759
Organometallics 1995, 14, 759-766
Successful Application of a “Forgotten”Phosphine in Asymmetric Catalysis: A 9-Phosphabicyclo[3.3.l]non-9-yl Ferrocene Derivative as Chiral Ligand Hendrikus C. L. Abbenhuis,t>tUrs Burckhardt,? Volker Gramlich,B Christoph Kollner,? Paul S. Pregosin,*>tRenzo Salzmann,? and Antonio Topi*?* Laboratory of Inorganic Chemistry and Institute of Crystallography and Petrography, Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092Zurich, Switzerland Received October 3, 1994@ The technical mixture “phobane”,containing the two isomers 9-phospha-9H-bicyclo[3.3.1]nonane (3a)and 9-phospha-9H-bicyclo[4.2.llnonane (3b)in a -2:l ratio was reacted with NJV-dimethyl-(S)-l-[(R)-2-(diphenylphosphino)ferrocenyllethylamine (4) in acetic acid. The clean amine substitution product is the new chiral biphosphine 5. When only 2 equiv of 3 were reacted with 4, a 4:l mixture of the two isomeric products Sa and 5b were obtained. However, the use of a 10-fold excess of 3 afforded the pure [3.3.1l-isomer Sa, 9-phospha-9[(S)-l-((R)-2-(diphenylphosphino)ferrocenyl}ethyll~3.3.llbicyclononane, in 68% isolated yield. (S)-(R)-Sacrystallizes in the orthorhombic space group P212121,Z = 4, a = 7.393(3) b = 19.261(5) and c = 19.546(8) S a was used in the asymmetric Pd-catalyzed alkylation of 1,3-diphenyl-3-acetoxypropenewith dimethyl malonate. Enantioselectivities up to 85% ee were obtained. The cationic Pd-allyl complexes [Pd(r3-C3H5)(5a)103SCF3 (6) and [Pd(r3PhCHCHCHPh)(5a)]03SCF3 ( 7 ) were prepared and characterized by X-ra diffraction. Complex 6 crystallizes in the monoclinic space group P21,Z = 2, a = 9.162(4) L l, b = 16.069( 5 ) A, c = 11.816(5) and ,8 = 96.86(3)”. Crystalline 7 was obtained as a CH2C12 monosolvate and belongs to the triclinic system: space group P 1 , Z = 1,a = 11.07(2) b = 11.216(14) c = 11.888(16) a = 62.37(9)”,/3 = 65.96(11)”,and y = 70.29(11)”. The ligand assumes very different conformations in its complexes, as compared to the free state. Multidimensional 31P,13C, and IH NMR studies reveal that 7 exists in solution as a mixture of four isomers. Aspects of the selective equilibria were elucidated using 31P-and lH-exchange spectroscopy.
A,
A,
A.
A, A,
A,
A,
Introduction We have recently shown that ferrocenyl derivatives containing two sterically and electronically different ligating phosphine units can impart high t o very high degrees of enantioselection to several transition-metalcatalyzed reactions.’ Thus, ligand 1 represents the Me I
N’4
7
suited for studying steric and electronic effects on stereoselectivity in asymmetric catalysis. The simple synthetic approach to such species also allows the incorporation of ligating fragments other than phosphines, with the recently reported pyrazole derivative 2 being an example.2 We also showed that an important feature of ligand 1 is its virtually constant conformation in its complexes with transition-metal ions, as found in the solid state.3 The same conformation, along with a site selective n-allyl isomerization of the corresponding cationic Pd(v3-allyl)complex, has been found in solution by 2D NMR ~ t u d i e s . ~ With a view to understanding the factors governing the steric and electronic properties of these ligands, we were interested in the use of a conformationally rigid, compact, and electron-rich dialklyphosphine for the synthesis of new ferrocenyl derivatives. “Phobane”, 9-phospha-9H-bicyclo[3.3.llnonane (3a),5 seemed a suitable starting material for further work in this field and could possibly be regarded as a sterically less bulky electronic equivalent of dicyclohexylphosphine. Furthermore it offers the advantage of being much less airsensitive and, as a solid, more convenient to work with than simple dialkylphosphines. From a steric point of
aPCy2 Me
l Fe
Me
I
ie
prototype of a class of chiral auxiliaries that are ideally Laboratory of Inorganic Chemistry.
* Present address: Eindhoven University of Technology, Laboratory +
of Inorganic Chemistry and Catalysis, Department of Chemistry, P.O. Box 513,5600 MB Eindhoven, The Netherlands. 5 Institute of Crystallography and Petrography. Abstract published in Advance ACS Abstracts, December 15,1994. (1)Togni, A.; Breutel, C.; Schnyder, A.; Spindler, F.; Landert, H.; Tijani, A. J.Am. Chem. SOC.1994,116,4062-4066, and references cited therein. @
0276-7333/95/2314-0759$09.00/0
(2)Schnyder, A.; Hintermann, L.; Togni, A. Angew. Chem.,in press. (3)Togni, A.; Breutel, C.; Soares, M. C.; Zanetti, N.; Gerfin, T.; Gramlich, V.; Spindler, F.; Rihs, G. Inorg. Chim. Acta 1994,222,213224. (4)Breutel, C.; Pregosin, P. S.; Salzmann, R.; Togni, A. J.Am. Chem. SOC. 1994,116,4067-4068.
