Insertion reactions reductive metalation

There are a number of amine-catalyzed carbonylation reactions which are catalyzed by cobalt carbonyl and iron carbonyl. It seems to me that these are insertion reactions of metal amides, where carbon monoxide is inserted and then some kind of a reduction or subsequent reaction gives the observed products, urea derivatives or carbamates in alcohols. We do not know the structure of the iron compound it is probably similar to the cobalt species shown. 

Facile isocyanide insertion reactions into metal-carbon, -nitrogen, -sulfur, -oxygen, - hydride, and - halide bonds have been found to readily occur. The insertion into metal-hydrides to give stable formimidines is particularly noteworthy since corresponding formyls (—CHO) are exceptionally difficult to synthesize and tend to be very unstable. There is a great deal of interest in carbon monoxide reductions, and the instability of the intermediate reduction products has made a study of the reduction process extremely difficult. Recently, however, the interaction of isocyanides with zirconium hydrides has allowed the isolation of the individual reduction steps of the isocyanide which has provided a model study for carbon monoxide reduction (39). 

The chemistry of metalated aziridines is far less developed than the chemistry of metalated epoxides, although from what is known [lb], it is obvious that their chemistry is similar. Like metalated epoxides, metalated aziridines can act as classical nucleophiles with a variety of electrophiles to give more highly substituted aziridines (Scheme 5.56, Path A). A small amount is known about how they can act as electrophiles with strong nucleophiles to undergo reductive alkylation (Path B), and undergo C-H insertion reactions (Path C). 

Formally, the metal oxidation number x increases to x+2, while the coordination number n of ML, increases to n+2. If such oxidative addition reactions are intended to be the first step in a sequence of transformations, which eventually will lead to a functionalization reaction of C-X, then the oxidative addition product 2 should still be capable of coordinating further substrate molecules in order to initiate their insertion, subsequent reductive elimination, or the like [1], This is why 14 electron intermediates MLu (1) are of particular interest. In this case species 2 are 16 electron complexes themselves, and as such may still be reactive enough to bind another reaction partner. 

Investigations in our laboratory by Rebecca Stimson have demonstrated that it is possible to combine the borane reduction of a metal acyl with the Lewis acid promoted CO insertion reaction which has been discussed earlier in this paper (29). In this reaction, which is presumed to proceed by equation 17, the... 

E) Sigma-bond metathesis. Dihydrogen is observed to react with transition-metal-alkyl bonds even when the metal lacks lone pairs. In this case the reaction cannot be explained in terms of the oxidative-addition or reductive-elimination motif. Instead, we can view this reaction as a special type of insertion reaction whereby the ctmr bond pair takes the donor role of the metal lone pair and donates into the cthh antibond. When the M—R bonds are highly polarized as M+R, the process could also be described as a concerted electrophilic H2 activation in which R acts as the base accepting H+. 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

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. 

Vaska s complex catalyzed the transformahon of aUenylcyclopropane into 2-alkenylidenecyclohex-3-enone under conditions of pressurized CO (Scheme 11.25) [38]. In this reaction, the jr-coordination to internal oleflnic moiety of the aUene brings the metal closer to the cyclopropane ring. Release of the cyclopropane ring strain then facilitates the oxidative addition of vinylcyclopropane moiety along with C-C bond cleavage, such that metallacyclohexene is obtained a subsequent carbonyl insertion and reductive elimination then provides the product Hence, the reaction can be recognized as a [5+1] cycloaddition of vinylcyclopropane and CO. 

The main steps in the catalytic MeOH carbonylation cyde which were proposed for the Co catalysed process [2] have served, with some modification perhaps in the carbonylation of MeOAc to AC2O, to the present day and are familiar as a classic example of a metal catalysed reaction. These steps are shown in Eigure 5.1. They are of course, (i) the oxidative addition of Mel to a metal center to form a metal methyl species, (ii) the migratory insertion reaction which generates a metal acyl from the metal methyl and coordinated CO and (iii) reductive elimination or other evolution of the metal acyl spedes to products. Broadly, as will be discussed in more detail later, the other ligands in the metal environment are CO and iodide. To balance the overall chemistry a molecule of CO must also enter the cycle. 

Metal Hydrides. The simplest reactions in this group are the various catalytic reduction reactions of carbon monoxide. Methane or higher hydrocarbons, methanol or higher alcohols, and a variety of other oxygenated organic compounds may be formed, depending upon the catalyst and reaction conditions (23). There is little evidence about the mechanism of these reactions, but the initial step in every example is probably a carbon monoxide insertion into a metal hydride, followed by reduction reactions. 

Metal Hydrides. It is likely that the reduction of aldehydes to alcohols by cobalt hydrocarbonyl (27) is an example of a carbonyl insertion reaction with a metal hydride. It is not clear which way the hydrocarbonyl adds to the carbonyl groups —whether it forms a cobalt-carbon bond (2), or a cobalt-oxygen bond (90). 

Figure C shows carbon monoxide insertion reactions. There are a number of reduction reactions of carbon monoxide catalyzed by transition metals, and these, I believe, all involve an insertion of carbon monoxide into a metal hydride as an initial step. Cobalt hydrocarbonyl reacts with carbon monoxide to give formate derivatives. This is probably an insertion reaction also.

The question arises as to whether the metalation is a one-step reaction or whether one or more intermediate species exist. The complexity of the reaction (insertion of the metal atom (M), oxidation to M2+ and parallel reduction of hydrogen, release of H2) makes a multistep reaction probable. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

Hydrogen cyanide can be added across olefins in the presence of Ni, Co, or Pd complexes (Scheme 56) (123). Conversion of butadiene to adiponitrile is a commercial process at DuPont Co. The reaction appears to occur via oxidative addition of hydrogen cyanide to a low-valence metal, olefin insertion to the metal-hydrogen bond, and reductive elimination of the nitrile product. The overall reaction proceeds with cis... 

The reaction mechanism commonly accepted to account for the double silylation of unsaturated substrates involves three key steps. First, the disli-lane undergoes oxidative addition to the metal center, forming a transition metal-bis(silyl) complex. The unsaturated moiety inserts into the metal-silyl bond, followed by Si-C reductive elimination to give the double sily-... 

Of the presently known reactions, production of the formate complex predominates. Catalysis of the hydrogen reduction of C02, which apparently involves insertion into a metal-hydrogen bond, is considered later. Here we consider the insertion reaction itself. 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

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