Synthesis of the New Chiral Aminodiphosphine Ligands (R)-and (S

Organometallics 1996, 14, 1489-1502

1489

Synthesis of the New Chiral Aminodiphosphine Ligands (R)-and (S)-(a-Methylbenzyl)bis(2-(diphenylphosphino)ethyl)amine and Their Use in the Enantioselective Reduction of a,/?-UnsaturatedKetones to Allylic Alcohols by Iridium Catalysis Claudio Bianchini,"??Erica Farnetti,$ Lionel Glendenning,?Mauro Graziani,*J Giorgio Nardin,$ Maurizio Peruzzini,? Eliana Rocchini,* and Fabrizio Zanobini? Istituto per lo Studio della Stereochimicu ed Energetica dei Composti di Coordinazione del CNR, Via J. Nardi 39, 50132 Firenze, Italy, and Dipartimento di Scienze Chimiche, Universita di Trieste, Via Giorgieri 1, 34127 Trieste, Italy Received December 7,1994@ Synthesis of the potentially tridentate, optically pure ligands C ~ H F , C * H ( M ~ ) N ( C H ~ C H ~ PPh2)2 (R)-(+)and (5')-(-)-PNP"; 5a,b)from their corresponding chiral primary amines was achieved with excellent yields. The catalytic hydrogen-transfer reduction of PhCH-CHCOMe and other a,p-unsaturated or unsymmetrical ketones in the presence of [Ir(COD)(OMe)l2 and 5a,b is reported. These catalytic systems show high activity and chemoselectivity for the reduction of the carbonyl group (up to 94%). Due to the presence of the stereogenic center on the PNP" ligands 5a,b, asymmetric induction was also achieved with enantioselectivities up to 54%. Discussion concerning the catalytic pathway and the intermediates involved is reported in conjunction with independent reactions of isolated, optically active compounds. Some of these are the iridium(1) cyclooctadiene complexes (R)-and (S)-[IrH(COD){C~HF,C*H(M~)N(CH~CH~PP~~)~)I (8a,b)and the iridium(II1) ortho-metalated dihydrides fuc-exo-(R)-fuc-exo-(SI-, fuc-endo-(I?)-, and fuc-endo-(S)-[IrH2{CsH&*H(Me)N(CH2CH2PPh2)2}1 10a,b and lla,b). X-ray crystal data are given for fuc-exo-(R)-[IrHz{CsH&*H(Me)N(CH2CH2PPh2)2)1(loa),a distorted octahedron. The (R)-PNP" ligand facially coordinates to the iridium metal center, with the two hydride ligands cis to one another. The sixth coordination site is occupied by the ortho carbon atom of the phenyl substituent bound to the stereocenter. Crystal data: monoclinic, P21,with u = 12.623(2) b = 18.265, = 92.55(1)", V = 3232 Hi3, 2 = 4, R = 0.037, and R, = 0.041. (3) A, c = 14.032(3) 9

A,

The development of a universally applicable homogeneous catalyst for use in the stereoselectivereduction of a variety of prochiral substrates (olefins, ketones, imines, enamines) is still a diverse and intriguing area of asymmetric synthesis. As phosphines are highly efficient ligands in many catalytic reactions, there is a continuing effort to develop and design new chiral phosphines and especially chelating diphosphines for use in asymmetric cata1ysis.l In step with these developments in chiral phosphine ligands, transition-metal complexes with chiral amine ligands have also been shown to have a number of significant applications in asymmetric catalysis.1,2 It is thus conceivable that a chiral ligand containing two ISSECC, CNR. Universita di Trieste. Abstract published in Advance ACS Abstracts, February 1, 1995. (1)(a) Morrison, J . D., Ed. Asymmetric Synthesis: Chiral Catalysis; Academic Press: London, 1985;Vol. 5. (b) Jardine, F. H. In Chemistry of the Platinum Group Metals; Hartley, F. R., Ed.; Elsevier: Oxford, U.K., 1991; pp 420-424. ( c ) Ojima, I., Ed. Catalytic Asymmetric Synthesis; VCH: New York, 1993. (d) Brunner, H.; Zettlemeier, W. Handbook of Enantioselectiue Catalysis with Transition Metal Compounds: Ligands; VCH: New York, 1993; Vol. 2. For recent applications of chiral diphosphine ligands in asymmetric catalysis see: Rajanbabu, T. V.; Casalnuovo, A. L. Pure Appl. Chem. 1994,66, 1535. Hayashi, T.; Ohno, A.; Lu, S.; Matsumoto, Y.; Fukoyo, E.; Yanagi, K. J . A m . Chem. SOC.1994,116,4221. Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A,; Lianza, F. Organometallics 1994,13,1607. +

