Selected stoichiometric examples

Casalnouvo, Calabrese and Milstein have described a good example of insertion of norbornene into the Ir-anilido bond of [IrCl(H)(NHPh)(PEt3)2] in the course of 

Ru and Os M-S bonds of a chelating dithiolate ligand are susceptible to insertion of alkynes to give metallacyclic structures as the one shown in Eq. 6.37 [210], 

Substrates that contain polarized C-Y unsaturated moieties (Y = heteroatom) have been observed to insert into M-X bonds to give new metal complexes. This is the case of CO2, COS, and PhNCO, that insert in the Pt-N bond of [PtH(NHPh)(PEt3)2] [205], or CS2 into the Pd-OR bond of [Pd(Me)(OR)(dppe)j to give [Pd(Me) /cCsC(S)OR (dppe)] (R = CH(CF3)Ph) [211], 

Synthetic applications that consist of metal catalyzed addition of X-X to carbon-carbon unsaturated substrates, generally alkynes, have been described for X = X = SR, SeR, TeR and X = SeR, X = PR2 as well as X = SR, X = BR2, SiR3 [201], These transformations generally involve oxidative addition of the X-X reactant to the metal center followed by insertion of the alkyne, into the M-X bond. Reductive elimination leads to the final disubstituted alkene, which shows cis stereochemistry. 

Metal mediated H-X addition to alkenes and alkynes is also a very interesting synthetic process that affords new phosphine derivatives (X = PR2, hydrophos-phination), sulfides (X = SR), and amines (X = NR2) [201a]. Although several mechanistic pathways can be envisaged for these reactions, insertion into the M-X bond has been proposed in the course of the addition of the heavier X groups, 

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A more complex notation is needed for non-stoichiometric phases. Selected simple examples are given below, and more detailed information will be reported when discussing crystal coordination formulae ... 

The potential for chiral ketones to effect the catalytic asymmetric epoxidation of olefins was first demonstrated by Curd in 1984 [104]. Since then,moderately selective catalytic [105,106] and stoichiometric examples have emerged [107,108,... 

Repeat the analysis in the example of 7.4.2, but for the one arbitrarily selected stoichiometric coefficient, use Vji = -1. Create a table of compositions vs. extent of reaction and compare it with the one in the example. Which quantities change and which remain unaffected by the way the reaction is balanced Discuss. 

To select stoichiometric coefficients so as to obtain whole non-codivisible numbers only. In the example given by equation (2.3) this condition would be satisfied by the first-named equation. 

The older methods have been replaced by methods which require less, if any, excess sulfuric acid. For example, sulfonation of naphthalene can be carried out in tetrachloroethane solution with the stoichiometric amount of sulfur trioxide at no greater than 30°C, followed by separation of the precipitated l-naphthalenesulfonic acid the filtrate can be reused as the solvent for the next batch (14). The purification of 1-naphthalenesulfonic acid by extraction or washing the cake with 2,6-dimethyl-4-heptanone (diisobutyl ketone) or a C-1—4 alcohol has been described (15,16). The selective insoluble salt formation of 1-naphthalenesulfonic acid in the sulfonation mixture with 2,3-dimethyl aniline has been patented (17). 

Zinc-tartrate complexes were applied for reactions of both nitrones and nitrile oxides with allyl alcohol and for both reaction types selectivities of more than 90% ee were obtained. Whereas the reactions of nitrones required a stoichiometric amount of the catalyst the nitrile oxide reactions could be performed in the presence of 20 mol% of the catalyst. This is the only example on a metal-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrile oxides. It should however be no-... 

For the performance of an enantioselective synthesis, it is of advantage when an asymmetric catalyst can be employed instead of a chiral reagent or auxiliary in stoichiometric amounts. The valuable enantiomerically pure substance is then required in small amounts only. For the Fleck reaction, catalytically active asymmetric substances have been developed. An illustrative example is the synthesis of the tricyclic compound 17, which represents a versatile synthetic intermediate for the synthesis of diterpenes. Instead of an aryl halide, a trifluoromethanesul-fonic acid arylester (ArOTf) 16 is used as the starting material. With the use of the / -enantiomer of 2,2 -Z7w-(diphenylphosphino)-l,F-binaphthyl ((R)-BINAP) as catalyst, the Heck reaction becomes regio- and face-selective. The reaction occurs preferentially at the trisubstituted double bond b, leading to the tricyclic product 17 with 95% ee. °... 

