A novel strategy for the synthesis of bis(ortho-amine)chelated monoanionic bisaminoaryl−metal complexes is presented. This method constitutes the exchange of a monoanionic, NCN-pincer ligand (NCN = [C6H3(CH2NMe2)2-2,6]-) between a stable arylgold(I) phosphine, [Au(η1-C-NCN)(PPh3)] (2), and a transition metal halide. This transmetalation results in the quantitative formation of [M(NCN)] organometallic compounds (M = AuIII, NiII, PdII, PtII, TiIV, and FeIII) and [AuCl(PPh3)] as the only coproduct that can be recovered.

2,6-Bis(oxazolinyl)phenylnickel bromide complexes [NiBr(R,R′-Phebox)] (2) were synthesized via two synthetic routes (A and B). In route A, selective bis-ortho lithiation of [R,R′-PheboxBr], followed by a transmetalation reaction with [NiBr 2 (PEt 3 ) 2 ], yielded not only complexes 2 with an η 3 -mer-N,C,N′-bonded Phebox ligand but also [NiBr(R,R′-Phebox)(PEt 3 ) 2 ], 7, where the nickel center is η 1 -C bonded to the intra-annular C ipso of the Phebox ligand. Coordination of two PEt 3 ligands completes the square-planar coordination sphere of the Ni center in 7. When R ) t-Bu, R′ ) H, only complex 7c was formed. Alternatively, when route B (oxidative addition with [Ni(cod) 2 ], cod ) cyclooctadiene) was followed, selective formation of complexes 2 was observed. X-ray crystal structures were obtained for [NiBr-(i-Pr,H-Phebox)] (2b) and [NiBr(bis(ketimine)phenyl)] (3). The Ni centers have square-planar geometries with a planar, η 3 -mer-N,C,N′ coordination of the terdentate ligand systems. Complexes 2 were found to be inactive as catalysts in the atom-transfer radical polymerization (ATRP) reaction of methyl methacrylate (MMA) and in the atom-transfer radical addition (ATRA, Kharasch addition) reaction of CCl 4 to MMA. This is ascribed to the relatively high oxidation potential of Ni II -Phebox complexes, which excludes the (reversible) formation of a d 7 -Ni III -Phebox complex, a crucial condition for subsequent reactions. Cyclovoltammetry (CV) experiments ((n-Bu) 4 NBr as supporting electrolyte) showed no electrochemical waves between -1.00 and +1.50 V (Ag/AgCl reference electrode, (n-Bu) 4 NBr as supporting electrode). Theoretical calculations showed that the energy (E ox ) needed for the oxidation reaction occurring during the CV experiments is considerably higher for [NiBr(Me,Me-Phebox)] (1.87 eV) and [NiBr(bis(ketimine)-phenyl)] (1.90 eV) than for [NiBr(NCN)] (1) (1.45 eV).

The treatment of 2,6-bis(oxazolinyl)phenyl bromide (Phebox-Br) with n-BuLi affords a Phebox-Li complex. Subsequent transmetalation with [SnClMe 3 ] affords a Phebox-Sn complex. The Phebox ligand can coordinate to a transition metal in various terdentate fashions; both the oxazoline oxygen and the imine nitrogen are perfectly positioned for chelation; "NCN", "OCO", or mixed terdentate coordination modes are theoretically possible using this ligand. The structural properties and NMR spectra of [Sn(Me,Me-Phebox)Me 3 ] (2) and [Li(R,R′-Phebox)] complexes 3a (R ) R′ ) Me), 3b (R ) iPr, R′ ) H), and 3c (R ) tBu, R′ ) H) were investigated. It was found that 2 exhibits no chelation of the Phebox ligand to the Sn center in this case. The [Li(R,R′-Phebox)] complex 3a has been crystallographically characterized and is in the form of a molecular dimer (i.e. [Li(Phebox)] 2 ), containing two formally three-center-two-electron bonds in a four-membered Li 2 C 2 ring. The formal Phebox anion is bonded to the lithium cation via the two ortho imine N centers and the intraannular aromatic C atom. The 13 C{ 1 H} NMR signal of C ipso , being a seven-line pattern with coupling constant 1 J( 13 C-7 Li) ) 18 Hz, confirms that the dimeric structure is maintained in solution at room temperature. Variable-temperature (VT) NMR studies of 3a indicate that a fluxional process is occurring at room temperature, which can be frozen out below -16°C (∆G q ) 56 kJ/mol). This fluxional process is not observed in VT-NMR studies on 3b,c. This is likely due to the presence of bulky (iPr or tBu) substituents that effectively shut down the pathways to rapid inversion of the puckering of the five-membered chelate ring.

A route for the synthesis of enantiopure allene-substituted lactams has been developed. The key-step involves the copper(I) mediated S N 2' substitution of propargylic tosylates by a (S)pyroglutamic acid derived organozinc reagent. Pd-catalysed reaction of these allenes with iodobenzene afforded enantiopure bicyclic enamides. Furthermore the unexpected formation of an interesting diene is reported.

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The synthesis of two organogold(I) complexes, [(Au(NCN))2(dppbp)] (6) and [(Au(Phebox))2(dppbp)] (9), and their application in subsequent transmetalating reactions are described. A trinuclear organogold(I) complex, [(AuCl)3(tdpppb)] (4) is also reported, which exhibits a surprisingly high solubility in dichloromethane. It was found that 6 and 9 can cleanly transfer the anionic NCN-([C(6)H(3)(CH(2)NMe(2))2-2,6]-) or Phebox-([2,6-bis(oxazolinyl)phenyl]-) moiety to Ti(IV) and Pd(II) centers, respectively. The coproduct [(AuCl)2(dppbp)] (3, dppbp is [4-Ph(2)PC(6)H(4)]2 (1)) formed during this transmetalation reaction, precipitates almost quantitatively from the reaction mixture (toluene) and can thus be separated by simple filtration. In comparison, [AuCl(PPh3)], formed as the coproduct in the transmetalation reaction of [Au(NCN)(PPh3)] with metal salts, has a higher solubility in apolar solvents and thus is more difficult to separate from the resultant organometallic complex. Digold complex 6 has been characterized by NMR spectroscopy and crystallographic analyses. These analyses show that the two gold units are essentially independent. The formation of a dimetallic transmetalating agent based on gold(I) had no effect on its transmetalating properties.

-037Palladium-Catalyzed Cyclization Reactions of Acetylenic Lactams.-The palladium-catalyzed cyclization of acetylenic lactams and oxazolidinones with a variety of organic halides proceeds with good yield and high cis stereoselectivity with respect to the nitrogen nucleophile onto the triple bond.

Palladium-Catalyzed Cyclization of Enantiopure Allenic Lactams Prepared from a Pyroglutamic Acid Derived Organozinc Reagent.-The organozinc reagent, prepared in situ from iodides (II), is successfully applied to the synthesis of enantiopure allene-substituted lactams. A following Pd-catalyzed cyclization in the presence of iodobenzene provides access to bicyclic enamides. -(KARSTENS, W. F. J.; STOL, M.; RUTJES, F. P. J. T.; HIEMSTRA, H.; Synlett (1998) 10, 1126-1128; Lab. Org. Chem., Inst. Mol. Stud., Univ. Amsterdam, NL-1018 WS Amsterdam, Neth.; EN)