@

0276-7333/95/2314-1489$09.00/0

A,

phosphorus and one nitrogen donor atom would have a diverse and wider use in catalysis. Also, the combination of soft (phosphorus) and hard (nitrogen) donor atoms in the same ligand framework may offer the advantage of providing free coordination sites along the catalytic pathway by the alternative decoordination of either the phosphorus or nitrogen donor, depending on the nature of the reactant and of the formal oxidation state of the metal. (2)Togni, A,; Venanzi, L. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 497-526. For recent applications of chiral diamine ligands in asymmetric catalysis, see: Brookhart, M.; Wagner, M. I.; Balavoine, G. A.; Haddou, H. A. J . A m . Chem. SOC.1994,116,3641.Nishiyama, H.; Itoh, Y.; Matsumoto, H.; Park, S.-B.; Itoh, K. J . A m . Chem. SOC. 1994, 116, 2223. For applications of P-N ligands in asymmetric catalysis, see: (a) Brunner, H.; Mokhlesur Rahman, F. M. Chem. Ber. 1984,117,710. (b) Mashima, K.; Akutagawa, T.; Zhang, X.; Takaya, H.; Taketomi, T.; Kumobayashi, H.; Akutagawa, S. J. Organomet. Chem. 1992,428,213-222.( c ) Mashima, K.;Akutagawa, T.; Zhang, X.; Takaya, H.; Taketomi, T.; Kumobayashi, H.; Akutagawa, S. J.A m . Chem. SOC.1993,115,3318. (d) Albinati, A,; Lianza, F.; Berger, H.; Pregosin, P. S.; Ruegger, H.; Kunz, R. W. Inorg. Chem. 1993,32,478486. (e) Brown, J . M. Book ofAbstracts, 9th International Symposium on Homogeneous Catalysis, Jerusalem, Israel, Aug 21-26, 1994; Abstract K-4, p 7. (0 Basoli, C.; Botteghi, C.; Cabras, M. A.; Chelucci, G.; Marchetti, M. Book of Abstracts, 9th International Symposium on Homogeneous Catalysis, Jerusalem, Israel, Aug 21-26,1994;Abstract A-11, p 106. (g) Burk, M. J.; Harlow, R. L. Angew. Chem., Int. Ed. Engl. 1990,29, 1462. (h) Burk, M. J.; Feaster, J . E.; Harlow, R. L. Tetrahedron: Asymmetry 1991,2,569.

0 1995 American Chemical Society

1490 Organometallics, Vol. 14, No. 3, 1995

The potentially tridentate aminodiphosphine ligands RN(CH2CH2PPh2)2(R= H, Me, PP;PNP) have received little attention in coordination ~hemistry.~ Recent studies, however, have shown that the use of these ligands allows the formation of many new types of transitionmetal complexes,4 exhibiting a number of unusual reactivity pattern^,^ and also provides novel applications in homogeneous catalysis.6 In a previous paper6 we reported the hydrogen-transfer reduction of benzylideneacetone, PhCH=CHCOMe, in the presence of [Ir(COD)(OMe)lz(COD cycloocta-1,Bdiene)and Pr"N(CH2CH2PPh& (PNPPP). The major product of this reaction was the allylic alcohol formed by reduction of the carbonyl group, whereas reduction of the C=C bond appeared t o be disfavored. Introduction of the stereogenic center (NC*H(Me)Phinto the PNP ligand framework was expected to form a catalytic system with reductive activity and chemoselectivity comparable to that exhibited before but also with the added desirable property of asymmetric induction. This paper details our endeavors t o design, synthesize, and characterize a new variety of optically pure ligand containing the PNP donor atom set for use in asymmetric catalysis. We also report our first results of the catalytic activity of new organometallic iridium species stabilized by C6H&*H(Me)N(CH2CH2PPh2)2 (Sa,b)in the chemo- and enantioselective reduction of a,p-unsaturated or imsymmetrical ketones uia hydrogen transfer from alcohols.

