Sergio Castillón

Highlights of Transition Metal-Catalyzed Asymmetric Hydrogenation of Imines

This Review deals with the asymmetric hydrogenation and asymmetric transfer hydrogenation of imines and some nitrogen‐containing heteroaromatic compounds to afford amines. Hydrogenation and transfer hydrogenation are covered in separate sections. Particular attention is devoted to the substrates, since this reaction is highly substrate dependent. In this way the general structural trends in imine hydrogenation are presented, highlighting the more challenging substrates. Since hydrogenation of acyclic and cyclic (including heteroaromatic) imines have some different features they are also presented separately. The catalytic systems are classified according to the metal, and focus on the most successful catalytic systems and metal–ligand combinations. The catalytic cycles for these hydrogenation processes are also proposed.

Synthesis of 2-substituted-benzothiazoles by palladium-catalyzed intramolecular cyclization of o-bromophenylthioureas and o-bromophenylthioamides

Abstract 2-Amino-, and 2-alkyl-benzothiazoles have been efficiently prepared by palladium catalyzed cyclization of o -bromophenylthioureas and o -bromophenylthiamides. Results were best with the Pd 2 (dba) 3 /monophosphine catalytic system.

Chiral diphosphites derived from D-glucose: new ligands for the asymmetric catalytic hydroformylation of vinyl arenes.

A series of novel diphosphite ligands derived from readily available D-(+)-glucose has been synthesized. These ligands have been applied to the Rh-catalyzed hydroformylation of vinyl arenes. Both excellent enantioselectivities (up to 91%) and regioselectivities (up to 98.8%) were achieved under mild conditions. The advantage of these ligands is that their modular natures allow facile, systematic variation in the configurations at the stereocenters [C(3), C(5)] at the ligand bridge and in the biphenyl substituents, enabling their effects on the stereoselectivity to be studied. Results show that the absolute configuration of the product is governed by the configuration at the stereogenic center C(3), while the level of the enantioselectivity is influenced by a cooperative effect between stereocenters C(3) and C(5). Replacement of the tert-butyl substituent by methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have characterized the rhodium complexes formed under CO/H2 by NMR techniques and in situ IR spectroscopy and have observed that there is a relationship between the structure of the [HRh(CO)2(PP)] species and their enantiodiscriminating performance in hydroformylation. Enantioselectivities were highest with ligands with a strong bis-equatorial coordination preference, while an equilibrium of species with bis-equatorial and equatorial-axial coordination modes considerably reduced the ees.

An Efficient and General Enantioselective Synthesis of Sphingosine, Phythosphingosine, and 4-Substituted Derivatives†

A general and efficient protocol for the enantioselective synthesis of sphingosine, phythosphingosine, and 4-substituted derivatives was established. These compounds were obtained from a common intermediate prepared from butadiene monoepoxide by a synthetic sequence involving enantioselective allylic substitution, cross-metathesis, and dihydroxylation.

Phosphine Ligands in the Palladium‐Catalysed Methoxycarbonylation of Ethene: Insights into the Catalytic Cycle through an HP NMR Spectroscopic Study

Novel cis-1,2-bis(di-tert-butyl-phosphinomethyl) carbocyclic ligands 6-9 have been prepared and the corresponding palladium complexes [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)] (L-L=diphosphine) 32-35 synthesised and characterised by NMR spectroscopy and X-ray diffraction. These diphosphine ligands give very active catalysts for the palladium-catalysed methoxycarbonylation of ethene. The activity varies with the size of the carbocyclic backbone, ligands 7 and 9, containing four- and six-membered ring backbones giving more active systems. The acid used as co-catalyst has a strong influence on the activity, with excess trifluoroacetic acid affording the highest conversion, whereas excess methyl sulfonic acid inhibits the catalytic system. An in operando NMR spectroscopic mechanistic study has established the catalytic cycle and resting state of the catalyst under operating reaction conditions. Although the catalysis follows the hydride pathway, the resting state is shown to be the hydride precursor complex [Pd(O(3)SCH(3))(L-L)][O(3)SCH(3)], which demonstrates that an isolable/detectable hydride complex is not a prerequisite for this mechanism.

A new and efficient catalytic method for synthesizing isocyanates from carbamates

Abstract Operationally simple, recyclable and environmentally friendly montmorillonite efficiently catalyses dealcoholysis of a wide range of mono- and dicarbamates to isocyanates.

Asymmetric hydroformylation of styrene using a rhodium catalyst with BDPP as the chiral ligand

Abstract Asymmetric hydroformylation of styrene is performed using the available rhodium [Rh(μ-OMe)(COD)] 2 complex with BDPP as the chiral auxiliary ligand. The enantiomeric excesses observed heavily depend on the excess of diphosphine used and ees of up to 60 % are achieved. The influence of different P and T conditions is also discussed.

Heterogenization of Pd–NHC complexes onto a silica support and their application in Suzuki–Miyaura coupling under batch and continuous flow conditions

The heterogenisation of a new family of Pd–NHC complexes is reported via a straightforward and efficient synthetic procedure. These silica-immobilised materials were successfully applied as catalysts in the Suzuki–Miyaura coupling of aryl chlorides and bromides under mild conditions. The materials exhibited improved stability when the catalytic reaction was run under anhydrous conditions and could be recycled up to five times without significant loss of activity. When the reaction was run within a continuous flow microreactor, these catalysts showed good activity after at least two hours on stream.

Chemo-, Regio-, and Stereoselective Silver-Catalyzed Aziridination of Dienes: Scope, Mechanistic Studies, and Ring-Opening Reactions

Silver complexes bearing trispyrazolylborate ligands (Tp(x)) catalyze the aziridination of 2,4-diene-1-ols in a chemo-, regio-, and stereoselective manner to give vinylaziridines in high yields by means of the metal-mediated transfer of NTs (Ts = p-toluensulfonyl) units from PhI═NTs. The preferential aziridination occurs at the double bond neighboring to the hydroxyl end in ca. 9:1 ratios that assessed a very high degree of regioselectivity. The reaction with the silver-based catalysts proceeds in a stereospecific manner, i.e., the initial configuration of the C═C bond is maintained in the aziridine product (cis or trans). The degree of regioselectivity was explained with the aid of DFT studies, where the directing effect of the OH group of 2,4-diene-1-ols plays a key role. Effective strategies for ring-opening of the new aziridines, deprotection of the Ts group, and subsequent formation of β-amino alcohols have also been developed.

Synthesis of d- and l-Carbocyclic Nucleosides via Rhodium-Catalyzed Asymmetric Hydroacylation as the Key Step

D- and L-carbocyclic nucleosides were obtained by a new procedure involving an enantioselective rhodium/duphos-catalyzed hydroacylation reaction as the key step. The 3-hydroxymethyl-cyclopentanol intermediate was obtained by stereoselective reduction of ketone and by dynamic kinetic resolution (DKR).