Phosphines aqueous-phase conditions

The ejfect of water on the conversion and selectivity of cohalt-catalyzed hydroformylations has long been noticed in industry [7,85,86], A systematic study [87] of this effect in hydroformylation of 1-octene with [Co2(CO)s] with and without P Bu3 revealed that addition of water, and especially when it formed a separate aqueous phase, significantly inaeased the hydrogenation activity of the phosphine-modified catalyst Under the same reaction conditions (190 °C, 56 bar CO H2 1 1, P Co 3 1), approximately 40 % nonanols were formed instead of 5 % observed with water-free solutions. No clear explanation could be given for this phenomenon, although the possible participation of water itself in the hydroformylation reaction through the water gas shift was mentioned. It was also established, that the [Co2(CO)g]-catalyzed hydroformylation was severly retarded in the presence of water. Under the conditions above, 95 % conversion was observed in 15 hour with no added water, while only 10 % conversion to aldehydes (no alcohols) was found in an aqueous/organic biphasic reaction. 

The same general principles and the same phosphines (18) can be used for still another variation of catalyst recovery which was demonstrated in the hydroformylation of 1-tetradecene [143], The reaction, catalyzed by the Rh/18 catdyst, was mn in a homogeneous methanolic solution and gave slightly better results than the Rh/PPhs catalyst under identical conditions. After the reaction most of the methanol was distilled off and the remaining solution was extracted with water. The catalyst-containing aqueous phase was evaporated to dryness, the catalyst was taken up in methanol and reused. No loss of activity and selectivity was observed in three recycles. 

Special mention has to be made of the use of surfactants. Aryl halides are insoluble in water but can be solubilized in the aqueous phase with the aid of detergents. A thorough study [24,25] established that the two-phase reaction of 4-iodoanisole with phenylboronic acid (toluene/ethanol/water 1/1/1 v/v/v), catalyzed by [PdCl2 Ph2P(CH2)4S03K 2], was substantially accelerated by various amphiphiles. Under comparable conditions the use of CTAB led to a 99 % yield of 4-methoxybiphenyl, while 92 % and 88 % yields were observed with SDS and n-Bu4NBr, respectively (for the amphiphiles see Scheme 3.11). Similar effects were observed with Pd-complexes of other water-soluble phosphines (TPPTS and TPPMS), too. 

A further evidence on the acceleration enjoyed by a typical Pd-catalysed reaction, the Heck reaction, in an ionic phase ( V-mcthyl-Y.Y. V.-trioctylammonium chloride or Aliquat 336) is found in a triphasic protocol developed by Tundo and coworkers. 7b.The arylation of electron poor olefins is catalysed by palladium supported on charcoal (Pd/C) and is carried out in the heterogeneous isooctane/Aliquat 336/water system (Figure 27). Under this multiphasic condition, Aliquat 336 forms a third liquid phase between the organic and the aqueous phase that traps the catalyst. The use of phosphines is not necessary. As a matter of fact, Aliquat 336 incorporates the solid-supported catalyst and ensures an efficient mass transfer between the bulk phases resulting in an increase of the reaction rate of an order of magnitude compared to the reaction in the absence of the ionic liquid. A determing role is played by the base while I LN drives the reaction towards the formation of ethyl cinnamate, reaction carried out in the presence of KOH lead to formation of Ullmann dimerisation products. 

The water-soluble phosphine, PTA, was grafted to dendrimers and reacted with [RuCl2(p-cymene)]2- The resulting dendritic complexes were applied to the redox isomerization of l-octen-3-ol in water/re-heptane biphasic systems at 75°C. Albeit the reactions were slow, there was a large positive dendritic effect The catalytic activity increased with increasing size of the dendritic support, and the catalyst derived from the third generation dendrimers afforded 98% conversion whereas use of the soluble monomeric catalyst led to only 38% conversion under the same conditions. The catalyst was recycled in the aqueous phase by decantation with no or negligible loss of the catalytic activity (138). 

The combination of water and an organic nitrile as the solvent system (aqueous bipha-sic system, ABS) permits one to separate the catalyst in the water solution, coordinated to a water-soluble phosphine, TPPTS, the trisodium salt of trisulfonated triphenylphos-phine.f" The groups of Sinou and Genet have studied this strategyy. " ° " ° Allylic carbonates are quite stable to the potentially hydrolytic conditions since the reactions occur in neutral medium and only traces of base are generated in the catalytic cycle. Organic-aqueous phase palladium catalysis has been reviewed extensively. 

The transfer of the phosphine-assisted catalytic processes to aqueous media prompts the development of specific hydrophilic ligands. The most important rationale for the application of such ligands is the development of phase-separation techniques. In the biphasic liquid-liquid technique, the hydrophilic phosphine works as an effective extractor of palladium to the aqueous phase. However, numerous recent works coming primarily from Genet s group (vide infra) show that many important Pd-catalyzed reactions can be made to run under very mild conditions in homogeneous aqueous media if carried out in the presence of hydrophilic phosphines—essentially aqueous phosphine-assisted catalysis. 

Efficient biphasic catalysis relies on the rapid mass transfer across the aqueous and organic phases. As indicated, this poses a problem for higher olefins because of their insolubility in water. To tackle the issue and thus to increase the hydroformylation rates, additives, such as co-solvents, surfactants, or modified cyclodextrins, have been explored. A water-miscible organic cosolvent such as an alcohol could increase the solubility of alkenes in the aqueous phase or the catalyst in the organic phase. For example, using [Rh(p-S Bu)(CO)(m-TPPTS)]2 as a catalyst, hydroformylation of 1-octene gave less than 24% conversion after 15 h in water at 80 °C but it reached 90% conversion in 10 h in water/methanol (3 1) [28]. Using Rh-1, 1-dodecene was hydroformylated with 42% conversion to aldehydes in a mixture solvent of water/propanol, while no hydroformylation was observed at all in water alone under identical conditions [29]. The same trend was observed in the reaction of 1-octene catalyzed by Co/BiphTS (BiphTS, trisulfonated tris(biphenyl)phosphine) [30]. 

The benzoic acid derivative 457 is formed by the carbonylation of iodoben-zene in aqueous DMF (1 1) without using a phosphine ligand at room temperature and 1 atm[311]. As optimum conditions for the technical synthesis of the anthranilic acid derivative 458, it has been found that A-acetyl protection, which has a chelating effect, is important[312]. Phase-transfer catalysis is combined with the Pd-catalyzed carbonylation of halides[3l3]. Carbonylation of 1,1-dibromoalkenes in the presence of a phase-transfer catalyst gives the gem-inal dicarboxylic acid 459. Use of a polar solvent is important[314]. Interestingly, addition of trimethylsilyl chloride (2 equiv.) increased yield of the lactone 460 remarkabiy[3l5]. Formate esters as a CO source and NaOR are used for the carbonylation of aryl iodides under a nitrogen atmosphere without using CO[316]. Chlorobenzene coordinated by Cr(CO)j is carbonylated with ethyl formate[3l7]. 

---