As we saw in Section L, titration involves the addition of a solution, called the titrant, from a buret to a flask containing the sample, called the analyte. For example, if an environmental chemist is monitoring acid mine drainage and needs to know the concentration of acid in the water, a sample of the effluent from the mine would be the analyte and a solution of base of known concentration would be the titrant. At the stoichiometric point, the amount of OH " (or 11,0 ) added as titrant is equal to the amount of H30+ (or OH-) initially present in the analyte. The success of the technique depends on our ability to detect this point. We use the techniques in this chapter to identify the roles of different species in determining the pH and to select the appropriate indicator for a titration. 

Because the pH changes rapidly near the stoichiometric point, an indicator is suitable for a titration if the pH at the stoichiometric point is within one unit of p K pHstoichiometric point P In 1 Table 18-2 lists a selection of acid-base indicators, and Example shows how to select an appropriate indicator. 

An intere.sting example in the context of waste minimization is the manufacture of the vitamin K intermediate, menadione. Traditionally it was produced by stoichiometric oxidation of 2-methylnaphthalene with chromium trioxide (Eqn. (8)), which generates 18 kg of solid, chromium containing waste per kg of menadione. Catalytic alternatives have been reported, but selectivities tend to be rather low owing to competing oxidation of the second aromatic ring (the. selectivity in the classical process is only 50-60%). The best results were obtained with a heteropolyanion as catalyst and O2 as the oxidant (Kozhevnikov, 1993). 

Olah et al. (1999) have been able to realize selective cyclisation of o-benzoyl benzoic acid to anthraquinone using dichlorobenzene as a solvent and Nafion-H as a catalyst. This may lead to avoidance of the Friedel-Crafts reaction using a stoichiometric amount of aluminium chloride and resulting in a lot of wa,ste. Many other examples of similar reactions have been reported. 

The preparation of ketones and ester from (3-dicarbonyl enolates has largely been supplanted by procedures based on selective enolate formation. These procedures permit direct alkylation of ketone and ester enolates and avoid the hydrolysis and decarboxylation of keto ester intermediates. The development of conditions for stoichiometric formation of both kinetically and thermodynamically controlled enolates has permitted the extensive use of enolate alkylation reactions in multistep synthesis of complex molecules. One aspect of the alkylation reaction that is crucial in many cases is the stereoselectivity. The alkylation has a stereoelectronic preference for approach of the electrophile perpendicular to the plane of the enolate, because the tt electrons are involved in bond formation. A major factor in determining the stereoselectivity of ketone enolate alkylations is the difference in steric hindrance on the two faces of the enolate. The electrophile approaches from the less hindered of the two faces and the degree of stereoselectivity depends on the steric differentiation. Numerous examples of such effects have been observed.51 In ketone and ester enolates that are exocyclic to a conformationally biased cyclohexane ring there is a small preference for... 

Unlike C—S insertion, the conversion of metallathiacycles to thiolato complexes via M—C bond cleavage, followed eventually by the formation of free thiols, is a quite rare reaction occurring both stoichiometrically and catalytically. Stoichiometric reactions proceed via either protono-lysis208,209,223 or hydrogenation of metallathiacycles.221 224 225 226-228 Selected examples are illustrated in Scheme 37 for (103),208 (102),221,224,225 and (104).223... 

Wong reported that stoichiometric amounts of copper(I) chloride alone can promote the intramolecular Stille coupling (equation 138)243. In fact, copper(I)-mediated reaction was cleaner and faster compared with that catalysed by Pd(0) species. Selected examples of intermolecular Stille coupling reactions leading to dienes (Table 25)236a,242b 244, polyenes (Table 26)245 and macrocyclizations (Table 27)246 are given in the respective tables. 

Attempts to employ allenes in palladium-catalyzed oxidations have so far given dimeric products via jr al lyI complexes of type 7i62.63. The fact that only very little 1,2-addition product is formed via nucleophilic attack on jral ly I complex 69 indicates that the kinetic chloropalladation intermediate is 70. Although formation of 70 is reversible, it is trapped by the excess of allene present in the catalytic reaction to give dimeric products. The only reported example of a selective intermolecular 1,2-addition to allenes is the carbonylation given in equation 31, which is a stoichiometric oxidation64. 

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

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