Experimental Section All the reactions and manipulations were routinely performed under a nitrogen or argon atmosphere, by using standard Schlenk-tube techniques. Propan-2-01and cyclopentanol were distilled over CaO. All the substrates werepurified by either recrystallization or distillation under an inert atmosphere prior to use. Optically pure (I?)-(+)- and (SI-(-)a-methylbenzylamine (la,b,respectively) were obtained from Aldrich and used without further purification. Diphenylchlorophosphine was purchased from Fluka AG and distilled prior t o use. All the other chemicals were reagent grade and were used as received by commercial suppliers. [Ir(COD)(OMe)l2 was prepared according to a published p r ~ c e d u r e . ~ Infrared spectra were recorded on a Perkin-Elmer spectrophotometer interfaced to a Perkin-Elmer 3600 data station. 'H, 13C,and 31PNMR spectra were recorded on a JEOL EX400 spectrometer (operating at 399.77, 100.54, and 161.82 MHz, respectively) or a Bruker AC 200P spectrometer (operating at 200.13, 50.53, and 81.01 MHz, respectively). 'H and 13C chemical shifts were reported relative to tetramethylsilane. 31PNMR chemical shifts were reported relative t o external 85% H3P04,with downfield values taken as positive. Chemical yields and ee's of the catalytic reactions were determined by GC-MS using a Hewlett-Packard 5971A mass (3) (a) Sacconi, L.; Morassi, R. J . Chem. SOC.A 1969, 2904. (b) Sacconi, L.;Morassi, R. J . Chem. SOC.A 1971,492. (c) Di Vaira, M.; Midollini, S.; Sacconi, L. Inorg. Chem. 1978,17, 816. (d) Bianchi, A.; Dapporto, P.; Fallani, G.; Ghilardi, C . A,; Sacconi, L. J . Chem. SOC., Dalton Trans. 1973, 1973. (e) Bertini, I.; Sacconi, L.; Morassi, R. Coord. Chem. Reu. 1973,11, 343. (4)(a) Bianchini, C.;Innocenti, P.; Masi, D.; Peruzzini, M.; Zanobini, F. Gam. Chim. Ital. 1992,122,461.(b) Marchi, A.;Marvelli, L.; Rossi, R.; Magon, L.; Uccelli, L.; Bertolasi, V.; Ferretti, V.; Zanobini, F. J . Chem. SOC.,Dalton Trans. 1993,1281. ( 5 ) Bianchini, C.; Glendenning, L.; Peruzzini, M.; Romerosa, A,; Zanobini, F. J . Chem. SOC.,Chem. Commun. 1994,2219. (6)Bianchini, C.;Farnetti, E.; Graziani, M.; Nardin, G.; Vacca, A,; Zanobini, F. J . Am. Chem. SOC.1990,112, 9190. (7) Farnetti, E.;Kaspar, J.; Spogliarich, R.; Graziani, M. J . Chem. SOC.,Dalton Trans. 1988,947.

Bianchini et al. detector coupled with a 589011 gas chromatograph, on a Chrompack CP-CYCLODEX B 236M capillary column (50 m x 0.25 mm, film depth 0.25 mm). Alternatively, chemical yields were determined by GLC on a Perkin-Elmer Sigma 3B gas chromatograph equipped either with a Supelcowax 10 wide-bore capillary column (30 m x 0.75 mm) or with a CpSil-5 CB wide-bore capillary column (25 m x 0.53 mm), and ee's were determined by optical rotation measurements on a Perkin-Elmer 241 polarimeter. Preparation of (R)-(+)-and (S)-(-)-(a-Methylbenzyl)bis(2-(diphenylphosphino)ethyl)amine(PNP*;5a,b). (i)

