Branched Selectivity in the Pd‐Catalysed Methoxycarbonylation of 1‐Alkenes

Abstract

The methoxycarbonylation of alkenes by palladium(II) complexes with P,O‐donor ligands [(2‐methoxyphenyl)diphenylphosphine (L‐2), bis(2‐methoxyphenyl) phenyl phosphine (L‐3) and tris(2‐methoxyphenyl) phosphine (L‐4)] has been investigated. The results show that the Pd complexes derived from these ligands provide high regioselectivity for the branched esters from 1‐pentene and 1‐hexene (>80 %). Various parameters (including temperature, pressure, acid concentration) were optimized to improve the performance of the catalyst system. Higher temperatures afforded higher regioselectivity; but effected rapid catalyst decomposition. Acceptable turnover frequencies, conversions as well as catalyst stability could be obtained at higher L/Pd ratios. The dramatic change in regioselectivity is rationalised on the basis of the hemi‐lability of the o‐methoxy moiety, which may lead to ligand dissociation from L2PdX2 (L=ligand, X=Cl) rather than the expected dissociation of X. In support of our hypothesis, direct evidence for the coordination of the o‐methoxy to the Pd centre was demonstrated by the crystal structure. To the best of our knowledge, this work provides the first reported route to valuable branched esters through the methoxycarbonylation of alkenes at suitable rates.

Structural and theoretical studies of the methoxycarbonylation of higher olefins catalysed by (Pyrazolyl‐ethyl)pyridine palladium (II) complexes

“…With respect to steric bulk, the relatively sterically demanding PCy 3 afforded lower conversions of 37%, compared to the less bulkier PPh 3 , which gave conversions of 91%. This trend may be assigned to a hindered substrate coordination to the palladium metal when the bulky PCy 3 phosphine is used . In addition, we noted that the more electron rich P (OMe) 3 group resulted in lower percentage conversion (56%) compared to PPh 3 (91%).…”

“…It has been widely reported that the use of different phosphine derivatives to stabilize the active palladium species significantly affect the catalytic performance of palladium complexes in methoxycarbonylation reactions . Complex 2 , was thus used to probe this feature using both chelating and monodentate phosphine groups, offering various electronic and steric properties (Table , entries 1, 7–9).…”

Sterically hindered (pyridyl)benzamidine palladium(II) complexes: Syntheses, structural studies, and applications as catalysts in the methoxycarbonylation of olefins

“…Such an observation has been made before and associated with enhanced isomerization reactions at higher temperatures. [ 47 ]…”

“…Such an observation has been made before and associated with enhanced isomerization reactions at higher temperatures. [47] The dependence of catalytic performance of complex Pd5 on CO pressure was scrutinized through varying it from 50 to 70 bar (Table 3, entries 5, 7, and 8). Expectedly, a higher pressure of 60 bar concomitantly led to enhanced yields of 94%, in comparison with percentage yields of 66%, observed at 50 bar.…”

“…[13,15] Similarly, the catalytic process has employed mostly in situ generated active catalysts from the reactions of various of palladium(II) metal salts and the phosphine-based ligands. [16][17][18][19][20] However, despite the success of these phosphine-based palladium(II) catalysts, they are not without any blemish. Key among the limitations is the relatively high costs of their establishment, in addition to their isolation and handling.…”