Mercè Rocamora

Coordination chemistry of oxazoline ligands

Abstract 4,5-Dihydro-1,3-oxazole ligands (commonly known as 2-oxazolines or simply oxazolines) have been used by many research groups as chiral auxiliaries in transition metal-catalyzed asymmetric organic syntheses. The oxazolines show a number of attractive characteristics: versatility of ligand design, straightforward synthesis of ligands from readily available precursors, and modulation of the chiral centers, which are located near the donor atoms. This review focuses on the transition metal coordination chemistry of oxazolines. It surveys data describing structural characterization in both solid state (X-ray diffraction) and solution (NMR spectroscopy).

Modular bis(oxazoline) ligands for palladium catalyzed allylic alkylation: Unprecedented conformational behaviour of a bis(oxazoline) palladium η3-1,3-diphenylallyl complex

New families of enantiopure bis(oxazolines) with 4,5-trans (5 a-g) or 4,5-cis (6 c) stereochemistry at the individual rings have been prepared in high yield. Their eta(3)-allyl palladium complexes (8 a-g, 9 c and 10) have been used as catalytic precursors in allylic alkylation reactions with excellent enantioselectivities (up to 96 %) for the trans oxazoline derivatives, while Pd/6 c system was inactive. NMR studies on palladium eta(3)-1,3-diphenylallyl intermediates (11 a, c and e) showed the presence of syn/syn- and syn/anti-allyl isomers in solution; this resembles the first example of eta(3)-eta(1)-eta(3) isomerism in Pd allylic complexes containing bis(oxazolines) derived from malonic acid.

Electrochemical cleavage of allyl aryl ethers and allylation of carbonyl compounds: umpolung of allyl-palladium species

Abstract The electrochemical, Pd-catalyzed cleavage of the CO bond of allyl aryl ethers has been examined; the method constitutes a new alternative for allyl ether deprotection. The allyl transfer from the allyl ether to the carbonyl group in 2-allyloxy benzaldehydes is reported and is an example of allyl-Pd reactivity umpolung. Pd(II) complexes, associated to several nitrogen ligands are efficient catalyst precursors for these electrochemical reactions.

Reactivity of [NiR(R′)L2] compounds and the crystal structure of [Ni(C2Cl3)(C6H2Me3-2,4,6)(PMe2Ph)2]

A series of compounds of the type trans-[NiR(R′)L2](L = PMe2Ph, R = C2Cl3, R′= Ph, C6H4OMe-4, C6H4Me-2, or -4, C6H2Me3-2,4,6; R = C6H4Cl-2, R′= C6H4Me-2 or C6H2Me3-2,4,6; R = C6H2Me3-2,4,6, R′= C6H4Me-2 or Ph; L = PEt3, R = C2Cl3, R′= C6H4Me-2, Ph, C6H4OMe-4, C6H4Cl-4, or C6H4Me-4; R = C6H4Cl-2, R′= Ph, C6H4Me-2 or -4; R = C6H4Me-2, R′= Ph, or C6H4Me-4; R = C6H2Me3-2,4,6, R′= C6H4Me-2) and cis-[Ni(C6H2Me3-2,4,6)(C6H4Me-2)(bipy)](bipy = 2,2′-bipyridine) have been prepared. The proton n.m.r. signals of the ortho methyls of the mesityl ligand in the most hindered compound of the series [Ni(C2Cl3)(C6H2Me3-2,4,6)(PMe2Ph)2] appear overlapped despite the asymmetry of C2Cl3. The crystal structure of this compound [triclinic, space group P, a=12.570(3), b=12.703(3), c= 9.352(2)A, α= 91.70(2), β= 90.02(2), γ= 102.65(2)°, Z= 2] indicates that this may be due to the phosphine aromatic rings. Ligandexchange reactions of PMe2Ph by PEt3 in benzene, [NiR(R′)(PMe2Ph)2]⇌[NiR(R′)(PMe2Ph)(PEt3)]⇌[NiR(R′)(PEt3)2], are complete in 0.5 h at room temperature, no intermediates being observed. The reverse process requires refiux for 10 h and in this case the intermediates [NiR(R′)(PMe2Ph)(PEt3)] can be obtained. If the groups R and R′ contain ortho substituents the reaction does not take place. In refluxing benzene under nitrogen the stability of the organometallics containing PMe2Ph is greater than those with PEt3. Decomposition takes place via reductive elimination to give R–R′ or via homolytic cleavage to give R–H and R′–H, the most favoured pathway depending on the size and electronegativity of the groups R and R′. The reductive-elimination process involves a previous step in which a phosphine group is lost. The addition of CO to solutions of [NiR(R′)L2](when R and R′ possess ortho substituents) results in decomposition of the organometallics giving RCOR′. The compound [Ni(C6H2Me3-2,4,6)(C6H4Me-2)(bipy)] is also decomposed but gives R-R′. When one of the ligands (R) contains an ortho substituent, the corresponding two isomers (syn and anti) of the acyl derivative [NiR(COR′)L2] can be detected. The compounds cis-[NiR(R′)(bipy)] show a favourable associative decomposition pathway when R and R′ are strong donor ligands.

