Organometallics 1995, 14, 869-874
869
Exploring Stereoselectivity through the Quantitative Analysis of Ligand Effects. Addition of Hydride to
(17-Cp)(CO)(PR3)FelC(OMe)(Me)l+ Beth A. Lorsbach, Dawn M. Bennett, Alfred Prock,* and Warren P. Giering" Department of Chemistry, Metcalf Science and Engineering Center, Boston University, Boston, Massachusetts 02215 Received July 25, 1994@ The stereoselectivity of the addition of hydride (from NaBH4) to 14 chiral cationic carbene has been studied as a function of the stereoeleccomplexes, q-Cp(CO)(PR3)Fe[C(OMe)Mel+, tronic properties (x,E,, and 6 ) of the phosphine ligands. The anti/syn (RS/SR:SS/RR)ratio which varies from 0.99 of the diastereomeric products, q-Cp(CO)(PR3)Fe*[C*H(OMe)Mel, to greater than 30, is independent of 8 below 145" and rises dramatically, becoming too large to measure by NMR methods above 153". The anti/syn ratio increases with increasing electron donor capacity of PR3 (smaller x) and decreases with increasing number of aryl groups (larger E a r ) attached to PR3. These results suggest that the addition of hydride to q-Cp(CO)(PR3)Fe[C(OMe)Mel+involves a steric threshold near 145" that must be exceeded in order to achieve significant stereoselectivity.
Introduction Many stereoselective reactions involve a chiral auxiliary or a stereogenic center t o which are attached pendent groups of the type AR3, where A = P,la ,b Si,lC or Sn.ld How variations of the stereoelectronic properties of these chiral auxiliaries affect the stereoselectivities of potentially asymmetric reactions has not been systematically explored, as far as we are aware, and is not understood. Since the stereoelectronic properties of these pendent groups can be varied almost continuously,2the results of experiments in which AR3 groups are systematically varied would afford insight into the factors influencing stereoselectivity and would allow, in principle, a "fine tuning" of the stereogenic center leading to a maximization of the stereoselectivity of the reaction. We are currently reexamining and expanding on a number of studies of stereoselective reactions involving AR3 groups that have been reported in the literature. These reactions include the addition of hydride (from NaBH4) to the family of cationic iron(I1) carbene comp l e x e ~~pCp(Co)(PR3)Fe[C(oMe)Mel+, ,~ (described herein) and the osmylation of the chiral acetoxy allyl silanes (R~S~)(ACO)CHCH=CH~,~ which we will describe in a future publication. Studies of these reactions are advantageous in that the preparation of the starting materials, the methodology of establishing relative stereochemistry, and the absolute configuration of the products have been worked out previou~ly.~,~ In addition, the conformationalpreferences of these and related ~~~~~~~~
Abstract published in Advance ACS Abstracts, January 1, 1995. (1) See for example the following references: (a) Kagan, H. B. Asymmetric Synthesu; Morrison, J. D., Ed.; Academic Press: London, 1985; Vol. 5, pp 1-39. (b) Davies, S.Aldrichim. Acta 1990,23, 31. (c) Ojima, I.; Hirai, K. Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: London, 1985; Vol. 5, pp 103-46. (d) Marshall, J. A.; Beaudoin, S.; Lewinski, K. J . Org. Chem. 1993, 58, 5876. (2) Wilson, M. R.; Liu, H.-Y.; Prock, A,; Giering, W. P. Organometallics 1993, 12, 2044. @