Preparation of (R)-(+)-and (S)-(-)-(a-Methylbenzyl)bis(2-hydroxyethy1)amine (2a,b). (R)-(+)-a-Methylbenzylamine (la) (10.0 g, 82.5 mmol) was dissolved in an ethanol/ water mixture (1:2, v/v, 75 mL) under nitrogen and the stirred reaction mixture cooled t o -10 "C. Ethylene oxide (9.5 g, 215 mmol, 2.6 equiv) was introduced directly into the reaction mixture (-2.0 h) at such a rate as to minimize loss of unreacted oxirane. The reaction mixture was warmed to room temperature and stirred for a further 18 h. Ethanol and water were removed by distillation, and the crude diol was purified by fractional distillation under reduced pressure (163-165 "C/ 1.0 mmHg) to give (R)-(+)-(a-methylbenzyl)bis(2-hydroxyethy1)amine (2a) as a clear oil (14.4 g, 84%). (S)-(-)-(a-Methylbenzyl)bis( 2-hydroxyethy1)amine(2b)was prepared from its corresponding primary amine by following a method similar to that described above. 2b was obtained as a clear oil (80%) (167-168 "C/l.O mmHg). Anal. Calcd for CIzH19N02: C, 69.0; H, 9.09; N, 6.70. Found: C, 69.2; H, 8.58; N, 6.15. MS ( d e ) : 206 (3%), 178 (511, 105 (loo), 77 (191, 74 (75). 'H NMR (CDCl3, 20 "C, 200.13 MHz; 6): 1.41, d, 3 J ~ i , ~ z * 6.8 Hz, 3 x H1; 2.55, dd, 3JHlra,H2. 5.4 HZ, 3 J ~ 1 , p , ~5.4 ~ , HZ, 4 X H2'; 2.57, dt, ' J H I , 13.7 ~,H Hz,~3 J~~ ~ y a , ~5.4 2 . Hz, 2 x Hl'a; 2.72, dt, 2JHl'a,Hl'p 13.7 Hz, 3 J ~ 1 , p , ~5.4 2 . Hz, 2 x Hl'B; 3.68, br s, 2 x OH; 3.97, q, 3 J ~ 1 , ~6.8 2 *Hz, 1 x H2*; 7.18-7.37, m, 5 x H Ph. 13C{1H)NMR (CDC13,20 "C, 50.53 MHz; 6): 16.1, 9, C1; 52.8, s, 2 x CY; 59.9, s, C2*; 60.9, 2 x Cl'; 127.7, s, Cpur,; 128.6, s, Cartho; 128.9, S, Cmeta; 143.4, S, Ctpso. (ii) Preparation of ( R H + ) -and (S)-(-)-(a-Methylbenzyl)bis(2-chloroethyl)ammonium Chloride (3a,b). To a stirred solution of thionyl chloride (11.3mL, 18.4 g, 155 mmol) in dry chloroform (15 mL) under nitrogen was added dropwise a solution of (I?)-(+)-(a-methylbenzyl)bis(2-hydroxyethyl)amine (2a; 10.0 g, 47.8 mmol) in dry chloroform (15 mL). The reaction mixture was refluxed for 10 min and then stirred at room temperature for 18 h. The solvent was then removed in uucuo, t o leave the crude salt (I?)-(+)-(a-methylbenzyl)bis(2chloroethy1)ammoniumchloride (3a)as a viscous oil. Removal of excess thionyl chloride was achieved by repeated addition and evacuation of chloroform (3 x 20 mL) and ethanol (2 x 10 mL). The purified 3a was dried to constant weight at the pump. 3a (11.0 g, 82%), as a clear oil, was stored under nitrogen prior to use in preparation iii. Correspondingly, (S)-(-)-(a-methylbenzyl)bis( 2-chloroethy1)ammonium chloride (3b; 75%) was obtained as a clear oil.