0 1995 American Chemical Society
760 Organometallics, Vol. 14, No. 2, 1995
Table 1. Pd-Catalyzed Alkylation of 1,3-Diphenyl-3-acetoxypropenewith Dimethyl Malonate
Scheme 1
-
2 eq "phobane"
(b
OAc PPh2
(&
Ph&Ph
1
Fe
\-
10 eq. "phobane"
1.0-0.1 mol%Pdcatr -78 to 20 *c
mol % of cat.
1
1.o
20
2
1.o
0
3 4
1.o 1.o
-20 -78
5
0.1
20
68% isolated yield a
view, although there are no data available: the "phobyl" fragment is probably less bulky than a diisopropylphosphine group. Compound 3a is inexpensively produced on a large scale from 1,5-cyclooctadiene and phosphine (PHd, a reaction that leads t o a product mixture that contains phobane together with its 14.2.11-isomer3b in a -2:l ratio (see eq l h 5 Since no convenient method
H (4.2.1pisomer 3b
exists for the purification of the technical phobane mixture, its synthetic applications in organometallic chemistry and homogeneous catalysis are still very limited.' We report herein the successful incorporation of stereochemically uniform phobane (3a) into the corresponding ferrocenyl ligand of type 1, as well as structural and catalytic studies on the new compound.
Results and Discussion Synthesis. The reaction of the (diphenylphosphin0)ferrocenylethylamine (S)-(R)-4with -2 equiv of the technical phobane mixture in acetic acid at 80 "C leads to clean substitution of the dimethylamino functionality by both bicyclic aliphatic phosphines 3a and 3b (Scheme 1,route i). An interesting observation, however, is that starting with the conventional 2:l mixture of 13.3.11- and [4.2.11-isomers, the ferrocenyl diphosphines 5a,b that ( 5 ) (a) Mason, R. G.;Van Winkle, J. L. U.S.Patent 3 400 163,Sept 3,1968 (assigned to Shell Oil Co.). (b) For a laboratory procedure for phobane, see: Harris, T. V.; Pretzer, W. R. Inorg. Chem. 1985,24, 4437-4439. See also: ( c ) Weferling, N. Phosphorus Sulfur 1987,30, 641-644. For similar additions of phosphine(s) to olefins, see, e.g.: (d) Vedejs, E.; Peterson, M. J. J . Org. Chem. 1993,58,1985-1986. (e) Wiseman, J.R.; Krabbenhoft, H. 0. J . Org. Chem. 1976,41,589-593. (0 Turnblom, E. W.; Katz, T. J. J . A m . Chem. SOC.1973,95,42924311. (6)For a review, see, e.g.: White, D.; Coville, N. J. Adu. Orgunomet. Chem. 1994,36,95-158, and references cited therein. (7)For recent uses of phobane, see: (a) Weferling, N. Z. Anorg. AZZg. Chem. 1987,548,55-62.(b) Klaui, W.; Song, C.-E. Inorg. Chem. 1989, 28, 3845-3849. No isomerically pure phobane derivatives were reported here. For a first obsewation of the incorporation of the 13.3.11isomer into ferrocenyl derivatives, see: ( c ) Abbenhuis, H. C. L.; Burckhardt, U.; Gramlich, V.; Togni, A,; Albinati, A,; Muller, B. Organometallics 1994,13,4481-4493.
T ("C)
entry
(S)-(R)-l3 3 11-5a
p.3.1]-isomer 3a
CH2(COOMe)2/BSA
&Le I
4 &e Fe I
H
-
Abbenhuis et al.
CH(COOMe)2
Ph
Ph
R- enantiomer
time (h)
conversion (%)
ee (%)
0.2 0.3 0.4 1 3 29 52 192 3.3 23 70
51 98 81 100 62 20 34 100 5 31 39
74 72 81 80 85 76 74 66 80 74 71
Catalytic experiments were carried out as described in ref 1.
are obtained show incorporation of the 13.3.11- and [4.2.1l-fragments in a 4:l ratio; Le., the final product is enriched in the r3.3.11-isomer. The two isomers 5a and 5b can best be distinguished from one another by 31P NMR (5a, 6 -26.8, -10.5; 5b, -27.3, 26.2 for the aromatic and aliphatic phosphines, respectively). Although the product mixture can be very easily crystallized from ethanol, separation of the ferrocenyl diphosphines Sa and 5b, by either repeated crystallization or column chromatography over A l 2 0 3 , was not feasible. Fortunately, this problem can be easily overcome when the conversion of the ferrocenyl amine 4 is performed with a greater excess of phobane. Thus reaction of 4 with a -10-fold excess of the phobane mixture leads to clean, kinetically controlled, formation of ferrocene 5a in 68% isolated yield, containing the 13.3.11-isomeric form of 3 exclusively (Scheme 1,route ii). The underlying reactivity differences between the two isomers 3a and 3b in the formation of 5 is very pronounced and has either not been observed in other reactions of phobane or not been put to use for the synthesis of isomerically pure phosphine derivatives. No obvious reason for such a drastically divergent reactivity can be put forward, however, in the less reactive 14.2.11isomer; the phosphorus atom is imbedded in a fivememberedheven-membered bicyclic system that will adopt a completely different conformation from that of the symmetric 13.3.13-isomer. Because of