Alkanediyl complexes

Fit . 8. NMR diagnosis of M-methylene complexes. The diagram indicates the chemical shift ranges (8CH, above 8CH, below) found for 95% of all metalcarbenes (type A), M-methylene complexes (type C), metal alkyls belonging to the metal carbonyl series (see text). 

ONE STEP BEYOND THE METHYLENE BRIDGE l (co,co )-ALKANEDIYL COMPLEXES... 

Complexes of type [LyM (CH2)IIX ], where n > 1, have been shown to be useful precursors for hetero- and homobimetallic n(a.,a>) alkanediyl complexes, [LjcM(CH2)bM L, ] (where ML, is not necessarily the same as M Lj,). Such hydrocarbon-bridged binuclear compounds have been proposed as models for intermediates in the Fischer-Tropsch reaction (18,19) and other significant catalytic processes (20-23). Some [LyM (CH2)BX ] complexes are precursors to cyclic carbene complexes (Section III), whereas others have been shown to have synthetic utility in organic chemistry (24). 

Reaction of the heterodinuclear alkanediyl complexes CpM (CH2)3pp (CO)3 (M = Mo, W) with R13C+ results in hydride abstraction to form the iron alkene complexes (CpFe Ti -CH2=CHCH2CH2M(CO)3Cp ]+. Reaction of CpMo(CO)3(CH2)4Mo(CO)3Cp with PPh3 affords the acyl derivatives CpMo(CO)3 C(=0)(CH2)4 Mo(CO)3Cp and... 

Similar reactions with a,(o-dihaloalkanes yield binuclear alkanediyl complexes, Eq. 5.3 ... 

The alkanediyl complex where n = 2 has been characterized by X-ray crystallography/ Figure 5.1 ... 

More recently, a similar approach was taken by Macartney and coworkers to synthesize [2]rotaxanes (Figure 4). Rotaxane 2 is self-assembled efficiently in solution from a-CD, [Fe(CN)5(H20)] and l,l -( f5 -alkanediyl)bis(4,4 -pyridyl-pyridium) ion. The [2]rotaxane is also formed by the addition of a-CD to a solution of the dumbbell component [(CN)5Fe(bpy(CH2)nbpy)Fe(CN)5]. This result infers a slow dissociation of the terminal metal complex, followed by formation of an intermediate pseudorotaxane and reassociation of the metal complex. 

More recently, we have developed an alternative chemoenzymatic route to the preparation of a variety of amino acid-based surfactants ranging from simple monoesters to more complex gemini amphiphiles [79], This approach was based on the observation that some lipases, notable immobilized Candida antartica and Rhizomucor miehei, readily accept N-Cbz amino acids as substrates and catalyze their esterification/amidation with a,(o-diols/a,to-diamines in excellent yields. The resulting alkanediyl-a,co-bis-(A -Cbz-amino acid) was further modified to obtain a range of amino acid-based gemini surfactants. Some examples of the products obtained in this work are shown in Fig. 8. 

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