Nucleophilic Addition to Ligands

The site of nucleophilic attack is commonly one of the atoms that donates directly to the metal, because it is the most electrophilic. However, the LUMO of the overall complex will dictate the site of attack, where the atom that contributes most to the LUMO will be the preferred site of attack. 

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There is not much to say about the mechanism of nucleophilic addition to ligands. The nucleophile typically adds in the rate-determining step, and in some cases a leaving group... 

Nucleophilic Addition to Coordinated lsocyanide and Vinylidene Ligands... 

II = Nucleophilic addition to coordinated isocyanide and vinylidene ligands. 

The effect of metal basicity on the mode of reactivity of the metal-carbon bond in carbene complexes toward electrophilic and nucleophilic reagents was emphasized in Section II above. Reactivity studies of alkylidene ligands in d8 and d6 Ru, Os, and Ir complexes reinforce the notion that electrophilic additions to electron-rich compounds and nucleophilic additions to electron-deficient compounds are the expected patterns. Notable exceptions include addition of CO and CNR to the osmium methylene complex 47. These latter reactions can be interpreted in terms of non-innocent participation of the nitrosyl ligand. 

B. Akermark, B. Krakenberger, S. Hansson, Ligand Effects and Nucleophilic Addition to (T)3-Allyl)palladium Complexes. A Carbon-13 Nuclear Magnetic Resonance Study, Organometallics, 1987, 6, 620-628. 

B. Akermark, K. Zetterberg, S. Hansson, B. Rrakenber-gei The Mechanism of Nucleophilic Addition to t]3-Allylpalladium Complexes the Influence of Ligands on Rates and Regiochemistry, J. Organometallic Chem. 1987,335,133-142... 

Neutral (cyclohexadienyl)manganese complexes 71, generated by nucleophilic addition to (arene)Mn(CO)3+ cations 65, undergo ligand substitution with nitrosyl hexafluorophosphate to give the corresponding (cyclohexadienyl)Mn(CO)2NO+ cations 72 (Scheme 17)93. Attack by a wide variety of nucleophiles on cations 72... 

Nucleophilic addition to acyclic (diene)Mo+ cations has been examined. For (isoprene) Mo(CO)2L (L = Cp, Cp, In), the regioselectivity for nucleophilic attack has been found to depend on the nature of the nucleophile, the ligand L, the reaction solvent and the temperature21,813 833 193. The generation and in situ reactivity of transoid acyclic (diene)molybdenum and tungsten cations with nucleophiles has been previously mentioned (Section IV.C.2). 

A formal asymmetric nucleophilic addition to carbonyl compounds is achieved by Trost and his co-workers in the allylic alkylation of acylals of alkenals. An excellent enantioselectivity is observed in this alkylation. The starting acylals are easily prepared by the Lewis-acid catalyzed addition of acid anhydrides to aldehydes, by use of Trost s ligand 118 (Scheme 13), where various carbon-centered nucleophiles are available (Scheme l4),101,101a-10lc Asymmetric synthesis of some natural products is achieved according to this procedure. 

Until then, only heterogeneous catalyst had been successful. However, in the mid-1980s, the work of Ito et al. led to an outstanding discovery in a catalytic asymmetric aldol reaction. In this case, enantioselectivity was given by a chiral ferrocene diphosphine ligand, with a carbon nucleophile addition to a carbonyl... 

It was found that the 15-membered macrocyclic complexes were significantly less acidic. Similar reversible protonations have been shown to occur in related macrocyclic complexes36 37 and this work has been developed by Busch into a major study of ligand reactions,38 which are summarized in equations (16)—(19). It is significant that these reactions include not only typical substitution reactions such as acylation and nitration (equations 16 and 19), but also nucleophilic addition to isocyanates (equation 17) and to a, 3-unsaturated esters (equation 18). 

Nucleophile addition to styrene derivatives (e.g. 75) coordinated with Cr(CO)3 is another example of addition-electrophile trapping.23,128 Addition of reactive anions is selective at the 3-position of the styrene ligand, leading to the stabilized benzylic anion (76). The intermediate reacts with protons and a variety of carbon electrophiles to give substituted alkylbenzene ligands (in 77) (equation 52). 

Exo addition is always observed. When the 6-exo substituent is very bulky, as in complex (132 equation 50), nucleophile addition to the dienyl ligand is completely suppressed. Instead, decomplexation is observed giving the dienone (133), possibly via attack of nucleophile at the metal (i.e. ligand exchange).42 Endo addition is not observed. 

Nucleophilic addition to these complexes is a little less predictable than for the dienyliron systems. For example, the nucleophiles studied add to C-3 of the acyclic dienyl ligand in (224), but to C-l of the cyclic system (225). Alkyllithiums add both to the cyclohexadienyl ligand, e.g. (227) gives (229), and to the cyclopentadienyl ligand, followed by interligand hydrogen transfer to give, e.g. (228). However, the fact that a hindered, very basic alkyllithium adds to a sterically hindered dienyl terminal carbon and does... 

The weakness of most metal-metal bonds compared with metal-ligand bonds makes cleavage of metal-metal bonds by nucleophiles a common process (15). In the case of metal-metal double bonds this corresponds to nucleophilic addition to the metal-metal bonded systems. Since unsaturated clusters exist which can be considered to contain metal - metal double bonds, this should be an important aspect of substrate activation by clusters. 

In contrast, those cluster compounds with an 18-electron environment at the central heterometal do not undergo nucleophilic addition reactions. Ligand substitution reactions at both the central heterometal and the peripheral gold atoms are observed to occur for both 16- and 18-electron cluster compounds, as in the reactions shown below ... 

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