compounds5 have been studied theoretically6-8 and spectroscopically.8 Qualitatively, it has been observed that these reactions are sensitive to the stereoelectronic properties of AR3,3d93194but the literature data are insufficient t o analyze quantitatively. Thus, the assessment of steric and electronic influences cannot be achieved. When we began these studies we asked three questions. (1)Can the stereoselectivity of a reaction be analyzed in terms of the stereoelectronic parameters of AR3 or will other factors not associated with AR3 (e.g. solvent properties) control the stereoselectivity? (2) If the stereoselectivity can be analyzed in terms of the stereoelectronic parameters, can we gain insight into the factors that control stereoselectivity? (3) Can we (3) (a) Brookhart, M.; Nelson, G. 0. J.Am. Chem. SOC.1977, 99, 6099. (b) Cutler, A. R. J.Am. Chem. SOC.1979, 101, 604. (c) Bodnar, T.; Cutler, A. R. J . Organomet. Chem. 1981,213, C31. (d) Brookhart, M.; Tucker, J. R.; Husk, G. R. J . Am. Chem. SOC.1983, 105, 258. (e) Brookhart, M.; Timmers, D.; Tucker, J. R.; Williams, G. D.; Husk, G. R.; Brunner, H.; Hammer, B. J. Am. Chem. SOC.1983,105,6721.(0 Baird, G. J.; Davies, S. G.; Maberly, T. R. Organometallics 1984, 3, 1764. (g) Davies. S. G.; Maberly, T. R. J.Organomet. Chem. 1985,296, C37. (h) Asycough, A. P.; Davies, S.G. J. Chem. SOC.,Chem. Commun. 1986, 1648. (i)Brookhart, M.; Liu, Y.; Buck, R. C. J. Am. Chem. Soc. 1988, 110, 2337. Brookhart. M.; Liu, Y. Organometallics 1989, 8, 1572. (k) Brookhart, M.; Buck, R. C. J . Am. Chem. SOC.1989, 111, 559. (1) Brookhart, M.; Liu, Y.; Goldman, E. W.; Timmers, D. A.; Williams, G. D. J . Am. Chem. SOC.1991, 113, 927. (4) Panek, J. S.; Cirillo, P. F. J . Am. Chem. SOC.1990, 112, 4973. (5) (a) Davies, S. G.; Seeman, J. I. Tetrahedron Lett. 1984,25,1845. (b) Davies. S. G.; Dordor-Hedgecock, I. M.; Sutton, K. H.; Whittaker, M. J.Organomet. Chem. 1987,320, C19.(c) Blackburn, B. K.; Davies, S. G.; Sutton, K. H.; Whittaker, M. Chem. SOC.Rev. 1988,17, 147. (d) Shambayati,S.; Crowe, W. E.; Schreiber, S. L. Angew. Chem., Int. Ed. Engl. 1990,29, 256. (6) (a) Seeman, J. I.; Davies, S. G. J . Chem. SOC.,Chem. Commun. 1984, 1019. (b) Seeman, J. I.; Davies, S. G. J. Am. Chem. SOC.1985, 107, 6522. (c) Seeman, J. I.; Davies, S. G. J . Am. Chem. SOC.1985, 107, 6525. (7) (a)Houk, K. N.; Moses, S. R.; Wu, Y.-D.; Rondan, N. G.; Jaeger, V.; Schohe, R.; Fronczek, F. R. J. Am. Chem. Soc. 1984,106,3880. (b) Jorgensen, K. A.; Hoffmann, R. J . Am. Chem. SOC.1986,108,1867.(c) Houk, K. N.; Duh, H.-Y.; Wu, Y.-D.; Moses, S. R. J . Am. Chem. SOC. 1986,108 2754. (d) Kahn,S. D.; Pau, C . F.; Chamberlain, A. R.; Hehre, W. J. J . Am. Chem. SOC.1987,109, 650. (8)Mackie, S. C.; Park, Y.-S.; Shurvell, H. F.; Baird, M. C. Organometallics 1991, 10, 2993.
e)
0276-733319512314-0869$09.00/0 0 1995 American Chemical Society
870 Organometallics, Vol. 14, No. 2, 1995
Lorsbach et al.
7 4 .
