Isomerization rhodium carboxylates

Carbonylation of 2,3-homo-l//-azepines has been effected by means of their metal and carbonyl complexes and provides a useful route to a variety of isomeric azabicyclo-nonadienones. For example, the tricarbonyliron complex with carbon monoxide at 80 °C and 160 atm yields the 9-oxo-2-azabicyclo[3.3.l]nona-3,7-diene (167) (57%) or the 9-oxo-2-azabicyclo[3.2.2]nona-3,6-diene (168) (60%) depending on the exo or endo configuration of the tricarbonyliron complex. A third isomer, namely ethyl 7-oxo-9-azabicyclo[4.2.1]nona-2,4-diene-9-carboxylate (169), is formed on heating (125 °C) the azepine with carbon monoxide under pressure in the presence of the rhodium carbonyl complex [Rh(CO)Cl2] (78CB3927). 

Asymmetric catalysis undertook a quantum leap with the discovery of ruthenium and rhodium catalysts based on the atropisomeric bisphosphine, BINAP (3a). These catalysts have displayed remarkable versatility and enantioselectivity in the asymmetric reduction and isomerization of a,P- and y-keto esters functionalized ketones allylic alcohols and amines oc,P-unsaturated carboxylic acids and enamides. Asymmetric transformation with these catalysts has been extensively studied and reviewed.81315 3536 The key feature of BINAP is the rigidity of the ligand during coordination on a transition metal center, which is critical during enantiofacial selection of the substrate by the catalyst. Several industrial processes currently use these technologies, whereas a number of other opportunities show potential for scale up. 

Because the unsaturated hydrocarbon has to bind to rhodium in the presence of bulky PPh3 groups, the catalyst favours unsubstituted double bonds (RCH=CH2 rather than RR C=CR r ). Since the alkyl intermediate is shortlived, there is little tendency to / -elimination with concomitant alkene isomerization. Although both alkene and alkyne functions are reduced, in general carbonyl or carboxylic groups and benzene rings are not, though aldehydes are frequently decarbonylated. Peroxides tend to oxidize and thus destroy the catalyst, so that substrates need to be purified carefully before use. 

Enantiomerically pure carboxylic acids are routinely obtained from N-acylsultams by Hydrogen Peroxide assisted saponification with Lithium Hydroxide in aqueous THF. 4 Alternatively, transesterification can be effected under neutral conditions in allyl alcohol containing Titanium Tetraisopropoxide, giving the corresponding allyl esters which can be isomerized/hydrolyzed with Wilkinson s catalyst (Chlorotris(triphenylphosphine)rhodium(I)) in Et0H-H20. This provides a convenient route to carboxylic acids containing base-sensitive functionality. Primary alcohols are obtained by treatment with L-Selectride (Lithium Tri-s-butylborohydride) in THF at ambient temperature. ... 

Alkyl-allyl complexes of isomeric systems can be interconverted and thus be used in isomerization of vinylcyclopropanes. Ethyl 4-azabicyclo[5.1.0]octa-2,5-diene-4-carboxylate (20) reacts with pentacarbonyliron to give complex 21, which photochemically rearranges to complex 23. Carbonylation of both products 21 and 23 leads to ethyl 9-oxo-2-aza-bicyclo[3.3.1]nona-3,7-diene-2-carboxylate (22). While complex 21 upon heating regenerates the starting material, complex 23 gives the isomeric product 24. In contrast to iron, with rhodium only the endo-complex 25 is formed. ... 

Callot [14,15] and Noels [16,17] examined the reactions of rhodium porphyrins [14, 15] and carboxylates [16, 17] with various alkanes. When n-alkanes 17 were utilized, complex mixtures of isomeric products were always obtained (Scheme 4). Functionalization at C2 (as in 18) was nearly always the major product, and, depending on the catalyst used, Cl (19) or C3 (21) could be the site of the second most common attack. Branched alkanes also led to multiple products, but C-H insertion at tertiary sites was generally favored. 

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