this, the secondary phosphine functionality is possibly less accessible. Asymmetric Palladium-CatalyzedAllylic Alkylation. With the new ligand (S)-(R)-Sa, containing the rare phobyl fragment in hand, selected applications in asymmetric catalysis have been addressed. Since the parent compound of this class of ligands, derivative 1, was previously shown to give high enantioselectivities in the Pd-catalyzed alkylation of 1,3-diphenyl-3-acetoxypropene with dimethyl malonate,l this same reaction was chosen in order to test the effectiveness of ligand 5a. Experiments have been carried out at different temperatures and different catalyst concentrations in CH2C12, as reported previous1y.l The results are collected in Table 1. As can be seen there, the selectivity reaches a maximum of 85% ee, when the reaction is carried out utilizing 1mol % of catalyst and at -20 "C. Although the ee is significantly lower than that obtained with 1 (93%),the activity of the catalyst is quite high,
A 9-PhosphabicycloC3.3.l]non-9-yl Fc Derivative
Organometallics, Vol. 14, No. 2, 1995 761
Table 2. Experimental Data for the X-ray Diffraction Study of 5a, 6, and 7 (S)-(R)-5a formula mol wt crystal dimens, mm data cool. T, “C cryst syst space group a (‘4) b (‘4) c (A) a (deg) P (deg) Y (deg) v (‘43) Z
e(calcd) ( g ~ m - ~ ) P (cm-‘)
F(0W diffractometer radiation measured reflcns 28 range (deg) scan type scan width (deg) bkgd time (s) max scan speed (degmin-I) no. of indepnt data coll no. of obsd reflcns (no) absorp correction transm coeff no. of params refined (n,) quantity minimized weighting scheme R O
RWb GOF‘
C32H36FePz 538.4 0.15 x 0.15 x 0.25 20 orthorhombic p2 12I21 7.393(3) 19.261(5) 19.546(8)
7
6 C~~H~IF~F~O~PZP~S 834.9 0.2 x 0.2 x 0.3 20 monoclinic p2 I 9.162(4) 16.069(5) 11.816(5)
C~SH~~F~F~O~PZP~SCH~C~Z 1072.0 0.08 x 0.15 x 0.16 20 triclinic P1 11.07(2) 11.216(14) 11.888(16) 62.37(9) 96.86(3) 65.96( 11) 70.29(11) 2783(2) 1727.1(12) 1174(3) 4 2 1 1.285 1.605 1.516 6.76 11.46 9.71 1136 852 548 Syntex P21 Syntex P21 Simens R3mN Mo K a (graphite monochrom), 1 = 0.710 73 ‘4 05h57,05k520,051520 05h59,05k517,-1251512 - 9 5 h 5 10,-95k510,051511 3.0-44.0 3.0-45.0 3.0-40.0 0 w w 1.10 1.oo 1.05 0.3 x scan time 0.3 x scan time 0.25 x scan time 1.0-14.0 in w 2.0-6.0 in w 2.0-15.0 in w 1993 2360 2178 1764 2268 2139 IFo21 4.0o(lF12) IF21 > 4.0o(lFI2) lFozl > 4 . 0 ~ ( l F / ~ ) N/A face-indexed numerical 0.7123-0.7891 0.7858-0.8662 3 16 423 551 CW(F0 - F C Y Ew(F0 - FJ2 Cw(F.3- F C Y w-1 = UZ(F) o.ooooF2 unit weights w-I = u2(F) 0.0015p 0.0334 0.0243 0.0708 0.0335 0.025 0.071 3.07 1.04 1.92
’
+
+
‘ R = E(llFol- ~ ~ ~ ~ ~ l ~ cb Rl Wl =) h ~~ ~ ll l ~Fo~~l l .~ ~ ~ ~ l ~ ~ I I ~ z GOF ~ ~= ~ w[Cw(lFo) l ~ o / -2 (l/k)lFc1)2/(no 1 1 ~ * . - nv)1’/2.
considering that complete conversion is attained even at -78 “C in less than 200 h with 1 mol % palladium catalyst. The selectivity shows a smooth dependence on the temperature, being at 20 “C and at -78 “C slightly, but significantly lower than at -20 “C. An important conversion dependence is also observed. This effect turns out to be more pronounced at -78 “C than at higher temperatures. Thus on going from 20 to 100% conversion, the enantioselectivity drops from 76 to 66% ee. The concentration of the catalyst does not seem to significantly influence the enantioface discrimination (compare entries 1 and 5 in Table 1). X-Ray Structures of Ligand Sa and Its Palladium(qs-allyl)Complexes 6 and 7. In order to confirm the incorporation of the 9-phosphabicyclo[3.3.1]non-9-yl (phobyl) fragment into the new ligand 5a, as well as to define conformational aspects in the solid state, an X-ray crystallographic study was carried out. Crystal and data collection parameters are given in Table 2, and a selection of bond distances and angles are provided by Table 3. A view of the molecule is shown in Figure 1. The structure turns out to be rather routine and all bonding parameters fall in the expected range.8 The conformation adopted by 5a in the solid state is comparable to the one previously found for ligand l;3however, both similarities and differences can be identified. The importance of the differences will become apparent later, when the conformational aspects (8)(a) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, 0.;Watson, D. G.; Taylor, R. J . Chem. SOC.,Dalton Trans. 1989,Suppl. Sl-SS3. (b) Allen, F. H.; Kennard, 0.;Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J . Chem. SOC.,Perkin Trans 2 1987,Suppl. S1-S19.