Anticlinal Anti
Synclinal
RR/SS Syn
SYn
RS/SR Anti
Figure 1. Newman projections showing the relative stereochemistry of the diastereomeric products (r-Cp(CO)(PR3)Fe[CH(OMe)Mel) resulting from the addition of hydride to the anticlinal (anti) and synclinal (syn) chiral cationic carbene complexes q-Cp(CO)(PRdFe[C(OMe)MeI+. use this insight to choose or design pendent groups that maximize stereoselectivity? In this paper we show that, indeed, the stereoselectivities of the addition of hydride to pCp(CO)(PR3)Fe[C(OMe)Mel+ can be analyzed in terms of t h e stereoelectronic parameters of PR3 and that t h e results can be interpreted in terms of concepts embedded in t h e “quantitative analysis of ligand effects”
Experimental Section The carbene complexes, pCp(CO)(PRdFe[C(OMe)(Me)I+, were prepared by methylation of the corresponding acyl complexes, q-Cp(CO)(PR3)Fe[C(O)Mel, by methyl triflate in methylene chloride according to the procedure reported by B r ~ o k h a r t .The ~ ~ carbene complexes were characterized by lH NMR and IR spectroscopy. The spectroscopic data, which are in agreement with those presented earlier,3dare displayed in Tables 1 and 2 of the supplementary material. The reduction of the carbene complexes was also carried out according to the protocol of B r ~ o k h a r t .The ~ ~ determination of the relative amounts of the resulting anti and syn diastereomers, q-Cp(CO)(PR3)Fe*[C*H(OMe)(Me)] (see Figure 1 for structural designations),were determined by integration of the lH methoxy resonances which were readily observed. For some systems (L = PMe3, PEt3, PPh3, P(p-CF3Ph)3, PCyPhz, P(i-Pr)s) the identity of the anti isomer was established by the observation of the long range couplin$d between the phosphorus and the hydrogen of the CH(OMe)(C&) ligand. The syn isomer does not show this coupling. This coupling is observed for the complexes containing the smallest ligands (Le. (9)(a) Golovin, M. N.; Fkhman, Md. M.; Belmonte, J. E.; Giering, W. P. Organometallics 1986,4 , 1981.(b) Dahlinger, K.;Falcone, F.; Poe, A. J. Znorg. Chem. 1986,25,2654. (c) Rahman, Md. M.; Liu, H.Y.; Prock, A.; Giering, W. P. Organometallics 1987,6,650-58.(d) Pee, A. J. Pure Appl. Chem. 1988,60, 1209.(e) Golovin, N.G.; Meirowitz, R. E.; Rahman, Md. M.; Liu, H.; Prock, A.; Giering, W. P.Organometallics 1987,6,2285.(0 Lezhan, C.; Poe, A. J. Znorg. Chem. 1989,28, 3641. (g) Rahman, Md. M.; Liu, H.-Y.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1989,8,1-7. (h) Eriks, K.; Liu, H.-Y.; Koh, L.; Prock, A.; Giering, W. P. Acta Crystallogr. 1989,C45, 1683. (i) Liu, H.; Fertal, D.; Tracey, A. A.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1989,8,1454-58.(i) Eriks, E.; Liu, H.-Y.; Prock, A.; Giering, W. P. Znorg. Chem. 1989,28,1759-63.(k) Brodie, N.M.; Chen, (1) Tracey, A.A.; Eriks, L.; Poe, A. J. Int. J.Chem. Kinet. 1988,27,188. K.; Prock, A.; Giering, W. P. Organometallics 1990,9, 1399.(m) Liu, H.-Y.; Eriks, E.; Prock, A.; Giering, W. P. Organometallics 1990, 9, 1758.(n) Panek, J.;Prock, A; Eriks, K; Giering, W. P. Organometallics 1990,9, 2175.( 0 ) Liu, H.-Y.; Eriks, E.; Prock, A.; Giering, W. P.Acta Crystallogr. 1990,C46,51.(p) Prock, A.;Giering, W. P.; Greene, J. E.; Meirowitz, R. E.: Hoffman,S. L.; Woska, D. C.; Wilson, M.; Chang, R.; Chen, J.; Magnuson, R. H.; Eriks, K. Organometallics 1991, 10, 3479-85. (9)Woska, D. C.; Wilson, M. R.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1992,11,3343.(r) Liu, H.; Eriks, K ; Prock, A.; Giering, W. P.. Acta Crystullogr. Sect. C: Cryst. Struct. Commun. 1992, C48,433.(s) Brown, T. L. Znorg. Chem. 1992,31,1286.(t) Woska, D. C.; Bartholomew, J.; Greene, J. E.; Eriks, K.; Prock, A.; Giering, W. P. Organometallics 1993,12,304. (u)Wilson, M. R.; Woska, D. C.; Prock, A.; Giering, W. P. Organometallics 1993, 12, 1742. (v) Choi, M.-G.; Brown, T. L. Znorg. Chem. 1993,32,5603.(w) Brown, T.L.; Lee, K J. Cooord. Chem. Rev. 1993,128,89. (x) Choi, M.-G.; Brown, T. L. Inorg. Chem. 1993,32,1548.(y) Fernandez, A. L.; Prock, A.; Giering, W. P. Organometallics 1994, 13, 2767. ( 2 ) White, D.; Coville, N. J. Adv. Organomet. Chem. 1994,36,95.(aa)Farrar, D. H.; Poe, A. J.;Zhang, Y. J.Am. Chem. SOC.1994,116,6252.
