Content deleted Content added
No edit summary |
No edit summary |
||
Line 11:
Chromate esters have been implicated in most oxidations of alcohols by chromium(VI)-amines. After formation of the chromate ester, either deprotonation or hydride transfer leads to the product carbonyl compound. Kinetic isotope effect studies have shown that C-H bond cleavage is involved in the rate-determining step.<ref>Banerji, K. K. ''J. Org. Chem.'', '''1988''', ''53'', 2154.</ref>
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(2)'''''</span><center>[[File:ChroMech1.png]]</center>
Oxidative annulation of alkenols to form six-membered rings may be accomplished with PCC. This process is postulated to occur via initial oxidation of the alcohol, attack of the alkene on the new carbonyl, then re-oxidation to a ketone. Double-bond isomerization
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(3)'''''</span><center>[[File:ChromeScopeCyc.png]]</center>
An important process mediated by chromium(VI)-amines is the oxidative transposition of tertiary allylic alcohols to give enones.<ref>Luzzio, F. A.; Moore, W. J. ''J. Org. Chem.'', '''1993''', ''58'', 2966.</ref> The mechanism of this process likely depends on the acidity of the chromium reagent. Acidic reagents such as PCC may cause ionization and recombination of the chromate ester (path A), while the basic reagents (Collins) likely undergo direct allylic transposition via sigmatropic rearrangement (path B).
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(
Oxidative cyclizations of olefinic alcohols to cyclic ethers may occur via [3+2], [2+2],<ref>Piccialli, V. ''Synthesis'' '''2007''', 2585.</ref> or [[epoxidation]] mechanisms. The exact mechanism has been debated, although a recent structure-reactivity study provided evidence for direct epoxidation by the chromate ester.<ref>Beihoffer, L.A; Craven, R.A.; Knight, K.S; Cisson, C.R.; Waddell, T.G. ''Trans. Met. Chem.'' '''2005''', ''30'', 582. </ref> Subsequent epoxide opening and release of chromium leads to the observed products.
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(
==Scope and Limitations==
Buffering agents may be used to prevent acid-labile protecting groups from being removed during chromium(VI)-amine oxidations. However, buffers will also slow down oxidative cyclizations, leading to selective oxidation of alcohols over any other sort of oxidative transformation. Citronellol, for instance, which cyclizes to pugellols in the presence of PCC, does not undergo cyclization when buffers are used.<ref>Fieser, L. F.; Fieser, M. ''Reagents for Organic Synthesis''; Wiley-Interscience, New York, 1979, '''7''', 309.</ref><ref name=whatup>Babler, J. H.; Coghlan, M. J. ''Synth. Commun.'' '''1976''', ''6'', 469.</ref>
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(
Oxidative cyclization can be used to prepare substituted tetrahydrofurans. Cyclization of dienols leads to the formation of two tetrahydrofuran rings in a ''syn'' fashion.<ref>McDonald, F. E.; Towne, T. B. ''J. Am. Chem. Soc.'', '''1994''', ''116'', 7921.</ref>
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(
Enones can be synthesized from tertiary allylic alcohols through the action of a variety of chromium(VI)-amine reagents. The reaction is driven by the formation of a more substituted double bond. (''E'')-Enones form in greater amounts than (''Z'') isomers because of chromium-mediated geometric isomerization.<ref>Majetich, G.; Condon, S.; Hull, K.; Ahmad, S. ''Tetrahedron Lett.'', '''1989''', ''30'', 1033.</ref><ref name=whatup></ref>
<span style="float:right;padding-right:50px;padding-top:10px;">'''''(
Suitably substituted olefinic alcohols undergo oxidative cyclization to give tetrahydrofurans. Further oxidation of these compounds to give tetrahydropyranyl carbonyl compounds then occurs.<ref>Schlecht, M. F.; Kim, H.-J. ''Tetrahedron Lett.'', '''1986''', ''27'', 4889.</ref>
<span style="float:right;padding-right:50px;padding-top:30px;">'''''(
In addition to the limitations described above, chromium(VI) reagents are often unsuccessful in the oxidation of substrates containing heteroatoms (particularly nitrogen). Coordination of the heteroatoms to chromium (with displacements of the amine ligand originally attached to the metal) leads to deactivation and eventual decomposition of the oxidizing agent.
Line 41:
===Example Procedure<ref>Guziec, F. S., Jr.; Luzzio, F. A. ''J. Org. Chem.'', '''1982''', ''47'', 1787.</ref>===
<span style="float:right;padding-right:50px;padding-top:10px;">'''''(
To a solution of ''p''-(3-hydroxypropyl)benzyl alcohol (165 mg, 1 mmol) in N,N-dimethylformamide (3 mL) was added imidazolium dichromate (705 mg, 2 mmol) and the mixture was stirred at room temperature (4 hours). After completion of the reaction, water (30 mL) was added to the reaction mixture and the product was extracted three times with diethyl ether. The ether extracts were washed with water and aqueous NaHCO3, dried over anhydrous MgSO4, and evaporated to dryness. The crude product was subjected to silica gel column chromatography with dichloromethane-ethyl acetate (4:1) as eluent to yield ''p''-(3-hydroxypropyl)benzaldehyde (112 mg, 68%); <sup>1</sup>H NMR (CDCl<sub>3</sub>): δ1.8–2.3 (m, 2 H), 2.77 (s, 1 H), 2.93 (t, 2 H, J = 6 Hz), 3.80 (t, 2 H, J = 6 Hz), 7.4–7.98 (m, 4 H), 10.10 (s, 1 H). IR (film) 1700 cm–1. δ and 3-(4-Formylphenyl)propanal (4 mg, 3%): <sup>1</sup>H NMR (CDCl<sub>3</sub>) = 2.45–3.21 (m, 4 H), 7.31–7.85 (m, 4 H), 9.81 (s, 1 H), 9.95 (s, 1 H).
| |||