Stoichiometric and Catalytic Reactions

A rational classification of reactions based on mechanistic considerations is essential for the better understanding of such a broad research field as that of the organic chemistry of Pd. Therefore, as was done in my previous book, the organic reactions of Pd are classified into stoichiometric and catalytic reactions. It is essential to form a Pd—C cr-bond for a synthetic reaction. The Pd— C (T-bond is formed in two ways depending on the substrates. ir-Bond formation from unoxidized forms [1] of alkenes and arenes (simple alkenes and arenes) leads to stoichiometric reactions, and that from oxidized forms of alkenes and arenes (typically halides) leads to catalytic reactions. We first consider how these two reactions differ. 

2 Stoichiometric Oxidative Reactions with Pd(II) Compounds in which Pd(II) is Reduced to Pd(0) 

The Pd—C cr-bond can be prepared from simple, unoxidized alkenes and aromatic compounds by the reaction of Pd(II) compounds. The following are typical examples. The first step of the reaction of a simple alkene with Pd(ll) and a nucleophile X or Y to form 19 is called palladation. Depending on the nucleophile, it is called oxypalladation, aminopalladation, carbopalladation, etc. The subsequent elimination of b-hydrogen produces the nucleophilic substitution product 20. The displacement of Pd with another nucleophile (X) affords the nucleophilic addition product 21 (see Chapter 3, Section 2). As an example, the oxypalladation of 4-pentenol with PdXi to afford furan 22 or 23 is shown. 

7r-Allylpalladium complex formation from alkene.s lakes place by the displacement of an allylic hydrogen of alkene with Pd(II) (see Chapter 3. Section 

Insertion of one of two double bonds of butadiene into Pd—X forms substituted a 7r-allylpalladium complex 24 (see Chapter 3, Section 4). 

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Palladation of aromatic compounds with Pd(OAc)2 gives the arylpalladium acetate 25 as an unstable intermediate (see Chapter 3, Section 5). A similar complex 26 is formed by the transmetallation of PdX2 with arylmetal compounds of main group metals such as Hg Those intermediates which have the Pd—C cr-bonds react with nucleophiles or undergo alkene insertion to give oxidized products and Pd(0) as shown below. Hence, these reactions proceed by consuming stoichiometric amounts of Pd(II) compounds, which are reduced to the Pd(0) state. Sometimes, but not always, the reduced Pd(0) is reoxidized in situ to the Pd(II) state. In such a case, the whole oxidation process becomes a catalytic cycle with regard to the Pd(II) compounds. This catalytic reaction is different mechanistically, however, from the Pd(0)-catalyzed reactions described in the next section. These stoichiometric and catalytic reactions are treated in Chapter 3. 

The most characteristic feature of the Pd—C bonds in these intermediates of both the stoichiometric and catalytic reactions is their reaction with nucleophiles, and Pd(0) is generated by accepting two electrons from the nucleophiles as exemplified for the first time by the reactions of 7r-allylpalladium chloride[2] or PdCl2-COD[3] complex with malonate and acetoacetate. It should be noted... 

Reactions of another class are catalyzed by Pd(II) compounds which act as Lewis acids, and are treated in Chapter 5 and partly in Chapter 4. From the above-mentioned explanation, the reactions catalyzed by Pd(0) and Pd(II) are clearly different mechanistically. In this book the stoichiometric and catalytic reactions are classified further according to reacting substrates. However, this classification has some problems, viz. it leads to separate treatment of some unit reactions in different chapters. The carbonylation of alkenes is an example. Oxidative carbonylation of alkenes is treated in Chapter 3 and hydrocar-bonylation in Chapter 4. 

Lin Y-S, Yamamoto A (1999) Activation of C-O Bonds Stoichiometric and Catalytic Reactions. 3 161-192... 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

The initial dominance of path b in the competitive reaction arises because the concentration ratio [6 )]/[4] is large enough to overwhelm the reactivity ratio j[rirX]/, which would favour the more reactive path a. This supports previous suggestions that the Pd(BINAP), which has never been isolated or observed spectroscopically, exists only fleetingly in stoichiometric and catalytic reaction networks. 

We have also underlined the potential of the OsHCl(CO)(P Pr3)2 complex in homogeneous catalytic reactions, showing that derived Os(t 2-H2) intermediates are formed under catalytic conditions. Stoichiometric and catalytic reactions involving the title complex have advanced together. 

Complexes 8 and 9 of cage ligands derived from 1,7-dioxa-4,10-diazacyclododecane can be used as anion activators both in stoichiometric and catalytic reactions and their activity is comparable with that found for lipophilic [k C (2.2.2,C )]Y cryptates 6 (28,32) (Table III). 

