Pilar Palma

Cleavage of Palladium Metallacycles by Acids: A Probe for the Study of the Cyclometalation Reaction

Useful mechanistic information about the cyclometalation reaction may be obtained by studying the reverse reaction, namely, the protonation of metallacycle 1 by acids of different coordinating anions (see scheme). Ar=3,5-(F3C)2C6H3; Tf=F3CSO2.

The chemistry of group 10 metalacycles

Abstract Metalacycles of Nickel, Palladium and Platinum in which the metal binds two carbon atoms form a rapidly growing class of compounds that find diverse applications in organic synthesis both in stoichiometric and catalytic reactions. We address herein the synthetic approaches used in the preparation of these complexes, and their chemical reactivity, with emphasis in processes such as the insertion of small unsaturated molecules into the metal–carbon bonds, which lead to the formation of organic products.

Synthesis of dialkyl, diaryl and metallacyclic complexes of Ni and Pd containing pyridine, α-diimines and other nitrogen ligands: Crystal structures of the complexes cis-NiR2py2 (R=benzyl, mesityl)

Abstract The alkylation of NiCl 2 py 4 or PdCl 2 py 2 with organomagnesium or organolithium reagents affords dialkyl complexes cis -MR 2 py 2 (R=Me, CH 2 SiMe 3 , CH 2 Ph, CH 2 CMe 2 Ph, 2,4,6-C 6 H 2 Me 3 ). The methyl and trimethylsilyl derivatives undergo ligand exchange reactions with chelating nitrogen ligands (α-diimines or 2-imidoylpyridines), yielding the corresponding dialkyl derivatives in good to excellent yields. A catalytic amount of PMe 3 induces the transformation of the nickel complex Ni(CH 2 CMe 2 Ph) 2 py 2 into the metallacyclic derivative NiCH 2 CMe 2 -o- C 6 H 4 ( py ) 2 .The latter, and the related palladacycle Pd ( CH 2 CMe 2 -o- C 6 H 4 ) ( cod ) , also undergo facile ligand exchange reactions.

Reactivity of an anionic Pd(II) metallacycle with CH2X2(X = Cl, Br, I): formal insertion of methylene into a Pd–Caryl bond

Whereas the reaction of the anionic palladium metallacycle [K[Pd(CH2CMe2-o-C6H4)(kappa2-Tp)]] with CH2Cl2 leads to the isolation of the stable Pd(IV) chloromethyl complex [Pd(CH2CMe2-o-C6H4)(kappa3-Tp)(CH2Cl)], the analogous reactions with CH2Br2 and CH2I2 give rise to the six membered metallacycles [Pd(CH2CMe2-o-C6H4(CH2))(kappa3-Tp)X](X = Br or I), as a result of the formal insertion of CH2 into the Pd-C(aryl) bond.

Well-defined alkylpalladium complexes with pyridine-carboxylate ligands as catalysts for the aerobic oxidation of alcohols

Neophylpalladium complexes of the type [Pd(CH(2)CMe(2)Ph)(N-O)(L)], where N-O is picolinate or a related bidentate, monoanionic ligand (6-methylpyridine-2-carboxylate, quinoline-2-carboxylate, 2-pyridylacetate or pyridine-2-sulfonate) and L is pyridine or a pyridine derivative, efficiently catalyze the oxidation of a range of aliphatic, benzylic and allylic alcohols with oxygen, without requiring any additives. A versatile method is described which allows the synthesis of the above-mentioned complexes with a minimum synthetic effort from readily available materials. Comparison of the rates of oxidation of 1-phenylethanol with different catalysts reveals the influence of the structure of the bidentate N-O chelate and the monodentate ligand L on the catalytic performance of these complexes.

Multiple insertion reactions of CNBut into nickel—alkyl bonds

Abstract Reaction of the trans square-planar complexes of nickel trans -[Ni(R)Cl(PMe 3 ) 2 ] with an excess of CNBu t (≥ 4 equiv.) gives the chelated polyimino compounds [ Ni (C(NBu t )C(NBu t )C(R)NBu t )Cl(CNBu t )] (R  CH 3 , I; R  CH 2 SiMe 3 , II; R  CH 2 C 6 H 4 - o -Me, III). Analogous complexes are formed for R  CH 2 CMe 3 and CH 2 CMe 2 Ph but could not be separated from other insertion products. The formation of the metallacyclic compounds IIII proceeds step-wise, as demonstrated by a study of the reaction of trans -[Ni(CH 3 )Cl(PMe 3 ) 2 ] with various proportions of CNBu t (from 1 to 4 equiv.) which revealed the formation of non-cyclic mono-, bis-, and tris-insertion products. The crystal structure of the o -methylbenzyl derivative III has been determined by X-ray crystallography. Complex III is triclinic, space group P 1 , with cell dimensions a 11.094(3), b 11.660(3), c 25.020(5) A, α 77.65(3), β 94.65(3), γ 90.04(2)°, V 3150.1 A 3 and Z  4. There are two independent molecules per asymmetric unit, each exhibiting distorted squareplanar geometry round the nickel atoms. The o -methylbenzyl group gives rise to considerable steric interactions, and there is a restriction of rotation around the CH 2 C(6) bond in this complex.

