Oxidative addition metallacycle formation

Scheme 8 displays reactions where 43 behaves as a source of a Zr(II) derivative.30 They can be formally viewed as oxidative additions to the [p-Bu -calix[4]-(0Me)2(0)2Zr] fragment. The main driving force in the case of ketones is the high oxophilicity of the metal, which induces the reductive coupling of benzophenone leading to 45, or the addition of dibenzoyl causing the formation of the dioxo-metallacycle in 46, which contain a C-C double bond. It has to be mentioned that... 

Scheme 7 comprises the following patterns First, a metallacycle gives rise to ketones by CO insertion and reductive elimination. Next, a nickel hydride inserts an unsaturated substrate L, followed by CO. The acyl intermediate can give rise to reductive elimination with formation of acyl halides or acids and esters by hydrolysis, or it can insert a new ligand with subsequent reductive elimination as before. Alternatively, there may be a new insertion of carbon monoxide with final hydrolysis. Third, an intermediate R—Ni—X is formed by oxidative addition. It can react in several ways It can insert a new ligand L, followed by CO to give an... 

Two essentially different mechanisms, (i) oxidative cyclization of two 7T-components (formation of metallacycle) and (ii) oxidative addition of reducing or alkylating agents followed by insertion of 7t-components, can operate in these three-component reactions.426 However, the aforementioned phenomena such as the reversal of regiochemistry and the crossover from reductive to alkylative manifolds remain unsolved. 

Pt(0), NVE 16/Pt(II), NVE 16). Oxidative additions generally occur most readily for low valent complexes, and for metals in the order 5d > Ad 3d. In addition to those that formally cleave C-X, H-X, and X-X (X = halide) bonds, oxidative addition reactions are also known where the metal is inserted into C-0, C-H, and some strained or activated C-C bonds. Another reaction which is effectively an oxidative addition is the formation of metallacycles from a low valent metal and an olefin (Equation 7). 

The oxidative addition of methylene- or allylidenecyclopropane to nickel(O) or palladium(O) can also lead to the formation of six-, seven-, and nine-membered metallacycles of nickeF " and palladium. Various substituted platinacyclobutanes (30) were obtained by oxidative addition of cyclopropanes to the complexes [CljPtLJj (Scheme... 

Metallacycles bearing cyclopropane units can be obtained by various methods. They are available via oxidative addition-type metallacyclopentane formation, using alkenes or alkynes with attached cyclopropane rings (such as vinylcyclopropanes, e.g. formation of 1). Methylenecyclopropanes and cyclopropenes can also be used vide infra). 

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

The presence of products derived not only from the reaction of the aliphatic palladacycle carbon, but also from the aromatic one as shown before (Eq. 9) [19b] and below [23, 25], suggested that an oxidative addition of a protonic acid HX to the palladium(II) metallacycle occurred with formation of palla-dium(IV) (Eq. 16). 

Other classes of nickel-catalyzed [3+2] cycloadditions include the addition of 2-haloacetophenone derivatives to alkynes to produce indenol derivatives (Scheme 3-36). This process likely involves initial oxidative addition to the haloaromatic, followed by alkyne insertion and carbonyl addition. In this case, zinc powder serves as reducing agent to regenerate the active nickel(0) catalyst. A mechanistically intriguing cycloaddition that proceeds without the action of reducing agents is the direct formation of bicyclic products from the addition of unsaturated ketones with alkynes. This process likely involves initial metallacycle formation followed by unusual rearrangement steps unique to the requisite doubly unsaturated carbonyl component. 

In addition to the syntheses of carbocycles in Scheme 3-38, oxygen-, silicon-, and aluminum-containing heterocycles have also been prepared by [2+2+1] cycloadditions. An interesting tetrahydrofuran synthesis involves stoichiometric metallacycle formation by oxidative cyclization of Ni(0) with two equivalents of norbomadiene, followed by oxygen atom insertion with N2O (Scheme 3-39). 

From these early achievements, it was found that the heteroatom in the directing group could saturate the coordination site on metal center facilitating the formation of a metallacyclic intermediate, which is able to proceed transmetalation with organometallic reagents (M /M catalytic cycle) or oxidative addition with... 

Two important points to note are that the basic reaction mechanism involves the formation of 4.39, a five-membered metallacycle from 4.38 (see Section 2.3.3). This may be considered an oxidative addition reaction as the formal oxidation state of M is zero and 2+ in 4.38 and 4.39, respectively. Second, conversion of 4.39 to 4.40 is the famiUar CO insertion into a metal-carbon bond, which is followed by reductive elimination of cyclopentenone. 

The stoichiometric head-to-head oxidative coupling of alkynes with CpRuBr(COD) affords a metallacyclic biscarbene complex [22], This process has been used to initiate catalytic formation of the RCH=CH-CH=C(Y)R backbone, to produce Junctional dienes from alkynes by addition of H-Y. The complex [Cp RuCl(COD)[ successfully catalyzes this new chemical transformation, involving the combination of two molecules of alkynes and one molecule of carboxylic acid to afford functional conjugated dienes (Scheme 4) [23]. 

The stereoselective synthesis of 1,4-disubstituted-l,3-dienes proceeds by head-to-head oxidative coupling of two alkynes with formation of an isolable metallacyclic biscarbene ruthenium complex [23], as shown in Scheme 6. Several key experiments involving labeled reagents and stoichiometric reactions and theoretical studies support the formation of a mixed Fischer-Schrock-type biscarbene complex which undergoes protonation at one carbene carbon atom whereas the other becomes accessible to nucleophilic addition of the carboxylate anion (Scheme 6) [23]. 

Addition of a diene 5.231 to these reagents results in the formation of a metallacycle 5.232 (Scheme 5.66). The carbon-zirconium bonds can then be cleaved by electrophilic or oxidizing agents. If the reaction is intramolecular, a reasonable degree of control of the new stereogenic centres is possible. The insertion can, however, be reversible. The stereochemistry of the product therefore depends on the reaction time and temperature, longer times yielding thermodynamically more stable complexes. ... 

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