Reaction insertion “oxidative addition

This index contains over 25 000 entries to the 6562 text pages of Volumes 1-6. The index covers general types of coordination complex, specific coordination complexes, general and specific organic compounds where their synthesis or use involves coordination complexes, types of reaction (insertion, oxidative addition, etc.), spectroscopic techniques (NMR, IR, etc.), and other topics involving coordination complexes, such as medicinal and industrial applications. 

The Ir11 dimer [Ir(oep)]2 (oep = octaethylporphyrin) has been prepared in low yield by photolysis of (oep)IrCH3 in C6D6.473 This preparation has been improved by Chan et al.474, as shown in Reaction Scheme 24, where TEMPO = 2,2,6,6-tetramethyl-l-piperidinyloxy, free radical. The dimer undergoes several organometallic reactions, including oxidative addition of alkyl C 11 bonds and alkene insertions.475... 

The insertion of 1 into element-element bonds is a crucial step in an extensive series of remarkable transformations. From the perspective of the particular substrate, such reactions are oxidative additions to a silylene-like center. The primary insertion products all possess reactive pentamethylcy-clopentadienyl-silicon cr-bonds. In this chapter only those insertion reactions... 

The abundance of accessible donor and acceptor orbitals in common transition-metal complexes facilitates low-energy bond rearrangements such as insertion ( oxidative-addition ) reactions, thus enabling the critically important catalytic potential of metals. 

Reductive elimination is simply the reverse reaction of oxidative addition the formal valence state of the metal is reduced by two (or one in a bimetallic reaction), and the total electron count of the complex is reduced by two. While oxidative addition can also be observed for main group elements, this reaction is more typical of the transition elements in particular the electronegative, noble metals. In a catalytic cycle the two reactions always occur pair-wise. In one step the oxidative addition occurs, followed for example by insertion reactions, and then the cycle is completed by a reductive elimination of the product. 

The main steps in the currently accepted catalytic cycle of the Heck reaction are oxidative addition, carbopalla-dation (G=G insertion), and / -hydride elimination. It is well established that both, the insertion as well as the elimination step, are m-stereospecific. Only in some cases has formal /r/ / i--elimination been observed. For example, exposure of the l,3-dibromo-4-(dihydronaphthyloxy)benzene derivative 16 and an alkene 1-R to a palladium source in the presence of a base led to a sequential intra-intermolecular twofold Heck reaction furnishing the alkenylated tetracyclic products 17 in good to excellent yields (Scheme 9). " In the rate-determining step, the base removes a proton in an antiperiplanar orientation from the benzylic palladium intermediate. The best amine base was found to be l,4-diazabicyclo[2.2.2]octane, which apparently has an optimal shape for this proton abstraction. 

By far the most common way for organic molecules to enter late transition metal catalyzed reactions is oxidative addition. In this process a low valent palladium(O)3 or nickel(O) atom inserts into a carbon-heteroatom bond, usually of an aryl halide or sulfonate (Figure 1-2). The formation of the carbon-metal bond is accompanied by an increase in the oxidation number of the metal by 2. There are a series of factors determining the speed of the process. 

Indolizines were arylated under similar conditions selectively in the 3-position (6.90.). A detailed mechanistic study of the transformation revealed that in this reaction the arylpalladium species, formed in the first step of the catalytic cycle, is attached to the indolizine core in an electrophilic substitution step, which is followed by reductive elimination. The presence of alternate routes such as Heck-type insertion, oxidative addition of the C-H bond, or transmetalation were excluded on the basis of experimental evidence.121... 

The carbanion is trapped with iodine to give 42. which makes a further functionali/aiion possible. Conversion of vinylic iodide 42 into a lactone is accomplished by palladium-cataly/ed carbonyla-tion under Stille conditions.13 This process ean be broken down into the following elementary reactions a) Oxidative addition of Pd° to vinylic iodide 42 with formation of 43 b) An insertion reaction of carbon monoxide with creation of the pallada-acyl species 44 c) Reaction of acid-chloride equivalent 44 with the alcohol to give lactone 13. 

The reaction sequence in the vinylation of aromatic halides and vinyl halides, i.e. the Heck reaction, is oxidative addition of the alkyl halide to a zerovalent palladium complex, then insertion of an alkene and completed by /3-hydride elimination and HX elimination. Initially though, C-H activation of a C-H alkene bond had also been taken into consideration. Although the Heck reaction reduces the formation of salt by-products by half compared with cross-coupling reactions, salts are still formed in stoichiometric amounts. Further reduction of salt production by a proper choice of aryl precursors has been reported (Chapter III.2.1) [1]. In these examples aromatic carboxylic anhydrides were used instead of halides and the co-produced acid can be recycled and one molecule of carbon monoxide is sacrificed. Catalytic activation of aromatic C-H bonds and subsequent insertion of alkenes leads to new C-C bond formation without production of halide salt byproducts, as shown in Scheme 1. When the hydroarylation reaction is performed with alkynes one obtains arylalkenes, the products of the Heck reaction, which now are synthesized without the co-production of salts. No reoxidation of the metal is required, because palladium(II) is regenerated. 

