Cationic metal carbonyls alkylation

The alkylation of an unsaturated atom or moiety bonded to a metal carbonyl produces an onium salt. This complex may be considered to be a further example of a cationic metal carbonyl, provided that the formal charge on the alkylated center can interact with the atomic orbitals of the metal. This may be justified on the basis of electronegativity differences between the metal atom and the alkylated center. It seems reasonable to argue that the location of the formal positive charge is on the electropositive metal atom, although the metal is probably only fractionally charged. 

Carbamoyl-metal complexes are prepared by the reaction between a cationic metal-carbonyl complex and NHj, or a primary or secondary alkyl (not aryl) amine ... 

Catalytic Reactions. As the techniques for solid-state n.m.r. continue to improve with the simultaneous improvement in sensitivity and hence speed, there will be a growing trend to look at chemical reactions occurring on or in catalysts. There have already been a number of instances where catalytically stimulated reactions have been studied by 13C n.m.r. - the alkylation of toluene by methanol on X zeolite, for example,138 in which the influence of the cation, Na+ or Cs+, on selectivity was deduced. The adsorption binding and decomposition of various metal carbonyls on A1203 or in zeolites has been studied,139 likewise, the nature and sites of interaction of CO and C02 on X and Y zeolites.140... 

The electron-transfer reactions of metal-carbonyl anions have been reviewed. Metal-carbonyl anions exhibit one-and two-electron reactions. The two-electron processes involving transfer of groups such as hydrogen, alkyl, and halogen between metal centers are related to the nucleophilicity of the anion involved. The one-electron processes are primarily outer-sphere electron transfers. However, in contrast to organic reactions, the metal-carbonyl anions can also undergo inner-sphere electron transfers. This is usually the case when an anion of low nucleophilicity transfers an electron to a metal-carbonyl cation or halide. 

Methods to generate the unsaturated intermediate that adds alkane include photolysis of metal-carbonyl or -dihydride complexes, thermolysis of alkyl hydride complexes, and abstraction of halide with a cationic reagent or protonation of an alkyl group with an acid containing a weakly coordinating anion. For example, the photolysis of Cp Ir(CO)j or Cp IrfPMejJHj generates Cp Ir(L) (L = CO or and photolysis of the... 

Fig. 5 Highly ordered transition state for the reaction between a metal carbonyl complex and alkyl halides in the presence of alkali metal cations...

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

Still another possibility in the base-catalyzed reactions of carbonyl compounds is alkylation or similar reaction at the oxygen atom. This is the predominant reaction of phenoxide ion, of course, but for enolates with less resonance stabilization it is exceptional and requires special conditions. Even phenolates react at carbon when the reagent is carbon dioxide, but this may be due merely to the instability of the alternative carbonic half ester. The association of enolate ions with a proton is evidently not very different from the association with metallic cations. Although the equilibrium mixture is about 92 % ketone, the sodium derivative of acetoacetic ester reacts with acetic acid in cold petroleum ether to give the enol. The Perkin ring closure reaction, which depends on C-alkylation, gives the alternative O-alkylation only when it is applied to the synthesis of a four membered ring ... 

Among isolable metal homoenolates only zinc homoenolates cyclize to cyclo-propanes under suitable conditions. Whereas acylation of zinc alkyls makes a straightforward ketone synthesis [32], that of a zinc homoenolate is more complex. Treatment of a purified zinc homoenolate in CDC13 with acid chloride at room temperature gives O-acylation product, instead of the expected 4-keto ester, as the single product (Eq. (22) [33]). The reaction probably proceeds by initial electrophilic attack of acyl cation on the carbonyl oxygen. A C-acylation leading to a 4-keto ester can, however, be accomplished in a polar solvent Eq. (44)-... 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

The isocyanide analogues of the above carbonyls, frans-[Mo(CNR)2(dppe)2] (R = alkyl or aryl) undergo a reversible one-electron oxidation in solution4,5 and oxidation with I2 or Ag1 salts allows isolation of the paramagnetic cation [Mo(CNC6H4Me)2(dppe)2]+ = 2.01 BM). With other isocyanides, substitution at the metal centre occurs on further oxidation.48... 

Organometallic crown ethers have also been synthesized.96-98 Recently, a crown-cation group was shown to interact with an appended transition metal acyl ligand (29)." Complexes of this type have potential applications in Lewis acid-accelerated alkyl migration to coordinated carbonyls. 

When the cation remains coordinated to the nucleophile, the reaction is under association control. Association-controlled reactions are usually slow, because the substrate is not activated and the nucleophile is deactivated. This general class can be subdivided into two groups. In the first (which occurs in the C-alkylation of enolates), the metal is not directly bound to the reactive site. If the transition state is acyclic, conjugate additions will dominate because there is no electrophilic assistance. If it is cyclic, chelation favors addition to the carbonyl ... 

C(O)CHCH) and 27.5 Hz (C(O)CHCH) shifts in the resonances for the matallacyclic ring H resonances can be observed upon the addition of 0.1 M Li[CF3S03] to a THF-d8 solution of 2c. Similarly, alkyl cations and H+ are known to attack the O atom of the metallacyclic ring carbonyl of 2b and 2c, and the product formed from the methylation of 2b has been crystallographically characterized (73). Furthermore, alkali cations, through an interaction similar to that proposed in equation (2), are known to inhibit the decarbonylation of transition metal acyl complexes (26). 

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