provides a convenient access to intermediates essential for the preparation of iminophosphoranes.14
1,4-Dihydropyridinedicarboxylates aromatize smoothly when treated by clayfen at rt (eq 8).15 Use of claycop rather than clayfen leads to slightly higher yields and is devoid of the risks accruing from the instability of the latter.
In this transformation promoted by NO+ ions, clayfen gives excellent results (77-100%) with aromatic substrates (eq 9)16 and offers a cheap alternative to Nitrosonium Tetrafluoroborate.17 1,2-Dioxa-4,5-dithiane six-membered rings are transient intermediates in these transformations.18
Clayfen nitrates phenols under extremely mild reaction conditions to produce exclusively ortho or para mononitration products (eq 10).3,19
The reaction is performed at rt in ether, in THF, or in toluene. The solvent is chosen so that it dissolves the nitro derivative formed. Aldehyde and nitrile groups are unaffected under these mild conditions. With meta-substituted phenols the reaction is nicely regioselective at the ortho position. For instance, the sole nitration product of estrone is 2-nitroestrone (eq 11).19
André Cornélis & Pierre Laszlo
Université de Liège, Belgium
Mark W. Zettler
Dow Chemical Company, Midland, MI, USA