Synthesis and reactivity studies on alkyl–aryloxo complexes of nickel containing chelating diphosphines: cyclometallation and carbonylation reactions

Nickel alkyl–aryloxo complexes of composition Ni(R)(OC6H32,6-Me2)(PP) (R= CH2SiMe3 ,C H 3 ,C H 2CMe2Ph; PPPPr2 (CH2)nPPr2 , n=2 (dippe) or 3 (dippp)) have been synthesized. While the (trimethylsilyl)methyl and the methyl derivatives are stable in solution at room temperature, the bis-neophyl (R = CH2CMe2Ph) complexes undergo a cyclometallation reaction that leads to the metallacycles Ni(CH2CMe2-o-C6 H4)(PP) together with 2,6-dimethylphenol. The alkyl–aryloxo complexes cleanly react with carbon monoxide giving products resulting from CO insertion and reductive elimination, i.e. Ni(CO)2(PP) and the corresponding 2,6-dimethylphenyl carboxylates quantitatively.

Mononuclear and binuclear o-xylylene derivatives of Pd(II) ☆

Abstract The synthesis of binuclear complexes of composition C 6 H 4 -o-( CH 2 PdX ( PMe 3 ) 2 ) 2 ( X = Cl , 4a ; Br , 4b ) that contain bridging o-xylylene units, can be accomplished by reaction of the corresponding α,α′-dihaloxylenes with the Pd(0) species [Pd(η2-CH2=CHCO2Me)(PMe3)2] (1). For X = Cl, partial oxidative addition takes place when the reaction is conducted at room temperature, leading to the mononuclear compound Pd(CH2C6H4-o-CH2Cl)Cl(PMe3)2 (3a) which undergoes a slow decomposition process in solution that involves oxidative addition of the chloromethyl group to palladium. The binuclear derivative 4a can also be obtained by reaction of the metallacycle (PMe3)2 (9) with PdCl2(PMe3)2.Complexes 4a,4b and 3a react with carbon monoxide and t-butyl isocyanide, giving rise to the corresponding acyl and imidoyl products. However, in the case of the imidoyl complex derived from 3a, (i.e., compound 13), the reactive chloromethyl group experiences a nucleophilic attack either by the metal or by the nitrogen atom of the imidoyl functionality to give respectively either unstable Pd(IV) species or the cationic amidocarbene complex 15. The structure of the cation of the 15·BPh4− salt has been determined by X-ray diffraction methods. Crystal data for 15·BPh4−: orthorhombic, Pna21, a = 19.826(5), b = 11.076(4), c = 19.131(5) A , Z = 4, R1 = 0.0544 and wR2 = 0.4141.

Dibenzyl and diallyl 2,6-bisiminopyridinezinc(II) complexes: selective alkyl migration to the pyridine ring leads to remarkably stable dihydropyridinates

Diorganozinc compounds (ZnR2) with R = CH2Ph or CH2CH=CH2 react with 2,6-bisiminopyridines ((iPr)BIP) to afford thermally stable dihydropyridinate(-1) complexes, and do not react if R = CH2SiMe3 or CH2CMe2Ph. NMR studies reveal that dibenzylzinc binds (iPr)BIP at -80 °C, yielding the unstable complex [Zn(CH2Ph)2((iPr)BIP)]. Above -20 °C, this undergoes selective alkyl migration to the remote 4 position of the central pyridine ring.

η2-Iminoethyl-C,N vs.η3-aminoethylene-C,C′,N formation in the thermally induced rearrangement of isocyanide alkyl complexes of molybdenum

Thermal rearrangement of the isocyanide alkyl complexes [Mo(η5-C5R5)(Me)(CO)2(CNBut)] 1, (R5= H5, 1a; H4Me, 1b; Me5, 1c) yields η2-iminoethyl-C,N complexes [Mo(η5-C5R5){η2-C(NBut)Me3}(CO)2]2, and/or η3-aminoethylene-C,C′,N complexes [Mo(η5-C5R5){η3-H2CC(H)NBut}(CO)2]3, in reactions whose course is dramatically solvent dependent.