9198
J . Am. Chem. SOC.1990, I 12, 9 198-92 12
Selective Activation of One Methyl Ketone Enantioface via a-Binding to a Chiral Transition-Metal Template: Synthesis, Structure, and Reactivity of Rhenium Ketone Complexes [ ( $-C5Hs)Re(NO)(PPh,)( q'-O-C(CH,)R)]+XDennis M. Dalton,'" Jesiis M. Fernindez,'" Kenneth Emerson,IbRaymond D. Larsen,Ib Atta M. Arif," and J. A. Gladysz*,'" Contribution from the Departments of Chemistry. University of Utah, Salt Lake City, Utah 841 12, and Montana State University, Bozeman, Montana 5971 7 . Received May I O , 1990
Abstract: Reactions of dichloromethane complex [(~S-CsHs)Re(NO)(PPh,)(CIC ti2CI)]+X-(2+X-: X- = BF4-, PI=,-) and O=C(CH3)R (R = (a) CH,, (b) CH2CH3,(c) CH(CH3)2,(d) C(CH,),; (e) c&) give u ketone complexes [(q5-C5H5)Re(NO)(PPh,)(s'-O=C(CH,)R)I+X(3a-e+X-, 79-8955). Crystal structures (h,e+PF,-) show N-Re-0-C torsion angles of 21-9', one C=O face to be shielded by the bulky PPh3 ligand, and rhenium cis to CH,.The E / Z methyl groups in 3a+RF; ~ 6 kcal/mol). Both K(sec-C4H9),BH and formyl complex ($-C,H,)Re(NO)(PPh,)(CHO) (7) rapidly cxchangc ( A G ' 1 3 3 = rcducc 3a-e'X- to alkoxide complexes (RS,SR)-(os-C5H5)Re(NO)(PPh,)(OCH(CH,)R) (5) in high yields and 75-99% de. Thc structure of (RS,SR)-Se is verified crystallographically and is consistent with hydride addition from a direction anti to the PPh3 ligand. Analogous reactions starting with optically active 2+BF4-give optically active ketone and alkoxide complexes with rctcntion of configuration a t rhenium. Reactions of (+)-(RS)-Sb-e, ( + ) - ( ~ ) - ~ 6 ~ 5 ( ~ ~ , ~ ) and ( ~ 43 ~ ) ~ ~ ( = ~ ) ~ (dimcthy1amino)pyridinegive esters (RS)-C6Hs(CH30)(F3C)CC(==O)OCH(CH,)R (95-99%) in de that closely match those of (+)-(RS)-Sb-e. Reactions of (+)-(RS)-Sc,dand (-)-(S)-C,H,(CH,O)( F3C)CC(=43)0H give alcohols (S)-HOCH(R)CH, JC6H5) (92-93%, 199% de). Reaction and carboxylatc complex (+)-( RS)-(~s-CsHs)Re(NO)(PPh3)(OC(=O)C(CF,)(OCH of (+)-(RS)-Se with HBF4.0(C2H5)2and then 2-butanone gives I-phenylethanol (71%) and (+)-(RS)-3b+BF4- (78%. 93% cc).
Unsaturated organic molecules are often activated toward nucleophilic addition upon coordination to a transition metals2 Surprisingly, however, there have been few if any systematic studics of rcactions of ketone complexes and nucleophiles-or for that matter the binding of ketone ligands to inorganic and organomctallic fragments.j This is remarkable in view of the considcrablc cffort that has been directed at the asymmetric reduction of unsymmetrical ketones to optically active secondary and tertiary alcohols.4d Many classes of chiral metal complexes are now readily available in optically active form' and would seem to offer excellent prospects for a general solution to this important problcm in organic synthcsis.
Scheme 1. Synthesis (of Ketone Complexes NO)(PPh&q'-O=C(CH,)R)]+BF; [(qS-CsHs)Re(
E
0002-7863/90/ 15 12-9 l98$02.50/0
R = a , CH3
Z
b, CHPCY -2,
(I)(a) University of Utah.
