N-Methylmorpholine-N-Oxide (NMO)
N-Methylmorpholine-N-Oxide (NMO) is available commercially as the monohydrate; it has a melting point of 70 °C, and is stable under normal conditions. This reagent is soluble in polar solvents, especially water.
NMO displays characteristic reactivity with various transition metals, which undergo oxidation with this reagent. Because of this, NMO is a preferred stoichiometric oxidant for transition metal-catalyzed oxidations.
The well-known combination of NMO and TPAP (tetrapropylammonium perruthenate) can, for example, be used in the preparation of aldehydes from primary alcohols, wherein the water produced must be taken up by molecular sieves. The presence of water fosters an equilibrium concentration of the aldehyde hydrate, which can undergo further oxidation to the carboxylic acid. (Review: S. Ley, J. Norman, W. P. Griffith, S. P. Marsden, Synthesis, 1994, 639. Abstract)
Name Reactions
Recent Literature
Tetrapropylammonium perruthenate enables oxidations of a wide range of molecules
including examples of both double oxidations and selective oxidations.
Mechanistic studies and general experimental procedures are reported. In
addition several interesting developments in the chemistry of this reagent are
outlined: heteroatom oxidation, cleavage reactions and use in sequential
reaction processes.
S. B. Ley, J. Norman, W. P. Griffith, S. P. Marsden, Synthesis, 1994,
639-666.
he mild instability of the Ley-Griffith catalyst (TPAP) creates preparation,
storage, and reaction reproducibility issues, due to unpreventable slow
decomposition. A set of readily synthesized, bench stable, phosphonium
perruthenates (ATP3 and MTP3) mirror the reactivity of TPAP, but avoid storage
decomposition issues.
P. W. Moore, C. D. G. Read, P. V. Bernhardt, C. M. Williams, Chem. Eur. J., 2018, 24, 4556-4561.
An efficient method for the oxidation of an olefin to the less
substituted carbonyl compound is described. This one pot conversion
includes hydroboration with borane dimethyl sulfide (BDMS), followed by
oxidation of the resulting alkylboranes with tetrapropylammonium
perruthenate (TPAP) and NMO.
M. H. Yates, Tetrahedron Lett., 1997, 38, 2813-2816.
The use of PhI(OAc)2 in dichloromethane enables a clean oxidative
cleavage of 1,2-diols to aldehydes. In the presence of OsO4 as
catalyst, NMO and 2,6-lutidine, olefinic bonds can be cleaved in acetone/water
to yield the corresponding carbonyl compounds.
K. C. Nicolaou, V. A. Adsool, C. R. H. Hale, Org. Lett., 2010,
12, 1552-1555.
In a simple, mild, and highly effective method for the direct conversion of
primary alcohols to carboxylic acids, TPAP serves as the catalyst, and NMO • H2O
plays a dual role, acting as the co-oxidant and as a reagent for aldehyde
hydrate stabilization. This previously unknown stabilizing effect of geminal
diols by N-oxides is the key for the efficiency of the overall
transformation.
A.-K. C. Schmidt, C. B. W. Stark, Org. Lett., 2011,
13, 4164-4167.
The ionic liquids [C4mim][PF6]
and [C8mim][PF6] as cosolvents in asymmetric
dihydroxylation give yields and enantioselectivity
comparable or higher than those of the conventional H2O/tert-BuOH
solvent system. After extraction of the reaction mixture with diethyl ether, the
contamination of the product by osmium was
remarkably low. The reuse of ionic liquid and catalyst is
possible.
L. C. Branco, C. A. M. Afonso, J. Org. Chem., 2004, 69, 4381-4389.
Resin-OsO4 are very efficient catalysts for the dihydroxylation
of various olefins to afford vicinal diols with high yields irrespective of
the cooxidant used. Resin-OsO4 is recovered quantitatively by a
simple filtration and reused for a number of cycles with consistent
activity. The high binding ability of the heterogeneous osmium catalysts
enables the use of an equimolar ratio of a chiral ligand such as (DHQD)2PHAL
to osmium to give excellent enantioselectives in the asymmetric
dihydroxylation.
B. M. Choudary, N. S. Chodari, K. Jyothi, M. L. Kantam, J. Am. Chem. Soc.,
2002, 124, 5341-5349.
Osmium tetroxide has been microencapsulated in a polyurea matrix. These
microcapsules have been effectively used as recyclable catalysts in the
dihydroxylation and the oxidative cleavage of olefins.
S. V. Ley, C. Ramarao, A.-L. Lee, N. Ostergaard, S. C. Smith, I. M.
Shirley, Org. Lett., 2003, 5, 185-187.
cis-Dihydroxylation of olefins with a catalytic amount of osmium
tetroxide and N-methylmorpholine N-oxide proceeds smoothly in an
organic solvent in the presence of dihydroxyphenylborane to give the
corresponding phenylboronic esters in good yields. The use of a diol captor
prevents further oxidation of the products and makes the isolation procedure
easier.
I. Nobuharu, K. Takashi, N. Koichi, Chem. Lett., 1988,
1721-1724.
The use of N-oxides in butanol as solvent enables a site-selective
oxidation of vicinal bis(boronates) with good efficiency and selectivity across
a range of substrates to provide 2-hydro-1-boronic esters, which are shown to be
versatile intermediates in the synthesis of chiral building blocks.
L. Yan, J. P. Morken,
Org. Lett., 2019, 21, 3760-3763.
The cis-dihydroxylation of olefin-containing potassium alkyl- and
aryltrifluoroborates proceeds readily in moderate to excellent yields. The
resulting diols are efficient coupling partners in Suzuki-Miyaura-type
reactions with both alkenyl and aryl bromides.
G. A. Molander, R. Figueroa, Org. Lett., 2006, 8, 75-78.
A new one-pot method is described for the removal of O- and N-allyl
protecting groups under oxidative conditions at near neutral pH. The
allyl group undergoes hydroxylation and subsequent periodate scission of
the vicinal diol. Repetition of this reaction sequence on the enole
tautomer of the aldehyde intermediate releases the deprotected
functional group.
P. I. Kitov, D. R. Bundle, Org. Lett., 2004, 3, 2835-2838.
