Download Molybdenum based catalysts as Oxidants and Reductants

Molybdenum based catalysts as Oxidants and Reductants
There are four major objectives which are to be covered under this project:1.
Synthesis of Dihydrobis(substituted cyclohexanone hydrazone)borato potassium,
KHzR
All preparations and manipulation will be carried out using standard Schlenk techniques
in nitrogen atmosphere. All the solvents and chemicals used will be dried prior to use using
standard techniques. The potassium salt of the dihydrobis(substituted cyclohexanone
hydrazone)borate anion, [HzR]- may be conveniently obtained in high yields by refluxing a
reaction mixture of potassium borohydride and substituted cyclohexanone hydrazone in 1:2
molar ratio in THF for about 18 hours as depicted in scheme 1.
The completion of the reaction may be ascertained by measuring the requisite amount of
hydrogen gas released. Generally these compounds exist in solution phase and can be recovered
in solid form by the partial evaporation of THF and addition of pentane. The purity of the
compounds may be checked by thin layer chromatography (TLC) using silica gel as the
stationary phase.
2.
Synthesis of [MoO2[dihydrobis(substituted cyclohexanone hydrazone) borate]],
MoO2(HzR)2
The compound MoO2Cl2 may be prepared in accord with the reported procedure [26]. A
solution of MoO2Cl2(THF)2 in dry THF may be treated with Dihydrobis(substituted
cyclohexanone hydrazone)borato potassium, KHzR in the same solvent with continuous mixing
to obtain the desired compound as shown in scheme 2.
Refluxing and stirring will be done according to the need and the reaction will be
monitored using TLC.
3.
Characterization
of
Dihydrobis(substituted
cyclohexanone
hydrazone)borato
potassium, and [MoO2[dihydrobis(substituted cyclohexanone hydrazone)borato
potassium]]
The synthesized potassium ligands, obtained by varying R and its dioxomolybdenum(VI) complexes may be synthesized by using elemental analysis, FT-IR, NMR (1H,
13
C and
31
P), Electron Spin Ionization-Mass Spectrometry, Gas Chromatography, Electron
Paramagnetic Resonance, Ultraviolet Spectroscopy and thermogravimetric analysis. Moreover, if
good quality crystals were obtained then they will be characterized by X-ray crystallography.
4.
Catalysis
Oxidation of triphenyl phosphine
Dioxomolybdenum(VI) complexes are extensively used as excellent catalysts for oxo-
transfer reactions. Generally the method developed by Holm and Berg [27] is used to evaluate
the oxo-tranfer ability of dioxomolybdenum complexes. This method is based on the oxidation of
triphenylphosphine. The reaction of the complexes, MoO2(HzR)2 with Ph3P may result in the
removal of one oxo-ligand leading to a two-electron reduction of the molybdenum centre from
+6 to +4 while the triphenylphosphine is oxidized to Ph3PO [28].
MoO2(HzR)2
+
PPh3
MoO(HzR)2
+
PPh3O
The reactions may be monitored with 31P NMR spectroscopy.
Oxidation of olefins
The synthesized complexes, MoO2(HzR)2 may be tested as catalysts for olefin epoxidation using
cyclooctene as a model substrate and tertiary butylhydrperoxide (t-BuOOH in decane) as oxygen
donor at 55 °C. Control experiments may be carried out in absence of catalyst or with the ligand,
Dihydrobis(cyclohexanone hydrazone) boratopotassium. The major role of the Mo(VI) centre is to
withdraw electrons from the peroxidic oxygen making it more susceptible to be attacked by
nucleophiles such as olefins [29] and we hope and propose a similar same reaction in steroidal
olefins. Moreover change of solvent, substituents of Mo-complexes, reaction condition and time of
reaction leads vicinal diols/ α-hydroxy ketones in steroidal system and in some cases it shows
allylic oxidation in different alkenes. These Mo-complexes may enhance the selectivity and rate of
reaction of different steroidal and nonsteroidal alkenes. The MoO2(HzR)2 will be employed as
selective catalysts for epoxidation. The major advantage of using transition Mo-complexes as
catalyst is due to the utilization of a variety of oxygen sources for the epoxidation reaction
(Scheme 3).
Moreover, the MoO2(HzR)2 can be used as a catalyst in the Baeyer-Villiger oxidation of
cyclic and acyclic ketones with hydrogen peroxide playing peroxy intermediates with metals
(scheme 4).
Reduction
Reductions of functional groups of organic compounds
Organic molecules having a number of unsaturated groups such alkenes, alkynes,
ketones, aldehydes, nitro, sulfoxides and aromatic ring etc can be reduced catalytically under
suitable conditions, although these groups are not all reduced with equal ease. Sometimes, one
group is protected, another reduced, however reduction process moved on handily under wide
range of condition, when selective reduction is needed, conditions become critical. The selection
of catalyst for reduction is ruled by the activity and selectivity. Usually, the more active the
metal complex the less acute it is in its action, but for greater selectivity the behavior of catalyst
is the lesser active under possible conditions. The reduction properties of catalyst can
considerably be modified in the presence of other metals. It is believed that the presence of
MoO2(HzR)2 will modify the selectivity and activity of other catalysts due to variable oxidation
state of Molybdenum.
The course of the reaction will be monitored using a gas chromatograph equipped with a capillary
column and a flame ionisation detector.
Biological Screening
After successful synthesis of the Dihydrobis(substituted cyclohexanone hydrazone)borato
potassium, and [MoO2[dihydrobis(substituted cyclohexanone hydrazone)borato] and their
subsequent characterization, these will be subjected to biological screening particularly for their
possible antibacterial and antifungal activities. Most notably against B. subtilis, S. pyogeres, S.
aureus, P. aeruginosa, S. typhimurium and E. coli.