(b) Montana State University. (2) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G.Principles and Applications of Organorransition Metal Chemistry; University Science Books: Mill Valley, CA; Chapter 7. (3) Huang. Y.-H.; Gladysz. J. A. J. Chem. Educ. 1988, 65, 298. (4) Reviews: (a) Midland. M. M. Chem. Rev. 1989, 89, 1553. (b) Grandbois, E. R.; Howard, S. I.; Morrison, J. D. I n Asymmetric Synthesis; Morrison, J. D., Ed.; Academic Press: New York, 1983; Vol. 2, pp 71-90. (c) Koenig, K. E. /bid. Vol. 5 ; pp 79-80. ( 5 ) This literature is extensive. Selected lead references to methods that employ stoichiometric quantities of the chiral reagent or auxiliary: (a) Noyori, R.; Tomino. I.: Tanimoto. Y.; Nishizawa, M. J. Am. Chem. SOC.1984, 106, 6709.6717. (b) Midland, M. M.; Kazubski, A. J . Org. Chem. 1982.47, 2495. (c) Itsuno. S.; Nakano, M.: Miyazaki, K.; Masuda, H.; 110, K.; Hirao, A,; Nakahama, S. J. Chem. SOC.,ferkin Trans. I 1 9 8 5 , 2039. (d) Imai, T.; Tamura. T.: Yamamuro, A,; Sato, T.; Wollmann, T. A,; Kennedy, R. M.; Masamunc, S. J. Am. Chem. SOC.1986. 108, 7402. (e) Brown, H. C.; Park, W. S.; Cho. B. T.: Ramachandran, P. V. J. Org. Chem. 1987.52, 5406. (9 Brown, H. C.; Chandrasekharan, J.; Ramachandran. P. V.J. Am. Chem. Soc. 1988. 110. 1539. (g) Brown. H. C.; Ramachandran, P. V. J . Org. Chem, 1989, 54, 4504. (6) Lead rcfcrenccs to methods that employ catalytic quantities of the chiral reagent or auxiliary: (a) Keinan, E.; Hafeli. E. K.; Seth, K. K.; Lamed. R. J . Am. Chem. Soc. 1986, 108, 162. (b) Kitamura. M.; Okhuma, T.; Inoue, S.; Sayo, N.: Kumobayashi. H.: Akutagawa. S.; Ohta, T.; Takaya, H.; Noyori, R. J. Am. Chem. SOC.1988. 110. 629. (c) Corey, E. J.; Bakshi, R. K.;Shibata. S . Ibid. 1987. 109. 5551. (d) Corey. E. J.: Bakshi, R. K.; Shibata, S.; Chen. C.-P.; Singh. V. K. Ihid. 1987. 109, 7925. (e) Corcy, E. J.; Chcn, C.-P.; Reichard, G . A. Tetrahedron Lett. 1989, 30, 5547. (f) Shzn, G.-W.; Wang, Y.-F.: Bradshaw. C.: WOnR. C.-H. J. Chem. SOC..Chem. Commun. 1990.677. (6) Takahashi. H.; Sakuraha. S.: Takeda, H.;Achiwa, K. J. Am. Chem. SOC. 1990. 112. 5876. (7) Scc' articlcs publishcd in "Organomctallic Compounds and Optical Activity". J. Orgonomet. Chem. 1989. 370; Brunner, H., volume editor.