Table 3. Selected Bond Distances (A): Angles (deg): and Torsion Angles (deg) for (S)-(R)-Sa P(l)-C( 1) P( 1)-C(22) P(2)-C(8) C(2)-C(6) Fe-Cp(
Bond Distances 1.823(6) P(1)-C( 16) P(WC(6) 1.839(6) 1.728(11) P(2)-C( 12) CW-C(7) 1.505(8) 1.643 Fe-c~(2)~
C(1)-P(1)-C(l6) C(16)-P(l)-C(22) P(1)-C( 1)-c(5) C(3)-C(2)-C(6) C(2)-C(6)-C(7) C(6)-P(2)-C(8) C(8)-P(2)-C(12)
Bond Angles 103.1(3) C(l)-P(l)-C(22) 103.1(3) P(l)-C(l)-C(2) 128.5(4) C(l)-C(2)-C(6) 124.9(5) C(2)-C(6)-P(2) 113.8(5) P(2)-C(6)-C(7) 111.0(4) C(6)-P(2)-C(12) 94.6(4) C~(l)-Cp(2)~
1.837(6) 1.875(7) 1.896(8) 1.535(9) 1.657 103.1(3) 123.9(4) 127.5(5) 108.0(4) 107.5(4) 103.5(3) 5.6
Torsion Angles C(2)-C(6)-P(2)-C(8) C(3)-C(2)-C(6)-C(7) C(5)-C(l)-P(l)-C(22)
174 -69 -17
C(5)-C(l)-P(I)-C(16) C(2)-C(6)-P(l)-C(l2)
86 73
Number in parentheses are ESDs in the least significant digits. Cp( 1) and Cp(2) are the planes of the “upper” (C(1-5)) and “lower” (C(1’-5’) cyclopentadienyl rings, respectively.
of the Pd(ally1)complexes are discussed. Thus, as in 1, the diphenylphosphino group in 5a is arranged in such a way that the “upper” phenyl is in a pseudoaxial and the “lower”in a pseudoequatorial position with respect to the upper Cp ring (torsion angles C(5)-C(l)-P(l)C(16) = 86” and C(5)-C(l)-P(l)-C(22) = -17”). In contrast t o the situation found in 1,the substituents a t the stereogenic carbon C(6) cannot be clearly classified in terms of axialfequatorial, since the plane of the upper
Abbenhuis et al.
762 Organometallics, Vol. 14, No. 2, 1995 GCil91
Figure 1. ORTEP view and atom numbering scheme of ligand (S)-(R)-5a. Thermal ellipsoids at the 50% probability level. Table 4. Selected Bond Distances (A); Angles (deg); and Torsion Angles (deg) for Complexes 6 and 7 6 Pd-P(l) Pd-P(2) Pd-C(28), [C(34)lb Pd-C(29), [C(35)lb Pd-C(30), [C(36)lb Fe-Cp( l)c Fe-c~(2)~ P( l)-Pd-P(2) P( 1)-Pd-C(28)[C(34)]6 P(2)-Pd-C(28)[C(34)lb P(2)-Pd-C( 30)[C(36)p P( 1)-Pd-C(30)[C(36)lb C(28)-C(29)-C(30) [C(34)-C(35)-C(36)lb C@)-P(2)-C( 12) CP( 1)-Cp(2) C(2)-C(6)-P(2)-C(8) C(2)-C(6)-P(2)-C( 12) C(5)-C( 1)-P( 1)-C( 16) C(5)-C( 1)-P( 1)-C(22) C(3)-C(2)-C(6)-C(7) P(2)-Pd-P( 1)-C( 1) P( 1)-Pd- P(2) -C( 6) Pd-P( 1)-C( 1)-C(2) Pd-P( 2)-C( 6) -C( 2)
Bond Distances 2.283(2) 2.32 8(2) 2.166(9) 2.165(9) 2.235(8) 1.673 1.653 Bond Angles 97.8(1) 93.0(2) 169.1(2) 102.9(2) 159.1(2) 124.7(8) 9533) 6.5 Torsion Angles -82 - 177 66 -42 -84
4 19 -12 -42
Figure 2. ORTEP view and atom numbering scheme of the cationic complex [Pd(773-C3HS)((S)-(R)-5a)l+ (triflate salt, 6). Thermal ellipsoids at the 50% probability level.
7
2.285(7) 2.301(5) 2.258( 15) 2.242(2 1) 2.225(26) 1.649 1.659 92.8(2) 99.4(6) 164.3(5 ) 103.4(5) 161.6(4) 118316) 94.0(9) 7.1 c:251
-75 - 175
Figure 3. ORTEP view and atom numbering scheme of the cationic complex [Pd(v3-PhCHCHCHPh)((S)-(R)-5a)l+ (triflate salt, 7). Thermal ellipsoids at the 50%probability level. The arrow shows the most probable site of preferred nucleophilic attack (see text).
39 -74 -70 33 -54 -9 54
a Number in parentheses are ESDs in the least significant digits. Numbering in brackets applies to 7 for corresponding atoms. Cp( 1) and Cp(2) are the planes of the "upper" (C(1-5)) and "lower" (C(1'-5')) cyclopentadienyl rings, respectively.
Cp ring roughly bisects the angle P(2)-C(6)-C(7).
