Bulky phosphite

Again, steric effects prevent more than four bulky phosphites coordinating [135],... 

Since carbohydrates constitute an inexpensive and highly modular chiral source for preparing chiral ligands," Claver et al. have reported the use of a series of thioether-phosphite" and thioether-phosphinite furanoside ligands" in the test palladium-catalysed allylic substitution reaction. In the first type of ligand, a systematic variation of the donor group attached to the carbon atom C5 indicated that the presence of a bulky phosphite functionality had a positive effect on the enantioselectivity. Indeed, the enantioselectivity was controlled mainly by the phosphite moiety. This was confirmed by the use of a ligand... 

Remarkably, Claver et al. showed that in a square planar rhodium carbonyl chloride complex, two bulky phosphite ligands (65) were able to coordinate in a trans orientation.214 Diphosphite ligands having a high selectivity for linear aldehyde were introduced by Bryant and co-workers. Typical examples are (67)-(70).215,216... 

Figure 8.1. Structures of ttpms, van Leeuwen s "bulky phosphite", and a highly stable, bulky...

Ligand design for fine chemical applications has been very limited and usually the ligands designed for large-scale applications are also tested for more complicated organic molecules. Tpp has been the workhorse in fine chemicals hydroformylation ever since Wilkinson s first examples [21,22], but also bulky phosphite [5], tppts and tppms [23] turned out to be very useful, and also diphosphites have been studied [24],... 

For carbonyl systems and bulky phosphite systems, the resting state of the catalysts (for 1-alkenes) is the acylrhodium species of type 8 and thus, the reaction with dihydrogen is rate limiting [47,48], The approximate rate equation is given below for type-II kinetics. We would like to stress that type-II kinetics is the exception rather than the rule. 

This complex easily looses CO, which enables co-ordination of a molecule of alkene. As a result the complexes with bulky phosphite ligands are very reactive towards otherwise unreactive substrates such as internal or 2,2-dialkyl 1-alkenes. The rate of reaction reaches the same values as those found with the triphenylphosphine catalysts for monosubstituted 1-alkenes, i.e. up to 15,000 mol of product per mol of rhodium complex per hour at 90 °C and 10-30 bar. When 1-alkenes are subjected to hydroformylation with these monodentate bulky phosphite catalysts an extremely rapid hydroformylation takes place with turnover frequencies up to 170,000 mole of product per mol of rhodium per hour [65], A moderate linearity of 65% can be achieved. Due to the very fast consumption of CO the mass transport of CO can become rate determining and thus hydroformylation slows down or stops. The low CO concentration also results in highly unsaturated rhodium complexes giving a rapid isomerisation of terminal to internal alkenes. In the extreme situation this means that it makes no difference whether we start from terminal or internal alkenes. 

Since hydroformylation of internal alkenes is also relatively fast with this catalyst the overall linearity obtained may become rather low (20-30%). This explains the low linearity of a non-optimised reaction using a similar bulky-phosphite R = 2,6-Me2C6H30 quoted in Table 8.5. 

Chart 6.1 Structure of bulky phosphites, 1 and 3, and an electron-poor phosphite, 2. 

Figure 1 Bulky phosphite and electron-poor phosphite ligands.

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

The Lewis acid-catalysed 3 + 2-cycloaddition of cyclopropanes with aldehydes yields tetrahydrofurans with high diastereoselectivity.22 The enantiospecific Sn(II)-and Sn(IV)-catalysed formal 3 + 2-cycloadditions of aldehydes with donor-acceptor (g) cyclopropanes produce optically active tetrahydrofurans23 The bulky phosphites and phosphoramidites are excellent ligands which promote the Pd-catalysed 3 + 2-intramolecular cycloaddition between alkylidenecyclopropanes and alkynes24 The... 

Also, bulky phosphite-modified rhodium catalysts are highly reactive for the hydroformylation of unsaturated fatty acid esters [23]. The catalyst was able to yield turnover numbers (TON) of 400-500 when moderate conditions with 20 bar synthesis gas pressure and 100°C were applied. These phosphites, like tris (2-ferf-butyl-methyl) phosphite, have higher activity than phosphines like triphenylphosphine. 

Muilwijk KF, Kamer PCJ, van Leeuwen PWNM (1997) A bulky phosphite-modified rhodium catalyst for the hydroformylation of unsaturated fatty acid esters. J Am Oil Chem Soc 74 223-228... 

Recently, Union Carbide has announced the replacement of the triphenylphos-phine ligand by bulky phosphites. The new catalysts lead to higher reaction rates, selectivity and catalyst stability and may also be of interest for the hydroformylation of 2-butenes. 

Very bulky phosphite ligands (see Fig. 6.3) also yield unstable rhodium complexes (sterically hindered phosphites are commercially available as antioxidants for polyalkenes). The cone angle of the ligand shown is as high as 175° and the complex formed with rhodium has the formula HRh(CO)sL apparently there is not enough space for the coordination of a second phosphite. (In a trigonal bipyramidal complex there is room for two bulky ligands at the two axial positions but this would leave an equatorial position for a a-bonded hydride... 

A system kinetically very similar to the phosphine-free rhodium carbonyl catalyst is obtained with bulky phosphites (Fig 6.3). At temperatures from 50 to 80°C, and CO and H2 partial pressures ranging from 10 to 70 bar, the rate of aldehyde formation is first order in H2 and approximately minus one order in CO. The reaction rate is independent of the concentration of 1-octene at conversions below 30%. The reaction was found to be first order in rhodium concentration and insensitive to the phosphite/rhodium ratio, provided that the absolute concentration was sufficiently high to generate a hydride complex from the pentanedionate precursor (reaction 9). 

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