Electrophilic attack insertion reactions with carbon

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

The oxazoliumcarboxylic acid (147) is easily decarboxylated via the ylide (148) the neutral compound (149) is much more stable due to the low equilibrium concentration of the zwitterionic tautomer (150 Scheme 7). Oxazolium salts lacking substituents at the 2-position react with dialkyl acylphosphonates in the presence of triethylamine to give mixtures of l,4-oxazin-3-ones and 2-azetidinones the reaction (see Scheme 8) proceeds by electrophilic attack of the phosphonate on an oxazolium ylide, e.g. (151), followed by insertion of oxygen into the carbon-phosphorus bond, ring-opening, and formation of the enolate anion (152) which can cyclize in two alternative ways with expulsion of the phosphonate group. 

Electrophilic attack at carbyne complexes may ultimately place the electrophile on either the metal or the (former) carbyne carbon, the two possibilities being related in principle by a-elimination/migratory insertion processes (Figure 5.39). The reactions of the osmium carbyne complex are suggestive of an analogy with alkynes. Each of these reactions (hydro-halogenation, chlorination, chalcogen addition, metal complexation see below) have parallels in the chemistry of alkynes. 

Ab initio molecular orbital theory has been applied by Olah and coworkers to investigate the reactions of NO and the protonitrosonium ion HNO with methane. The reaction path was found to involve attack of NO on carbon instead of C-H bond insertion in accord with the studies of Schreiner et al. It was, however, pointed out that this is the consequence of the ambident electrophilic nature of NO and does not represent a general electrophilic reaction pathway for the reactions of methane. In fact, Schreiner and coworkers suggested that the electrophilic substitution of methane occurs by substitution of the nonbonded electron pair of methane instead of insertion of the electrophile into a C-H bond via 3c-2e bonding. Nonbonded electron pair formation in methane, however, can be considered only when methane would tend to flatten out (58) from its tetrahedral form, but this would be prohibitively energetic (>100 kcal mol ) and thus unlikely. 

For electrophilic attack, Markovnikov addition is that in which the positive portion of the reagent adds to the least substituted carbon atom of the double bond undergoing reaction.) This may result from a steric preference for the least-substituted metal alkyl intermediate formed by insertion of olefin into the metal hydride bond . Vinylarenes comprise an exception, where interaction of nickel with the aromatic ring stabilizes the precursor of the branched nitrile, leading primarily to a Markovnikov addition product . 

Reactivity modes of the poly(pyrazolyl)borate alkylidyne complexes follow a number of recognised routes for transition metal complexes containing metal-carbon triple bonds, including ligand substitution or redox reactions at the transition metal centre, insertion of a molecule into the metal-carbon triple bond, and electrophilic or nucleophilic attack at the alkylidyne carbon, C. Cationic alkylidyne complexes generally react with nucleophiles at the alkylidyne carbon, whereas neutral alkylidyne complexes can react at either the metal centre or the alkylidyne carbon. Substantive work has been devoted to neutral and cationic alkylidyne complexes bearing heteroatom substituents. Differences between the chemistry of the various Tp complexes have previously been rationalised largely on the basis of steric effects. 

Nitrenes, like carbenes, are immensely reactive and electrophilic, and the same Wolff-style migration (insertion into an adjacent C—C bond) takes place in which the substituent R migrates from carbon to the electron-deficient nitrogen atom of the nitrene. The product is an isocyanate. Isocyanates are unstable to hydrolysis attack by water on the carbonyl group gives a carbamlc acid, which decomposes to an amine. Alternatively, reaction with an alcohol gives a carbamate. If the alcohol is BnOH, the product is a Cbz-protected amine. 

As discussed in Chapter 3, olefins and dienes bind to electron-poor metal centers by a flow of electrons from the olefin iT-system to the metal and from the metal to the olefin t -system. Thus, olefins bound to electron-rich and strongly backbonding metal centers react with protons and electrophiles directly at the metal-carbon bond. However, olefins and dienes coordinated to electron-poor metal centers are less reactive toward electrophiles than those bound to electron-rich metal centers or even free olefins and dienes. However, electron-poor olefin and diene complexes do imdergo reactions with electrophiles at the coordinated ligand by an indirect pathway. This indirect pathway occurs by insertion of the olefin or diene into the bond formed by attack of the electrophile at the metal. 

The reactions of benzyne complexes of zirconium " also occur by electrophilic attack at an M-C bond. The isolated phosphine adduct of a zironocraie-benzyne complex reacts with ketones to imdergo insertion into one of the M-C bonds and with alcohol to make an aryl alkoxo complex, as shown in Equation 12.67. An electron-rich ruthenium-benzyne complex also reacts with electrophiles, such as borzaldehyde or carbon dioxide, to form products from insertion, as shown at the top of Equation 12.68. It also reacts with weak acids, such as aniline, to form products from formal protonation at the Ru-C bond, as shown at the bottom of Equation 12.68. - This reaction with aniline could occur by initial protonation at the metal, followed by C-H bond-forming reductive elimination, or by direct protonation of the M-C bond. Initial protonation of the metal center was proposed. 

It has been observed that this reaction proceeds with inversion at the alkyl carbon bonded to Fe, and the mechanism is believed to involve electrophilic attack of SOj at this C followed by rearrangement to the 0-bonded isomer. An analogous pathway has been found d for SOj insertion into W(CO)5(Y(CH3)3)", where Y= Si or Sn. 

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