(iii) Preparation of (R)-(+)-and (S)-(-)-(a-Methylbenzyl)bis(2-(diphenylphosphino)ethyl)amine (5a,b). To an aqueous solution of sodium hydroxide (10 mL, 7 M) was added (I?)-(+)-(a-methylbenzyl)bis(2-chloroethyl)ammoniumchloride (3a;11.0 g, 38.7 mmol) in water (20 mL). The two immiscible layers which form were separated, and the organic layer, (I?)(+)-(a-methylbenzyl)bis(2-chloroethyl)amine(4a), was dried over sodium hydroxide pellets. Anal. Calcd for C12H17NClz: C, 58.8; H, 6.94; N, 5.71. Found: C, 57.9; H, 6.60; N, 5.94. M+ + 11, 245 (1.1, M+), 231 (3.5), 197 MS ( d e ) : 246 (l.l%, (12.51, 105 (loo), 77 (12.7),63 (10.6). 'H NMR (CDC13,20 "C, 200.13 MHz; 6): 1.44, d, 3 J ~ 1 , ~6.8 2 * Hz, 3 x H1; 2.72-2.85, , ,HZ, ~ ~ 4 X H2'; 3.97, g, 3 J ~ 1 , ~ z * m, 4 X HI'; 3.47, t, 3 J ~ i6.7 6.8 Hz, 1 x H2*; 7.25-7.47, m, 5 x H Ph. 13C{lH}NMR (CDC13, 20 "C, 50.53 MHz; 6): 15.0, S, C1; 41.6, S, 2 x C2'; 52.2, 5, C2*; 58.9, S, 2 x C1'; 125.9, S, Cpara; 126.3, S, Cartho; 127.0, S, C,,t,; 141.9, S, Cipso.

New Chiral Aminodiphosphine Ligands A solution of potassium diphenylphosphide in tetrahydrofuran (130 mL) under nitrogen was prepared in situ from diphenylchlorophosphine (12.1 g, 55.0 mmol) and potassium metal (4.4 g, 112 mmol). To this stirred solution was added dropwise (R)-(+)-(a-methylbenzyl)bis(2-chloroethyl)amine(4a; 6.8 g, 27.5 mmol) and the temperature maintained at 50 "C during the addition. During the course of the addition the initial red solution gradually changed to a creamy color. The reaction mixture was then refluxed for 5 min and cooled and water (100 mL) added. The two immiscible layers were separated, and the aqueous layer was extracted with ether (2 x 100 mL). The combined organic layers were dried (NazS04) and filtered, and the ether was removed by distillation to leave a cleadivory oil. The crude product was purified by recrystallization from acetone/ethanol (1:l v/v) t o give (R)-(+)-(amethylbenzyl)bis(2-(diphenylphosphino)ethyl)amine(5a) as white crystals (78%). By a method similar to that described above, (S)-(-)-(amethylbenzyl)bis(2-(dipheny1phosphino)ethyl)amine(5b)was obtained as white crystals (78%). Anal. Calcd for C36H37NPz: C, 79.3; H, 6.79; N, 2.57. Found: C, 78.8; H, 6.75; N, 2.57. MS ( d e ) : 360 (59%), 346 (44), 185 (68), 105 (100). lH NMR (CDzClz, 20 "C, 200.13 MHz; 6): 1.23, d, 3 J ~ i , ~ 6.8 z * Hz, 3 x H1; 2.10, dd, 3J~1'a,~z. 8.3 Hz, 3 J ~ l f p , ~8.3 z . Hz, 4 x H2'; 2.59, dt, 2 J ~ i ! a , ~ 113.2 , p Hz, 3 J ~ ~8.8 , aHz, , ~2 ~x Hl'a; 2.65, dt, 2 J ~ l ' a , ~ i ,12.7 p Hz, 3 J ~ 1 , p , ~8.8 ~ , Hz, 2 x Hl'P; 3.85, q, 3 J ~ 1 , ~ 6.8 Hz, 1 x H2*; 7.26-7.39, m, 25 x H Ph. l3C{lH) NMR (CDzC12, 20 "C, 50.53 MHz; 6): 18.3, S, C1; 26.8, d, l J p , c z . 12.2 Hz, 2 x C2'; 46.7, d, ' J p , c i , 23.2 Hz, 2 x Cl'; 59.8, S, C2*; 128.8, S, Cp,; 129.0, S, 133.1, d, 2 J p , ~ , e h o3.7 Hz, 2 x COrtb; 133.5, d, ' J P , C , ~ ~ ~ 3.7 , Hz, 2 x Cortho'; 139.2, d, ' J P , C , ~ ~ 13.0 ~ Hz, Cipso; 139.4, d, 2Jp,c,p,,. 13.0 Hz, CLpso'.31P{1H}NMR (CDC13, 20 "C, 81.01 MHz; 6): -20.0, s, 2 x P. 5a: [ a ] =~ +12.0" ~ ~ (c = 1, CHC13). 5b: [ a ] ~ "= -11.2" (C = 1, CHC13).

