Green Chemistry
A traditional concept in process chemistry has been the optimization of the time-space yield. From our modern perspective, this limited viewpoint must be enlarged, as for example toxic wastes can destroy natural resources and especially the means of livelihood for future generations. In addition, many feedstocks for the production of chemicals are based on petroleum, which is not a renewable resource. The key question to address is: what alternatives can be developed and used? In addition, we must ensure that future generations can also use these new alternatives. "Sustainability" is a concept that is used to distinguish methods and processes that can ensure the long-term productivity of the environment, so that even subsequent generations of humans can live on this planet. Sustainability has environmental, economic, and social dimensions.
Paul Anastas of the U.S. Environmental Protection Agency formulated some simple rules of thumb for how sustainability can be achieved in the production of chemicals - the "Green chemical principles":
- Waste prevention instead of remediation
- Atom economy or efficiency
- Use of less hazardous and toxic chemicals
- Safer products by design
- Innocuous solvents and auxiliaries
- Energy efficiency by design
- Preferred use of renewable raw materials
- Shorter syntheses (avoid derivatization)
- Catalytic rather than stoichiometric reagents
- Design products to undergo degradation in the environment
- Analytical methodologies for pollution prevention
- Inherently safer processes
Implementing these Green Chemical Principles requires a certain investment, since the current, very inexpensive chemical processes must be redesigned. However, in times when certain raw materials become more expensive (for example, as the availability of transition metals becomes limited) and also the costs for energy increase, such an investment should be paid back as the optimized processes become less expensive than the unoptimized ones. The development of greener procedures can therefore be seen as an investment for the future, which also helps to ensure that the production complies with possible upcoming future legal regulations.
A typical chemical process generates products and wastes from raw materials such as substrates, solvents and reagents. If most of the reagents and the solvent can be recycled, the mass flow looks quite different:
Thus, the prevention of waste can be achieved if most of the reagents and the solvent are recyclable. For example, catalysts and reagents such as acids and bases that are bound to a solid phase can be filtered off, and can be regenerated (if needed) and reused in a subsequent run. In the production of chemical products on very large scale, heterogeneous catalysts and reagents can be kept stationary while substrates are continuously added and pass through to yield a product that is continuously removed (for example by distillation).
The mass efficiency of such processes can be judged by the E factor (Environmental factor):
Whereas the ideal E factor of 0 is almost achieved in petroleum refining, the production of bulk and fine chemicals gives E factors of between 1 and 50. Typical E factors for the production of pharmaceuticals lie between 25 and 100. Note that water is not considered in this calculation, because this would lead to very high E factors. However, inorganic and organic wastes that are diluted in the aqueous stream must be included. Sometimes it is easier to calculate the E factor from a different viewpoint, since accounting for the losses and exact waste streams is difficult:
In any event, the E factor and related factors do not account for any type of toxicity of the wastes. Such a correction factor (an “unfriendliness” quotient, Q) would be 1 if the waste has no impact on the environment, less than 1 if the waste can be recycled or used for another product, and greater than 1 if the wastes are toxic and hazardous. Such discussions are at a very preliminary stage, and E factors can be used directly for comparison purposes as this metric has already been widely adopted in the industry.
Another attempt to calculate the efficiency of chemical reactions that is also widely used is that of atom economy or efficiency. Here the value can be calculated from the chemical equation:
Atom efficiency is a highly theoretical value that does not incorporate any solvent, nor the actual chemical yield. An experimental atom efficiency can be calculated by multiplying the chemical yield with the theoretical atom efficiency. Anyway, the discussion remains more qualitative than quantitative, and does not yet quantify the type of toxicity of the products and reagents used. Still, atom economy as a term can readily be used for a direct qualitative description of reactions.
Considering specific reactions, the development of green methods is focused on two main aspects: choice of solvent, and the development of catalyzed reactions. By way of example, the development of catalyzed reactions for dihydroxylations have made possible the replacement of the Woodward Reaction in the manufacture of steroids, in which huge amounts of expensive silver salts were used and produced, and thus had become an economic factor:
The Woodward reaction can be replaced through the use of stoichiometric quantities of OsO4, but osmium tetroxide is both very toxic and very expensive, making its use on a commercial scale prohibitive. Only in its catalytic variant, which employs N-methylmorpholine-N-oxide as the stoichiometric oxidant and catalytic quantities of OsO4, can this be considered a green reaction that can be used on industrial scale.
Some systems have already been reported in which H2O2 is used to reoxidize the N-methylmorpholine, allowing this material also to be used in catalytic amounts. Considering the atom efficiency using H2O2 as the terminal oxidant, H2O as the stoichiometric byproduct is much better than N-methylmorpholine. Notably, catalytic systems are available in which the osmium catalyst is encapsulated in a polyurea matrix or bound to a resin, so that the catalyst can be more easily recovered and reused. An additional advantage of such polymer-bound catalysts is the avoidance of toxic transition metal impurities, for example in pharmaceutical products.