Table 1. Stereoelectronic Properties and Ratio of Diastereomers q-Cp(CO)(L)Fe[CH(OMe)(Me)],Formed by the NaB& Reduction of tl-Cp(CO)(L)Fe[C(OMe)(Me)l+ (I
entry
phosuhine
Xb
&
E,d
1 2 3 4
PMe3 PMezPh PEt3 PBu~ PMePhz PEtzPh PEtPhz P(i-Bu)s P(p-MeOPh)3 PPhs P(p-FPh)s P(p-ClF’hh P(p-CF3Ph)3 PCyPhz P(i-Pr)3 PCyzPh PCY3 P(t-Bu)3
8.55 10.6 6.3 5.25 12.1 9.3 11.3 5.7 10.5 13.25 15.7 16.8 20.5 9.3 3.75 5.35 1.4 0.0
118 122 132 136 136 136 140 143 145 145 145 145 145 153 160 161 170 182
0
5 6 7 8 9 10 11 12
13 14 15 16 17 18
1 0 0 2
1 2 0 2.7 2.7 2.7 2.7 2.7 2 0 1 0 0
antusyn
2.W 2.86 4.60‘ 5.13 1.96
1.33 1.32e 1.07 0.99 1 .oo 9.6 > 30 >30 > 30
a Entries 6-8 and 18 are listed for use in the discussion of analysis of alternate mechanisms (see text). Reference 10. ‘Reference 11. dReference 2. Reference 3d.
PMe3) as well as complexes containing some of the largest ligands (P(i-P&). In a few instances (L= PBm, PMePhz, PCy2Ph, PCy3) the CH(OMe)(CH3)resonance was obscured by the resonances of the pendent groups on the phosphine ligand. In other complexes (L = PMezPh , P(p-FPh)a, P(p-ClPh)s, P(pMeOPh)3) this coupling was not observed for either diastereomer. Since where we can determine the antilsyn ratio, we found that the ratio is never less than unity, we assumed that the anti isomer was always the predominant isomer. The spectroscopic data for q-Cp(COXPR3)Fe*[C*H(OMeXMe)l,which are in harmony with the literature data for the known compounds,3d are presented in Tables 3 and 4 of the supplementary material. The anti/syn ratio of the stereoisomers formed in the reduction of the carbene complexes bearing phosphines with 0 > 153” is very large and is reported as ’30. All of the reductions were done in triplicate, and the results agree t o within 10%. The average of the three runs is presented in Table 1.
Results Eleven new cationic iron carbene complexes of t h e
type ~-Cp(CO)(PR3)Fe[C(OMe)Mel+ were prepared (along with one, L = PPh3, previously examined by Brookh a ~ - t ~and ~ J )then reduced with sodium borohydride in methanol at -78 “C according to t h e protocol of B r ~ o k h a r t (Scheme ~~J 1). These data supplement t h e d a t a (PR3 = PMe3 and PEt3) reported The reduction affords the anti and syn diastereomers, q-Cp(CO)(L)Fe*[C*H(OMe)(Me)l. The terms anti and syn (10)Bartik, T.; Himmler, T.; Schulte, H.-G.; Seevogel, K. J. Organomet. Chem. 1984,272,29. (11)Tolman, C . A. Chem. Rev. 1977,77,313.
