Being a part of the environment, we do not have right to harm it if we cannot save. Due to our deeds in the past we deserve humiliation and were humiliated. Having been rightly instigated by the hostility of chemical wastes and reckless disposal of unsafe chemical agents causing the environment to be unsafe for the human habitat and ecology, it became incumbent upon experimental chemists to address the need of Clean Chemistry and its application. This area of chemistry is attracting increasing attention involving quite a challenging problem that chemists and some chemical industries are now encountered with. Philosophically, the implied issue falls in the regime of triple bottom line concept. Recently, the clean chemistry’s bandwagon started and chemists started looking into the possibility where they can contribute. Catalysis is not only one of the important tenets but also an integral part of it. The invention of a clean and appropriate catalyst or catalytic system for a chosen transformation is extremely important which have already been highlighted.
Chapter 1
16
It is known that Michael reactions serve as synthetic routes to essential intermediates of γ-amino alcohols, diamines, β-amino acid derivatives, β-lactam antibiotics, β-acylvinyl cation, homoenolate anion equivalents, β-calcium antagonist diltiazem and natural products.
Consequently, the development of novel protocols for the Michael reactions has attracted a great deal of attention in synthetic organic chemistry. Notably the success of conjugate addition reactions lie in the use of either acidic or basic conditions which, if not selected judiciously, can be detrimental to the desired synthesis allowing unwanted side reactions to contaminate the product.
Moreover, the possibility of poisoning of metal-based catalyst by thiols, alkyl or aryl amines cannot be completely ruled out. In order to alleviate some of these problems the Michael reaction has undergone metamorphosis over the years involving a number of reagents and catalysts and alternative procedures. One of the rational alternatives would be to use an appropriate catalyst soluble in either ionic liquid or water as the solvent of choice. Importantly, both the solvents provide pseudo homogeneous medium for the reaction and provide easy recycling of the catalysts.
In the realm of oxidation chemistry, the partial oxidation of organic molecules (typically sulfides and alcohols) is a diverse and widely used area of chemistry with applications in almost all of the fine and speciality chemicals industries including those manufacturing pharmaceuticals, agrochemicals and monomers, for example (Scheme 1.10):
H
N N
N S
MeO Cl
H
N N
N S
Cl MeO O
S S
O
OH CHO
CH3 CHO
TH-482_03612207
Chapter 1
17
OH
O
Scheme 1.10. Some important partial oxidation reactions
Significantly, chromium(VI) and manganese(VII) are perhaps the best known oxidizing agents in chemistry commonly used in both bench scale as well as on large scale partial oxidation reactions. Endeavor is on to replace them by O2 or H2O2 and catalyst derived from Ti, V, Mo, W, etc. An important point to be made here is that the use of Cr (VI) and Mn(VII) oxidant on a large scale leads to a rather large volume of toxic metal waste. Thus, interests on the efficacious catalytic systems are certainly going to be in demand and are likely to draw special attention.
Research addressing the problems highlighted above is not only of topical importance but also rewarding.
Oxidative brominations and the other investigations related to the understanding of VBrPO and its activity is unquestionably very important which have already been highlighted. Equally important is to develop a VBrPO mimetic catalyst. For this reasons vanadium(V) compounds containing appropriate heteroligands are considered suitable candidates for VBrPO mimetic studies. [V(O)2(OH)(H2O)(imz)], (Imz = imidazole), which has been considered as the resting form of VHPO cofactor, was recently studied by DFT calculation to derive information on catalytic reactivity. Similar vanadium(V) species containing 3,5-dimethyl pyrazole (dmpz) as the heteroligand deserves the attention of vanadium chemists since such complexes are in all probability likely to be as VHPO mimic as the analogous imidazole containing vanadium(V) cofactor is. This might be all the more rational because of the structural similarity betwen dmpz and imidazole. In addition, studies involving dmpz as a ligand may not be very trivial because dmpz complexes of vanadium(IV) or (V) seems to have been very little worked on. Thus, the coordination chemistry of V(V)–dmpz deserves attention.
As a sustained scientific curiosity in reaction chemistries under microwave irradiation, our attention was drawn to Friedlaender synthesis of quinolines. This reaction is an important chemical process especially when it is used to generate highly useful intermediates for natural products and drugs, copolymers for electronic and optoelectronics. General method is a two-step process where isolation of o-aminobenzaldehyde suffers from the problem of polymerization and condensation
Chapter 1
18
with enolizable ketone is achieved by the use of Lewis acids. Thus, development of improved process for the synthesis of quinolines is desirable.
The present overview, including a critical assessment of the state-of-art of the problems addressed therein, provide reasons that are persuasive enough to undertake studies addressing the identified problems. In resonance with this, the present Ph.D research was initiated in 2003 in order to investigate some of the above-mentioned problems. The nature of the problems is such that there have been significant developments in nearly every couple of months thereby rendering it rather difficult to keep pace with. The outcome of our endeavor has finally led to the following end results:
(i) The development of clean aza- and thia-Michael reactions involving Cu(acac)2
immobilized in ionic liquids (IL), boric acid in water and borax in water as the catalysts.
While investigating the thia-Michael reactions using B(OH)3 or Na2B4O7.10H2O as the catalysts, some of the condensed products attracted our attention as prospective candidates for studying their self-assembled H-bonding interactions in the crystal lattice. Accordingly, X-ray crystallography of three β-sulfidocarbonyls was carried out and some interesting observations were made. These results constitute the subject matter of Chapter 3.
(ii) The selective oxidation of sulfides has been achieved with a newly synthesized vanadium complex, [VO2F(dmpz)2], borax or phosphate as the catalyst and H2O2 as the oxidant. As a logical extension, oxidative desulfurization of diesel was attempted with [VO2F(dmpz)2]–H2O2 and reasonable success has been achieved. The results obtained from this exercise constitute the subject matter of Chapter 4.
(iii) An improved synthesis of highly peroxygenated vanadium(V) species, [V(O2)3]– has been achieved and it was considered quite apt to use this as a precatalyst for oxidative brominations, and selective oxidation of alcohols with H2O2. While we were engaged in the triperoxovanadate(V) catalyzed reactions, it was observed that our [VO2F(dmpz)2] catalyst very efficiently catalyzed the oxidation of bromide to tribromide, (Br3– ), by H2O2. This encouraged us to try out the oxidative extraction of bromide from seawater using this catalyst. A comprehensive account of this work has been presented in Chapter 5.
(iv) Chapter 6, the concluding chapter of the thesis, gives an account of efficient solvent-free one-pot synthesis of quinolines achieved from o-nitrobenzaldehyde and enolizable ketones using SnCl2.2H2O and subjecting them to microwaves.
TH-482_03612207
Chapter 1
19
In addition to this Chapter (i.e., Chapter 1) which introduces the identified problems for the Ph.D.
research, provides an overview of the status of the problem, and also pin points the scope of work, Chapter 2 describes the sources of chemicals and solvents that were used in the work, methods of preparation of a few starting materials, details of the methods of chemical analyses, and particulars of various instruments and equipment used for physico-chemical studies and characterization of the reported compounds. Each chapter from 3 through 6 has been deliberately designed to be self-contained having a brief introduction and sections on results and discussion, and experimental followed by bibliography. While some of the results have been published, manuscripts based on the rest are either under communication or under preparation.