A key point is still the choice of solvent, as this is the main component of a reaction system by volume (approx. 90%). Chlorinated solvents should be avoided, as many of these solvents are toxic and volatile, and are implicated in the destruction of the ozone layer. Alternative solvents include ionic liquids, for example, which are non-volatile and can provide non-aqueous reaction media of varying polarity. Ionic liquids have significant potential, since if systems can be developed in which the products can be removed by extraction or distillation and the catalyst remains in the ionic liquid, theoretically both the solvent and the catalyst can be reused. The solvent of choice for green chemistry is water, which is a non-toxic liquid but with limited chemical compatibility. On the one hand, reactions such as the Diels-Alder Reaction are often even accelerated when run in an aqueous medium, while on the other hand, many reactants and reagents, including most organometallic compounds, are totally incompatible with water. There is thus a great need to develop newer methods and technologies that would make interesting products available through reactions in water or other aqueous media. For a short review of reactions in water, please check: S. Varma, Clean Chemical Synthesis in Water, Org. Chem. Highlights 2007, February 1. Chemical reactions run under neat conditions (no solvent) and in a supercritical CO2 medium can also be considered as green choices. Other possible improvements can be considered, such as for example replacement of benzene by toluene (as a less toxic alternative), or use of solvents that can be rapidly degraded by microorganisms.
It is quite astonishing to consider the progress that has been made in the development of greener alternatives to traditional reactions. Some examples can be seen in the recent literature section at the end of this page, and this resource is continuously being updated. A good introduction to Green Chemistry with a focus on catalyzed reactions is offered in the book edited by Sheldon, Arends and Hanefeld (Green Chemistry and Catalysis, Wiley-VCH Weinheim, 2007, 1-47.), on which this article is partly based.
Links of Interest
Green Chemistry Conferences
Introduction
to the Concept of Green Chemistry
More Links on Green Chemistry
Books on Green Chemistry
Green Chemistry and Catalysis
Roger A. Sheldon, Isabel Arends, Ulf Hanefeld
Hardcover, 434 Pages
First Edition, 2007
ISBN-13: 978-3-527-30715-9
Wiley-VCH
Chemistry in Alternative Reaction Media
D. J. Adams, P. J. Dyson, S. J. Taverner
Paperback, 268 Pages
First Edition, November 2003
ISBN: 0-471-49849-1
Wiley
Recent Literature
A highly active, air- and moisture-stable and easily recoverable
magnetic-nanoparticle-supported palladium catalyst enables the Suzuki
cross-coupling reaction of alkynyl bromides with organoboron derivatives in very
good yields in ethanol. The supported palladium catalyst can be recovered and
reused up to 16 times without significant loss of catalytic activity.
X. Zhang, P. Li, Y. Ji, L. Zhang, L. Wang, Synthesis, 2011,
2975-2983.
The use of a planetary ball mill enables a solvent-free method for the addition
of amines to dialkylacetylendicarboxylates or alkylpropiolates. Conversion of
educts was quantitative within five minutes without use of any catalyst or base.
Beside the E-/Z-isomers, no side products were formed.
R. Thorwirth, A. Stolle, Synlett, 2011,
2200-2202.
Both poly(3,4-ethylenedioxythiophene) and poly(pyrrole) mediate a pinacol
rearrangement of 1,2-diols to give ketones or aldehydes in comparable yields to
those observed for treatment with mineral or Lewis acids. The two-phase reaction
medium in hydrocarbon solvents allows facile recovery of the products. Both the
polymer and the hydrocarbon solvent may be reused.
C. Pavlik, M. D. Morton, M. B. Smith, Synlett, 2011,
2191-2194.
In situ preparation of an active Pd/C catalyst from Pd(OAc)2 and
charcoal in methanol enables a simple, highly reproducible protocol for the
hydrogenation of alkenes and alkynes and for the hydrogenolysis of O-benzyl
ethers. Mild reaction conditions and low catalyst loadings, as well as the
absence of contamination of the product by palladium residues, make this a
sustainable, useful process.
F.-X. Felpin, E. Fouquet, Chem. Eur. J., 2010,
12440-12445.
CuI/DIPEA/HOAc is as a highly efficient catalytic system for CuAAC. In this
novel acid-base jointly promoted formation of 1,2,3-triazoles, HOAc was
recognized to accelerate the conversions of the C-Cu bond-containing
intermediates and buffer the basicity of DIPEA. As a result, all drawbacks
occurring in the popular catalytic system CuI/NR3 were overcome
easily.
C. Shao, X. Wang, Q. Zhang, S. Luo, J. Zhao, Y. Hu, J. Org. Chem., 2011,
76, 6832-6836.
In a method for the Friedel-Crafts-type insertion reaction of acetylene with
acid chlorides in chloroaluminate ionic liquids, the use of ionic liquids not
only serves to avoid the use of carbon tetrachloride or 1,2-dichloroethane but
also suppresses side reactions, and enables a simpler purification procedure,
giving a range of aromatic and aliphatic β-chlorovinyl ketones in high yield and
purity.
D. J. M. Snelders, P. J. Dyson, Org. Lett., 2011,
13, 4048-4051.
