Nucleophilic Attack on a Ligand

Hydroamination of alkenes can be catalyzed by lanthanide alkyls via insertion, as shown in Eq. 14.79.  

As we saw in Section 8.2. the binding of a polyene or polyenyl ligand to a metal can suppress the reactivity toward electrophiles usually seen for the free polyene and encourages attack by nucleophiles instead. This reversal of the normal reactivity pattern (umpolung) has been very widely used in organic synthesis. 

Of all the applications of nucleophilic attack, that on an allyl group coordinated to palladium is perhaps the one that has been most widely applied to oiganic synthesis. The allyl group is usually formed either ftom PdCl2 and an alkene 

The palladium selectively attacks an allylic acetate with inversion, even in the presence of other reactive groups, such as a C—Hal bond nucleophilic attack then occurs exclusively at the allyl group, showing the strongly activating effect of the metal (Eq. 14.81)  

The nucleophile usually attacks the exo face of the allyl group (the one opposite the metal), and at the least hindered terminus of the allyl group (although this preference can be reversed with suitable ligands). The stereochemical consequences of this sequence have been used to define the relative stereochemistries of two chiral centers five carbons apart in an acyclic system, during the synthesis of 

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It now appears that at least two mechanisms exist for the base-promoted homogeneous water gas shift reaction, differing in the method of hydride formation. The "associative mechanism", first proposed by Pettit and co-workers (1,4), involves nucleophilic attack on a carbonyl ligand and it has two variations. 

Nucleophilic attack on a rt-allyl ligand of a metal complex occurs in general at one of the terminal carbons to afford allylated products. The attack, however, may be directed to the central carbon atom of the 7i-allyl group to produce cyclopropyl derivatives by appropriate choice of nucleophile, metal ligand and reaction conditions (equation 33). A variety of nucleophiles (pA"a > 20) including ester and ketone enolates and a-sulfonyl carbanions react with... 

Identify reaction steps in Fig. 8.2 that illustrate the concepts of (a) the trans effect, (b) nucleophilic attack on a coordinated ligand, (c) insertion of alkene into an M-H bond, and (d) intramolecular electron transfer. 

Reactions involving nucleophilic attack on coordinated ligands are promoted by very polar solvents that stabilize anions. Metal carbonyls are also attacked by strong nucleophiles (RLi) to give the Fischer carbene complexes qv below. Section A 1.4),... 

The key synthetic routes for Fischer carbenes fall into one of three general categories, illustrated by equations (1-3). In equation (2) an acyl or similar species (often but not always formed by a nucleophilic attack on a CO or a similar ligand) is treated with an electrophile to give a Fischer carbene. In equation (3), an H (Fischer case) or an H+ (Schrock case) is abstracted from the a-position of an alkyl and in equation (5) a traditional carbene source is used. 

The synthetic limitations of metal-carbonyl hydrolysis arise from the requirement for initial nucleophilic attack on a carbonyl ligand. The susceptibility of carbonyl to such attack varies inversely with the extent to which it is serving as a tt acceptor and can be predicted from its IR carbonyl stretching frequency and force constant . Whereas some electron-poor cationic carbonyls can be attacked by H O, as in Eq. (j). 

Functionalization of a thiocarbonyl ligand occurs during the nucleophilic attack on a cyclopentadienyl iron complex by aziridine. 

Carbon-Carbon Bond Formation via Nucleophilic Attack on a n Ligand... 

The cation [(C5H5)Mo(CO)(NO)(allyl)] exists as an equilibrating mixture of isomers VII and VIII. Nucleophilic attack on a terminal carbon of cation VII would lead to the olefin complex IX if attack occurred onto the carbon trans to the NO ligand, or to complex XI if attack occurred onto the carbon trans to the CO ligand. Cation VIII, however, would give complex X by attack on the carbon atom trans to CO and complex XII by attack onto the carbon trans to NO. The olefin complexes IX and X rapidly equilibrate by rotation about the Mo-olefin bond as do the olefin complexes XI and XII. Nucleophilic attack by hydride or cyanide shows little regioselectivity, and both diastereoisomeric pairs IX X and XI XII are formed . 

Formation of Ni—CH3 species by the more likely biological route of Ni-based nucleophilic attack on a methyl electrophile has been demonstrated. The Ni complex, (40), supported by thioether and phosphine ligation, was reduced to the Ni adduct in situ. It was then oxidized to the Ni CH3 and Ni (C(0)CH3) species by reaction with CH3I and CH3C(0)C1, respectively. These low-spin, square-pyramidal complexes contain apical organic ligands and are related by CO insertion/extrusion, which was rapid even at -60 °C, precluding kinetic analysis. 

Nucleophilic attack on coordinated ligands is a widely encountered type of reaction. For example, carbonyl complexes are readily attacked by various nucleophiles, including OH , OR , NR3, NR, and CH. A well-known example is the base reaction of carbonyl complexes (Eq. 2-74). 

Electrophilic attack on a ligand is often observed for complexes of olefins and aromatie eompounds. The eleetrophihc or nucleophilic behavior of these tt ligands ean be piedieted on the basis of the a/tt bonding model. The olefin reacts not only as a a donor but also as a it acceptor. 

Nucleophilic attack of the aryl amine on the complex could lead to displacement of the metal with direct formation of the product or to formation of the intermediate carbamoyl ligand. The former mechanism is ruled out because it would produce the wrong product, a carbamate. The formation of the carbamoyl ligand by nucleophilic attack on a metal carbonyl or on a methoxycarbonyl cannot be dismissed by any of our data. Unfortunately the site of nucleophilic attack cannot be established by our kinetics or any of our direct evidence. In a separate study of the transesterification of Ru(dppe)(C0)2[C(0)CXIH3]2, the reactivity patterns could be best explained by invoking nucleophilic attack on a metal carbonyl.(18) It is reasonable to expect a similar mechanism of nucleophilic attack with the aryl amine. 

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