This
is also illustrated by the two torsion angles C(3)-C(2)-
C(6)-P(2) and C(3)-C(2)-C(6)-C(7) of -50" and 69", respectively. The two six-membered rings of the phobyl moiety assume a nearly perfect chair conformation. This is also the case for 6 and 7,discussed below. The two cationic complexes [Pd(v3-C3H&5a)l03SCF3 (6) and [Pd(y3-PhCHCHCHPh)(5a)103SCF3 (7)were also studied by X-ray diffraction. Crystallographic parameters for both complexes are presented in Table 2, and a selection of bond distances and angles and torsion angles is provided in Table 4. The overall geometries of the cations 6 and 7 are depicted in Figures 2 and 3, respectively. The most interesting aspects of these two structures relate to the conformation of the chelate ring
and the relative position of the palladium center with respect to the upper Cp ring, other bonding parameters falling in the expected range. In both complexes, the Pd atoms show a pseudo-square-planar coordination geometry. However, due to the presence of the 1,3phenyl substituents, a significant deviation from the ideal geometry is observed in derivative 7. Thus, the plane Pd/l?(l)Ip(2) forms an angle of 16.6" with the plane defined by the metal center and the two terminal allylic carbon atoms (PdlC(34)/C(36)), such that there is a clockwise rotation of the allyl fragment around the Pdallyl axis, when looking from the ligand toward the metal center. The corresponding angle in complex 6 is 4.5". Furthermore, the allylic fragment C(34)-C(35)C(36) is bent away from the PdP2 plane, as illustrated by the angle of 111" between these two planes. One of the allylic phenyl groups shows an important stacking interaction with one of the phenyl substituents on P(1). The two rings are almost ideally parallel, the angle and the distance between the corresponding planes being only 1.9" and -3.3 A, respectively. This stacking
A 9-Phosphabicyclo~3.3.lhon-9-yl Fc Derivative
Organometallics, Vol. 14,No. 2, 1995 763
Scheme 2. Four Observable Isomers of 7 H
I
Ph
l+ Ph
I
1'
'Ph Me
I
Fe
Fe I
7b Ph
Figure 4. Superimpositionof the structures of Sa, 6, and 7, showing the complete lack of correspondence of the conformations of free and coordinated ligand (Pd(allyl) fragments of 6 and 7 are omitted for clarity). 7a
7c a
7a
76
The configuration of 7c,d has been chosen arbitrarily.
interaction may be interpreted as a driving force for the rotation discussed above. The most apparent difference between the two complexes is the relative position of the palladium atom. Whereas in the n-allyl derivative 6 the metal center is located slightly below (0.29 A) the upper Cp ring, in complex 7 it is clearly above (1.16 A). The conformation of the chelate ring in the latter compound is best described as a typical twisted chair. On the other hand, in 6, five of the six atoms belonging to the metallacycle are coplanar within 0.07 A. Carbon atom C(6)is thereby the only center clearly out of this plane (distance 0.54 A). The observed differences between 6 and 7 are mainly a consequence of the conformational flexibility of the phobyl-containing side chain of the ligand. Because the 9-phosphabicyclononyl fragment is relatively small and very compact, it does not contribute in determining a particular, rigid conformation of that part of the molecule. Thus, its position with respect to the plane of the upper Cp ring varies from "all-above" in the free ligand t o "all-below" in the Pd(ally1)complexes under study, as illustrated in the superimposition of the three structures in Figure 4. This is not the case for ligand 1, which contains the more bulky dicyclohexylphosphino group and which is able to maintain a virtually constant c~nformation.~ NMR Studies of 7. slP NMR. Complex 7 shows interesting solution characteristics. The room-temperature 31PNMR spectrum reveals several sharp and several broad components. A series of variable-temperature measurements in CDCl3 between 223 K and ambient temperature showed that two components, 7a and 7b, were sharp throughout, a third complex became and stayed fairly sharp, 7c, and the fourth, 7d (see Scheme 21, became reasonably sharp with the optimum overall spectrum reached at 273 K, as shown in Figure 5. Each of the complexes shows an AX (or AB) spin system. The observed populations of these compounds are temperature dependent, and a summary of the pertinent 31PNMR data is given in Table 5.
7d
7d
~
"
"
PPm
~
"
~
'
G
1
"
"
~
'
'
'
15
'
~
~
'
'
10
Figure 6. 31PNMR spectrum of 7 (200 MHz, CDCl3,273 K). The four isomers are clearly visible. Table 5. 31PN M R Data for 7a-d (CDCb, 200 MHz) % isomer at given T
nucleus
6(ppm)
U(P,P)(Hz) 7a 86.3
P(Ph)2 P(phoby1)
15.4 16.6
P(Ph)z P(phoby1)
12.7 13.7
87.4
P(Ph)2 P(phoby1)
10.6 19.4
85.9
P(PW2 P(phoby1)
9.5 15.2
92.7
297 K
273 K
223 K
70
69
67
6
4
1
7
9
13
11
18
19
7b
IC
7d
For each of these complexes we assign the highfrequency resonance signal to the phobyl P atom and the low-frequency absorption to the PPh2 moiety. For 7a and 7b, this assignment is supported by a 31P,lH correlation; Le., individual aromatic and aliphatic resonances correlate t o these spins. Given the assignment, it is interesting to note that the PPh2 31Pspin in 7d is that which remains dynamic at 273 K (and still so even a t 223 K). A 31P 2D exchange spectrum a t ambient temperature reveals selective exchange processes, with 7a and 7b in exchange, and 7c and 7d in exchange, with the former pair illustrated in Figure 6. 7a is not in observable equilibrium with either 7c or 7d; at this
Abbenhuis et al.
