Hydroformylation of 1-Hexene Utilizing Homogeneous Rhodium Catalysts Regioselectivity as a Function of Conversion Brian E. Hanson Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Mark E. Davis Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
Since the appearance of the landmark paper by Evans, Oshorn, and Wilkinson (1) in 1968 on the hydroformylation of alkenes by homogeneous rhodium catalysts, research in this area has grown phenomenally and the level of activity remains high.' Undoubtedly a great deal of the interest in rhodium hydroformylation is related to the industrial significance of the hydroformylation of olefins and the commercial success of the Union Carbide, Davy International Ltd., and Johnson Matthey process for the hydroformylation of propylene (2). This process involves HRh(CO)(PPh3)3, which is generated in situ as the catalyst precursor. The hydroformylation reaction converts an olefin to an aldehyde via the addition of Hg and CO, formally as HzCO, thus the origin of the reaction's name. In the early literature, the reaction is referred to as the OX0 process (3). The reaction may be expressed in general terms for the hydroformylation of 1-alkenes according to
I
+ ... .
(1)
CHO where n 2 0. Typically, i t is the linear product that is of the greatest use; thus the distribution of aldehyde products is usually reported as a ratio of linear to branched products with high nlb ratios desired. Side reactions include hydrogenation and isomerization of the 1-alkene, hydrogenation of aldehyde products, and aldol condensation of the aldehydes. Cobalt catalysts generally are used under conditions that lead directlv to alcohols (4). Alkene isomerization &der hydroformylation conditions mav vroceed without vieldina free alkenes for cobalt catalysts-while for rhodium catalysts isomerization generally leads to production of free internal alkenes (5). In the presence of phosphines, isomerization is suppressed for rhodium catalysts (la). The generally accepted mechanism for rhodium hydroformylation is given in the figure (6-9). Also shown are mechanisms for hydrogenation and alkene isomerization. The isomerization and hydrogenation pathways in the figure clearly have much in common with the hydroformylation reaction. I t has been shown that isomerization is favored by
'Over 2,200 papers on hydroformylation have appeared from 1967 to 1986. Of these. 840 deal with rhodium catalysts. in 1985 alone 55 reports were published on rhodium hydroformylation catalysts. 928
Journal of Chemical Education
takes place under hydroformylation conditions and each of these species may act as the precursor to a rhodium complex that can operate in the catalytic cycles of the figure (4,lO). Isomerization is thought to he catalyzed primarily by the complexes to the right in this equilibrium, thus explaining the observed inhibition by PPh3. The CO dependence on isomerization is best explained by an increase in the rate of migratory insertion (4 5 6) thus minimizing isomerization. Examination of the figure reveals that the pathways to linear and hranched aldehvdes involve anti-Markownikov and Markowniknv nddition, respectively, of the metal hydridc ro the olefin (lo). Two factors which govern this addition were discussed in detail by Evans, ~ s b b mand , Wilkinson (la); these are the polarity of the Rh-H bond the steric wnstramts oi thr liaanhs lxmhed t(, the metal. Suhstituents on rhodium thnt increase the acidity of the Rh-H functiunality would lead to Markownikov addition and thus favor the hranched aldehyde. This is consistent with a large body of experimental results a t high olefin conversion (1,4,5,7, l l a , 12). However, the sterically demanding ligands such as PPh3 would be expected to favor anti-Markownikov addition,
--
~ H O
H-(CHz)&HCH&H3
low partial pressures of CO (5,lO). In the absence of CO, of course, hydrogenation occurs. Isomerization is inhibited both by high pressures of CO and excess Lewis base such as triphenylphosphine (la, 5,lO). Results from many laboratories suggest that the equilibrium represented by
The results for propylene hydroformylation are in good agreement with this hypothesis. Thus a 10-fold excess of PPh3 using HRh(CO)(PPh3)3as the catalyst precursor gives an nlb ratio of 2.3 (13), while a 600-fold excess of PPh3 (the reaction is run in neat PPh3) leads to an nlb ratio of 15.3 (14). Commercially the hydroformylation of propylene is accomplished a t an nlb ratio of approximately 911 (15).
