Green Chemistry for Chemical Synthesis
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PERSPECTIVE Green chemistry for chemical synthesis Chao-Jun Li*† and Barry M. Trost†‡ *Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, QC, Canada H3A 2K6; and ‡Department of Chemistry, Stanford University, Stanford, CA 94305-5080 Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved July 3, 2008 (received for review May 5, 2008). Green chemistry for chemical synthesis addresses our future challenges in working with chemical processes and products by invent- ing novel reactions that can maximize the desired products and minimize by-products, designing new synthetic schemes and appa- rati that can simplify operations in chemical productions, and seeking greener solvents that are inherently environmentally and ecologically benign. atom economy ͉ synthetic efficiency ͉ sustainable chemical feedstocks ͉ green solvents ver the past two centuries, quantity of product isolated addition incorporates all atoms of the Reaction Yield = x100% fundamental theories and re- theoretical quantity of product starting materials into the final product. activities in chemistry have Recognizing this fundamental phenome- been soundly established. molecular wt. of desired product non, in 1991 (4) Trost presented a set of O Atom Economy = x100% coherent guiding principles for evaluat- Such theories and reactivities have pro- molecular weight of all products vided the foundations for the chemical ing the efficiency of specific chemical enterprise that generates critical living Fig. 1. Definition of the fundamental difference processes, termed the atom economy, needs such as food for the world’s popu- in the manner in which the reaction and the atom which has subsequently been incorpo- lation, achieves various medical wonders economy yields are generated. rated into the ‘‘Twelve Principles of that save millions of lives and improve Green Chemistry’’ and has altered the way many chemists design and plan their people’s health, and produces materials feedstocks, reactions, solvents, and syntheses. Atom economy seeks to maxi- essential to the present and future needs separations. of mankind. Just less than two centuries mize the incorporation of the starting ago, organic compounds were believed Chemical Feedstocks materials into the final product of any given reaction. The additional corollary to be only accessible through biological Presently, the main feedstock of chemi- is that, if maximum incorporation can- processes under the influence of ‘‘vital cal products comes from nonrenewable not be achieved, then ideally the quanti- forces’’ (1). Today, many molecules of petroleum that is being depleted rapidly great complexity can be synthesized both for chemical and energy needs. ties of side products should be minute readily. The total syntheses of natural However, nature provides a vast amount and environmentally innocuous. There is products with extremely high complexity of biomass in the renewable forms of a fundamental difference in the manner in which a reaction yield and the atom such as vitamin B12 (2) and palytoxin carbohydrates, amino acids, and triglcer- (3) in the laboratory are testimonials ides to obtain organic products (9), but economy yield is calculated (Fig. 1). of achievements comparable to the con- a major obstacle to using renewable The reaction yield is only concerned struction of the great pyramids at the biomass as feedstock is the need for with the quantity of the desired product molecular scale. However, despite such novel chemistry to transform the large that is isolated, relative to the theoreti- enormous achievements, we are facing amounts of biomass selectively and effi- cal quantity of the product. Atom econ- great challenges in future chemical syn- ciently, in its natural state, without ex- omy takes all used reagents and unwanted side products into account along with the thesis. The present state-of-the-art pro- tensive functionalization, defunctional- desired product. For example, substitu- cesses for synthesizing chemical products ization, or protection. tions and eliminations represent the vast are highly inefficient. The concept of Reactions majority of uneconomical classical reac- atom economy (4, 5) was created to tions in which inherent wastes are un- emphasize the importance of this ineffi- Reactions play the most fundamental avoidable (Scheme 1). Simple additions or ciency. The E factor (6) provided a quan- role in synthesis. The ideology of Green cycloadditions and rearrangements repre- tifiable measure of such inefficiency and Chemistry calls for the development of new chemical reactivities and reaction sent desired modes of reactivities (Scheme showed that, for every kilogram of fine conditions that can potentially