Organometallics, Vol. 14, No. 3, 1995 1491 dd, J c , p 17.6 Hz, Jc,p 3.2 Hz, 2 x CH (COD); 127.0-134.2, m, 30 x c Ph. 31P{1H}NMR (C&, 20 "C, 81.01 MHz; 6): -2.49, d, V p , p 23.1 Hz, 1 x P; 1.24, d, V p , p 23.1 Hz, 1 x P. 8a: [ ~ I D ~ O = +12.9" (C = 1,CHC13). 8b: [ a ] ~ "= -13.0" (C = 1,CHC13). Preparation of (22)- and (SI-[(a-Methylbenzyl)bis(Z(diphenylphosphino)ethyl)amineliridium Dihydride (fhcexo-[IrHz{CsH&*H(Me)N(CH2CH2PPh2)2}1; 10a,b). [IrH(COD)(PNP*)] (8a,b; 540 mg, 1.0 mmol) was dissolved in a minimum amount of tetrahydrofuran prior to addition of propan-2-01 (30 mL) under nitrogen. The resulting solution was refluxed for 9 h. The reaction mixture was concentrated t o half its volume and n-hexane added t o induce precipitation of an off-white solid. This product was filtered and washed with propan-2-01 and n-hexane to give a mixture of the fucexo iridium dihydride (10a,b)and the fuc-endo iridium dihydride (lla,b)in a 1 O : l ratio (videinfra). Recrystallization of this crude reaction mixture from dichloromethane and propan2-01 gave pure fuc-exo-[IrHz{C~H~C*H(M~)N(CHZCHZPP~Z)Z}] (10a,b;300 mg, 70%) as a cream-colored solid. IR spectrum (Nujol mull): v(1r-H) 1975 (s), 2079 (s) cm-l. Anal. Calcd for C36H38IrNPz: C, 58.5; H, 5.02; N, 1.90. Found: C, 58.8; H,5.12;N, 1.85. 1HNMR(C6D6,200C,399.77MH~;6): -18.4, td, 'JH hydnde,P 11.8Hz, 'JH hydnde,H hydnde 4.9 Hz, 1X Hapeal; -7.7, ddd, 'JH hydnde,Pa,,, 139.2 HZ, 'JH hydnde,P,,, 22.0 HZ, 'JH hydnde,Hh y b d e 4.9 HZ, 1 X Hmendlonal. l3C{'H) NhfR (C&, 20 "c, 100.54 6): 12.7, S, C1; 34.3, d, l J p , c z ' 12.2 Hz, 1 x C2'; 39.3, d, ~ MHz; * 'Jp,cz. 23.2 Hz, 1 x C2'; 53.4, S, 1 x Cl'; 58.8, 1 x Cl'; 76.6, S, C2*; 120.0 t o 141, 29 x C Ph; 154.9, dd, zJpf_n~,c,,iho 82.5 Hz, zJp,.,,~,h, 10.3 Hz, 1 x Corth. 31P{1H}NMR (C&, 20 "C, 161.82 MHz; 6): 13.9, 2 J p , p 7.7 Hz, 1 x P; 17.7, ' J p , p 7.7 Hz, 1 x P. ~ (c = 1, CHC13). lob: [a1d5= -123.8' loa: [ a ] ~=' +124.0" (C = 1, CHC13).