For propylene, of course, isomerization does not lead to a new olefin and thus will not affect the observed nlb ratios. This is not the case for hieher olefins. I t is i m ~ l i e dand in some cases explicitly stated in the literature that the name factors affect h~droformvlationnlb ratios for hirher olefins as for propylene. The problem of isomerization of higher olefins was recognized by Evans, Osborn, and Wilkinson (I). However, since Pph3 suppresses isomerization, this pathway was not considered to govern nlb ratios. This is true for phosphine modified catalysts, but for unmodified rhodium catalysts, that is, precursors that lead to HRh(C0)d (161, isomerization is sienificant. Because of this. a fundamental misconception ahout the hydroformylation of higher olefius is often presented in review articles and textbooks. Specifically that HRh(CO)(PPh& is inherently more regioselective in its addition to l-alkenes than HRh(C0)s. (By inherent regioselectivity we mean the relative probability of antiMarkownikov vs. Markownikov addition.) In fact, although phosphine modified catalysts give higher nlb ratios a t 100% conversion the relative probability of anti-Markownikov vs. Markownikov addition for binary rhodium carbonyls is obscured by alkene isomerization for alkenes with five or more carbon atoms. The table presents results for l-hexene hydroformylatiou as a function of conversion for rhodium catalyst precursors at 50 and 125 "C and 300 psig pressure (17). The reactions listed in the table as 1.. 4.. 5.. and 6 were followed with time and thus give information on selectivity as a function of conversion. Reactions 2 and 3 are shown only at 100% conversion.
At 50 "C and a 20-fold excess of PPh3 (reaction 3) an nlb ratio of 2.77 is observed for hexene hydroformylation. Only aldehyde products are observed and no 2-ethylpentanal is produced which would have required l-hexene isomerization. This is in good agreement with published reports. In the absence of added PPh8 (reaction 2), the nlb ratio drops to 0.82 a t 100% conversion and significant quantities of 2ethylpentanal are produced. This is also consistent with the literature and suggests that the equilibrium shown in eq 2 lies far to the right. When Rh6(C0)16 is used as the catalyst precursor the results shown in reaction 1 are obtained a t 50 OC. At low conversion, 23%, an nlb ratio of 2.67 results and no 2-ethylpentanal is observed. This is very close to the nlb ratio in reaction 3 a t 100% conversion. The selectivity for hydroformylation is low, and the major reaction pathway is isomerization to 2-hexene. The internal alkene undergoes hydroformylation a t aslower rate than the terminalolefin. Thusat 99% conversion the selectivity to hydroformylation is 37.7% and the regioselectivity, nlb, has dropped to 1.66. When the selectivity has risen to >97% (i.e., when 2-hexene has been nearly completely consumed) the nlb ratio has dropped to 0.81. Similar results are obtained a t 125 OC (reactions 4, 5, and 6). However, i t is clear that i t is more difficult to suppress isomerization by adding phosphine a t the higher temperature. Thus for l-hexene hydroformylation the inherent n f b ratio for binary rhodium carhonyl catalysts and phosphine substituted catalysts appears to he nearly the same. The key step in the figure is a t compound 4 after Markownikov addi-
Schematic representation of posslble reaction pathways for isomerization, hydroformylation, and hydrogenation of l-alkenes using HRh(CO)Ls catalyst Precursors. (L may be phosphine or CO). Hydrogenation is nota significant pathway in the presence of CO. Hydroformylation is represented by the two outside loops: isomerization is represented by the inside loop through complex 8 on the len.
Volume 64
Number 11 November 1987
929
Homogeneous Hydrotormylation01 I-Hexene 01 300 pslg CO and H, In a 1:l Ratlo
Reaclion
Catalyst
I('c)
Time (h)
Conversion'
Seleclivlty'
n/b
Aldehyde Dinribution (mol %) 2-methyl14hylheptanal hexanal pentanal
sFractlon of l h e x e n e Mnsumed. 'Aldehydsa prmucedllhsxene consumed W a n d 3hexene side products). OOMP = dimemylph"nylph0sphine.
tion to the alkene. When L = CO, the secondary alkyl ligand is more likely to undergo p-hydride elimination to yield coordinated 2-alkene (complex 8) than to insert CO to yield the ac~lcom~'ex, 6' If p-h~dride from yield is fast, then elimination from 4 and 4' to regenerate 3 and must also be fast. Thus all information about the relative probabilities of Markownikov vs. anti-Markownikov addition is lost when the rate of isomerization is fast. From this discussion i t is clear that added phosphine enhances the production of linear aldehydes only in part by controlling the regiochemistry of addition of Rh-H to the l-alkene, Of equal importance is the role the phosphine plays in suppressing alkene isomerization. Decreasing nlb ratios with increasing conversion has been noted previously in the literature for some modified rhodium catalysts but not for binary rhodium carbonyls. For example, H R ~ ( C O ) ( N B Z NBz ) ~ , = tribenzylamine, shows an nlb from 13'3 at 24' to 4'9 at 62% cOnversion for 1-octene hydroformylation a t 20 and 30 OC, respectively (18).The major side reaction is shown to he isomerization. Literature Cited 1. (a) Evans, 0.: Oaborn. J. A,: Wilkinson, G.J. Chem. Soc. (A) 1968,3133. The above
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Journal of Chemical Education
paper gives the firat complete amount of homogeneous rhodium hydmfarmyktion;
however, soma result9 wereeommuniested asriier by (b) osbarn,J. A,; Young. J. F.; wilkinson, G. J. cham. so