The product and state which reactions are oxidative addition and which are insertion reaction, predicted ... 

The mechanism of this polymerization has been discussed by Cundy et al. (9). The first step is apparently the insertion of a low-valent metal into the strained C—Si bond to give a silametallacyclopentane. Metallacycles 4 and 5 have in fact been isolated from the reactions of Fe2(CO)9 and CpMn(CO)3 with 1, respectively (10, 11). Complex 4 when treated with phosphines gives polymer and LFe(CO>4. If the metallacycle resulting from insertion (6) is unstable, repeated insertions (oxidative additions) and reductive eliminations lead to polymer. Chain termination results from reductive elimination of =SiH [Eq. (13)]. 

The probable catalytic cycle for the carbonylative coupling reaction involves oxidative addition, carbon monoxide insertion into the R-Pd complex, transmetala-tion, tran -cw-isomerization, and reductive elimination. 

A prototypical reaction involving oxidative addition of a four-membered ring to a transition metal is seen in the Cr(CO)6 mediated transformation of biphe-nylene to 9-fluorenone [24]. The product can be viewed as arising from the insertion of chromium into the central C-C bond bridging the two aromatic rings, subsequent carbonylation, and reductive elimination. 

A plausible mechanism for the above reaction involves oxidative addition of the heteroaryl chloride to Pd(0) to provide Pd(II) intermediate 75, which subsequently inserts into benzoxazole to form the arylpalladium(II) complex 76. P-Hydride elimination of 76 would afford 11 with concomitant regeneration of Pd(0) for the next catalytic cycle. 

The mechanisms and catalysts used in this Buchwald-Hartwig chemistry mirror those of coupling reactions involving oxidative addition, transmetallation, and reductive elirruna-tion. The first step, as usual, is oxidative insertion of Pd(0) into the aryl-halogen bond. The Pd(ll) complex now adds the amine so that both coupling partners find themselves bonded to the same palladium atom. The base eliminates H—1 from the complex and reductive elimination forms the Ar—N bond. 

HR consists of three elemental reactions (1) oxidative addition of an organic halide to form arylpalladium halides (2) insertion of an alkene to form the alkyl-palladium 17 (or carbopalladation of alkene) and (3) dehydropalladation (jS-H elimination) to give the arylalkenes 18 or conjugated dienes. 

The cycloisomerization of 1,6-enynes proceeds smoothly in the presence of AcOH or HCO2H and the reaction is explained by the following mechanism (hydridopalladium acetate mechanism) [45]. Most importantly, oxidative addition of AcOH to Pd(0) generates H-Pd-OAc 143, and the cyclization of 1,6-enynes starts by insertion of the triple bond to 143 to afford the alkenylpalladium 144. Subsequent intramolecular insertion of the double bond gives the alkylpalladium 145. The termination step is (i-R elimination and either the diene 136 or 138 is formed with regeneration of H-Pd-OAc. It should be noted that the alkenylpalladium 144 is a similar species formed in a Heck reaction by oxidative addition of alkenyl halide to Pd(0). Based on this reaction, alkyne is a useful starter in domino cyclization of polyenynes. 

The suggestion is then made that the stereoconlrolling step in asymmetric Mizoroki-Heck reactions is oxidative addition (via dynamic kinetic resolution) rather than alkene association or migratory insertion. The implication is that only substrates capable of a dynamic kinetic resolution may cyclize with high enantioselectivity. This would limit the substrate scope of the asymmetric intramolecular Mizoroki-Heck reaction. While the dynamic kinetic resolution during the oxidative addition may be a component of the overall stereoselectivity, it does not rule out contributions from later events in the mechanistic pathway and does not explain the effect of additives on selectivity. What has been shown is that the axial chirality of the o-iodoanilides (as with any enantioenriched isomer of a chiral precursor) influences the stereochemical outcome of their reactions. 

The catalytic cycle suggested for this reaction involves oxidative addition of to Pd(0) with formation of Pd disulfide or diselenide, alkyne coordination and insertion into the Pd-Z bond and reductive elimination (Scheme 3.60). 

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