Stoichiometric inorganic reagents

As noted above, a prime cause of waste generation is the use of stoichiometric inorganic reagents. Hence, the solution is simple replacement of antiquated stoichiometric methodologies with cleaner catalytic alternatives, e.g. catalytic hydrogenations, catalytic oxidations with O2 or H2O2 and catalytic carbonylations. 

Thus, in the fine chemicals industry, reduction of ketones and aldehydes relies mainly on the use of complex metal hydrides that require time-consuming workup of reaction mixtures and produce significant amounts of inorganic and organic wastes. Similarly, the oxidation of alcohols into carbonyls is traditionally performed with stoichiometric inorganic oxidants, notably Cr(VI) reagents or a catalyst in combination with a stoichiometric oxidant [1]. 

TEMPO-catalysed oxidation of alcohols to carbonyl compounds with buffered aqueous NaOCl has found broad application, even in large-scale operations. Indeed, this selective methodology involves the use of safe, inexpensive inorganic reagents under mild reaction conditions. A new supported catalyst PEG-TEMPO 11, soluble in organic solvents such as CH2CI2 and AcOH, but insoluble in ethers and hexanes, was prepared and proved to be an effective catalyst for the selective oxidation of 1-octanol with various stoichiometric oxidants. When 11 was used at 1 mol% as a catalyst in combination with KBr (10 mol%) a slight excess of buffered bleach (pH = 8.6) as the terminal oxidant, partial over-oxidation of 1-octanol to octanoic acid was observed (91% yield in... 

Until recently, noncatalytic stoichiometric oxidations of arenes with toxic inorganic reagents, such as CrOj and K Cr O., MnO and KMnO, OsO, Pb(OAc), TllNOj), nitric acid, ceric ammonium nitrate, and some others, were the main route for the production of oxygenated aromatic compounds [1, 2, 6, 51]. A classic example is the manufacture ( 1500t/year) of vitamin via stoichiometric oxidation of 2-methylnaphthalene (MN) with carcinogenic CrO in sulfuric acid (Eq. 14.14) [52]. 

Once the major cause of the waste has been recognized, the solution to the waste problem is evident the general replacement of classical syntheses that use stoichiometric amounts of inorganic (or organic) reagents by cleaner, catalytic alternatives. If the solution is so simple, why are catalytic processes not as widely used in fine and specialty chemicals manufacture as they are in bulk chemicals. One reason is that the volumes involved are much smaller, and thus the need to minimize waste is less acute than in bulk chemicals manufacture. Secondly, the economics of bulk chemicals manufacture dictate the use of the least expensive reagent, which was generally the most atom economical, for example O2 for oxidation H 2 for reduction, and CO for C-C bond formation. 

As is clear from Table 1.1, enormous amounts of waste, comprising primarily inorganic salts, such as sodium chloride, sodium sulfate and ammonium sulfate, are formed in the reaction or in subsequent neutralization steps. The E factor increases dramatically on going downstream from bulk to fine chemicals and pharmaceuticals, partly because production of the latter involves multi-step syntheses but also owing to the use of stoichiometric reagents rather than catalysts (see later). 

The oxidation of primary and secondary alcohols into the corresponding carbonyl compounds plays a central role in organic synthesis [1, 139, 140]. Traditional methods for performing such transformations generally involve the use of stoichiometric quantities of inorganic oxidants, notably chromium(VI) reagents [141]. However, from both an economic and environmental viewpoint, atom efficient, catalytic methods that employ clean oxidants such as 02 and H202 are more desirable. 

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