Half-sandwich alkyl complexes

Scheme 48 A direct, enantioselective alkylation of pyridines by a cationic half-sandwich scandium complex.

In recent years, catalytic applications of half-sandwich metal complexes have been extensively developed. A series of (p-cymene)Ru(II) complexes containing pyrazole-based iV-heterocyclic carbine (pyrazoUn-3-ylidene) ligands (Fig. 21.13a and b) showed excellent catalytic y3-alkylation of secondary alcohols with primary alcohols and the dimerization of phenylacetylene [30]. 

K. C. Hultzsch, T. P. Spaniol, J. Okuda, Half-sandwich alkyl and hydrido complexes of yttrium convenient synthesis and polymerization catalysis of polar monomers. Angew. Chem. Int. Ed. 38, 1999 227. 

In contrast to the chemistry of the zinc complexes [TpRR]ZnR, the cadmium alkyl derivatives have not been used to prepare an extensive series of half-sandwich [TpRR]CdX complexes. However, several [Tp JCdX complexes supported by sterically demanding tris(pyrazolyl)hydroborato ligands, e.g., [TpBut]CdCl (98), [TpBut]CdI (91), [Tp lCdl (91), and [TpBut,Me]Cd(Tj2-02N0) (91), have been synthesized by metathesis between CdX2 and either K[TpRR ] or Tl[TpRR ] [Eq. (15)]. 

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

Bis(alkyl) complexes, with mercury, preparation, 2, 428 Bis(alkylidene)s, in Ru and Os half-sandwiches, 6, 583 Bis(alkylimido) complexes, with chromium(VI), 5, 346 Bis(rj2-alkyne)platinum(0) complexes, preparation, 8, 640 Bis(alkynyl) complexes in [5+2+l + l]-cycloadditions, 10, 643 with manganese, 5, 819 with mercury, preparation, 2, 426 mononuclear Ru and Os compounds, 6, 409 with platinum, 12, 125 with platinum(II), 8, 539 with titanium(IV), 4, 643 with zirconium, 4, 722... 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Mononuclear osmium half-sandwiches, with rf-cyclopentadienyls and 7]5-indenyls alkenyls and allenyls with t/-M-C bonds, 6, 558 alkenyl vinylidenes, 6, 593 alkyl, aryl, acyl complexes, 6, 552 with alkylidyne complexes, 6, 599 alkynyl and enynyl complexes, 6, 567 allenylidene and cumulenylidene complexes,... 

Cationic sandwich complexes of the type CpCo(arene) + were first prepared by hydride abstraction from cyclohexadi-enyl cations (Section 7.1). They are accessible in broader variation from the reaction of CpCoX half-sandwich complexes with arene in the presence of AICI3. Their electrochemical reductions to the corresponding 19-electron monocations and to 20-electron neutral complexes have been studied. The stability of electron-rich sandwich complexes increases with increasing alkyl substitution in either ring despite the more negative redox potential mass spectrometry studies of bond dissociation energies of (arene)Co+ complexes corroborate these results. However, neutral sandwich complexes are not very stable in the polar solvents necessary for the reduction of mono- or dications and have been isolated only from alkyne trimerization with CpCo precursors in nonpolar solvents (Section 5.1.4). 

A few half-sandwich complexes containing a CpCo unit are known some of them are of synthetic utility and will be discussed below. The [CpCo(CO)]2 anion (19 ) can be methylated with Mel to give a dimethyl derivative, [CpCoMe(CO)]2. This complex decomposes to produce acetone in high yield, a reaction that is reminiscent of the use of Fe(CO)4H in the carbonylation see Carbonylation) of alkyl halides. 

Half-sandwich lanthanide alkyl complexes and, subsequently oranolanthanide amides were found to be highly efficient catalysts for the cross-coupling reactions of carbodiimides with alkynes and amines, respectively [136, 137]. Although the half-sandwich lanthanide alkyl complexes can also catalyze the dimerization of alkynes, no homodimerization product is observed in the reaction of alkynes with carbodiimides. These reactions offer a wide scope for the substrates of terminal alkynes and amines, respectively [138]. 

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