Hydroformylation regioselective, alkenes

Regioselective reactions belong to the most important applications of homogeneous catalysis. An example is the hydroformylation of alkenes, which is a very important industrial reaction ... 

Abstract Recent advances in synthetic aspects of the rhodium-catalyzed hydroformylation of alkenes are reviewed. Emphasis is given to practical improvements, efficient new catalysts for regioselective and enantioselective hydroformylation, and to applications of the reaction in organic synthesis. Furthermore, new developments in directed hydroformylation are covered as well as new approaches toward efficient hydroformylation catalysts employing the concept of self-assembly. 

The regioselectivity of the hydroformylation of alkenes is a function of many factors. These include inherent substrate preferences, directing effects exerted by functional groups as part of the substrate, as well as catalyst effects. In order to appreciate substrate inherent regioselectivity trends, alkenes have to be classified according to the number and nature of their substitution pattern (Scheme 3) [4]. 

Recently, a new bidentate hemispherical chelating bisphosphite ligand based on a calixarene backbone has been designed for linear selective hydroformylation of alkenes (Scheme 9) [54], Excellent levels of regioselectivity have been observed, and even the intrinsic branched-selective hydroformylation of styrene could be overruled by this system. However, the system suffers from low catalytic activity. 

Three generations of dendritic phosphines have been prepared from 3,5-diaminobenzoylglycine and 9-fluorenylmethoxycarbonyl-L-phenylalanine. The dendrimers were then attached to MBHA resin, treated with CH20 and Ph2PH, and converted to their Rh complexes. The polymer-supported complexes are excellent catalysts for the hydroformylation of alkenes, which could be recycled.283 The bidentate diphosphine A,A-bis-(P-(phosphabicyclo[3.3.1] nonan) methyl)aniline was prepared by phosphanomethylation of aniline. It forms a Rh-complex which is a highly regioselective catalyst in the hydroformylation of citronellene.284... 

Breit, B. and Seiche, W. (2003) Hydrogen bonding as a construction element for bidentate donor ligands in homogeneous catalysis Regioselective hydroformylation ofterminal alkenes. J. Aw. Chem. Soc., 125, 6608-6609. 

Hydroformylation. Rhodium catalysts are generally preferred to cobalt catalysts by industry for hydroformylation of alkenes to give the homologous aldehydes because of the former s greater activity and higher regioselectivity. ... 

Miscellaneous. - Several new optically active tervalent phosphorus acid derivatives have been prepared for use as ligands in asymmetric metal catalysed reactions. These include the cyclic diaminophosphines 57, the cyclic bisamino-phosphine 58, and the compounds 59, 60, 61, and 62 containing a 1,1 -binaphthalene group as the chiral inducer. A new diphosphoramidite (63) has been used for improved regioselectivity of rhodium-catalysed hydroformylations of alkenes. A new sterically hindered chiral phosphite (64) derived from glucose and a Cu(I) complex of 64 have been prepared. ... 

Hydroformylation regioselectivity can be established by either four-coordinate or five-coordinate intermediates. In one possible mechanism, the regioselectivity is controlled by the relative concentrations of four-coordinate intermediates, 3c, 3t, 10c and lOt, which undergo irreversible formation of alkene hydride complexes, 4. Accordingly, this mechanism requires that interconversion of isomeric pairs 4ee/4ae and 4e/4a be slow relative to irreversible migratory insertion of alkene. 

Os-2-alkenes react a few times faster than tra 5-2-alkenes. Steric influences, both from the complex part and the alkene, play a dominant role in determining the fate of hydroformylation of alkenes not containing functional groups. For alkenes such as styrene, 3,3,3-trifluoropropene, acrylates, and vinyl esters, the electronic properties of the alkene determine to a large extent the regioselectivity. 

An interesting catalyst behaviour has been observed in the hydroformylation of alkenes here, the iridium complex (135) first activates hydrogen via the hydrogenolysis of the Ir-O bond upon the formation of (136), which then reacts with the alkene and CO to produce MeCH2CH=0. Notable is the participation of the side arm of the ligand and no change of the oxidation state of the metal. However, the regioselectivity of the reaction (branched/linear) is rather poor (Scheme 9). ... 

The hydroformylation of alkenes is one of the most extensively applied homogeneous catalytic processes in industry. More than 9 million tons of aldehydes and alcohols are produced annually [13]. Many efforts have been devoted in the last few years to the development of systems with improved regioselectivity toward the formation of the industrially more important linear aldehyde. Both phosphine- and phosphite-based systems giving high regioselectivities to linear aldehyde for the hydroformylation of terminal and internal alkenes have been reported [1,2,14—16] (Scheme 1.1). 

These atom-economic reactions use only carbon monoxide and an appropriate alcohol or water as reagents, to install a carboxyl group onto an alkene via the formation of a new carbon-carbon bond. As with hydroformylation, regioselectivity is of clear importance for the generation of a chiral center, and high enantioselectivity is plainly desired for the reaction to be truly efficient. 

Scheme 3 Regioselectivity trends on hydroformylation of different alkene classes...

Much progress has been made on regioselective hydroformylation of terminal alkenes in favor of the linear product. In particular bidentate phosphine or phosphite ligands, which have a natural bite angle 9 of about 110°, will favor the linear product. The most successful ligand types are BISBI [49, 50], BIPHEPHOS [51,52], and XANTPHOS systems (Scheme 8) [53]. 

Scheme 8 Bidentate ligands for regioselective hydroformylation of terminal alkenes...

The major problem remains control of regioselectivity in favor of the branched regioisomer. While aryl alkenes as well as heteroatom-substituted alkenes favor the chiral branched isomer, for aliphatic alkenes such an intrinsic element of regiocontrol is not available. As a matter of fact branched-selective and asymmetric hydroformylation of aliphatic alkenes stands as an unsolved problem. In this respect regio- and enantioselective hydroformy-... 

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