(3+BF4')
CH(CH3)2
4 C(CM e, C S Y
We have shown that "chiral-at-rhenium" complexes of the general formula [($-C,H5)Re(NO)(PPh,)(L)]n+ are easily prepared in optically pure form8 and undergo a variety of reactions in which new ligand-bas'ed stereogenic centers are formed with high stereoselectivity.e" In particular, the corresponding cationic aldehyde complexes have been found to undergo highly stereoselective nucleophile add itions to give complexes of chiral alkoxides.I0 Thus, we sought to synthesize and study the reactivity of analogous ketone complexes. (8) Merrifield, J. H.: Strouse, C. E.; Gladysz, J. A. Organometallics 1982,
I. 1204.
(9) Some lead references: (a) OConnor, E. J.; Kobayashi, M.; Floss, H. G.; Gladysz, J. A. J. A m . Chem. SOC. 1987, 109,4837. (b) Bodner, G . S.; Smith, D. E.; Hatton, W. G.: He.ah, P. C.; Georgiou, S.; Rheingold, A. L.; Geib. S. J.; Hutchinson, J. P.; Glatlysz, J. A. /bid. 1987. 109, 7688. (c) Senn, D. R.; Wong. A.; Patton, A. T.; Mimi, M.: Strouse, C. E.; Gladysz. J. A. fbid. 1988. 110, 6096. (d) Crocco, G . L.; Lee, K. E.: Gladysz, J. A. Organomelallics 1990, 9, 2819. (e) Pen&:. T.-S.;Gladysi, .I A. Tetrahedron Lett. 1990. 31. 4417. (IO) (a) Fernlndez, J. M.; Eme:rson, K.; Larsen, R. D.; Gladysz, J. A. J . Am. Chem. SOC.1986, 108, 8268. (b) Garner, C. M.; FernBndez, J. M.; Gladysz, J. A. Tetrahedron Lett. 1989, 30, 3931. (c) Garner, C. M.; Qui& MEndcz, N.; Kowalczyk, J. J.; Ferngndez, J. M.; Emerson, K.; Larsen. R. D.; Gladysz. J. A . J. A m . Chem. SOC.1990, 112, 5146. ( I I ) Dewey, M. A,; Arif, A. M.: G ladysz, J. A. Manuscript in preparation.
0 I990 Anicrican Chemical Society
Synthesis oJ
C5H5iRe(NOJIPPhJ(v'- O=C(CHJR)]+X
J . Am. Chem. SOC.,Vol. 112, No. 25, 1990 9199
The optically active methyl complex (qS-CSHs)Re(NO)(PPh3)(CH3)(1) is easily converted to the optically active dichloromethane complex [(~S-C5Hs)Re(NO)(PPh3)(CICH2Cl)]+XON ,Re\ I (2+X-).I2 Complex 2+X- reacts with a variety of donor ligands * o (L) between -50 and -30 "C to give substitution products H3C,C// [(q5-C5H,)Re(NO)(PPh3)(L)]+X-in high yields and with reI tention of configuration at rhenium. Hence, 2+X- serves as the *CH3 functional cquivalcnt of the chiral Lewis acid [($-C,H,)Re(NO)(PPh,)]+ ( I ) . I n this paper, we report ( I ) the conversion of raccmic and optically active 1 to racemic and optically active u methyl ketone complexes [(~5-CsHs)Re(NO)(PPh3)(~1-O=C(CH,)R)]+X- (3+X-) in high chemical and optical yields, (2) crystal structures of the acetone and acetophenone complexes, which show one C=O face to be shielded by the bulky PPh3 ligand, (3) dynamic propcrties of the ketone ligands in solution, (4) highly diastcrcoselcctive and enantioselective reductions of 3+X to raccmic and optically active alkoxide complexes ($CsH 5 ) Re( N 0)( PPh,) (OCH (C H3)R), (5) reactions of optically active alkoxide complexes that give esters or alcohols and rhenium complexes in high chemical and optical yields, and (6) mechanistic