provide 1). Reaction Mass Efficiency (RME) chemical and pharmaceutical products benefits for chemical syntheses in terms and Mass Intensity (MI) are additional produced, 5–100 times that amount of of resource and energy efficiency, prod- concepts to evaluate the efficiency of chemical waste is generated. Such low uct selectivity, operational simplicity, synthetic reactions to take into account efficiency in state-of-the-art organic and health and environmental safety. the reaction yield (10). syntheses presents great challenges in Recently, innovative reactions with resource conservation and draws envi- Atom Economy. Conventionally, attaining such inherent advantages have been de- ronmental and health concerns related the highest yield and product selectivity veloped with the aid of chemical and to the chemical wastes. were the governing factors of chemical Since its birth over a decade ago the synthesis. Little consideration was given field of Green Chemistry has been spe- to the usage of multiple reagents in stoi- Author contributions: C.-J.L. and B.M.T. wrote the paper. cifically designed to meet such chal- chiometric quantities, which often were The authors declare no conflict of interest. lenges in chemical synthesis (7, 8). To not incorporated into the target mole- This article is a PNAS Direct Submission. address these challenges, innovative and cule and would result in significant side †To whom correspondence may be addressed. E-mail: fundamentally novel chemistry is needed products. However, in a balanced chemi- [email protected] or [email protected]. throughout the synthetic processes: cal reaction, a simple addition or cyclo- © 2008 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0804348105 PNAS ͉ September 9, 2008 ͉ vol. 105 ͉ no. 36 ͉ 13197–13202 Downloaded by guest on September 26, 2021 H Krische and coworkers (16), in which N N + OH + + H2O primary alcohols were added stereos- electively to alkenes, which provides an Atom economy % = (42.08/119.21) x 100% = 35.3% atom-economic version of the classical reaction where a Grignard reagent is added to an aldehyde (Scheme 6). Claisen Rearrangement: Direct Conversion of C–H Bonds. Direct O OH 200oC transformation of the C–H bonds of or- ganic molecules into desired structures H without extra chemical transformations Atom economy % = (134.18/134.18) x 100% = 100% represents another class of major desir- able reactions (17–23). In nature, a vari- Scheme 1. ety of organic compounds can be oxi- dized easily by molecular oxygen or other oxygen donors in the cells of bac- cat. teria, fungi, plants, insects, fish, and Ru Ph3P Cl mammals (24–26). It is worth noting the Ph P O HO O OH 3 O 6 6 O important advances in biomimetic ap- 6 6 cat. In(OSO CF ) 2 3 3 71% proaches to such oxidations (27–36). H H Hydroxylation of linear alkanes or meth- 2steps ane to generate terminal alcohols is very O useful in the synthesis of chemicals and HO 6 OH 6 fuels (37). ADOCIACETYLENE B However, the direct conversion of Scheme 2. C–H bonds into C–C bonds leads to more efficient syntheses of complex products with reduced synthetic opera- tions (38). Recently, great progress has H3CO2C H H3CO2C been made in transition-metal-catalyzed OTIPS R=CH2OTIPS H activation and further reaction of C–H 85% bonds (39). Li (40) and others have H3CO2C H + developed various methods to generate H3CO2C - Ru(NCCH3)3 PF6 C–C bonds directly from two different H C–H bonds in the presence of an oxidiz- R CHO ing reagent through a cross-dehydroge- R=CHO H3CO2C H native coupling (CDC) catalyzed by H3CO2C H transition metals. For example, (NH)- 80% indoles and tetrahydroisoquinolines were converted directly into alkaloids by us- Scheme 3. ing such a coupling (Scheme 7) (41). Recently, an elegant cross-coupling of two aryl C–H bonds to form arene– 2+ O Ru(H O) (tos) O O OMe arene coupling products was reported by OMe 2 6 2 m OMe + (H O) Ru OMe Fagnou, Sanford, and others (Scheme 8) H O 2 5 n 2 (42, 43). MeO OMe (n-1) equiv. Synthesis Without Protections. Because of Scheme 4. the nature of classical chemical reactiv- ity, organic synthesis extensively utilizes protection–deprotection of functional biological catalysts. Some representative thetic strategies to ones of ideal atom groups, which increases the number of examples are the following. economy (Scheme 3) (12). steps in synthesizing the desired target Isomerizations. Isomerization of propargyl Ring-opening metathesis polymerization. The compounds. Novel chemistry is needed alcohols into conjugated carbonyl com- living ring-opening metathesis polymer- to perform organic synthesis without protection and deprotection. Recently, pounds provides an atom-economic ization developed by Grubbs and co- progress has been made on this subject. means for synthesizing