764 Organometallics, Vol. 14, No. 2, 1995 3'P
'H
1
'H 3' P
il"
He
1 1 7 1 I 16.5
" 14
16
/
Ii
PPm
Figure 6. Section of the 31P2D exchange NMR spectrum at ambient temperature. The strong cross-peaksbetween 7a and 7b are readily observed, even though the ondiagonal signals for 7b are too weak to be seen in this presentation. Table 6. 'H NMR Data for 7a (CDCIJ, 500 MHz, 297 K) "upper"
1 2 3 4 5
5.81 5.89 5.31 6.65 7.39
6 7 CHCH3 CHCH3 CP
6.21 8.00 3.37 1.09 3.67
temperature, we see no exchange with mixing times of 0.5-1.0 s. Presumably, there is slow exchange, at least between 7b and 7c, as these two isomers show the most marked population change between 223 and 297 K. For the major isomer, the 31P, lH correlation mentioned above also allows us to (a) assign the two terminal allyl protons and (b) assign the two sets of ortho PPhz protons, and both of these are important for the NOESY discussion that follows. 'H NMR and NOESY for 7a. The IH spectrum for 7 at ambient temperature is dominated by 7a (see Table 6). Using simple inspection, the 31P,IH correlation, and a lH NOESY we could make the necessary assignments such that the major isomer, 7a, could be recognized. Complex 7a has the structure found in the solid state. Specifically, from the NOESY cross-peaks we note: (a) that NOE from the v5-Cpto the lower set of ortho PPhz protons differentiates these two phenyl rings. (b) an NOE from the methyl group to the upper PPhz ortho protons supporting the assignment in (a). This latter NOE also shows the conformation of the chelate ring, i.e., the methyl group lies above the substituted Cp plane. (c) an NOE from one terminal allyl proton t o the ortho protons of the lower PPhz phenyl, suggesting these
iiP
P ~
ppm 6.0
Figure 7. Section of the IH NOESY for 7a. The arrows indicate the cross-peaks due to (a) NOE from one terminal allyl proton to the ortho protons of the lower PPhz phenyl, suggesting these anti protons are down and thus that the central allyl proton is up (lower right arrow), and (b)NOE from the ortho allyl phenyl protons near to the phobyl interacting with the upper PPhz ortho resonances, thus indicating some allyl rotation (middle left arrow). The empty spaces, indicated by the rectangles, are where one should find cross-peaksif there were no stacking of the allyl and upper PPh group.
anti protons are "down" and thus that the central allyl proton is %pn. This is the key observation for the selection of 7a as the correct structure. (d) there is an interaction from the CH3 t o one set of ortho phenyl protons from the 1,3-diphenyl allyl. (e) intra-allyl NOES, which allow us to assign the second set of ortho phenyl protons from the 1,3-diphenyl allyl. Given all of these interactions (and of course the assignments), we can suggest further subtle solutionstructural aspects: (0 there must be n-stacking of the upper PPhz phenyl and the immediately adjacent allyl phenyl. We note that the appropriate PPh2 ortho resonances are at relatively low frequency (an anisotropic effect of the allyl phenyl group) and that there is no NOE between these two phenyl groups. This latter is more or less impossible unless the rings are stacked, in which case they are >3.3 A apart. Interestingly, the rings must be rotating synchronously, as there is no restricted rotation, at ambient temperature, about either of the appropriate P-C or C-C bonds. (g) there is probably some clockwise rotation of the allyl group (seen from behind the allyl toward the Pd atom), as the ortho allyl phenyl protons near t o the phobyl fragment show a weak NOE to the upper PPhz ortho resonances. Figure 7 shows a section of the NOESY for 7a in which points b, f, and g are emphasized; all three of these are supported by the solid-state structure of 7a. The lH NOESY spectrum was measured in phasesensitive mode so that both NOE and exchange can be
A 9-Phosphabicyclo[3.3.llnon-9-yl Fc Derivative
observed. We do find exchange between 7a and 7b, and this allows us to find the central allyl proton of 7b. From the form of its signal (two relatively large spin-spin interactions), and the observation of two NOES t o the two different sets of ortho allyl phenyl protons, we can assign 7b a syn/syn arrangement for the allyl phenyl groups. Unfortunately, the signals of 7c and 7d are too broad, and frequently under the signals of 7a, for us t o say anything definite with respect to structure. We assume that the two remaining isomers have the syn/ anti arrangements, as indicated in Scheme 2; however, we cannot distinguish between the various syn/anti possibilities (or even exclude antifanti isomers). Nevertheless, it is important to recognize the presence of these isomers, since they will possibly contribute to the overall enantioselectivity of the catalytic reaction. The allyl 13C chemical shifts for 7a are interesting: 6 110.8 (central), 100.7 (trans to phobyl-P), and 80.8 (trans t o PPh2-PI. Discounting steric effects, the 19.9 ppm difference between the two terminal carbons is a hint as t o the electronic structure of 7a and specifically, as to which terminal allylic center has more double character (that trans to the phobyl P). The cationic complex 7 will be formed as an intermediate in the course of the catalytic alkylation of 1,3diphenyl-3-acetoxypropene. Having shown that the configuration of this Pd(ally1) complex observed in the solid state corresponds to the preferred one existing in solution (7a), and because the absolute configuration of the product is known (R), one can determine the preferred site of nucleophilic attack. As indicated in Figure 3, this corresponds to the allylic carbon atom in pseudotrans position with respect t o the phobyl group (attack on the other allyl terminus would lead to the other enantiomer). A tentative explanation for this observation is as follows. Since the aliphatic phosphine (the phobyl fragment) exerts a higher trans influenceg than the diphenylphosphino group, the Pd-carbon bond trans to it should be weaker than the cis one. Such a weakening will increase the olefinic character and hence the electrophilicity of that carbon atom. This is also supported by the 13C NMR data discussed above. However, care must be exercised in applying such ground-state criteria to a problem that is eminently kinetic in nature, i.e., enantioselectivity. Whether or not an enhanced ground-state electrophilicity will be paralleled by a low-energy path for nucleophilic attack cannot be decided on the basis of our data.