analyses of the preceding reactions. Portions of this study have becn comm unica tcd .I Results 1. Synthesis of Ketone Complexes. Methyl complex (osCs M & Re( NO) ( PPh3)(C H 3) ( 1) l 4 and H B F4-0(C2HJ were combined in CH2CI2at -80 "C as previously described to give the dichloromet hane complex [ ( qS-C5H5)Re(NO)(PPh3)(CICH2CI)]+BF4-(2+BF4-).I2 Then the methyl ketones (a) acetone, (b) 2-butanone, (c) 3-methyl-f-butanone, (d) 3,3-dimethyl-2-butanone. and (e) acetophenone were added (3 equiv; Scheme I ) . Workup gave u ketone complexes [($-C,H,)Re(N 0)( PPh 3) (q'-O=C (C H 3) K)]+BF4 (3a-e+BF4-) in 79-86% yiclds as orange or red powders. The hexafluorophosphate salts 3a.b.e+PF6- were analogously prepared from 1 and HPF6.0(CzHs)2.i5 When the preceding reactions were monitored by 31PNMR, ketone complexes 3+X- were observed to form over the course of several hours at -40 "C. A kobsfor the appearance of 3a+BF4was measured at -36 OC in a reaction with 4 equiv of acetone (2.7 X s-I). This gave a half-life of 43 min. Complexes 3a-e+X- were characterized by microanalyses (Experimental Section) and by IR and N M R (IH, 13CliH],jlP{'HI) spectroscopy (Table I). The u ketone coordination mode was assigned from the medium-strong IR uc4 at 1554-1625 cm-l and characteristic carbonyl "C NMR resonances at 216-240 ppm (Table I)., Interestingly, reactions of 2+BF4- and oliphafic aldehydes'O gave exclusively ?r aldehyde complexes [($-C,H,)Re(NO)(PPh,)(s2-O==CHR)]+BF4(4+BF4-).I6 The IR uN0 of ketone complexes 3a-e+BF4- (1677-1697 cm-l) were lower than those of aldehyde complexes 4+BF4- (1729-1740 cm-I). Also, the PPh3 31PNMR resonances of 3a-e+BF4- were downfield of those of 4+BF4- (18.4-18.9 vs 10.0-10.2 ppm). The cyclopentadienyl 'H and 13CN M R resonances were upfield of those of 4+BF4- (6 5.59-5.71 vs 5.83-5.96 and 92.9-93.3 vs 98.3-99.3 PPm). Solutions of the aliphatic ketone complexes 3a-d+BF4- were orange. while those of acetophenone complex 3e+BF4-were red. The UV/visible spectra showed intense UV absorptions that tailed ( I 2) Fcrniindcz. J . M.; Gladysz, J . A . Organomerallics 1989, 8, 207. (13) (a) FernAndez. J . M.; Emerson, K.; Larsen, R. L.; Gladysz, J . A. J . Chew. Soc.. Chem. Commun. 1988, 37. (b) Dalton, D. M.; Gladysz, J. A. J Organomer. Chem. 1989, 370, C17.
(14) Tam, W.; Lin, G.-Y.; Wong, W.-K.; Kiel, W. A,; Wong. V. K.; Gladysz, J . A . J. Chem. Sor. 1982, 104, 141. (15) At an early stage of this study. the quality of HPF,.0(C,Hs)2 available from commercial sources declined dramatically, and 2+PF,- could only bc gencratcd in low spectroscopic yields. Accordingly, we standardized on tctrafluoroborate salts 3+BF4- for reactivity studies. (16) (a) In aromatic aldehyde complexes [(ps-CsHS)Re(NO)(PPh,!(0= CHAr)]+BF4-. either the I or u binding mode can dominate. This IS a sensitive function of arene substituents. temperature, and solvent.'" (b) Quirds MEndez. N.: Arif. A. M.;Gladysz, J . A. Angew. Chem., I n i . Ed. Engl., in prcss.
