Stoichiometric catalyst

Since amines react more readily than alcohols in noncatalyzed reactions with anhydrides, the reaction is more difficult and initially required stoichiometric catalyst loadings [107], but could be performed in a catalytic sense with an O-acylated azlactone as acylating agent, which does not react with a benzylic amine at —50°C, but is capable of acylating the catalyst [108, 109]. Depending on the buUdness of the substrate, selectivities ranged from S = 11 to 27 (s = [ enantiomer l]/[ enantiomer 2])-... 

Interesting information about the catalytic systems was obtained by studying the stereochemistry of the reaction of the catalyst precursor 1 with methacrylonitrile. The water molecule of complex 1 is readUy displaced by methacrylonitrile rendering complex [(t -C5Me5)Ir (/ )-Prophos (methacrylonitrile)] (SbFg)2 (9) as a mixture of the two possible epimers at metal, namely, R iJic and 5ir,/ c, in 34% diastereomeric excess in the former (Scheme 20). In acetone, at 50°C, the R r,Rc isomer slowly epimerizes to the thermodynamically preferred epimer. From the solution, pure samples of the latter can be isolated that have been employed as stoichiometric catalysts for the DCR between methacrylonitrile and nitrones IV and V. 

Pyrazole and 3,5-dimethylpyrazole were effective stoichiometric catalysts in the Baylis-Hillman reaction of cyclo-pentenone 892 with /i-nitrobenzaldehyde 893 in basic media to give adducts 894 in good yields (Equation 190) <2004TL5171>. An asymmetric borane reduction of ketones catalyzed by AT-hydroxyalkyl-Z-menthopyrazoles has been reported <2000JHC983>. 3-Aryl-/-menthopyrazoles 895 were assessed for their catalytic activity for asymmetric Diels-Alder reactions <2002JHC1235, 2003JHC773>. 

Many catalysts are only required in trace amounts in order to affect the rate of the reaction. Some, however, are required in similar quantities to the reagents, in which case they are called stoichiometric catalysts. This often indicates that the catalyst is forming a complex with one of the reagents, and it is this complex that is the attacking species in the reaction. An example of this is the Lewis acid catalysis of the Friedel-Crafts reaction. 

The ammonia is the base, while the boron trichloride is the acid, because it accepts the lone pair from the ammonia. This definition is of great use to the organic chemist in, for example, the Friedel-Crafts reaction. Here, the reagent, an alkyl halide, reacts with an aromatic compound such as toluene, in the presence of a catalyst such as aluminium trichloride. There must be approximately the same amount of the catalyst as there is alkyl halide, and not just trace amounts of the catalyst, i.e. the aluminium trichloride is acting in this reaction as a stoichiometric catalyst. The reason becomes apparent when... 

Stoichiometric catalyst Used to describe a catalyst that is required in approximately the same concentration as the reagents it is catalysing, e.g. the A1C13 catalyst in the Friedel-Crafts reaction. 

This was an extraordinary result when it was published in 1974 as it was well ahead of its time. It is still one of the best asymmetric reactions with catalytic amounts of a chiral reagent. An example of an asymmetric Robinson annelation using stoichiometric catalyst is in the workbook. 

Table 8.1. Examples of Sharpless AE Reactions. These reactions were carried out under catalytic conditions (<10 mol % of Ti(OR)4 and tartrate), except for entry 8 (done using stoichiometric catalyst).

The epoxidation reaction is normally best carried out with only 5-10 mol% of the titanium catalyst in the presence of activated molecular sieves. These conditions avoid the traditional use of stoichiometric catalyst and provide a mild and convenient method (although often at the expense of a slight reduction in enantioselectivity and rate of reaction). Numerous examples of highly enantioselective epoxidations of allylic alcohols by this procedure have been reported. For example, the allylic alcohol 44 was converted selectively into the epoxides 45 and 46 (5.56). 

The fiinctional dependence of tire reaction rate on concentrations may be arbitrarily complicated and include species not appearing in the stoichiometric equation, for example, catalysts, inliibitors, etc. Sometimes, however, it takes a particularly simple fonn, for example, under certain conditions for elementary reactions and for other relatively simple reactions ... 