Metal catalysed hydrovinylation

An overview of the hydrovinylation reaction catalysed by transition metal organometallic complexes is given. The addition of ethylene to another CC double bond of vinylarenes, dienes or strained olefins is highlighted as an elegant methodology to generate a new stereogenic centre. In addition, the introduction of a new vinyl group allows further functionalisation. The parameters that control the selectivity of the formal codimerisation reaction and some selected applications are discussed.

Kinetico-mechanistic insights on the assembling dynamics of allyl-cornered metallacycles: the Pt-Npy bond is the keystone.

The square-like homo- and heterometallamacrocycles [{Pd(η(3) -2-Me-C3 H4 )(L(n) )2 }2 {M(dppp)}2 ](CF3 SO3 )6 (dppp=1,3-bis(diphenylphosphino)propane) and [{Pd(η(3) -2-Me-C3 H4 )(L(1) )2 }2 {M(PPh3 )2 }2 ](CF3 SO3 )6 [py=pyridine, M=Pd, Pt, L(n=) 4-PPh2 py (L(1) ), 4-C6 F4 PPh2 py (L(2) )] containing allyl corners were synthesised by antisymbiotic self-assembly of the different palladium and platinum metallic corners and the ambidentate N,P ligands. All the synthesised assemblies displayed a complex dynamic behaviour in solution, the rate of which is found to be dependent on the electronic and/or steric nature of the different building blocks. A kinetico-mechanistic study by NMR line shape analysis of the dynamics of some of these assemblies was undertaken in order to determine the corresponding thermal activation parameters. Both an enhanced thermodynamic stability and slower dynamics were observed for platinum-pyridine-containing species when compared with their palladium analogues. Time-dependent NMR spectroscopy in combination with ESI mass spectrometry was used to study the exchange between the assemblies and their building blocks, as well as that occurring between different metallamacrocycles. Preliminary studies were carried out on the activity of some of the metallamacrocyclic compounds as catalytic precursors in the allylic substitution reaction, and the results compared with that of the monometallic allylic corner [Pd(η(3) -2-Me-C3 H4 )(L(1) )2 ](+) .

Intramolecular allyl transfer reaction from allyl ether to aldehyde groups: experimental and theoretical studies.

The intramolecular transfer of the allyl group of functionalized allyl aryl ethers to an aldehyde group in the presence of Ni0 complexes was studied from chemical, electrochemical and theoretical points of view. The chemical reaction involves the addition of Ni0 to the allyl ether followed by stoichiometric allylation. The electrochemical process is catalytic in nickel and involves the reduction of intermediate eta3-allylnickel(II) complexes.

Preparation of four-membered phosphonickelocycles. Unusual facile stabilization of five-co-ordinate complexes

