Oxidative addition/insertion

As a typical example, the catalytic reaction of iodobenzene with methyl acrylate to afford methyl cinnamate (18) is explained by the sequences illustrated for the oxidative addition, insertion, and /3-elimination reactions. 

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

The acrylate complex 10 was suggested to be the major solution species during catalysis, since the equilibrium in Scheme 5-11, Eq. (2) lies to the right (fQq > 100)-Phosphine exchange at Pt was observed by NMR, but no evidence for four-coordinate PtL, was obtained. These observations help to explain why the excess of phosphine present (both products and starting materials) does not poison the catalyst. Pringle proposed a mechanism similar to that for formaldehyde and acrylonitrile hydrophosphination, involving P-H oxidative addition, insertion of olefin into the M-H bond, and P-C reductive elimination (as in Schemes 5-3 and 5-5) [11,12]. 

D) Oxidative addition (insertion). In the limit of strong synergism, the H—H bond can be broken and two new M—H bonds formed. Although termed oxidative addition, this reaction does not necessarily deplete the metal of electron density (i.e., oxidize the metal). Rather, this reaction is fundamentally an insertion of the metal into the H—H bond. In the process, a lone pair of electrons at the metal and the electron pair of the H—H bond are uncoupled and then recoupled to form two M—H bonds. Whether synergistic H2 coordination or insertion prevails depends largely on the relative... 

Odle, R. Blevins, B. Ratcliff, M. Hegedus, L. S. Conversion of 2-Halo-V-allylanilines to Indoles via Palladium(O) Oxidative Addition-Insertion Reactions, J. Org. Chem. 1980,45, 2709-2710. 

The mechanism for the reaction is believed to be as shown in Eq. 15.170 (start with CH3OH, lower right, and end with CHjCOOH, lower left).180 The reaction can be initiated with any rhodium salt, e.g., RhCl3, and a source of iodine, the two combining with CO to produce the active catalyst, IRItfCO y. The methyl iodide arises from the reaction of methanol and hydrogen iodide. Note that the catalytic loop involves oxidative addition, insertion, and reductive elimination, with a net production of acetic acid from the insertion of carbon monoxide into methanol. The rhodium shuttles between the +1 and +3 oxidation states. The cataylst is so efficient that the reaction will proceed at atmospheric pressure, although in practice the system is... 

Palladium(0)-catalyzed transformations generally involve three steps oxidative addition, insertion or transmetallation (really a special type of insertion), and reductive elimination. Together they comprise a pathway for the formation of new carbon-carbon bonds. Oxidative addition takes place when a coordinatively unsaturated Pd(0) species cleaves a covalent bond to give a new complex in which die palladium is oxidized to Pd(II). Typically dissociation of two phosphine ligands to a 14-electron complex is file first step followed by oxidative addition to give a 16-electron Pd(II) complex. 

Carbonylation of alkyl halides is rare. As an exception, AcOH is produced commercially by the Monsanto process from MeOH and CO using Rh as a catalyst in the presence of HI. In this process (Scheme 3.10), Mel is generated in situ from MeOH and HI and undergoes oxidative addition. Insertion of CO generates an acetylrhodium intermediate, and nucleophilic attack of water produces AcOH, regenerating the Rh catalyst and HI (or reductive elimination to give acetyl iodide and hydrolysis). 

Scheme 1. Palladacycle formation through a sequence of oxidative addition, insertion, and electrophilic aromatic substitution. L= phosphorous or nitrogen ligands, solvent, or coordinating species.

Both amido and pinacol derivatives of B-Si compounds 125 and 126 added to terminal and internal alkynes in the presence of a palladium244-246 or platinum(O) catalyst247 by a mechanism involving an oxidative addition-insertion process (Equation (39)).248 On the other hand, phosphine-free nickel(O) catalyst resulted in the dimerization of alkynes giving a Z,Z-isomer of l-silyl-4-borylbutadiene derivatives.249 Since the palladium-catalyzed cross-coupling at the C-B bond is faster than the G-Si bond of 137, a silylboration-cross-coupling sequence provided a method for the synthesis of 1-alkenylsilanes.246... 

Oxidative addition inserts metal atoms into single bonds... 

In Figs. 7.1 and 7.2 identify oxidative addition, insertion, and reductive elimination steps. Show the formation of 7.7 from 7.6 by an electron pair (curly arrow) pushing formalism. 

