2-Hexenal chemoselectivity

Alternatively, Ballini devised a new strategy to synthesize tri-alkylated pyrroles from 2,5-dialkylfurans and nitroalkanes <00SL391>. This method involves initial oxidation of 2,5-dimethylfuran with magnesium monoperoxyphthalate to cA-3-hexen-2,5-dione (6). Conjugate addition of the nitronate anion derived from the nitro compound 7 to 6 followed by chemoselective hydrogenation of the C-C double bond of the resulting enones 8 (obtained by elimination of nitrous acid from the Michael adduct) completes the conversion to the alkylated y-diketones 9. Final cyclization to pyrroles 10 featured improved Paal-Knorr reaction conditions involving reaction of the diketones with primary amines in a bed of basic alumina in the absence of solvent. 

The hydroformylation of 1-hexene catalyzed by rhodium carbonyl has recently been studied by Lazzaroni and coworkers [21]. They were particularly interested in the influence of reaction parameters on the regioselectivity and the chemoselectivity (to aldehyde and 2-hexene). To understand their results we have to extend Scheme 6.1 by taking account of the formation of linear and branched aldehydes, as well as of isomerization. This is shown in Scheme 6.2. 

The rhodium-catalyzed hydroformylation of 1-hexene using a polar-phase solvent system (30% water by volume in acetone) has resulted in improvements in rate and chemoselectivity for catalysts supported by the tetraphosphine ligand et,ph-P4. A cationic rhodium dimer comprising... 

Logothetis studied the thermal intramolecular azide-alkene cycloaddition (IAAC) of 5-azido-5-methyl-1-hexene and observed the formation of a mixture of imines and aziridines <65JA749>. The first case of utilization of silica gel in the selective decomposition of triazoline (35) to pyridines and aziridines (X = CH2—CH2, (CH2)3) was reported by Murthy and Hassner (Equation (17)) <87TL97>. The role of silica gel in the chemoselective reaction is still obscure, but it is likely that acidic surface... 

Kaspar et al. demonstrated the reduction of a,P-unsaturated ketones to allylic alcohols with /-PrOH in the gas phase over MgO as fixed bed catalyst at 250°C [7]. The MgO was formed in situ by heating Mg(OH)2 at 350°C in an air current during 4 hours. Regeneration of the catalyst was done in the same way. In a subsequent paper the chemoselective reduction of the carbonyl group of 4-hexen-3-one over various solid catalysts was reported [8]. MgO was found to show the highest chemoselectivity. However, as a result of its high basicity several side reactions were also observed. Doping of the MgO catalysts with HCl afforded solid catalysts with improved selectivity. 

The biphasic hydroformylation of 1-hexene with carbon dioxide (4 MPa)/hydrogen (4 MPa) can also be catalyzed by Ru3(CO)i2 in ILs/toluene giving high yields and good chemoselectivity in heptanols with relatively low amounts of hexane (11%) (Scheme 17). ... 

The dienamines derived from enals undergo hydroboration with 9-BBN, with high regio- and chemoselectivity at the enamine carbon-carbon double bond and place the boron atom on a carbon having higher electron density. The high selectivity is confirmed by the hydroboration of a 1 1 mixture of 1-hexene and 1-morpholino-l-octene with 1 equiv of 9-BBN. The reaction mixture affords, after the addition of methanol, a 1 1 mixture of 1-hexene and 1-octene, and no 1-hexanol is detected. Consequently, this process is used to convert unsaturated aldehydes to dienes. Thus, enamine of citronellal on hydroboration-elimination affords a chiral non-conjugated diene, p-citronellene (Scheme 24.5) [1]. 

The water-soluble pyrazolato complex (Rh(p-Pz)(CO)(TPPTS)]2 was used as precursor for olefin hydroformylation in an aqueous heptane solvent system 9]. Without additional ligand, olefin isomerization dominated hydroformylation at 7 bar CO/H2 (1 1) giving large amounts of 2-hexene. Isomerization was suppressed most effectively at 49.8 bar, leading to aldehyde chemoselectivities >90% and a 2.4-3.S njiso ratio. It was demonstrated that aerobic recycling of the aqueous catalyst phase by... 

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