Norbornene hydroformylation

In the case of aliphatic or alicyclic olefins only norbornene hydroformylation with Rh/(—VDIOP does not fit into the picture. The contrasting results between (Z)-2-butene and bicyclo[2.2.2]oct-2-ene obtained with Co/(—)-DIOP and Ru/ (—)-DIOP catalytic systems have been attributed to extensive isomerization of (Z)-2-butene to (E)-2-butene during hydroformylation16>. In the case of the only phenyl-substituted substrate investigated, the prediction of the unsaturated carbon atom preferentially formylated is not correct. This type of exceptions is found also for styrene, as it will be discussed later. 

Hydroformylation of 2-phenylsulfonyl-substituted norbornene (73) and norbornadi-ene derivatives, catalysed by the unmodified (acac)Rh(CO)2, has been reported to give, under standard conditions, exo-norbornane- and exo-norborncnc-carboxaldehydes. The steric properties of the sulfonyl substituent, rather than its electronic effects, are believed to control the regioselectivity of the process.102... 

A highly regio- and enantio-selective hydroformylation of alkenes, such as PhCH= CH2, CH2=CHCH2CN, and CH2=CHOAc, catalysed by ruthenium complexes with (g) 2,5-disubstituted phospholane ligands has been reported. With (83) as the ligand, the turnover rates over 4000 h-1 at 80 °C, have been attained.108 (Acac)Rh(CO)2-TangPhos [Tangphos = (84)] has been developed as a new enantioselective catalyst for asymmetric hydroformylation of norbornene and other [2.2.1]-bicyclic alkenes (55-92% ee).109... 

The last contribution quoted in this section is a phosphine-free hydroformylation process based on a liquid triphasic system consisting of isooctane, water and trioctylmethylammonium chloride (TOMAC). The hydroformylation of model olefins required neat RhCls only as catalyst precursor. In the triphasic system, the catalyst is confined in the TOMAC phase, likely in the form of an ion pair. Products are obtained in excellent yields (> 90% at 80 °C) and high regioselectivity (>98%) in favour of the branched aldehyde in the case of styrene, while the exo isomer was obtained in >90% selectivity in the case of norbornene. The products were easily removed and the catalyst was recycled several times, with no leaching of rhodium into the organic phase. 

The Rh/TPPTS catalyst system is only applicable to the hydroformylation of terminal linear alkenes. With branched or internal alkenes as substrates only very low conversion rates are achieved. Exceptions include strained cyclic alkenes such as cyclopentene and norbornene, which are hydroformylated at moderate rates under Ruhrchemie/Rhone-Poulenc conditions. 

Even hydroformylation of symmetrically substituted aliphatic alkenes. such as (E)- and (Z)-2-butene. norbornene and bicyclo[2.2.2]oct-2-ene, where no problems of regioselectivity arise, often give only unsatisfactory results of asymmetric induction (Table 3). 

An exception is norbornene, which after 84% conversion in the presence of PtCl2/SnCl2/(—)-BPPM gives exclusively < .TO-2-formylbicyclo[2.2.1]heptane with 98.7% aldehyde selectivity and 60 % ee12. The product can be converted to the corresponding carboxylic acid upon oxidation without loss of enantiomeric purity. If the hydroformylation reaction is carried out in triethyl orthoformate as solvent, the aldehyde is trapped as the acetal. Thus, higher temperatures with higher conversions and yields can be applied without racemization of the product. 

But these protocols are not suitable for synthetic purposes. It was therefore a breakthrough when in 2015 Dong and coworkers [4] discovered the rhodium-catalyzed dehydroformylation of aldehydes in the presence of olefins, which corresponds to a net transfer hydroformylation. The new methodology operates at ambient reaction conditions and use a Rh(Xantphos) catalyst in low concentrations (Scheme 8.16). As formyl acceptors, NBD, norbornene, and benzonorbornadiene were screened, which were added in 1—6equiv relative to the aldehydic substrate. Benzonorbornadiene developed sufficient activity even at ambient temperature. A broad range of aldehydes could be so converted into olefins, among which were cychc and acyclic compounds with and without functional groups. 

Little is known on the stereochemistry of the addition of hydrogen and the formyl group in hydroformylation. However, the results obtained in the hydroformylation of 3.4-di-O-acetyl-D-xylal [920] of 3.4.6-tri-O-acetyl-D-glucal [921] and of norbornene [1040] where deuterium was used instead of hydrogen demonstrate a cis-addition. 

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