Diene-metal complexes, protonation

The protonation of neutral diene metal complexes giving jr-enyl metal cations is another preparative route, closely related to those above (see p. 66). 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

Similar bridging hydrogen exchange processes have been obsa-ved for neutral and cationic (i -aUyl-/i-H)MLn complexes 25 (Scheme 3)23,58,63 any of these complexes are prepared by the protonation of the corresponding anionic or neutral (i -diene)MLn complexes 26 (see Section V.C.l). Migration of the metal about a cychc i -aUyl ligand (i.e. 25a to 25b) is proposed to involve an ( -diene)MLn hydride intemediate/transition state 27. 

An important pathway to formation of A3-allyl complexes is by protonation of (tetrahapto-1,3-butadiene)metal species. This may occur in a variety of ways (1) By addition of an acid such as HC1 to a butadiene complex30 (reaction 23-6) (2) by reversible addition of a proton only31 (reaction 23-7) or (3) by reaction of a diene with a metal complex containing a proton ligand32 (reaction 23-8). The weight of evidence suggests that the proton... 

The protonation of the Ti -cyclohexa-1,3-diene complex in Equation 12.64 illustrates a mechanism for the reaction of an electrophile at a coordinated diene via initial protonation of the metal and shows the stereochemistry that results from this pathway. The initially formed cationic hydride complex transfers the hydride onto the endo face of the cyclo-hexadiene ring by insertion of the C=C double bond into the Rh-H bond. The resulting Rh-C single bond is then part of the ligand in the product of the migration. Migra-... 

Reviews.—Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds, transition-metal complexes of olefins and acetylenes, transition-metal-catalysed homogeneous olefin disproportionation, rhodium(i)-catalysed isomerization of linear butenes, catalytic olefin disproportionation, the syn and anti steric course in bi-molecular olefin-forming eliminations, isotope-elfect studies of elimination reactions, chloro-olefinannelation, Friedel-Crafts acylation of alkenes, diene synthesis by boronate fragmentation, reaction of electron-rich olefins with proton-active compounds, stereoselectivity of carbene intermediates in cycloaddition to olefins, hydrocarbon separations using silver(i) systems, oxidation of olefins with mercuric salts, olefin oxidation and related reactions with Group VIII noble-metal compounds, epoxidation of olefins... 

A number of metal complexes of cyclo-octa-1,5-diene are reported from cobalt, palladium, rhodium, iridium, and copper(i) systems. Bis(cyclo-octa-l,5-diene)nickel has been used to couple alkenyl halides. An X-ray study of (356), obtained by proton abstraction from tricarbonylcyclo-octa-l,5-diene-... 

N3-coordinated complexes containing platinum group metal ions have also been synthesized and studied (56,60,61). Steric hindrance has been used to direct binding to N3 in a series of Pd- and Pt-containing complexes of 6,6,9-trimethyladenine (60). Platinum modification was found to have a pronounced effect on the basicity of the adenine moiety. The protonation constants (log Kh values) for the twofold protonation of 6,6,9-trimethyladenine are 4.15 and -0.75, with the initial protonation occurring at N1 followed by N7. The equivalent values for the formation of [Pt(dien)TMA-A3)H]3+ and [Pt(dien)TMA-2V3)H2]4+ are 0.3 and —1.2, respectively. Moreover, the site of initial protonation was found to be N7 (Fig. 19). These observations are supported by theoretical studies (62). 

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