ZrOCl2 • 8 H2O is a highly effective, water-tolerant,
and reusable catalyst for the direct condensation of carboxylic acids and N,N′-dimethylurea
under microwave irradiation to give the corresponding N-methylamides in moderate
to excellent yields. ZrOCl2 • 8 H2O is a useful green catalyst
due to its low toxicity, easy availability, low cost, ease of handling, easy
recovery, good activity, and reusability.
D. Talukdar, L. Saikia, A. J. Thakur, Synlett, 2011,
1597-1601.
A strategy for the asymmetric Michael addition of aldehydes to nitroolefins with
a catalytic system of an organocatalyst in combination with
ionic-liquid-supported benzoic acid gives excellent diastereo- and
enantioselectivities. A notable feature of this organocatalytic system is that
the catalyst can be recycled more than 12 times without significant loss of
enantioselectivity.
D. Sarkar, R. Bhattarai, A. D. Headley, B. Ni, Synthesis, 2011,
1993-1997.
Copper iodide is an effective cocatalyst for the olefin cross-metathesis
reaction. It has both a catalyst stabilizing effect due to iodide ion, as well
as copper(I)-based phosphine-scavenging properties. A variety of Michael
acceptors and olefinic partners can be cross-coupled under mild conditions in
diethyl ether and in water using the new surfactant TPGS-750-M.
K. Voigtritter, S. Ghorai, B. H. Lipshutz, J. Org. Chem., 2011,
76, 4697-4702.
A mild, environmentally friendly, and efficient process enables the synthesis of
2-imidazolines in high yield by reaction of aldehydes with ethylenediamine using
hydrogen peroxide as an oxidant in the presence of sodium iodide and anhydrous
magnesium sulfate.
G.-y. Bai, K. Xu, G.-f. Chen, Y.-h. Yang, T.-y. Li, Synthesis, 2011,
1599-1603.
Methanesulfonic anhydride promotes Friedel-Crafts acylations of aryl and alkyl
carboxylic acids. This reagent allows the preparation of aryl ketones in good
yield with minimal waste containing no metallic or halogenated components,
clearly differentiating it from other available methodologies.
M. C. Wilkinson, Org. Lett., 2011,
13, 2232-2235.
Magnetically Recoverable Pd/Fe3O4-Catalyzed Hiyama Cross-Coupling of Aryl
Bromides with Aryl Siloxanes
B. Sreedhar, A. S. Kumar, D. Yada, Synlett, 2011,
1081-1084.
Recyclable Catalysts for Suzuki-Miyaura Cross-Coupling Reactions at Ambient
Temperature Based on a Simple Merrifield Resin Supported
Phenanthroline-Palladium(II) Complex
J. Yang, P. Li, L. Wang, Synthesis, 2011,
1295-1301.
Carbonyl compounds were obtained in very good yields after treatment of oximes
with 2 molar equivalent of CuCl2 • 2 H2O at reflux in
acetonitrile and water (4:1). In addition, cupric salt was readily recovered in
an almost quantitative yield via the complete precipitation of Cu(OH)2
• 2 H2O.
N. Quan, X.-X. Shi, L.-D. Nie, J. Dong, R.-H. Zhu, Synlett, 2011,
1028-1032.
Nanosized sulfated titania was prepared by a sol-gel hydrothermal process. The
nanoparticles showed high catalytic activity in a direct amidation of fatty
acids as well as benzoic acids with various amines under solvent-free conditions.
M. Hosseini-Sarvari, E. Sodagar, M. M. Doroodmand, J. Org. Chem., 2011,
76, 2853-2859.
A true Click catalytic system is based on commercially available [CuBr(PPh3)3].
This system is active at room temperature, with catalyst loadings of 0.5 mol %
or less, in the absence of any additive, and it does not require any
purification step to isolate pure triazoles.
S. Lal, S. Diez-González, J. Org. Chem., 2011,
76, 2367-2373.
An efficient and simple protocol for phosphine-free Heck reactions in water in
the presence of a Pd(L-proline)2 complex as the catalyst under
controlled microwave irradiation conditions is versatile and provides excellent
yields of products in short reaction times. The reaction system minimizes costs,
operational hazards and environmental pollution.
B. K. Allam, K. N. Singh, Synthesis, 2011,
1125-1131.
Efficient Aqueous-Phase Heck Reaction Catalyzed by a Robust Hydrophilic
Pyridine-Bridged Bisbenzimidazolylidene-Palladium Pincer Complex
Z. Wang, X. Feng, W. Fang, T. Tu, Synlett, 2011,
951-954.
Microwave heating enables a Borrowing Hydrogen strategy to form C-N bonds from
alcohols and amines, removes the need for solvent and reduces the reaction times,
while the results are comparable with those using thermal heating.
A. J. A. Watson, A. C. Maxwell, J. M. J. Williams, J. Org. Chem., 2011,
76, 2328-2331.
A simple protocol for the efficient synthesis of 3(2H)-furanones by
cycloisomerization of allenic hydroxyketones is achieved in water and in the
absence of any expensive metal catalysts.
M. Poonoth, N. Krause, J. Org. Chem., 2011,
76, 1934-1936.
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