Bis[1,3-bis(diphenylphosphino)propane]rhodium Tetrafluoroborate1

[70196-21-3]  · C54H52BF4P4Rh  · Bis[1,3-bis(diphenylphosphino)propane]rhodium Tetrafluoroborate  · (MW 1014.61)

(catalytic decarbonylation of aldehydes2,3)

Solubility: sol CH2Cl2, acetone, MeCN, m-xylene, toluene; slightly sol MeOH; insol diethyl ether.

Analysis of Reagent Purity: 31P NMR: d = 6.4, JRh-P = 132 Hz (ambient temp, acetone-d6, downfield from 85% H3PO4). Electronic absorption spectroscopy: lmax, nm (log εmax) 406 (3.49).

Preparative Method: all manipulations are carried out under a nitrogen atmosphere with use of standard Schlenk techniques. Rh2Cl2(1,5-cyclooctadiene)2 (400 mg, 1.63 mmol) is dissolved in 50 mL of degassed acetone to which Silver(I) Tetrafluoroborate (337 mg, 1.73 mmol) is added to precipitate AgCl. The mixture is stirred for 1 h and filtered. The filtrate is added to an acetone solution of dppp (1,3-Bis(diphenylphosphino)propane) (1.58 g, 3.84 mmol). The acetone is partially evaporated under vacuum leaving a precipitate of orange-yellow flakes of [Rh(dppp)2]BF4 in high yield.

Purification: the product [Rh(dppp)2]BF4 is recrystallized from a solvent mixture of CH2Cl2 and Et2O by a standard layering technique. The orange-yellow flakes of [Rh(dppp)2]BF4 are dissolved in a minimum amount of CH2Cl2 in a Schlenk tube equipped with a rubber septum. A volume of Et2O (ca. 3 times the CH2Cl2 volume) is carefully layered on top with use of a syringe or cannular tube. The solvents are allowed to slowly diffuse, and large red crystals are isolated. The red crystals are washed with Et2O and dried under vacuum.4

Handling, Storage, and Precautions: the catalyst is air stable in the solid state. However, it is recommended that it be stored under N2. Decarbonylation reactions must be carried out in the absence of oxygen.

Aldehyde Decarbonylation.

The catalyst [Rh(dppp)2]BF4 decarbonylates aldehydes in a homogeneous solution phase with rates that are more than two orders of magnitude faster than the stoichiometric reagent Chlorotris(triphenylphosphine)rhodium(I), or the catalytic reagent Carbonyl(chloro)bis(triphenylphosphine)rhodium(I). A wide variety of aldehydes can be decarbonylated under mild thermal conditions, as shown in Table 1.5 This decarbonylation reaction occurs with high product yields (based on aldehydes), and total turnovers in excess of 100 000 have been achieved. The catalyst is stable for days, and the reactions are highly selective. A typical reaction with benzaldehyde or heptanal is carried out with approx. 20 mg of [Rh(dppp)2]BF4 dissolved under an N2 atmosphere in 30 mL of either neat aldehyde or about 2 mL of aldehyde in m-xylene or toluene. The homogeneous solution is stirred at constant temperature and continuously purged with N2 gas. The products can be collected continuously in a cold trap connected to a reflux condenser.

The activity of the catalyst is dependent on the steric size and electronic inductive properties of the aldehyde substrate. Compared to other catalysts, [Rh(dppp)2]BF4 is significantly more active for the decarbonylation of benzaldehyde. Rh(PPh3)3Cl6 at 150 °C demonstrates a catalytic activity of 0.60 moles benzene (mol catalyst)-1 h-1 while the catalytic activity of [Rh(dppp)2]BF4 at the same temperature is 1.0 × 102. Focusing on the steric size of the substrate, there is a significant rate decrease when the a-carbon atom of the aldehydes is substituted (Table 2). The decarbonylation of tertiary aldehydes is of no synthetic utility as it occurs very slowly, even at 180 °C. Para substitution on benzaldehyde (either with electron-withdrawing or electron-donating groups) decreases the catalytic activity. This is in contrast to that observed using Rh(PPh3)3Cl (stoichiometric reaction at 100 °C), where a linear Hammet plot was obtained with a positive ρ value.7

An important characteristic of [Rh(dppp)2]BF4 is its excellent selectivity. For example, the yield for benzene production from benzaldehyde is 100%. Note that 1-heptanal is converted into 86% hexane and 14% 1-hexene by Rh(PPh3)3Cl (stoichiometric reaction, 25 °C); while using [Rh(dppp)2]BF4 as the catalyst, the only volatile product is hexane. Additionally, only ethylbenzene is produced from 2-phenylpropanal although the possible unsaturated product, styrene, is very stable. Rh(PPh3)3Cl is also known to isomerize double bonds during decarbonylation.8 For example, a-methylcinnamaldehyde is converted into 91% cis- and 9% trans-b-methylstyrene at 140 °C. No trans isomer is produced using [Rh(dppp)2]BF4 at 150 °C.

The effect of added reagents on the catalytic activity of [Rh(dppp)2]BF4 is worth noting. The catalyst will tolerate molar concentrations of ketones, acids, alkenes, and chlorinated aromatics with only a moderate decrease in activity.1 In general, reagents that can compete for a coordination site on the rhodium atom (i.e. CO, nitriles, phosphines, acid chlorides) will effectively retard the reaction rate. Most importantly, the presence of O2 irreversibly decomposes the catalyst in solution. Thus all decarbonylations must be carried out under an inert atmosphere such as N2 gas.

There have been several reports of catalytic aldehyde decarbonylation systems based on RuII and FeII tetraphenylporphyrin compounds.9,10 These catalysts are thought to involve radical pathways and offer an alternative to [Rh(dppp)2]BF4.

1. Doughty, D. H.; Anderson, M. P.; Casalnuovo, A. L.; McGuiggan, M. F.; Tso, C. C.; Wang, H. H.; Pignolet, L. H. Adv. Chem. Ser. 1982, 196, 65.
2. Doughty, D. H.; Pignolet, L. H. JACS 1978, 100, 7083.
3. Doughty, D. H.; McGuiggan, M. F., Wang, H. H.; Pignolet, L. H.; Fundam. Res. Homogeneous Catal. 1979, 3, 909.
4. Preparation adapted from Slack, D. A.; Baird, M. C. JOM 1977, 142, C69.
5. Doughty, D. H. Ph.D. Thesis, University of Minnesota, Minneapolis, 1979.
6. The active catalyst is actually trans-RhCl(CO)(PPh3)2 which forms via the stoichiometric decarbonylation. (a) Lau, K. S. Y.; Becker, Y.; Huang, F.; Baenziger, N.; Stille, J. K. JACS 1977, 99, 5664. (b) Egglestone, D. L.; Baird, M. C.; Lock, C. J. L.; Turner, G. JCS(D) 1977, 1576.
7. Stevens, D. J.; Nelson, D. A. Abstracts of Papers, 177th National Meeting of the American Chemical Society, April, 1979; INOR 47.
8. Ohno, K.; Tsuji, J. JACS 1968, 90, 99.
9. Domazetis, G.; James, B. R.; Tarpey, B.; Dolphin, D. ACS Symp. Ser. 1981, 152, 243.
10. Belani, R. M.; James, B. R.; Dolphin, D.; Rettig, S. J. CJC 1988, 66, 2072.

Mark A. Aubart & Louis H. Pignolet

University of Minnesota, Minneapolis, MN, USA

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