Samarium , direct bonding

The fourth chapter gives a comprehensive review about catalyzed hydroamina-tions of carbon carbon multiple bond systems from the beginning of this century to the state-of-the-art today. As was mentioned above, the direct - and whenever possible stereoselective - addition of amines to unsaturated hydrocarbons is one of the shortest routes to produce (chiral) amines. Provided that a catalyst of sufficient activity and stabihty can be found, this heterofunctionalization reaction could compete with classical substitution chemistry and is of high industrial interest. As the authors J. J. Bmnet and D. Neibecker show in their contribution, almost any transition metal salt has been subjected to this reaction and numerous reaction conditions were tested. However, although considerable progress has been made and enantios-electivites of 95% could be reached, all catalytic systems known to date suffer from low activity (TOP < 500 h ) or/and low stability. The most effective systems are represented by some iridium phosphine or cyclopentadienyl samarium complexes. 

Samarium iodide selectively cleaves trifluoracetylated hydrazines. Since direct cleavage of the N-N bond in 30 did not work with samarium iodine, it was necessary to trifluoracetylate the carbamate first. 

The amino derivatives of rare-earth metals follow a distinct mechanism involving direct insertion of the allene or alkene into the metal-nitrogen bond (Scheme 6.73). With the less-reactive lanthanum complex 6.215, amine 6.214 underwent insertion of only the allene, giving a pyrrolidine product 6.216 as the trans isomer. On the other hand, using the more-reactive samarium complex 6.217, tandem allene and alkene insertion occurred giving, after hydrogenation, the pyrrolizidine alkaloid, xenovenine 6.189, A closely related synthesis of this alkaloid is in Scheme 6.59. Other syntheses may be found in Scheme 6.65, and Scheme 9.46. 

Stilbene is an unusual arene as a ligand in that it offers the possibility of coordination to an arene ring or to the double bond. In spite of the abundance of rare earth naphthalene complexes in hterature (Bochkarev, 2002 Bochkarev et al., 1997 Fryzuk et al., 2000), ( )-stilbene complexes are rare. Evans et al. reported the synthesis of [(C5Me5)2Sm]2((E)-stilbene) from the direct reaction of (C5Me5)2Sm and ( )-stUbene (Chart 23 Evans et al., 1990). Due to the poor quality of the crystal, the structure of the molecule could not be unambiguously determined but the authors suggested an unsym-metrical coordination mode based on coimectivity. The analogous samarium styrene and butadiene complexes (styrene complex shown in Chart 23) showed two-electron reduction of the C=C bond and concomitant oxidation of Sm(ll) to Sm(lll) (Evans et al., 2001). Related yttrium and lutetium complexes of the readily available tetraphenylethylene dianion have been reported with similar stmctural features (Chart 23 Roitershtein et al., 1998, 2004). 

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