Three different types of organometallic compounds [[graphic omitted]R2)-2}2](R = Ph 1 or Et 1′), [[graphic omitted]R2)-2}L](R = Ph 2 or Et 2′) and [NiCl{C6Cl4(PR2)-2}L2](R = Ph 3 or Et 3′) have been obtained from 1 equivalent of PR2(C6Cl5)(R = Ph or Et), [Ni(cod)2](cod =cis,cis-cycloocta-1,5-diene), and L = PMe2Ph a, PEt3b, P(CH2Ph)3c or PPh3d. Complexes 2 evolve in solution, either to 1 and [NiCl2L2], or to 3 by breaking of the Ni–P bond of the four-membered ring by free phosphine. The selective preparation of compounds 1 or 3 can be achieved by performing the oxidative-addition reaction in the absence or with 2 equivalents of L respectively. When 1 equivalent of a diphosphine was used in the oxidative-addition reaction a mononuclear five-co-ordinate complex was obtained, [[graphic omitted]R2)-2}-(L–L)]4[L–L = Ph2P(CH2)nPPh2, n= 2 or 3]. However, dppm (Ph2PCH2PPh2) acts as a monodentate ligand to give the five-co-ordinate compound [[graphic omitted]Ph2)-2}(dppm)2]. Complexes 2, 2′ show preferentially a cis geometry, 1 is trans, and 3, 3′ have the L ligands in trans position. Insertion of CO or alkynes into the Ni–C bond was not observed. Compounds 1 and 1′in the presence of neutral ligands L = CO or PR3(PR3= PMe2Ph a or PEt3b) gave five-co-ordinate complexes [[graphic omitted]Ph2)-2}2L] without cleavage of the Ni–P bond of the ring. Stabilization of the four-membered ring is achieved when two bidentate ligands are present or in the five-co-ordinate compound [[graphic omitted]Ph2)-2}(dppm)2]. Two bidentate ligands are also needed to stabilize the formation of five-co-ordinate complexes. The molecular structures of complexes 1′, 3b, and [[graphic omitted]Ph2)-2}2(PEt3)] were determined by single-crystal X-ray diffraction.

Preparation and reactivity of five-membered phosphonickelocycles

The complexes trans-[[graphic omitted])L][L = PMe2Ph 1a, PEt31b, P(CH2Ph)31c or PPh31d], and [{[graphic omitted])}2] containing the bidentate anionic ligand o-C6H4CH2PPh2(P–C) have been prepared. The molecular structure of 1c has been determined by single-crystal X-ray crystallographic methods: space group P21/c, a= 20.641(6), b= 9.794(3), c= 17.047(5)A, β= 102.05(3)° and Z= 4. Complexes 1 showed a conformational interconversion of the metallocyclic ring. Different amounts of cis and trans isomers were observed in the solutions of [[graphic omitted])L] compounds by NMR spectroscopy, depending on the size of the phosphine and the anionic ligands. With [[graphic omitted]){P(CH2Ph)3}] only the cis product is obtained. The ionic compounds [[graphic omitted])(PMe2Ph)2]BF4 and [[graphic omitted])(MeCN){P(CH2Ph)3}]BF4 were synthesised by reaction of [[graphic omitted]] with 1 equivalent of TIBF4 in the presence of free ligand. The five-co-ordinate compound [[graphic omitted])(PMe2Ph)2] was obtained from 1a and PMe2Ph. The reactivity of the Ni–C bond of compounds 1 in the presence of CO, SO2, CS2, CO2, alkynes and olefins was studied. Acyl or S-sulfinate compounds containing six-membered rings [[graphic omitted]Ph2)L][L = PEt3 or P(CH2Ph)3] and [[graphic omitted]Ph2){P(CH2Ph)3}] were obtained from the reactions with CO and SO2. The reaction with diphenylacetylene or ethyl phenylpropiolate of compounds 1a and 1c gave one isomer of [[graphic omitted]Ph2}L][L = PEt3 or P(CH2Ph)3] and [[graphic omitted]Ph2}{P(CH2Ph)3}], with the alkenyl fragment cis with respect to the substituents of the alkyne. The geometry of the latter was confirmed by single-crystal X-ray structure determination: space group P21/c, a= 11.386(3), b= 14.715(3), c= 26.000(5)A, β= 90.03(4)° and Z= 4. Decomposition products after insertion of ethylene under pressure in 1c and [[graphic omitted]){P(CH2Ph)3}] were also observed. No evidence of insertion reactions was obtained with CS2 and CO2 even under pressure.

Synthesis and catalytic properties of neutral and cationic rhodium- and iridium-containing carbosilane dendrimers

The reaction of phosphanyl-terminated carbosilane dendrimers containing only one phosphorus ligand per arm with [MCl(cod)]2 (M = Rh, Ir; cod = cycloocta-1,5-diene) resulted in the grafting of MCl(cod) moieties on the surface of the dendrimer. However, dendrimers displaying two phosphorus ligands per arm gave very unstable species which were not isolated. On the other hand, the last group of dendrimers reacted with [MCl(cod)]2, in the presence of silver salts, to afford cationic dendrimers in which a metal fragment is attached simultaneously to both phosphorus atoms of the same arm. The substitution of cod with 1,1′-bis(diphenylphosphino)ferrocene (dppf) in one example of the latter group yielded a bimetallic Rh/Fe layer-containing dendrimer. The new metallodendrimers were tested as catalysts in the hydrogenation of 1-hexene.