Stoichiometric Experiments

Hersh et al. found that the cationic complex [CpFe(CO)2(THF)]BF4 (23) can accelerate the [4 + 2] cycloaddition of acrolein and cyclopentadiene [32]. However, the catalytic activity was higher than expected from rate constants determined in stoichiometric experiments, indicating that a Brpnsted or Lewis acid impurity might accelerate this process and generating doubts about the role of 23. 

Consistent with the mechanism proposed earlier (Section 5.2.2, Scheme 5-8), in stoichiometric experiments, the intermediates phosphide hydride 12 and diastere-omeric mixture of alkyl hydrides 13 could be isolated and observed by NMR at -20°C, respectively (Scheme 5-14). 

The carbomethoxy cycle starts with the attack of a methoxy group at a coordinated carbonyl group or a migratory insertion of CO in a palladium methoxy bond. Any type of methoxy species will have a low concentration in the acidic medium of the reaction. In Figure 12.20 many details of these reactions, discussed above in section 12.2, have been omitted and only a shorthand notation is presented. Subsequently insertion of ethene takes place. It is known from stoichiometric experiments that both reactions are relatively slow. In the final step a formal protonation takes place, which as we saw before, may actually involve enolate species. 

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

In the synthesis of alkylidenecyclobutenes from propargyl alcohols, stoichiometric experiments show that the first step involves [2+2] oxidative head-to-head coupling of the alkynes, leading to an isolable cyclobutadiene-ruthenium complex. Addition of acid generates a cyclobutenyl metal intermediate which undergoes carboxylate addition on the less substituted allylic carbon atom (Scheme 7). 

In the past 200 years a great deal of experimental evidence has accumulated to support the atomic model. This theory has proved to be both extremely useful and physically reasonable. When atoms were first suggested by the Greek philosophers Democritus and Leucippus about 400 B.c., the concept was based mostly on intuition. In fact, for the following 20 centuries, no convincing experimental evidence was available to support the existence of atoms. The first real scientific data were gathered by Lavoisier and others from quantitative measurements of chemical reactions. The results of these stoichiometric experiments led John Dalton to propose the first systematic atomic theory. Dalton s theory, although crude, has stood the test of time extremely well. 

Materials. The method of purifying distilled water has been described (6, 13, 14). Except for Baker and Adamson s 70% perchloric acid and Baker s inhibitor-free hydrogen peroxide, all chemicals used in stoichiometric experiments were reciystallized at least once from purified water. Buffer solutions were prepared by adding purified KH2PO4 or Na4P207 to solutions of HCIO4 in purified water. 

As shown in stoichiometric experiments treated in Sect. 2.4, Eq. 13 hexahydromethanobiphenylenes are formed by reductive elimination from palladacycles. These compounds are often present as secondary products in the catalytic reactions shown. Path b of Scheme 1 is an example. In the absence of competitive reactants the palladacycle ehminates a hexahydromethanobiphenylene, forming palladium(O). A catalytic process was thus worked out starting with an aryl iodide or bromide, norbornene, Pd(OAc)2, and K2CO3 [23]. Yields were good to excellent with o-substituted iodo- or bromobenzene (94% with... 

The knowledge acquired from stoichiometric experiments on the ortho effect allowed a series of new sequential reactions leading to biphenyl-derived structures to be worked out. 

Stoichiometric experiments were conducted with the functionalized alkenes to further explore iron-substrate interactions and gauge relative coordination affinities. Several bis(imino)pyridine iron amine and ketone compounds were isolated and studied using a combination of X-ray diffraction, NMR and Mossbauer spectroscopy and established electronic structures similar to that for ( PDI)Fe(N2)2-In the absence of H2, diallyl ether and allyl ethyl ether underwent facile C-0 bond cleavage and yielded a near equimolar mixture of the corresponding iron allyl and alkoxide complexes [89]. Our group has recently published a comprehensive study on these types of reachons and discovered rare examples of C-0 bond cleavage in saturated esters [89]. 

A similar complex could be isolated by the reaction of palladiumbis-(acetylacetonate) with two moles of triisopropyl phosphine and two moles of 6-lactone (Equation 7). By ring opening of the lactone a palladium complex with two long-chain carboxylate substituents was formed. It is remarkable that the same ring opening reaction which is discussed in the catalytic cycle could be observed in a related stoichiometric experiment. 

Mechanistic investigations can also provide insight into unexpected side reactions. For example, in Ir(III)-catalyzed indole synthesis, Messerle and coworkers noted an unexpected side product, N-vinyhndole, when 2-(2-phenylethynyl)anihne was used as substrate with acetone as the solvent By probing solvent effects, deuterium-labeling experiments, and isolation of dormant species formed in stoichiometric experiments, two possible reaction pathways were considered. In collaboration with Eisenstein [303], these pathways were compared computationally, yielding a proposed reaction mechanism that proceeds via the initial formation of imine between the aniline and acetone solvent, followed by imine nucleophilic attack of coordinated alkyne for indole formation. 

We have shown, in stoichiometric experiments, that reaction of copper(I) with TEMPO affords a piperidinyloxyl copper(II) complex. Reaction of the latter with a molecule of alcohol afforded the alkoxycopper(II) complex and TEMPOH. Reaction of the alkoxycopper(II) complex with a second molecule of TEMPO gave the carbonyl compound, copper(I), and TEMPOH. This mechanism resembles that proposed for the aerobic oxidation of alcohols catalyzed by the copper-dependent enzyme, galactose oxidase, and mimics thereof. Finally, TEMPOH is reoxidized to TEMPO by oxygen. We have also shown that copper in combination with PIPO affords an active and recyclable catalyst for alcohol oxidation [18]. 

However, it must be pointed out that the results reported here have been obtained in stoichiometric experiments under very particular conditions. It is not sure whether these isomerizations occur under technical hydroformylation conditions because other reactions may be faster. 

Three independent stoichiometric experiments were carried out hydrogenation of 158 at -90°C and reaction of 156a and 156b with MAC at low temperatures. Despite the difference in the experimental set-ups and in the temperature regimes of hydrogenation, the product 72 obtained in these experiments was always of >99% ee (S) (Scheme 1.38). 

The first and important evidence came from the group of Hammond [14]. They used allenic esters 5. Stoichiometric experiments with the cationic gold(l) species... 

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