De I squale reported that a series of coordina lively unsaturated nickel complexes, such as Ni(PPli3)2 or Ni(PCy3)2, act as excellent catalysis. A mechanism is proposed consisting of a sequence of oxidative addition, insertion and reductive elimination steps which involve an oxometallocyclobutane intermediate [225], The decisive step is Uie insertion of carbon dioxide mto a metal -oxygen bond. 

A possible mechanistic scheme for carbocupration may involve initial complexa-tion of the RCu(I)MgBr2 species with the triple bond, oxidative addition (insertion) of RCu to the activated triple bond, transfer of the R to the vinylic carbon, reductive elimination of Cu(I), and finally metal-metal exchange to furnish the 2,2-disubstituted vinylcopper intermediate. 

We have just been considering the formation of aryl-lithiums by deprotonation with the more basic alkyl-lithiums, normally BuLi, and you are also aware that protons may be removed from many molecules to give lithium derivatives. We shall not discuss this further in this chapter, but concentrate instead on three other approaches, oxidative addition (insertion), transmetallation, and hydrometallation. 

In the foregoing, the formation of organic molecules on transition metal complexes is explained by stepwise processes of oxidative addition, insertion, and reductive elimination. One typical example, which can be clearly explained in this way, are the carbonylation and decarbonylation reactions catalyzed by rhodium complexes 10-137). Tsuji and Ohno found that RhCl(PPh3)3 decarbonylates aldehydes and acyl halides under mild conditions stoichiometrically. Also this complex and RhCl(CO) (PPh3)2 are active for the catalytic decarbonylation at high temperature. 

P-H bonds in H-phosphonates and secondary phosphine oxides could also be added to special aUcene derivatives, cyclopropenes, with Pd catalysts [25]. Scheme 15 shows the proposed mechanism, including P-H oxidative addition, insertion into the Pd-H bond, then P-C reductive elimination. The observed... 

Oxidative addition inserts metai atoms into singie bonds... 

Having seen steps such as oxidative addition, insertion, and reductive elimination in the context of transition metal-catalyzed hydrogenation using Wilkinsons catalyst, we can now see how these same types of mechanistic steps are involved in a mechanism proposed for the Heck-Mizoroki reaction. Aspects of the Heck-Mizoroki mechanism are similar to steps proposed for other cross-coupling reactions as well, although there are variations and certain steps that are specific to each, and not all of the steps below are involved or serve the same purpose in other cross-coupling reactions. 

Following this oxidative addition, insertion of the olefin or alkyne occurs into the platinum-hydride bond to form an alkyl or vinyl hydride complex. The regioselectivity of the insertion step and the chemistry of the alkyl silyl complexes control the overall regioselectivity of the hydrosilylation of olefins. Recall that these platinum catalysts form terminal alkylsilane products. This regioselectivity indicates that insertions of terminal olefins occur in order to generate a linear alkylplatinum intermediate. Apparently, the insertions of styrene and acrylic acid derivatives into the platinum hydride in these catalysts occur with the same regiochemistry. 

Interestingly, this Heck-type palladium-catalyzed oxidative addition/insertion manifold can also be applied to the actual formation of the carbon-heteroatom bond. This was illustrated by Narasaka in the reaction of olefin-tethered oxime derivatives. This chemistry can be considered to arise from oxidative addition of the N—O bond to palladium (30) followed by the more classical olefin insertion and (3-hydride elimination, ultimately allowing the assembly of pyrroles (Scheme 6.58) [79]. The nature of the OR unit was found to be critical in pyrrole formation, with the pentafluorobenzoylimine leading to selective cyclization and rearrangement to the aromatic product. An analogous approach has also been applied to pyridines and imidazoles [80]. 

In the reduction of an acyl derivative, Pd undergoes oxidative addition inserting between the acyl group and X. Hydrogen or a substituted hydride adds to the palladium(II) followed by reductive elimination to regenerate Pd(0) and release the aldehyde (Eq. 1 Scheme 1). 

B.i.e. Hydrophobic Effect. Hydrophobic organic molecules are prone to fold inside to populate more compact conformations when put into aqueous environments. The same can be said about the transition states. Thus, water may have an effect similar to the effect of high pressure, so that it may accelerate those steps in catalytic cycles that have a negative entropy of activation (oxidative addition, insertion, etc.), particularly in intramolecular cyclization reactions. Currently, it is not clear what the actual contribution of the hydro-phobic effect is in the positive influence of water noted in many Pd-catalyzed reactions. This problem deserves a separate study. 

---