Highly Regioselective Hydroformylation with Hemispherical Chelators

Abstract

The hemispherical diphosphites (R,R)- or (S,S)-5,11,17,23-tetra-tert-butyl-25,27-di(OR)-26,28-bis(1,1'-binaphthyl-2,2'-dioxyphosphanyloxy)calix[4]arene (R=OPr, OCH(2)Ph, OCH(2)-naphtyl, O-fluorenyl; R=H, R'=OPr) (L(R)), all with C(2) symmetry, have been synthesised starting from the appropriate di-O-alkylated calix[4]arene precursor. In the presence of [Rh(acac)(CO)(2)], these ligands straightforwardly provide chelate complexes in which the metal centre sits in a molecular pocket defined by two naphthyl planes related by the C(2) axis and the two apically situated R groups. Hydroformylation of octene with the L(Pr)/Rh system turned out to be highly regioselective, the linear-to-branched (l:b) aldehyde ratio reaching 58:1. The l:b ratio significantly increased when the propyl groups were replaced by -CH(2)Ph (l:b=80) or -CH(2)naphthyl (l:b=100) groups, that is, with substituents able to sterically interact with the apical metal sites, but without inducing an opening of the cleft nesting the catalytic centre. The trend to preferentially form the aldehyde the shape of which fits with the shape of the catalytic pocket was further confirmed in the hydroformylation of styrene, for which, in contrast to catalysis with conventional diphosphanes, the linear aldehyde was the major product (up to ca. 75 % linear aldehyde). In the hydroformylation of trans-2-octene with the L(benzyl)/Rh system, combined isomerisation/hydroformylation led to a remarkably high l:b aldehyde ratios of 25, thus showing that isomerisation is more effective than hydroformylation. Unusually large amounts of linear products were also observed with all the above diphosphites in the tandem hydroformylation/amination of styrene (l:b of ca. 3:1) as well as in the hydroformylation of allyl benzyl ether (l:b ratio up to 20).

Micellar Effects in Olefin Hydroformylation Catalysed by Neutral, Calix[4]arene‐Diphosphite Rhodium Complexes

“…We noted that the rate increase was not accompanied by a significant modification of the regioselectivity. As previously observed in reactions performed at higher temperatures, [9,10] the regioselectivities were lower with 4 than 5 ( Table 1, entries 3 and 4). For the catalytic study carried out in water, only the precatalyst giving the best results in toluene, namely complex 5 (with benzyl substituents) was used.…”

“…[9,10] The high regioselectivity observed with these ligands toward the production of linear aldehydes was attributed to the ability of the diphosphites to form upon chelation a tight molecular pocket able to control the shape of the outcoming product. The H NMR spectrum displays two AB systems for the ArCH 2 Ar methylenic protons.…”

“…For the catalytic study carried out in water, only the precatalyst giving the best results in toluene, namely complex 5 (with benzyl substituents) was used. The experiments were done in the presence of various upper rim-sulfonated calixarenes (7)(8)(9)(10). The calixarenes 8-10, bearing linear alkyl chains at the lower rim, were prepared according to a general method reported by Shinkai starting from the tetrasulfonated calix [4]arene 7.…”

“…[10] We have no rationale for this change in behaviour. Note that in the presence of sulfonated calixarenes bearing H, n-C 4 H 9 , or n-C 6 H 13 substituents at the lower rim, the endo selectivity was close to 100% (Table 4, entries 1-3).…”

Rhodium-Biphephos-Catalyzed Tandem Isomerization–Hydroformylation of Oleonitrile

“…Rhodium-catalyzed tandem isomerization-hydroformylation of middle-chain alkenes (<10 C) has been widely studied over the last two decades, generally with bulky ligands [7,8]. Regioselectivities in the range l/b = 95:5-98:2 were obtained with 2-alkenes (e.g., 2-hexene, 2-octene) [9][10][11][12][13][14], while with 4-octene regioselectivities up to l/b = 89:11 were observed [15,16]. On the other hand, the hydroformylation of long-chain alkenes (>10 C), with a C=C bond in a deep internal position (>5 C away), remains poorly documented.…”