J. Carles Bayón

Homogeneous catalysis with transition metal complexes containing sulfur ligands

Abstract Transition metal complexes with sulfur ligands are active catalysts in a considerable number of homogeneous reactions, although they have been less investigated than complexes with other donor atoms. We review the application of complexes containing sulfur ligands in hydrogenation, hydrogen transfer, isomerization, hydrosilylation, carbonylation, hydroformylation, polymerization, Heck reaction, allylic alkylation, Grignard cross-coupling, conjugate addition and oxidation with O2. The use of different types of sulfur ligands and the characteristics of the coordination complexes are described together with the results of the catalytic reactions.

Hydroformylation of myrcene: metal and ligand effects in the hydroformylation of conjugated dienes

The hydroformylation of myrcene catalyzed by Rh and Pt/Sn catalysts containing different P-donor ligands leads to the formation of a number of mono- and dialdehydes. Nine major products of the reaction have been characterized, showing that they arise from the n-alkyl and η3-allyl intermediates, formed through the reaction of the metal catalysts with the less substituted CC bond of the substrate. Thus, 4-methylene-8-methyl-7-nonenal is the major aldehyde formed with Pt/Sn catalysts, regardless of the P-donor ligand used. This aldehyde is also the main product of the reaction catalyzed by the Rh/xantphos system (xantphos = 9,9-dimethyl-4,6-bis(diphenylphosphino)xantene). However, with ligands such as bisbi (bisbi = 2,2′-bis((diphenylphosphino)methyl)-1,1′-biphenyl), also with bite angles around 120°, but with more flexible backbones than xantphos, rhodium catalysts yield mainly cis- and trans-3-ethylidene-7-methyl-6-octenal. These two aldehydes are also formed in the reactions catalyzed by Rh and P-donor monodentate ligands or the bidentante ones with bite angles around 90° (dppe, dppp). For the last type of ligands, an increase in the flexibility of the backbone reduces the selectivity for the β,γ-unsaturated aldehydes.

Rhodium-diphosphine catalysts for the hydroformylation of styrene : the influence of the excess of ligand and the chelate ring size on the reaction selectivity

Abstract This study discusses the hydroformylation of styrene using rhodium systems containing four structurally related diphosphines: 1,2-bis(diphenylphosphine)ethane 1 (dppe), 1,3-bis(diphenylphosphine)propane 2 (dppp), ( R , R )-2,3-bis(diphenylphosphine)butane 3 (chiraphos) and ( S , S )-2,4-bis(diphenylphosphine)pentane 4 (bdpp). A systematic analysis of the effect of the pressure, temperature and the ligand to metal molar ratio for these catalytic systems shows that the five- and six-membered ring chelating diphosphines behave different. The regio- and enantioselectivity observed provide evidence of the catalytic species involved in the process. By analyzing the selectivity of the catalytic systems formed by mixing PEtPh 2 and the chiral diphosphine ligands, we propose a model describing the equilibria among the catalytic species.

Diastereoselective hydroformylation of Δ4-steroids with rhodium–phosphite catalysts

Abstract The hydroformylation of two steroidal substrates, namely 17β-acetoxyandrost-4-ene 1 and 3β,17β-diacetoxyandrost-4-ene 2, with a rhodium tris(O-tert-butylphenyl)phosphite catalyst was investigated. In both cases, the major reaction product was 4β-formyl-17β-acetoxy-5β-androstane 3, which was isolated and characterized by X-ray diffraction and NMR techniques. This reaction is the first example of catalytic carbonylation to the β face of a steroid backbone. The effect of reaction temperature, the pressure at which the reaction was completed and the ligand:Rh ratio on the regio- and stereoselectivity of the reaction is also discussed.

Palladium catalytic systems with hybrid pyrazole ligands in C–C coupling reactions. Nanoparticles versus molecular complexes

This paper reports the comparison of the chemoselectivity of two different Pd catalytic systems, namely molecular and colloidal systems, in C–C coupling reactions. For this purpose, new hybrid pyrazole derived ligands containing alkylether, alkylthioether or alkylamino moieties have been synthesized and used to form Pd(II) complexes and to stabilize Pd nanoparticles (Pd NPs). With the aim of studying the coordination mode of the ligands and further to understand their role in catalysis, both types of Pd species were characterized by appropriate techniques. In C–C coupling reactions promoted by different Pd colloidal systems, several reports evidenced that active species are molecular catalysts leached from Pd NPs. The most important feature of this work relies on the differences observed in the output of C–C coupling reactions, depending on the colloidal or molecular nature of the catalyst employed. Thus, molecular systems carry out typical Suzuki–Miyaura cross-coupling, together with the dehalogenation of the substrate in different proportions. In contrast, Pd NPs catalyze either Suzuki–Miyaura or C–C homocoupling reactions depending on the haloderivative used. Interestingly, Pd NPs catalyze the quantitative dehalogenation of 4-iodotoluene. Differences observed in the chemoselectivity of these two catalytic systems support that reactions carried out with Pd NPs stabilized with the hybrid pyrazole ligands employed here take place on the surface of the colloids.