Studies regarding the nature of the catalytically active species for NHC complexes in Heck-type reactions have focused on the Mizorvki—Heck reaction and have consistently revealed a palladium(O) species as the active catalyst. The induction period is shortened upon addition of a reducing agent,and postulated intermediates of the reaction were isolated and characterized as well as employed in stoichiometric and catalytic reactions. Theoretical studies using DPT calculations showed the mechanism for NHC complexes to most likely he in agreement with phosphine chemistry. ... 

As discussed in this chapter, the fundamental host-guest chemistry of 1 has been elaborated to include both stoichiometric and catalytic reactions. The constrained interior and chirality of 1 allows for both size- and stereo-selectivity [31-35]. Additionally, 1 itself has been used as a catalyst for the sigmatropic rearrangement of enammonium cations [36,37] and the hydrolysis of acid-labile orthoformates and acetals [38,39]. Our approach to using 1 to mediate chemical reactivity has been twofold First, the chiral environment of 1 is explored as a source of asymmetry for encapsulated achiral catalysts. Second, the assembly itself is used to catalyze reactions that either require preorganization of the substrate or contain high energy intermediates or transition states that can be stabilized in 1. 

Both stoichiometric and catalytic reactions of allylic compounds via 7r-allyl complexes are known. Reactions of nucleophilic 71-allyl complexes with electrophiles involve oxidation of metals and hence constitutes stoichiometric reactions. 7i-Allyl complexes of Ni, Fe, Mo, Co and others are nucleophilic and undergo the stoichiometric reaction with electrophiles. However, electrophilic 71-allyl complexes react with nucleophiles, accompanying reduction of metals. For example, 71-allylnickel chloride (2) reacts with electrophiles such as aldehydes, generating Ni(II), and hence the reaction is stoichiometric. In contrast, electrophilic 7i-allylpalladium chloride (3) reacts with nucleophiles such as malonate and Pd(0) is generated. Thus repeated oxidative addition of allylic compounds to Pd(0) constitutes a catalytic reaction. 

Both stoichiometric and catalytic reactions involving 7r-allylpalladium complexes are known. Reactions involving 7r-allylpalladium complexes become stoichiometric or catalytic depending on the preparative methods of the 7r-allylpalladium complex. Preparation of the 7r-allylpalladium complexes 6 by the oxidative addition of various allylic compounds 5, mainly esters to Pd(0), and their reactions with nucleophiles are catalytic. This is because Pd(0) is regenerated after the reaction with the nucleophile, and the Pd(0) reacts again with allylic compounds to form the complex 6. These catalytic reactions are treated in Section 4.3. However, the preparation of 7r-allyl complexes 6 from alkenes 7 requires Pd(II) salts. Subsequent reaction with nucleophiles generates Pd(0). As a whole, Pd(II) is consumed, and the reaction ends as the stoichiometric process, because in situ reoxidation of Pd(0) to Pd(II) is not attainable in this case. Also, 7i-allylpalladium complex 9 is formed by the reaction of conjugated dienes 8 with Pd(II), and the reaction of 9 with nucleophiles is stoichiometric. 

We have restricted ourselves to discussing stoichiometric and catalytic reactions which clearly involve 7r-allylnickel complexes and have not considered polymerization reactions, the Reppe synthesis, or template reactions. Fortunately there is an abundance of books and reviews which cover these fields as well as the similarities, and dissimilarities, with the other transition metals (1-8). The literature up to the end of 1968 has been surveyed but no attempt has been made to include all the material available. 

The field of acetylene complex chemistry continues to develop rapidly and to yield novel discoveries. A number of recent reviews 1-10) covers various facets including preparation, structure, nature of bonding, stoichiometric and catalytic reactions, and specific aspects with particular metals. The first part of this account is confined to those facets associated with the nature of the interactions between acetylenes and transition metals and to the insertion reactions of complexes closely related to catalysis. Although only scattered data are available, attempts will be made to give a consistent interpretation of the reactivities of coordinated acetylene in terms of a qualitative molecular orbital picture. 

Instead, it is the goal of this chapter to highlight some conceptually novel developments in the field of HAT reactions. We will concentrate on complexes of borane with N-heterocyclic carbenes (NHCs), on organometallic hydrogen atom donors in stoichiometric and catalytic reactions, and on the activation of water and alcohols for HAT. 

Several good reviews (3-10) and books (11,12) have been published on phase-transfer catalysis, should the reader desire a more detailed examination of the phase-transfer process. Although hundreds of examples of the application of phase-transfer catalysis to organic chemistry have appeared in the literature (13), there were, prior to 1976, no such examples in organometallic chemistry despite acceptance of the pivotal role played by organometallic anions in many stoichiometric and catalytic reactions. Since the first publication on organometallic phase-transfer catalysis (14), the field has developed sufficiently rapidly to justify an account at this time. A brief review was published by Cassar (15), and another by the same author is in press. 

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