Conclusions We have shown that an important reactivity difference between the two isomers of 9-phospha-9H-bicyclononane exists, when the technical 2:l mixture is used as nucleophile. Thus, the new ferrocenyl phosphine 5a, containing the [3.3.1l-fragment exclusively, could be prepared. It is t o hope that, by exploiting such reactivity difference, the symmetric phobyl group will be incorporated in other chelating phosphine systems when a nonbulky aliphatic PR2 group is needed. This will allow avoidance of the use of the air-sensitive and more expensive PMez and PEt2 synthons. The application of ligand 5a in the asymmetric allylic alkylation has shown it to be inferior t o other chiral (9) For a review, see: Appleton, T.G.; Clark, H. C.; Manzer, L. E. Coord. Chem. Rev. 1973,10,335-422.
Organometallics, Vol. 14, No. 2, 1995 765 ligands for that particular reacti0n.l This may be attributed to the existence of a t least four configurational isomers of the intermediate Pd(ally1) complex, as shown by our NMR studies of 7. In a broader sense, the relatively low enantioselectivities observed may also be correlated to the high conformational freedom ligand 5a displays. In other words, because Sa can assume significantly different conformations, it is not able to create a well-defined chiral environment around the Pd center during catalysis.
Experimental Section General Considerations. All reactions with air- or moisture-sensitive materials were carried out under Ar using standard Schlenk techniques. Freshly distilled, dry, and oxygen-free solvents were used throughout. Technical grade phobane (2:l mixture of L3.3.11- and [4.2.1l-isomers together with -30% oxides) was obtained by courtesy of Prof. A. Salzer, RWTH Aachen, and was used as received. Routine IH (250.133 MHz), (62.90 MHz), and 31PNMR (101.26 MHz) spectra were recorded with a Bruker AC 250 spectrometer. Chemical shifts are given in ppm and coupling constants (J) are given in hertz. The detailed NMR study of 7 has been carried out using CDC13 solutions and a Bruker AMX 500 spectrometer. Standard pulse sequences10-'2were used for the NOESY, 31P, 31P exchange, 13C, 'H, and 31P, lH spectra. NOESY spectra were measured using a 0.8 s mixing time, while two measurements for the 31P,31Pexchange were done using a 0.5 and 1s mixing time. Merck silica gel 60 (70-230 mesh) was used for column chromatography. Optical rotations were measured with a Perkin-Elmer 241 polarimeter using 10 cm cells. Elemental anslyses were performed by the Mikroelementar-analytisches Laboratorium der ETH. Catalytic experiments and analyses of reaction products were carried out as previously described.' Z)-Phospha-O-[(S)-l-{ (R)-2-(diphenylphosphino)ferrocenyl}ethyllbicyclo[3.3.llnonane ((S)-(R)-5a). A yellow solution of NJVN-dimethyl-(S)-l-[(R)-2-(diphenylphosphino)ferrocenyllethylamine (4; 1.77 g, 4.01 mmol) and HPCsH14 (6.88 g, 31 mmol of 2:l L3.3.11- and [4.2.1l-isomers) in acetic acid (-50 mL) was heated at 80 "C for 2 h. The solvent was subsequently removed in UUCUO and the sticky, smelly, residue subjected t o flash chromatography over A 2 0 3 using toluene (with 2% NEt3) as the eluent. The phosphine oxides still contained in the raw material are not eluted and remain on the column. Subsequent crystallization from hot EtOH (50 mL) gave 1.46 g (68%) of analytically pure, orange, product that crystallized as large needles in two successive crops of 1.27 and 0.19 g respectively: [aIz2~ = +336 (c = 1.1,CHCl3); 'H NMR (CDCl3,298K) 6 7.18-7.63 (m, 10H, PPhZ), 4.58,4.37, , (s, 5H, CsHs), 3.09 (dq, lH, 4.03 (all br s, 3H, C E H ~ )3.92 CH(Me)P, 3J(H,H) = 7.7, V(H,P) = 3.51, 1.59 (dd, 3H, CH(Me)P, 3J(H,H) = 7.5, 3J(P,H) = 13.01,0.91-2.17 (m, 14H, PC8H14); 13C{1H) NMR (CDC13, 298 K) 6 140.1-127.8 (Ph), 74.0,69.9 (C5H3),69.2 (C5H5),32.0 (CH(Me)P),25.4-21.6 (CH(Me) and PC8H14): 31P NMR (CDC13, 298 K), 6 -10.5 (s, P C B H ~ ~-26.8 ), (s, PPh2). Anal. Calcd for C32H36PzFe: C, 71.38; H, 6.74. Found: C, 71.83; H, 7.14. [Pd(r13-CsHs)((S)-(R)-5a)][CF3S031 (6).To a magnetically stirred solution of (S)-(R)-Sa (158 mg, 0.294 mmol) and [Pd(r3-C3H5)(pC1)]2(54 mg, 0.15 mmol) in CHzClz (10 mL) was added a solution of AgCF3S03 (77 mg, 0.29 mmol) in MeOH (1.5 mL). The resulting suspension was stirred for 1h in the dark and filtered over Celite, followed by removal of the (10)Jeener, J.; Bachmann, P.;Ernst, R. R. J. Chem. Phys. 1979, 71.4545-4553. -, ~ ~ -
(ll)Summers, M. F.; Marzilli, L. G . ; Bax, A. J . Am. Chem. S O ~ . 1986,108,4285-4294. (12) Sklener, V.; Miyashiro, H.; Zon, G.; Miles, H.T.; Bax,A. FEBS Lett. 1986,208,94-98.