Quantitative Analysis of Ligand Effects
Organometallics, Vol. 14,No. 2, 1995 871
Scheme 1
Scheme 2
OMe racemic
0
mixture of syn and and Isomers
refer t o the position of the carbonyl and methoxy groups in the most stable conformation of the organoiron compounds (Figure 1). For example, in q-Cp(CO)(PR3)Fe[CH(OMe)Mel the anti isomer has the two groups opposite to each other. The anti diastereomer has the RSISR configurations about the stereocenters. In the syn isomer, the methoxy and carbonyl groups are on the same side and these stereocenters have the RRISS configurations (Figure 1). In keeping with this terminology, we will refer to the anticlinal and synclinal conformations of the carbene complexes as the anti and syn conformations, respectively. The lH NMR spectra of the reaction mixtures immediately after work up showed only q-Cp(CO)(PR3)Fe[CH(OMe)Mel (with four exceptions; vide infra) so that actual yields of the reduction product appear to be quite high and greater than 90%. This crude material was used to determine the antihyn ratio of diastereomers. These ratios, which vary from 1.0 to greater than 30, along with the stereoelectronic properties of the ancillary phosphine ligands are displayed in Table 1. Since the addition of hydride was done at -78 "C and the product mixtures were analyzed a t room temperature, we investigated the configurational stability of one of the product mixtures over this temperature range. In this experiment a CDzC12 solution of the carbene was added complex q-Cp(CO)(PMePhz)Fe[C(OMe)Me]+ to a CD30D solution of NaBH4 and NaOCD3 at -78 "C. The lH NMR spectrum taken a t -78 "C showed that the reduction of the carbene complex was complete upon mixing, affording a 2:l mixture of anti and syn isomers, respectively. This ratio remained invariant as the mixture was warmed to room temperature. Thus, the invariance of the isomer ratio to change in temperature suggests that the anti and syn isomers once formed are configurationally stable under the conditions of the experiment and that the ratio is not thermodynamically determined.lZ In a separate experiment we found that the isomer ratio was independent of the relative concentrations (over a 4-fold range) of carbene complex and NaBH4 for L = PPhzMe , which supports the notion that there is rapid equilibration of the conformers of q-Cp(C0)(PMePh2)Fe[C(OMe)Mel+.Thus, the Curtin-Hammett principle is applicable. Although the reduction is unexceptional for most of the complexes, the carbene complexes containing the largest ligands (entries 14-17 in Table 1) undergo reduction and subsequent loss of methanol to form (12) If we assume that the 2:l antikyn ratio is the equilibrium constant at -78 "C and that there is no significant entropy change in the equilibration of the isomers, then we would expected about a 30% decrease in the equilibrium constant on warming to room temperature. This change would be easily observable spectroscopically as the temperature of the mixture was raised.
complexes which appear to be the vinyl complexes, q-Cp(CO)(PR3)Fe(CH=CH2),when the reduction was carried out in the manner described by B r ~ o k h a r t (Scheme ~~J 2). Although these vinyl complexes have not been isolated and fully characterized, their existence is inferred by the observation of 1 proton multiplet resonances (for PR3 = P(Cy2Ph)) at 8.24, 6.44, and 6.08, which are characteristic of the vinyl group. A control experiment with the PCyzPh complex showed us that the formation of the vinyl complex was effected by methoxide and was not attributable to intramolecular decomposition of the initial reduction product. In this was alexperiment ~-C~(CO)(PC~ZP~)F~[CH(OM~)M~I+ lowed to react with NaBH4 for 3 minutes at -78 "C and then rapidly warmed to 22 "C. Half of the mixture was immediately worked up whereas the other half was allowed t o stand at 22 "C for an additional 0.5 h. The sample that was worked up immediately was free of the vinyl complex whereas the other portion of the reaction mixture was completely converted to the vinyl complex. We monitored a sample of q-Cp(CO)(PCy2Ph)Fe[CH(OMe)Mel, in the absence of methoxide, and found the complex to be stable a t 24 "C over a period of at least 1 h in either acetone or benzene as evidenced by its invariant 'H NMR spectrum. We analyzed the data (log anti/syn) for complexes containing PR3 with 8 160" by regression analysis according to eq 1 to yield eq 2. In eq 1,x and E,, are log(antilsyn) = ux
+ b(8 - 8& + cE, + d
(1)
log(anti/syn) = -0.022J o.oss(e- 1450)~- 0 . 1 3 7 ~ ~0.77 (2) io.00
+
fO.010
n = 11
f0.034
+
r2 = 0.968
electronic parameters,2J08 (Tolman's cone angle1') is a steric parameter, 8,t is a steric threshold,2 and ;1 is a switching function2 that is zero for 8 < est and is 1for 8 > est. (The standard errors are written directly below the corresponding coefficients in eq 2. A discussion of the estimate of the error in the steric threshold is presented in ref 2.) As a check on the internal consistency of the analysis, we note that the coefficient of x in eq 2 must be statistically indistinguishable from the coefficient of x obtained when only the data for the triarylphosphine complexes are analyzed (eq 3).2 Inspection of eqs 2 and 3 shows that this is indeed the case. log(anti/syn) = -0.0150~ f0.005
n =5
+ i0.071 0.28
r2 = 0.78
(3)
872 Organometallics, Vol. 14, No. 2, 1995
Lorsbach et al.
