Electron metal complexes

The first class of amine-based nucleophilic catalysts to give acceptable levels of selectivity in the KR of aryl alkyl. yec-alcohols was a series of planar chiral pyrrole derivatives 13 and 14, initially disclosed by Fu in 1996 [25, 26]. Fu and co-workers had set out to develop a class of robust and tuneable catalysts that could be used for the acylative KR of various classes of. yec-alcohols. Planar-chiral azaferrocenes 13 and 14 seemed to meet their criteria. These catalysts feature of a reasonably nucleophilic nitrogen and constitute 18-electron metal complexes which are highly stable [54-58]. Moreover, by modifying the substitution pattern on the heteroaromatic ring, the steric demand and hence potentially the selectivity of these catalysts could be modulated. 

In addition to this Lewis-acid behavior, 16- and 18-electron metal complexes can act as Lewis bases, i.e., they possess accessible electron pairs. This Lewis basic character depends strongly on the donor and acceptor strengths of the ligands and is very pronounced in complexes of strong donors such as trialkylphosphines. For example, whereas CpCo(CO)2 shows little basic character and little tendency to react with electrophiles such as CH3I, CpCo(PMe3)2 is a strong metallic base .10 Such compounds are particularly reactive towards oxidative addition reactions ... 

Substitution reactions of 18-electron metal complexes may be associative if a pair of electrons can be delocalized from the metal to a ligand (NO, cyclopentadienyl, etc.), making available a vacant low-energy orbital for nucleophilic attack on the metal. 

Clearly, the first step in the carbonylation of allylic and benzylic derivatives to 3,4-unsaturated carboxylie acid derivatives and arylacetic acid derivatives, respectively, requires activation of a C-X bond at an sp -hybiidized carbon atom. Such activation could proceed either via nueleophilic attack with anionic, 18-electron metal complexes, whieh has been described, e. g., with cobalt [2]... 

Addition of an alkene to 16-electron metal complexes is exemplified by reaction 23.72 ethene readily dissociates from Ir(CO)Cl(q -C2H4)(PPh3)2, but the related complex Ir(CO)Cl(q -C2(CN)4)(PPh3)2 is very stable. 

Similar reactions are also known for other coordinated molecules. The majority of isomerization reactions involving unsaturated hydrocarbons are catalyzed by -electron metal complexes. In the transition state, hydrido complexes are usually formed ... 

Metalaracficafs A very recent approach to CO activation by Wayland" is the use of odd-electron metal complexes, such as the 17e species [Rh(TMP)j (where TMP = tetramesitylporphyrin). This reacts with CO to give [(TMP)Rh(n.-CO)Rh(TMP)] and [(TMP)Rh(M,-CO-CO)Rh(TMP)], presumably via an intermediate [(TMP)Rh(CO)] that behaves like an acyl radical (R—C O) and either dimerizes or combines with the starting metalaradical [Rh(TMP)J. 

Oxidative addition reactions usually involve a coordinatively unsaturated 16-electron metal complex or a five-coordinate 18-electron species, and take the following general form ... 

The two generally recognized routes by which an organometallic complex can catalyze the hydrogenation of alkenes are referred to as the olefin or unsaturated route and the hydride route, as shown in Scheme 5.26. Both pathways start from a coordinatively unsaturated (16-electron) metal complex, M(L)4, which might be formed by ligand dissociation, as shown. The two routes differ in the first step which is olefin complexation or oxidative addition of Hj. Both routes lead to the key dihydride-olefin species in the center of the Scheme. 

This new organometallic process should allow the synthesis of other catalysts via activation of alkyl propargyletiier by 16-electron metal complexes. 

These lightly stabilized complexes easily undergo substitution processes in which the weakly coordinated ligand is displaced by other uncharged 18-electron metal complexes. This kind of reaction will be referred to in the next section in discussing displacement reactions. 

Reduction of TpW(NO)(PMe3)(Br) (Tp=hydrido-tris(pyrazolyl)borate) with sodium in the presence of thiophene yielded a mixture of tungsten-containing complexes, exhibiting distinct chemical properties, which could be partially hydrogenated to dihydrothiophene complexes with combined yields of 22% in a 1 1 ratio of two diastereoisomers (Scheme 130). The coordinate binding fashion is less common for 16-electron metal complex than the or S-bound form [144]. 

Likewise, monoelectronic oxidation of a neutral 18-electron metal complex to a 17-electron cationic complex before insertion also considerably facilitates this reaction. Whereas the 18-electron complex p eCp(CO)2Me] only very slowly undergoes CO insertion into the Fe-CH3 bond at ambient temperature, the 17-electron cation rapidly gives the acetylated complex in MeCN at -78°C, the solvent playing the role of the nucleophile that coordinates to the iron center. ... 

Bis-7r-2-methyl allyl nickel reversibly forms an adduct, 2.8a, with tri-ethylphosphine [31]. The related 14-electron metal complex w-CsHsNiMe has been reported it decomposes at — 35 C [56]. The catalytic properties of bis-TT-allyl nickel are discussed in Chapter 9. 

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