Isomerization of fac-ao-(R)-[IrH2{CdLC*H(Me)N(CH2CH*Ph&}] (loa) to fa~-endo-(R)-[IrH2{CsH4C*H(Me)Preparation of (R)-(+)-(a-Methylbenzyl)bis(Z-(diphe- N(CH&H2PPh2)2}] (lla). Reflwingfu~-exo-(R)-[IrHz{C6H4C*H(M~)N(CHZCHZPP~Z)Z}~ (loa; 31P{1H} NMR (CsD&D3, ny1phosphino)ethyl)ammonium Chloride (6a). Concen8.4 Hz) in propan161.82 MHz) 6 12.7, V p , p 8.7 Hz; 16.2, VP,P trated hydrochloric acid (4.0 mL, 10 M) was slowly added to 2-01 under nitrogen gave fa~-endo-(R)-[IrH~{C6H4C*H(Me)(R)-(+)-(a-methylbenzyl)bis(2-(diphenylphosphino)ethyl)N(CHZCHZPP~Z)Z}] (lla; 15%) after 2 h, with the following amine (5a; 270 mg, 0.50 mmol) t o give a viscous oil. Slow spectroscopic properties. lH NMR (CeD6,20 "C, 399.77 MHz; addition of water (10mL) induced the precipitation of the (R)6): -20.4, dt, 'JH hydnde,P 12.1 Hz, 'JH hydnde,H hydnde 5.2 Hz, 1 X (+)-(a-methylbenzyl)bis(2-(diphenylphosphino)ethyl)ammonium chloride salt. This white precipitate was filtered and washed with water until the filtrate was neutral. (R)-(+)-(amethylbenzyl~bis(2-~diphenylphosphino)ethyl)ammonium chloride (6a;288 mg, 90%) was used without further purification. lH NMR (CDCl3, 20 "C, 200.13 MHz; 6): 1.71, d, 3 J ~ 1 ,6.8 ~ ~ * Hz, 3 x H1; 2.00-3.50, m, 4 x H2' and 4 x Hl'; 4.12, qd, 3 & ~ , ~ 2 *6.8 Hz, 3 J ~ ~ 6.6, Hz, ~ ~1 x* H2*; 7.14-7.54, m, 25 x H Ph; 12.52, br s, 1 x HN. 31P{1H}NMR (CDC13,20 "C, 81.01 MHz; 6): -19.7, S, 1 x P; -20.0, S, 1 x P. Preparation of (R)- and (S)-[(~-Methylbenzyl)bis(2(diphenylphosphino)ethyl)amine]iridium Hydride Cyclooctadiene [IrH(COD)(PNP*)];8a,b). [Ir(COD)(OMe)lz (663 mg, 1.0 mmol) was dissolved in dichloromethane (20 mL) under nitrogen. PNP* (5a,b; 1.09 g, 2.0 mmol) was added, and the reaction mixture was stirred at room temperature for 1h. The reaction mixture was concentrated and methanol (15 mL) added to induce precipitation of an off-white crystalline solid. This precipitate was filtered under nitrogen and washed with methanol and pentane to give [IrH(COD)(PNP*)I(8a,b; 740 mg, 69%). IR spectrum (Nujol mull): v(1r-H) 2108 (s) cm-'. Anal. Calcd for C44H5oIrNPz: C, 62.4; H, 5.95; N, 1.65. Found: C, 62.8; H, 5.80; N, 1.57. 'HNMR (C6D6, 20 "c, 200.13 MHz; 6): -13.44, t, ' J Hhydride,p 22.0 Hz, 1 x H hydride; 1.12, ~ ~ Hz, * 3 x H1; 1.65-3.25, m, 4 x H2' and 8 x H d, 3 J ~ 1 ,6.7 COD; 3.46, q, 3 J ~ 1 ,6.3 ~ ~Hz, * 1 x H2*; 4.02, m, 4 x Hl'; 7.20 t o 8.10, m, 28 x H Ph. l3C{'H) NMR (C&, 20 "c, 100.54 MHz; 6): 19.9, S, C1; 31.7, d, J c , p 5.1 Hz, 2 x CHz (COD); 33.8, d, J c , p 23.6 Hz, 1 x CHz-P; 34.3, d, J c , p 23.6 Hz, 1 x CHz-P; 36.1, dd, J c , p 10.8 Hz, J c , p 3.2 Hz, 2 x CHz (COD); 46.1, d, J c , p 2.5 Hz, 1 x CH2-N; 47.7, d, J c , p 3.2 Hz, 1 x CH2-N; 47.9, dd, J c , p 27.0 Hz, J c , p 6.4 Hz, 2 x CH (COD);61.4, S, C1*; 79.2,