Scheme 3 rlcp(CO)(Pph3)Fe[C(OMe)Etlt
3 tl-Cp(CO)(Pph~)Fe[CH(OMe)Etl' syn and anti isomers
110
130
,
I
150
170
0 Figure 2. Steric profile of log(anti/syn) for addition of hydride (from NaBH4) to the cationic iron(I1) carbene complexes T~CP(PR~)(CO)F~[C(OM~)(M~)I+. The complexes indicated with a bar are lower limits based on our use of log(anti/syn) = 30. The arrow indicates that the actual value of log(anti/syn),tis probable larger.
The relatively large number of data for eq 2 and the small standard errors give 95%confidence limits which indicate that the coefficients are all statistically significant. This, coupled with the high correlation coefficient, reveals an excellent fit of the data to eq 1. There is only one point for 8 greater than 145" in our analysis; thus, the steric threshold could lie anywhere between 145 and 153". The value of 145" used for the steric threshold in our analysis is simply taken as a lower bound, and the coefficient of (8 - &)A is also a lower bound. We regard this steric threshold as real because there are three more data above the steric threshold with anti/syn ratios greater than 30. Clearly, steric effects become extremely important above 145". (Since there is only point above the steric threshold, we cannot explore the use of other steric parametersgx in this analysis.) Often the goodness of fit is shown by plotting the calculated property (e.g. via eq 2) versus the actual experimental data (log(anti/syn)). We find it more useful t o generate a steric p r ~ f i l e which ,~ also shows graphically the goodness of fit and gives a snapshot of the dependence of the property on the size of the ligand. The steric component (log(anti/sy&) of log(anti/syn) was generated by subtracting the electronic terms and the constant of eq 2 from the experimental data (eq 4). A plot of log(anti/syn),t versus 8 gives the steric profile shown in Figure 2. log(anti/syn),, = log(anti/syn),,,
+ ( 0 . 0 2 2 ) +~ (0.137)E,
- (0.77)
(4)
Discussion The stereochemistry of iron(II1) carbene and alkoxy carbene complexes of the type pCp(CO)(L)Fe[CHRl+ and v-Cp(CO)(L)Fe[C(OMe)Rl+ has been studied extensively. Brookhart predicted,3don the basis of calcula-
tions by Hoffmann,13that the most stable conformation of the alkylidene complexes would appear to be the one where the plane of the carbene ligand is perpendicular to the bond between iron and the ligand which is the best electron donor. On the basis of molecular mechanics calculations and by analogy to the acyliron complexes, Davies furthermore predicted that the related carbene complexes should exist primarily in the least congested (anti) c~nformation.~~ Schreiber also suggested that this should be the most stable conformation but because of electronic factors.5d In Davies' report of the addition of hydride (from NaBH4) to v-Cp(CO)(L)Fe[C(OMe)Et]+,it was assumed, quite reasonably, that the addition of hydride occurred to the face opposite the phosphine ligand.4f In Brookhart's work on the addition of hydride to the methoxymethyl carbene complexes, 7-Cp(CO)(L)Fe[C(OMe)Me]+, it was found that the dominant product results from addition of hydride to the less stable syn conformer. As we shall see, it is relevant to this work that Davies also observed that the a-methoxypropylcomplex v-Cp(CO)(PPh3)Fe[CH(OMe)Et1 was readily transformed to the 1-propenyl complex (Scheme 3). Br0okha1-t~~ determined the barrier to rotation about the Fe-carbene bond in v-Cp(CO)(PR3)Fe[C(Ph)H]+to be less than 7 kcaymol at -114 "C. Presumably, the rotation about the Fe-carbene bond in q-Cp(CO)(PR& Fe[C(OMe)Me]+will be even faster because of the better ~d bonding capacity of the methoxy group as compared to that of the phenyl group. If the rate of rotation about the iron-carbene bond in v-Cp(CO)(PRS)Fe[C(OMe)Me]+ is rapid compared to the rate of addition of hydride, the ratio of stereoisomers will be unaffected by changes in the relative concentrations of the reactants. This is what we observe (vide supra). Thus, we employ the Curtin-Hammett principle to interpret our data (see Figure 3): the anti/syn ratio is determined by the ratio of rate constants for the addition of hydride to the two carbene conformers and the equilibrium constant between the two conformers. Our analysis shows that steric effects appear to be nonlinear. For example, the addition of hydride (from NaBH4) to the cationic carbene complexes, r-Cp(CO)(PR3)Fe[C(OMe)Mel+,shows only modest stereoselectivity up to 8 = 145" after which the stereoselectivity rises steeply to a point ( 8 = 160") where we can no longer observe the syn isomer (anti/syn > 30). Clearly steric effects become disproportionately important for the larger phosphines. Three Possible Explanations of the Origins of Steric Thresholds. The appearance of a steric threshold in the analysis of kinetic data involvingligand effects is not rare.g Although it can be interpreted as the onset (13) (a) Schilling, B. E. R.; H o f h a n n , R.; Lichtenberger, D. L. J. Am. Chem. SOC.1979, 101, 585. (b) Goddard, R. J.; Hoffmann, R.; Jemmis, E. D. J. Am. Chem. SOC.1980, 102 7667. (c) See also: Hoffmann, R.; Eisenstein, 0. W. Unpublished results as reported in ref 3d.
