Interplay of bite angle and cone angle effects. A comparison between o-C₆H₄(CH₂PR₂)(PR′₂) and o-C₆H₄(CH₂PR₂)(CH₂PR′₂) as ligands for Pd-catalysed ethene hydromethoxycarbonylation

Abstract

The following unsymmetrical diphosphines have been prepared: o-C6H4(CH2PtBu2)(PR2) where R = PtBu2 (L3a); PCg (L3b); PPh2 (L3c); P(o-C6H4CH3)2 (L3d); P(o-C6H4OCH3)2 (L3e) and o-C6H4(CH2PCg)(PCg) (L3f) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions has been investigated in the presence of Pd complexes of each of the ligands L3a–f and the results compared with those obtained with the commercially used o-C6H4(CH2PtBu2)2 (L1a). The Pd complexes of the bulkiest ligands L3a, L3b and L3f are highly active catalysts but the Pd complexes of L3c, L3d and L3e are completely inactive. The crystal structures of the complexes [PtCl2(L1a)] (1a) and [PtCl2(L3a)] (2a) have been determined and show that the crystallographic bite angles and cone angles are greater for L1a than L3a. Solution NMR studies show that the seven-membered chelate in 1a is more rigid than the six-membered chelate in 2a. Treatment of [PtCl(CH3)(cod)] with L3a–f gave [PtCl(CH3)(L3a–f)] as mixtures of 2 isomers 3a–f and 4a–f. The ratio of the products 4:3 ranges from 100:1 to 1:20, the precise proportion is apparently governed by a balance of two competing factors, steric bulk and the antisymbiotic effect. The palladium complexes [PdCl(CH3)(L3b)] (5b/6b) and [PdCl(CH3)(L3c)] (5c/6c) react with labelled 13CO to give the corresponding acyl species [PdCl(13COCH3)(L3b)] (7b/8b) and [PdCl(13COCH3)(L3c)] (7c/8c). Treatment of [PdCl(13COCH3)(L)] with MeOH gave CH3(13)COOMe rapidly when L = L3b but very slowly when L = L3c paralleling the contrasting catalytic activity of the Pd complexes of these two ligands.

Catalyst Activity and Selectivity in the Isomerising Alkoxycarbonylation of Methyl Oleate

“…To identify the byproducts, these were enriched by removing the linear dimethyl nonadecane-1,19dioate (L) from the crude reaction mixture by crystallisation from methanol and column chromatography of the supernatant to remove the starting material (MO; for details, see www.chemeurj.org the Supporting Information). NMR analysis ( 1 H, 13 C, 1 H, 1 H COSY, 1 H, 13 C HSQC, 1 H, 13 C HMBC) of this purified reaction mixture revealed the presence of dimethyl 2-methyloctadecane-1,18-dioate (B1), dimethyl 2-ethylheptadecane-1,17-dioate (B2) and dimethyl 2-propylhexadecane-1,16dioate (B3). Longer-chain branched diesters, dimethyl 2-butylpentadecane-1,15-dioate (B4) and dimethyl 2-pentyltetradecane-1,14-dioate (B5), were also found.…”

“…[12] Beyond these findings, no further insights into the requirement of diphosphine ligands to promote this unusual catalytic isomerising alkoxycarbonylation have been reported. To this end, we chose the o-tolyl backbone [13,26] (Figure 2) as this allows for more straightforward and less hazardous preparation of diphosphines in comparison with 1,2-(tBu 2 PCH 2 ) 2 C 6 H 4 . More importantly, a large range of unsymmetrically substituted, bulky electron-rich diphosphines are also accessible selectively.…”

“…More importantly, a large range of unsymmetrically substituted, bulky electron-rich diphosphines are also accessible selectively. [13,14] We report herein on the corresponding neutral complexes suitable as single-component catalyst precursors, and their selectivity in isomerising methoxycarbonylation reactions.…”

An Empirical Study of Phosphine Ligands for the Methoxycarbonylation of Medium‐Chain Alkenes

“…In this case, the electron‐poorer phosphine also has an extreme steric requirement that favors the formation of MeP 10a. A further demonstration of the efficacy of unsymmetrical ligands was presented the following year, for which Pringle20d et al. conducted a study by using a variety of unsymmetrical phosphines based on o ‐diphosphinoxylenes.…”

Polymer precursors from catalytic reactions of natural oils

“…111 The field of hydro/methoxycarbonylation of ethylene to methyl propanoate using BTDPMB-based systems and variations of the ligand has been recently reviewed by Fanjul and co-workers. [112][113][114] Drent and Jager have shown the BTDPMB/palladium system to give methyl pentanoate from 2-butene in 97% selectivity at 65 bar and 100℃, 115 showing the methoxycarbonylation of a carbon chain longer than ethylene was possible. The same authors desired to increase the selectivity to the linear ester and found that using a mixture anisole/methanol : 2/1 as a solvent for the methoxycarbonylation of 2-butene and 1-octene yielded a full conversion to the ester with high selectivity to the linear species (97%).…”