Abbenhuis et al.
766 Organometallics, Vol.14,No. 2, 1995 solvents in vacuo. The orange residue is dissolved in warm CH2C12, and hexane is added till the onset of cloudiness. The warm solution is allowed to cool to room temperature over night, causing the product to crystallize as orange needles: yield 245 mg (94%), [ a ] 2 2=~ 315 (c = 1.2, CHClS), 'H NMR (CDC13,298 K) major isomer 6 7.1-7.8 (m, lOH), 5.95 (m, lH), 5.01 (m, 1H),4.25-4.65 (m, 2H), 4.22 (s,IH), 3.80 (s,2H), 3.72 (s, 5H), 3.26 (m, lH), 1.21 (dd, 3H, 3J(H,H) = 7.1, 3J(P,H) = 15.1), 0.87-2.57 (m, 14H); lH NMR (CDC13, 298 K) minor isomer 6 7.1-7.8 (m, lOH), 5.32 (m, lH), 4.70 (m, lH), 4.254.65 (m, 2H), 4.16 (s, lH), 3.74 (s, 2H), 3.66 (s, 5H), 2.85 (m, lH), 1.06 (dd, 3H, 3J(H,H)= 7.2, 3J(P,H)= 15.6), 0.87-2.57 (m, 14H); 13C{IH}NMR (CDC13, 298 K) 6 134.3-122.0, 78.0, 76.0, 71.7-72.5, 70.8, 30.9, 30.6, 29.6, 24.5-23.1, 22.0, 21.1, 31P NMR (CDC13, 298 K)major isomer 6 18.5 (d, 2J(P,P) = 57), 9.2 (d, 2J(P,P)= 57); 31PNMR (CDCl3,298K) minor isomer 6 17.8 (d, *J(P,P)= 60), 10.4 (d, *J(P,P)= 59.8). Anal. Calcd for C ~ ~ H ~ I O ~ F ~ C, P ~51.78; S F ~H,P 4.95. ~ : Found: C, 51.68; H, 5.18. [Pd(q3-PhCHCHCHPh)((S)-(R)-5a)l[CF3SO~l (7). The procedure is the same as that for 6 except that the phosphine (S)-(R)-Sa (102 mg, 0.19 mmol), [Pd(y3-PhCHCHCHPh)@Cl)1z (64 mg, 0.095 mmol), and AgCF3S03 (49 mg, 0.19 mmol) reacted to give 128 mg (68%) of orange, crystalline product: [ a ] 2= 2 ~- 134 (c = 0.73, CHC13); 'H NMR (CDC13, 298 K)6 6.2-8.1 (m, 20H), 5.86 (m, lH), 5.81 (m, lH), 5.36 (m, lH), 4.40 (s, 2H), 4.01 (s, lH), 3.68 (s, 5H), 3.35 (m, lH), 1.09 (dd, 3H, 3J(H,H) = 6.9, 3J(H,H) = 14.7), 0.9-2.7 (m, 14H); 13C('HI NMR (CDC13, 298 K) 6 138.6-129.1 (Ph), 110.7, 100.5, 80.8, 73.5, 70.8, 70.1, 70.7, 30.8-20.9; 31PNMR (CDC13, 298 K) 6 15.4 (d, *J(P,P)= 87), 14.2 (d, 2J(P,P) = 87.) Anal. Calcd
+
for C48H4903F3P$3FePd*CH2Cl2:C, 54.78; H, 4.79. Found: C, 54.85; H, 4.85. X-ray CrystallographicStudies of Racemic (S)-(R)-5a, 6, and 7. Selected crystallographic and relevant data collection parameters are listed in Table 2. Data were measured with variable scan speed to ensure constant statistical precision on the collected intensities. One standard reflection was measured every 120 reflections; no significant variation was detected. The structures were solved either by direct (5a,7) or Patterson (6)methods and refined by full-matrix least squares using anisotropic displacement parameters for all nonhydrogen atoms. The contribution of the hydrogen atoms in their idealized position (Riding model with fixed isotropic U = 0.080 A2) was taken into account but not refined. All calculations were carried out by using the Siemens SHELXTL PLUS system.
Acknowledgment. We thank Dr. N. Weferling, Hoechst AG, and Professor Salzer, RWTH Aachen, for a generous gift of phobane. Supplementary Material Available: Tables of atomic coordinates, complete listing of bond distances and angles, tables of anisotropic displacement coefficients, and coordinates of hydrogen atoms for 5a, 6, and 7 (19 pages). Ordering information is given on any current masthead page. Table of calculated and observed structure factors (24 pages) may be obtained from the authors upon request. OM940765D
