Metallacycles coupling reactions

Carbonylation is an exceedingly broad subject, but the main reaction patterns can be easily rationalized by recalling the classification used earlier for coupling reactions involving (a) metallacycles (b) hydride-promoted reactions and (c) oxidative addition of organic halides to zero-valent nickel. In fact, one or other of these steps is necessary to form a species able to undergo carbonylation. 

Louie J, Hartwig JF (1996) A route to PdO from Pdll metallacycles in amination and cross-coupling chemistry. Angew Chem Int Ed Engl 35 2359-2361 Mucalo MR, Coouey RP (1989) F.T.I.R. spectra of carbon monoxide adsorbed on platinum sols. J Chem Soc Chem Commun 94—95 Munoz MP, Martin-Matute B, Fernandez-Rivas C, Cardenas DJ, Echavarren AM (2001) Palladacycles as precatalysts in Heck and cross-coupling reactions. Adv Synth Catal 343 338-342... 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

Reactions other than Lewis acid-base associations/dissociations are frequently observed wit donor molecules, leading notably to solvolysis, oxygen or sulfur abstraction, insertion reaction and carbon-carbon coupling reactions. The tendency to form metal-element multiple bonds i remarkable in this respect the activation of dinitrogen by tantalum or niobium is unique. Th formation and chemistry of constrained reactive metallacycles open another promisin fast-developing area, on the frontier with organometallic chemistry. 

Matrix 3. Evaluation matrix for the oxidative coupling reaction affording a five-membered metallacycle. For explanation, see Matrix 1. 

The (butadiene)zirconocene template coupling reaction has found application in organic synthesis. Coupling of [(butadiene)ZrCp2] with [W(CO)6] followed by the reaction of the steroid ketone 3-methoxy-l,3,5(10)-trien-17-one (108) gave the corresponding metallacyclic... 

Metallacycles have been claimed to play pivotal roles in many transition metal-mediated multi-component coupling reactions [1]. For example, [2 -i- 2 -i- 2] alkyne cyclo-trimerization leading to benzenes - the Reppe reaction - has been considered to proceed via metallacyclopentadiene and elusive metallacycloheptatriene intermediates ("common mechanism ), while metallacyclopentenes have been proposed as intermediates for the [2 -i- 2 -i- 1] cyclo-coupling reactions of an alkyne, an alkene, and CO leading to a cyclopentenone (the Pauson-Khand reaction). A metallacyclic compound - which is defined here as a carbocyclic system with one atom replaced by a transition metal element - can be generally formed by oxidative cyclization of two unsaturated molecules with a low-valent transition metal fragment [2-4]. Alter-... 

Scheme 30. Heterobimetallic carbon-carbon coupling reactions that yield metallacyclic, head-to-head 1,3-diyne products.

Ni(0) complex are formed upon heating this complex. Hoberg and co-workers fiirther studied the C-C coupling reactions of CO2 with alkynes, alkenes (including cycloalkenes), and 1,2- or 1,3-dienes (4-10). Unfortunately, most of these reactions produce stable five-membered metallacycle complexes and the catalytic reactions, involving insertion of activated allies (or other reagents) into the five-membered metallacycle followed by reductive elimination, have not been realized. 

Besides the reversible processes just mentioned, yS-SiRs ehmination from a silyl substituted metal alkyl or metallacycle is a well-documented process [185,186]. This reaction accounts for an easy loss of a silyl group that can be used to generate a M-S1R3 moiety in catalytic processes such as dehydrogenative silation reactions (see below). This reaction, and the analogous yS-SnRs elimination, may also be involved in the loss of regioselectivity found for some C-C coupling reactions of vinyl silanes or vinyl tin derivatives (cine substitution in Hiyama and Stille... 

Demerseman and coworkers studied recently the coupling reaction between an unsaturated alkynetitanium(II) complex and carbon dioxide [84]. The titanium-diphenylacetylene complex (n -C5H5)(n -C5Me5)Ti-(PhC=CPh) adds readily CO2 to give a five-membered metallacyclic compound via carbon-carbon bond formation (Equation 14). This reaction occurs smoothly in hexane under atmospheric pressure and is supported by the well known tendency of titanium to form stable Ti-0 bonds. 

In ethylene oligomerization, oxidative addition plays no role. In the insertion-elimination mechanism, the metal oxidation state is constant throughout the catalytic cycle. In the metallacycle mechanism, the oxidative step is an oxidative coupling reaction (see below). 

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