Quantitative Analysis of Ligand Effects
Organometallics, Vol. 14,No.2, 1995 873
i
L
OC
-
L
RS/SR (anti)
Figure 3. Reaction coordinate diagram for the NaBH4 reduction of the carbene complexes pCp(CO)(PR3)Fe[C(OMe)(Me)]+.This diagram is based on information given in ref 3i,k. Scheme 4 5-
A
uB
3-
k2
of steric interactions when the ligand reaches a critical size, it can also arise, in principle, where steric effects are linear and continuously operative. We take this opportunity to expand briefly on these ideas. Consider, for example, if two parallel pathways (Scheme 4) to a product exist, each of which have different stereoelectronic dependencies, conditions might be right to produce a steric threshold. The following expressions (eqs 5 and 61, for example, give the dependence of the individual rate constants on the two stereoelectronic parameters x and 8:
+
(6)
where the difference in the constant terms, C1 - C2 is -30. This difference makes the rates the same when parameters x and 8 are 10 and 155",respectively. (This is the mid range of each parameter.) The total rate is taken as the sum of the individual rates. The logarithm of total rate was calculated for a group of 18 phosphorus(111)ligands; see Table 1. This set of calculated data was analyzed by means of linear regression in the usual manner. The resulting equation is
+
I
I
I
I
110
130
150
170
e Figure 4. Steric profile for a process involving two competitive reactions as described by eqs 5 and 6. Scheme 5 A
A
B
L
C
(5)
+ 0.208 + C2
In 12 = 0 . 0 9 1 ~ 0.237(8 - 151")A
-1
k.1
In 12, = 0 . 1 0 ~ C, In K, = -0.10%
1-
+ 0.156 r2 = 0.992 ( 7 )
The range of k is a factor of 90. The steric profile was obtained by subtracting the electronic term from the calculated data and is shown in Figure 4. We see that In k is virtually constant up to 8 = E l " , followed by a sharp and nearly linear increase with increasing 6. Thus, we have a steric threshold. It turns out that we can produce similar steric profiles even if either coefficient of x is made to vary over a significant range, as long as the coefficient of 8 is large enough to make steric factors dominate for large 6. The second example is for the case of sequential reactions shown in Scheme 5, where for small cone angles k2 is large (thus, the first step is rate determining) and for large cone angles the second step becomes very slow. For illustration, let k1 = k-1 = 1.0 and let
874 Organometallics, Vol. 14, No. 2, 1995 In k, = 0 . 1 0 ~- 0.208
+ 29
The apparent constant is selected to make k 1 =
Lorsbach et al. (8) k2 for
x = 10 and 8 = 150. We defined the rate constant, k, for the reaction as the inverse of the time for product C to reach half its final value. In k was then calculated for the set of 18 ligands (see Table 1))and the results were analyzed by means of linear regression to yield In k = 0.0452 - 0.165(8 - 14l")A - 0.132
(9)
r2 = 0.987 The range of k is 150. The steric profile was obtained as described above and is shown in Figure 5. The steric threshold is clear. Steric profiles with these shapes have been noted previo~sly.~ They can also be explained in terms of a single unvarying rate-determining step where the steric effect is nonlinear. Thus, whatever its origins, the steric threshold is real and its inclusion must be provided for in the analysis of kinetic data. Steric thresholds resulting from changes in the rate-determining step (sequential reactions) or changes in mechanism (parallel reactions) are more likely to occur in more complicated reaction mechanisms. For simple, one step reactions, the observation of a steric threshold is likely attributable to nonlinear steric effects. For correlation of thermodynamic data, mechanism does not enter the picture and the existence of a steric threshold must necessarily indicate a nonlinear steric effect. It seems reasonable to expect that the addition of hydride to q-Cp(CO)(L)Fe[C(OMe)Mel+is most likely a one step process, the mechanism of which remains invariant over the entire range of the ligands employed in