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Development of environmentally reactions has been of central importance in recent synthetic organic chemistry. Water is among most environmentally friendly and least expensive solvents, and therefore reaction mediated by water should constitute the ideal green chemistry. Since many organic compounds have limited solubility in water, water mediated reactions with high selectivities and reactivities are still relatively limited, and there is a need for close examination of vast research areas.
Therefore topic of issue will focus on review articles of recent development of water mediated organic reactions.
Stereoselective reactions in aqueous media
Regioselective reactions in aqueous media
C-C bond formations in aqueous media
Chemical characteristics of water:
Waters are complex aqueous solutions, e.g., having different dissolved solids and gases
Controlling factors: Chemical state parameters
Redox potential (dissolved oxygen content)
For a given aquifer or substrate matrix, the chemical state parameters control the chemical composition of the water in equilibrium with surroundings
Impact of aquifer matrix on groundwater composition
Water accelerated organic reactions:
These are useful in synthesis-
Although it is not practical to run many chemical reactions in water, if small quantities of water add it seem to be beneficial in some organic and organometallic transformations. According to all the surveys water accelerated reactions and highlighted the effects of small amounts of water on reaction rate, product yield, and Regioselective, Diastereoselective, and enantioselectivity. They noted, however, that the contribution of water in these reactions is poorly understood. They emphasized the role of a small amount of water as
a hydrolyzing agent which leads to secondary product that served as catalysts or promoters.
an internal quenching agents to drive chemical equilibria.
an Lewis acid activator or co-activator.
Usefulness Of water accelerated organic reactions-
1. Environmental biotechnology creates efficient biological systems with less secondary waste and more energy efficiency. Municipal waste is ~80% biowaste, which adds to agricultural waste to make a principal resource for biodevelopment.
2. Biotech methods can remove organic solids from waste water and control the release of phosphorus and nitrogen. Techniques such as bioconversion, bioaugmentation, phytoremediation, biostimulation, and biodestruction use bacteria, microalgae, fungi, and yeasts. The enzymes of these organisms allow conversion of biowaste into bioproducts.
3. Almost 70% of municipal solid waste consists of lignocellulose (35-50% cellulose, 20-35% hemicellulose, 15-25% lignin). Converting cellulose into sugars and EtOH requires opening up its crystalline structure with pretreatments. Recent approaches to this problem include acid-catalyzed steam explosion, CO2 explosion, sequential cellulase treatment, and the use of cellulases from different sources.
Water Compatible Reactions:
These are useful in synthesis Organocatalytic Michael Reactions of C-Centered Nucleophiles with Enals:
In a pool of water compatible catalysts, chiral prolinol based catalyst 1, been developed for the highly enantioselective Carbon Carbon bond formation of the Michael reactions in water. The synthesis of (S)-Rolipram, a type IV phosphodiesterase inhibitor, was also demonstrated in this reactions.
Current gene therapy knowledge is setting the stage for cancer vaccines:
Replacing missing or defective genes is a compelling idea, but vectors such as weakened viruses, used to transport the new genes into patients, are highly inefficient. Even when the transfer is successful, the gene activity typically falls off after a short time. A. Abbott reports that researchers in immunotherapy are using this short-term or transient expression of the transferred genes to develop cancer vaccines. The transient activity only needs to last long enough to kick-start the immune response and allow the immune system to sustain it.
One of the big challenges is activating specific T cells targeted to molecular signatures (antigens) carried by the tumor cells. Earlier ideas of tailoring therapy to individuals by genetically manipulating their own cells now appears too time-consuming to be a realistic option. Researchers are instead focusing on cell lines maintained in cultures that share common antigens with the patient's tumor type. Results have been mixed, although several pharmaceutical companies are conducting clinical trials targeting melanoma as well as kidney, prostate, pancreatic, and lung cancers. The author also discusses the gene therapy approach to revascularizing tissue in the vicinity of blocked arteries-one company is starting phase III studies in this area.
Stereoselective reactions in aqueous media:
An stereoselectivity is the properties of a chemical reaction and in which a single reactant form un-equal mixture of stereoisomer during the nonstereospecific creations of new stereo-center or during the nonstereospecific transformation of a pre-existing one. The selectivity arises from the differences in steric- effect and electronic effect in the mechanistic pathway leading to the the different product, stereoselectivity can vary in degree but it can never totally since the activation energies of different pathways is in finite. However, the minor stereoisomers may not be detected by the analytic methods used.
Some point related to stereoselectivity reactions with example-
We have seen many examples of substrate control in nucleophilic addition to the carbonyl group (Felkin-Ahn & chelation control).
If molecule does not contain a stereogenic centre then we can use a chiral auxiliary.
The chiral auxiliary can be removed at a later stage.
Opposite diastereoisomer can be obtained from reduction of the ketone.
There is lower diastereoselectivity in the second addition as the nucleophile, 'H-' is smaller.
Regioselective reactions in aqueous media:
An regioselectivity is the one directional chemical bonding making or breaking over all possible direction, it can often applied to which many possible position of a reagent will affect, the proton of a strong base will abstracted from an organic molecule, where on a substituted benzene ring is further substituent will add.
Some point related to regioselectivity reactions with example in a halohydrin formation reaction with 2-propenylbenzene-
The preference for the formations of one product over another, the reactions are selective. This reaction is called regioselective reactions because its selectively generates one constitutional isomer rather than the others.