In principle, Chen, given the flux relations there is no difficulty in constructing differencial equations to describe the behavior of a catalyst pellet in steady or unsteady states. In practice, however, this simple procedure is obstructed by the implicit nature of the flux relations, since an explicit solution of usefully compact form is obtainable only for binary mixtures- In steady states this impasse is avoided by using certain, relations between Che flux vectors which are associated with the stoichiometry of Che chemical reaction or reactions taking place in the pellet, and the major part of Chapter 11 is concerned with the derivation, application and limitations of these stoichiometric relations. Fortunately they permit practicable solution procedures to be constructed regardless of the number of substances in the reaction mixture, provided there are only one or two stoichiomeCrically independent chemical reactions. 

A proper resolution of Che status of Che stoichiometric relations in the theory of steady states of catalyst pellets would be very desirable. Stewart s argument and the other fragmentary results presently available suggest they may always be satisfied for a single reaction when the boundary conditions correspond Co a uniform environment with no mass transfer resistance at the surface, regardless of the number of substances in Che mixture, the shape of the pellet, or the particular flux model used. However, this is no more than informed and perhaps wishful speculation. 

In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) comple.xes, are used. Pd(II) compounds are used either as stoichiometric reagents or as catalysts and Pd(0) complexes as catalysts. Pd(Il) compounds such as PdCh and Pd(OAc)2 are commercially available and widely used as... 

The intramolecular version for synthesizing cyclic and polycyclic compounds offers a powerful synthetic method for naturally occurring macrocyclic and polycyclic compounds, and novel total syntheses of many naturally occurring complex molecules have been achieved by synthetic designs based on this methodology. Cyclization by the coupling of an enone and alkenyl iodide has been applied to the synthesis of a model compound of l6-membered car-bomycin B 162 in 55% yield. A stoichiometric amount of the catalyst was used because the reaction was carried out under high dilution conditions[132]. 

Standard Heck conditions were used to introduce the dchydroalanine side-chain with 4-bromo-3-iodo-l-(4-methylphenylsulfonyl)indole[12]. Using 4-fluoro-3-iodo-l-(4-methylphenylsulfonyl)indole as the reactant, Merlic and Semmelhack found that addition of 2 eq, of LiCl or KCl improved yields in reactions carried out with 10% Pd/C as the catalyst[13]. The addition of the dehyroalanine side chain can also be done by stoichiometric Pd-mediated vinylation (see Section 11.2). A series of C-subslituled dehydro tryptophans was prepared in 40-60% yield by this method[14]. 

Although stoichiometric ethynylation of carbonyl compounds with metal acetyUdes was known as early as 1899 (9), Reppe s contribution was the development of catalytic ethynylation. Heavy metal acetyUdes, particularly cuprous acetyUde, were found to cataly2e the addition of acetylene to aldehydes. Although ethynylation of many aldehydes has been described (10), only formaldehyde has been catalyticaHy ethynylated on a commercial scale. Copper acetjlide is not effective as catalyst for ethynylation of ketones. For these, and for higher aldehydes, alkaline promoters have been used. 

Secondary acetylenic alcohols are prepared by ethynylation of aldehydes higher than formaldehyde. Although copper acetyUde complexes will cataly2e this reaction, the rates are slow and the equiUbria unfavorable. The commercial products are prepared with alkaline catalysts, usually used in stoichiometric amounts. 

The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

Other Methods. A variety of other methods have been studied, including phenol hydroxylation by N2O with HZSM-5 as catalyst (69), selective access to resorcinol from 5-methyloxohexanoate in the presence of Pd/C (70), cyclotrimerization of carbon monoxide and ethylene to form hydroquinone in the presence of rhodium catalysts (71), the electrochemical oxidation of benzene to hydroquinone and -benzoquinone (72), the air oxidation of phenol to catechol in the presence of a stoichiometric CuCl and Cu(0) catalyst (73), and the isomerization of dihydroxybenzenes on HZSM-5 catalysts (74). 

Water hydroly2es pure diketene only slowly to give acetoacetic acid [541-50-4] which quickly decomposes to acetone and carbon dioxide, but increasing the pH or adding catalysts (amines, palladium compounds) increases the rate of hydrolysis. The solvolysis of diketene in ammonia results in aceto acetamide [5977-14-0] if used in stoichiometric amounts (99), and P-arninocrotonarnide [15846-25-0] if used in excess (100). 

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