the study. If this is true, then the steric threshold is attributable to nonlinear steric effects in the formation of the anti and/or the syn isomer, and we interpret the analysis in the following manner. Interpretation of the QALE Analysis. The regression eq 2 shows us that the antilsyn ratio is dependent on x and E, but not on 8 for 8 1145". In this domain we find that variations in x and E,, account for 55% and 45% of the variations in log antilsyn, respectively. (In ref 9s we discuss our method of determining the extent of involvement of each parameter.) Enhanced electron donor capacity (smaller x) increases log antikyn whereas increasing the number of aryl groups tends to decrease log antilsyn. It is important to note that the aryl effect plays a significant role in determining the stereoselectivity of this reaction. Since the relative rates of formation of the anti and syn isomers are dependent on the equilibrium constant between the two conformers of the carbene complex (vide supra), and since this equilibrium constant could be sensitive to steric factors, the lack of a significant steric dependence for small ligands suggests that the antilsyn ratio is not determined by steric factors either in the ground or transition state. This observation supports Schreiber's suggestion that the anti conformer of the carbene complexes is more stable because of electronic factors.5d Above the steric threshold there is a dramatic increase in the anti/syn ratio. Thus, the antilsyn ratio for the PCyPhz system (8 = 153") is 9.6 and increases to greater than 30 for the three systems bearing ligands with 8 greater than 160". These observations indicate
110
,
I
I
130
150
170
I
e Figure 5. Steric profile for a process involving two sequential reactions as described in the text. that the datum for the PCyPh2 system is not an outlier and indeed the steric threshold is real. It is interesting to note that q-Cp(CO)(PRS)Fe[CH(0Me)Mel compounds that contain phosphines with 8 greater than 145" undergo a facile loss of methanol in the presence of methoxide to form the vinyl complexes (Scheme 2). Since this reaction is not observed for complexes containing smaller ligands (under our reaction conditions), we conclude that the elimination of methanol is sterically accelerated for the large ligands. Thus, there must be a steric threshold in the ground state of this reaction; i.e., steric effects turn on in q-Cp(CO)(L)Fe[CH(OMe)Melabove 145". It important to note that Davies observed that the a-methoxypropyl complex, (q-Cp(CO)(L)Fe[CH(OMe)Etl,underwent loss of methanol when PPh3 is the ancillary ligand.4f This is consistent with our results since the a-methoxypropyl ligand is larger than the a-methoxyethyl ligand and, thus, should have a smaller steric threshold for the onset of this elimination reaction.
Conclusions The high quality of the fit of the stereoselectivities to eq 2 suggests that, indeed, the stereoselectivities of the addition of hydride to (q-Cp(CO)(L)Fe[C(OMe)Mel+ (Scheme 1)can be analyzed in terms of the phosphorus(111)stereoelectronic parameters x,8, and E,. According to our analysis, below 8 = 145" the stereoselectivity is determined by electronic factors with contributions from x and E, having about equal weight. There can be little doubt that steric effects are ushered in by a steric threshold above 145". We recognize that we cannot distinguish between two models, one that has a single mechanism involving discontinuous steric effects, which is operative over the entire range of ligands, and a model involving a more complex mechanism with continuous steric effects. Even with this caveat we can make a very important statement concerning stereoselectivity: Steric control of stereoselectivity is not necessarily linear, and to achieve maximum stereoselectivity a set of ligands must be chosen with 8 greater than the steric threshold of the reaction. Supplementary Material Available: Tables of IR and NMR data (3 pages). Ordering information is given on any current masthead page.
OM9405870
