Tris- -lanthanoid complexes

A series of trivalent lanthanoid complexes, scandium, and ytterbium tris-(R)-(-)-1,1 -binaphthyl-2,2 -diyl phosphonate, have been introduced as new chiral and stable Lewis acids for the asymmetric hetero Diels-Alder reaction of Danishefsky s diene and aldehydes. 2,6-Lutidine was found to be an effective additive to improve the enantioselectivity up to 89% ee [66] (Eq. 8A.42). 

The first report of the use of enantiomerically pure lanthanoid complexes as chiral Lewis acids for Diels-Alder-type reactions appeared in 1983.90 As shown in Figure 37, in the presence of (+)-Eu(hfc)3(tris(3-(heptafluoropropylhydroxy-methylene)-(+)-camphorato)europium(III)), usually utilized as a chiral shift reagent for NMR study, the hetero-Diels-Alder reaction of benzaldehyde (54) with compound 167 has been found to give 168 in 50% ee. 

O6Q2H24, 1,4,7,10,13,16-Hexaoxacyclooc-tadecane, 25 126 lanthanoid complexes, 23 149 OjCoNsCgHu, Cobalt(III), tris(gIycinato)-fac- and mer-, 25 135 OgCoNsQHig, Cobalt(III), tris(alaminato)-fac- and mer-, 25 137 OJC0P2C11H23, Cobalt(I), (Ti -cyclopen-tadienyl)bis(trimethyl phosphite)-, 25 162... 

Monomeric tris-(r/2-pyrazolate)-lanthanoid complexes with the bulky 3,5-di(/-butyl)pyrazolate ligand, of general formula [M(r-Bu2pz)3(thf )2] (M = La, Nd, Gd. or Er), were obtained by reaction of Hg(C6F3)2 and 3.5-/-BupzH with an excess of lanthanoid metal in THF at room temperature. The X-ray crystal structure of the neodymium derivative was also reported (191). 

The introduction of substituted cyclopentadienyl ligands has only been successful in two instances. Thus, all lanthanoides form tris-(methylcyclopentadienyl)lanthanoide complexes, Ln(C5HI CH3)3, as well as niimerous adducts of them with Lewis bases (28,37 to 4o). Furthermore, lanthanum, praseodymium and neodymium form compoimds of the type Ln(C5Hif-i-C3H7) 3 (4l). With the more bulky ligands, such as the penta-methylcyclopentadienyl- or even the bis(trimethylsilyl)cyclopentadienyl ligand, only the disubstituted derivatives can be obtained. 

Novel Blnuclear Tris(cyclopentadlenyl) Lanthanoid Complexes of the General Type [Cp3Ln(y-X)Ln Cp3] and Their NMR Spectroscopic Properties... 

The tris(silylamide) complexes, [(Me3Si)2N]3Ln, are isolated from a pentane solution as needle crystals readily hydrolysed on air. They are well soluble in common organic solvents and posses a rather high volatility. These properties as well as the simplicity of their synthesis and the high reactivity of Ln-N bonds make REM amides one of most suitable starting material for the preparation of other organic and inorganic compounds of lanthanoids. The last method can be used as well for the preparation of mixed chloride/amides of lanthanoids [29]. 

Figure 4.27 (a) Ortep plot of one molecule and (b) stick bond model projection of Ce(OEP)2 [42]. (Reprinted with permission from J.W. Buchler, et ah, Metal complexes with tetrapyrrole ligands. 40. Cerium(IV) bis(octaethylporphyrinate) and dicerium(III) tris(octaethylporphyrinate) parents of a new family of lanthanoid double-decker and triple-decker molecules, Journal of the American Chemical Society, 108, no. 13, 3652-3659, 1986. 1986 American Chemical Society.)... 

The attempts to isolate in analytically pure state the products of these reactions have failed, apparently because of the partial complex formation of CpLi with tris(oligobutadienyl)lanthanoids. 

---