Electrophilic metallation mechanism

By synthesizing bromoalkyl-sulfur and -oxygen based heterocycles, Lautens was able to extend the strategy to include the synthesis of polycyclic thiophenes and furans [83, 84], Under similar reaction conditions to those used for nitrogen-based heterocycles, polycyclic products were obtained in good to excellent yields (Scheme 35). The electronic nature of the aryl iodide had a large impact on the observed yields, as electron-deficient aryl iodides worked well, while little to no product was obtained with electron-rich ones. Based upon these results, Lautens proposed that the mechanism of direct arylation of thiophenes occurs through an electrophilic metalation mechanism. 

The stability to oxidative degradation of palladium/NHC complexes has made possible the synthesis of complexes previously unknown or much less explored. (NHC)Pd(K-OAc)(OAc) shows a unique coordination of the acetate units that stabilizes a distorted square planar geometry around the metal center. These complexes are stable in acidic media and can activate C-H bonds of simple arenes by an electrophilic metalation mechanism (Scheme 32). 

The first step of both mechanisms is the same, namely the addition of pyridine at the electrophilic metal atom of the triorganotin halide to give a pentacoordinate adduct. 

It was found in the case of O-benzyl systems that palladium oxide is much more effective than palladium metal. No such effect was observed with the N-benzyl system.8 It is possible that the N-compounds can poison the electrophile metal ions, and the hydrogenolysis of the N-benzyl bond can take place only by the hydrogenolytic cleavage instead of the insertion mechanism. This is supported by the experimental finding that the product amine can inhibit the catalyst, and this can be minimized by buffering at a pH less than 4. 

Many different types of 1,3-dipoles have been described [Ij however, those most commonly formed using transition metal catalysis are the carbonyl ylides and associated mesoionic species such as isomiinchnones. Additional examples include the thiocar-bonyl, azomethine, oxonium, ammonium, and nitrile ylides, which have also been generated using rhodium(II) catalysis [8]. The mechanism of dipole formation most often involves the interaction of an electrophilic metal carbenoid with a heteroatom lone pair. In some cases, however, dipoles can be generated via the rearrangement of a reactive species, such as another dipole [40], or the thermolysis of a three-membered het-erocycHc ring [41]. 

Consequently, we were faced with the task of formulating a widely acceptable and consistent definition of bond activation . Our research, discussions, and analyses led to a conclusion that bond activation should refer to a process of increasing the reactivity of a bond in question and as such encompasses an entire spectrum of possible mechanisms. Also, we argue that activation is not equivalent to reaction or, in other words, that activation of a bond is not the same as cleavage of a bond. For the latter process we proposed the general term bond transformation . It should be emphasized that both bond activation and bond transformation are general terms and, therefore, information about the reaction and mechanism category should be specified by additional descriptors (cf. C-H bond arylation via electrophilic metalation, C-H bond metalation via concerted metal insertion). 

Whereas uncatalyzed substitution reactions of organozinc compounds are limited to very reactive electrophiles, metal-transmetallated organozinc compounds are able to perform substitution reactions on various electrophiles. In the case of conjugated electrophiles, these zinc copper reagents can follow a Sn2 or Sn2 mechanism. 

The importance of the electrophilic character of the cation in organo-alkali compounds has been discussed by Morton (793,194) for a variety of reactions. Roha (195) reviewed the polymerization of diolefins with emphasis on the electrophilic metal component of the catalyst. In essence, this review willattempt to treat coordination polymerization with a wide variety of organometallic catalysts in a similar manner irrespective of the initiation and propagation mechanisms. The discussion will be restricted to the polymerization of olefins, vinyl monomers and diolefins, although it is evident that coordinated anionic and cationic mechanisms apply equally well to alkyl metal catalyzed polymerizations of polar monomers such as aldehydes and ketones. 

You met borohydride in Chapter 6, where we discussed the mechanism of its reactions. Sodium borohydride will reduce only in protic solvents (usually ethanol, methanol, or water) or in the presence of electrophilic metal cations such as Li+ or Mg2+ (LiBH4 can be used in THF, for example). The precise mechanism, surprisingly, is still unclear, but follows a course something like this with the dotted lines representing some association, perhaps coordination or bond formation. 

The addition of organic halides to electrophilic metal centers can follow several different mechanisms. For a concerted m-addition mechanism, a 3-center transition state of low polarity (21-XI) may be expected. Other possibilities include an SN2 type mechanism, involving either inversion (structure 21-XIIa) or retention (structure 21-XIIb) at the C atom, followed by dissociation of X" to give an ionic intermediate ... 

The cleavage of the C—H bond by direct participation of a transition metal ion proceeds via an oxidative addition mechanism or an electrophilic substitution mechanism. Metals in low oxidation states undergo oxidative addition while high oxidation state metals take part in electrophilic substitutions. Another function of the metal complex in these reactions consists of abstracting an electron or a hydrogen atom from the hydrocarbon, RH. The RH radical ions or R radicals which are formed then interact with other species, such as molecular oxygen which is present in the solution or in one of the ligands of the metal complex (21). 

The cyclopropanation is initiated by the interaction of the electrophilic metal-carbene species with the jr-system of the olefin (Scheme 4). Two different mechanisms have been proposed for the formation of the cyclopropane ring a concerted pathway (a) or a two-step process via a metallacyclobutane (b). The first pathway (a) resembles the mode of addition of free carbenes to (C=C) double bonds [33] and has been proposed for reactions of metal carbenoids by various authors [7,11]. The principal bonding interaction in this case initially develops between the electrophilic carbenoid C-atom and the Ti-system of the C-C double... 

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