Lewis super-acid catalyzed cyclizations: a new route to fragrance compounds.

This review deals with the application of Lewis super acids such as AlIII, InIII, and SnIV triflates and triflimidates as catalysts in the synthesis of fragrance materials. Novel catalytic reactions involving CC and Cheteroatom bond‐forming reactions, as well as cycloisomerization processes are presented. In particular, SnIV and AlIII triflates were employed as catalysts in the selective cyclization of unsaturated alcohols to cyclic ethers, as well as in the cyclization of unsaturated carboxylic acids to lactones. The addition of thiols and thioacids to non‐activated olefins, both in intra‐ and intermolecular versions, was efficiently catalyzed by InIII derivatives. SnIV Triflimidates catalyzed the cycloisomerization of highly substituted 1,6‐dienes to gem‐dimethyl‐substituted cyclohexanes bearing an isopropylidene substituent. The hydroformylation of these unsaturated substrates, catalyzed by a RhI complex with a bulky phosphite ligand, selectively afforded the corresponding linear aldehydes. The olfactory evaluation of selected heterocycles, carbocycles, and aldehydes synthesized is also discussed.

A chiral diphosphine containing hemilabile ether donor groups and its use in rhodium asymmetric hydroboration of styrene

Abstract The new homochiral diphosphine (R,R)-o-C6H4[CH2OCH2CH(Me)PPh2]2 (1) was prepared from ethyl lactate in five steps. Because of the two hemilabile ether groups, the ligand can be visualized as a diphosphine containing two intramolecular solvent molecules. The cationic Rh(I) complex [Rh(1)](BF4) was prepared and used as the catalyst in the asymmetric hydroboration of styrene with catecholborane. The catalytic system is chemo- and regioselective, but it yields low enantiomeric excess.

Strong π-acceptor sulfonated phosphines in biphasic rhodium-catalyzed hydroformylation of polar alkenes

A new series of sulfonated triarylphosphines with a strong π-acceptor character were synthesized by direct sulfonation of trifluoromethylated neutral phosphines. Due to the deactivating character of the trifluoromethyl group, high oleum concentration and the use of boric acid to prevent phosphine oxidation were required for the sulfonation step. The new sulfonated phosphines are water-soluble and more inert toward oxidation than classical sulfonated phosphines. The use of these trifluoromethylated and sulfonated phosphines as ligands in the biphasic hydroformylation of vinyl acetate and allyl cyanide increases the rate of the reaction up to 4 times, compared to the results obtained with the non-trifluoromethylated counterparts, TPPMS, TPPDS and TPPTS. Moreover, it is possible to recycle the catalyst without a significant loss of the system activity.

Cationic dinuclear rhodium complexes as catalyst precursors for the hydroformylation of alkenest

The dimeric cationic complexes [M2(µ-HL)2L′2(L″)2]2+[M = Rh or Ir; HL = 4-mercapto-1-methylpiperidine; L′= L″=½ cyclo-octa-1,5-diene (cod) or CO or L′= CO and L″= PPh3] have been prepared as chloride and tetrafluoroborate salts. The activity of these complexes as hydroformylation and hydrogenation catalysts under mild conditions has been tested, in an attempt to elucidate the importance of functional groups in the ligand and the effect of counter ions. In hydroformylations, best results have been obtained with [Rh2(cod)2(µ-HL)2][BF4]2 plus added PPh3 or P(OPh)3, with turnover rates of ca. 2.7 mmol of olefin per mmol of catalyst per minute, and conversions up to 80%; when P(OMe)3 was used, the activity decreased but the normal : iso-aldehyde ratio increased. An important anion effect was observed, namely that all the chlorides were found to be inactive. In hydrogenation, activities are only moderate but unaffected by the nature of the counter ion. However, the [BF4]– salts caused isomerization of the olefins, while the chlorides did not. The iridium complexes were found to be inactive.

Extended linear metal-metal interactions in an anionic rhodium(I) complex: X-ray structure of NMe4[Rh(ox)(CO)2] (ox= oxalato)

The complex NMe4[Rh(ox)(CO)2](2)(ox = oxalato) was prepared from NMe4[Rh(ox)(cod)](1)(codcyclo-octa-1,5-diene) by bubbling carbon monoxide and, complex (1) was prepared from Rh2Cl2(cod)2, oxalic acid, and NMe4OH; the solid state structure of (2) consists of infinite chains of metal atoms [Rh ⋯ Rh ⋯ Rh 175.01(3)°] with equal rhodium–rhodium distances of 3.243(1)A, the shortest distance found in a stacked rhodium complex.