-1 44 9 "C
BF; PPh,
-1 40 4 'C
n I , , i l n I , t r i
39 ppm
26
Figure 1. Variable-temperature "C N M R spectra of labeled acetone complex [ ( ~ s - C ~ H S ) R e ( N O ) ( P P h l ) ( ~ ~ - O = C ( ~ l C H l ) z )(3a+]+BF~ I1CZBFL) in CDFC12.
into the visible. The spectrum of acetone complex 3a+BF4(CH2C12)exhibited small shoulders at 312 and 364 nm (e 2200, 1600 M-'cm-I). That of acetophenone complex 3e+BF4- showed a distinctive visible absorption at 433 nm (c 5300) and a UV shoulder at 260 nm (e 18700). 2. Ketone Ligand Exchange and Isomerism. Unsymmetrical ketone complexes 3b-e'X- can exist as either E or Z C=O geometric isomers. In the former, the rhenium is cis to the smaller methyl substituent. Thus, it was anticipated that E geometric isomers would predominate. Accordingly, 3b-e'X- appeared homogenous. No "doubling" or structure associated with the IR uN0 or uc4 was noted. Also, no decoalesence phenomena were noted in ' H N M R spectra recorded a t -95 "C. However, acetone complex 3a+BF4- exhibited only one methyl IH and I3C N M R resonance, when in theory separate signals should be observed for the groups cis and trans to the rhenium. Hence, some process must equivalence the E / Z methyl groups. One possibility would be acetone ligand dissociation. Thus, the rate of reaction of 3a+BF4- and acetone-d6 (16 equiv, CD2CI2, 28 "C) was measured. Deuterioacetone complex 3a+-d,-BF4formed with kob = 1.05 f 0.05 X IO4 s-l, corresponding to a half-life of 1.8 h." This shows that the acetone ligand in 3a+BF, does not undergo particularly rapid exchange at room temperature. Efforts to decoalesce the methyl 'H and "C N M R resonances of 3a+BF4-were unsuccessful. Thus, the bis-13C-labeledacetone complex [ (vS-C,HS)Re(NO)( PPh& T$-O=C( I3CH3)J'J+BF,(3a+-I3C2BF4-) was prepared from 2+BF4- and acetone-I,3-l3C2. Next, 13C N M R spectra of 3a+-I3C2BF4-were recorded in CDFCI2, as shown in Figure 1. This solvent (mp -1 35 "C) has bcen found to easily supercool to .-150 "C.18 The methyl resonances decoalesced at -140 "C, indicating a AG*,33Kof only 6.0 f 0.1 k ~ a l / m o l . ' ~ Hence, E / Z C=O geometric isomers of unsymmctrical ketone complexes 3b-e'X- should interconvert with similarly low barriers and would not be distinguishable under conventional N MR conditions. We also sought to assay for ?r isomers of 3+X-. Careful inspcction of the IR uN0 region (see above; CH2Clzor CH3NO2)I6 (17) The first-order kob is consistent with either a dissociative or associative exchange mechanism: Espenson, J . H. Chemical Kinetics and Reaction Mechanisms; McGraw Hill: New York, 1981; pp 50-55. Experiments to distinguish these possibilities are planned. (18) Siegel. J . S.;Anet, F. A . L. J . Org. Chem. 1988, 53, 2629. (19) Sandstrom, J . Dynamic NMR Spectroscopy; Academic Press: New York, 1982; Chapter 7.
9200 J . Am. Chem. Soc., Vol. 112, No. 2 j S1990
Dalton et al. c5
Chart In
I
E - I1
E- m
Z-n Z-m 1: Pyramidal rhenium fragment [(q5-C5H5)Re(NO)(PPh3)]+ with d orbital HOMO. 11 and 111: Newman-type projections of possible R e - 0 conformations of u ketone complexes 3+X-. E and Z designate
Figure 2. Structure of the cation of acetone complex [(qJ-C5H5)Re(NO)(PPh,)(q'-o=C(CH3)~)]+PF,- (3a'PFc).
geometric isomers about the C=O bond.
showed no absorptions or shoulders that could be attributed to ir isomers, within the limits of detection (14%). Also, ir isomers would exhibit characteristic C-0 I3CN M R chemical shifts (70-1 00 ppm)3*10.'6 and increase in concentration upon c001ing.l~ Thus, the carbonyl-labeled acetone complex [ ($-C,H,)Re(NO)( P P ~ , ) ( v ' - O = ' ~ C(CH3),)]+BF4- (3a+-13CBF