-9.9, ddd, 'JH hydnde,Pt,, 140.9 Hz, 'JH hydnde,P,, 22.8 Hz, 4.8 HZ, 1 X Hmendlonal. 31P{1H}NMR (C6D5CD3, 20 "C, 161.82 MHz; 6): 13.2, ' J p , p 7.3 Hz, 1 x P; 15.8, 2Jp,p 7.3 Hz, 1 x P. The equilibrium concentration between 10a and l l a does not appreciably change after a further 8 h of refluxing in propan-2-01, which indicates the attainment of the equilibrium system to a stationary state. Hapical;

2 J hydnde,H ~ hydnde

Preparation of (23)- and (S)-[(a-Methylbenzyl)bis(Z(dipheny1phosphino)ethyl)amineliridiumCarbonyl Hydride ([IrH(CO){(C&I&*H(Me)N(CH2CHSPh2)2)1;14a,b). ~U~-[I~H~{C~H~C*H(M~)N(CHZCHZPP~Z)Z)I (10a,b; 200 mg, 0.27 mmol) was dissolved in toluene (40 mL). The resulting clear solution was placed in an autoclave and charged with carbon monoxide (5.0 atm). The autoclave was then heated to an internal temperature of 80-90 "C for 3 h. The resulting deep yellow solution was removed and the solvent evaporated in vacuo to leave a yellow powder. [IrH(CO){C6H5C*H(Me)N(CH~CH~PP~Z ) ~ } ] 200 mg, 97%). IR spectrum (Nujol (14a,b; mull): Y 1906 (s), 1957 (s) cm-l. IR spectrum (CHC13): v 1914 (s), 1971 (s) cm-l. Anal. Calcd for C U H ~ E I ~ N O P C,Z57.9; : H, 4.99; N, 1.83. Found: C, 57.0; H, 4.85; N, 1.77. IH NMR (THF-ds, 20 "C, 200.13 MHZ; 6 ) : -11.03, t, 3 5 H hydnde,P 28.4 Hz, 1 x H hydride; 1.15, d, 3 J ~ 1 7.0 , ~ ~ Hz, * 3 x HI; 2.40-3.10, , ~ ~ Hz, * 1 x H2*; 7.12m, 4 x H1' and 4 x H2'; 3.75, q, 3 J ~ 1 6.9 7.81, m, 25 x H Ph. 13C{lH}NMR (THF-ds, 20 "C, 50.53 MHz; 6): 21.2, S, C1; 31.9, d, l J p , c z ' 26.6 Hz, 2 x C2'; 48.1, S, 2 x C1'; 62.6, 9, C2*; 128.9-135.1, 30 x C Ph; 185.7, t, 2 J p , c 10.6 Hz, 1 x CO. 31P{1H}NMR (THF-ds, 20 "C, 81.01 MHz; 6): -7.0, S, 2 x P. Catalytic Experiments with the System in Situ. In a typical experiment, a three-necked thermostated glass reactor

1492 Organometallics, Vol. 14,No. 3, 1995 equipped with a condenser, a n argon inlet, a rubber septum, and a magnetic bar was charged under an argon flow with propan-2-01(or cyclopentanol) (50 mL). Addition of [Ir(COD)(0Me)ln (6.63 mg, 0.01 mmol) was followed by addition of the PNP* ligand (Sa,b;10.91 mg, 0.02 mmol). The resulting pale yellow solution was heated to the desired temperature in the instances where the reaction was performed at 83 or 60 "C, whereas catalytic reactions at temperatures lower than 60 "C were performed by heating the solution at 60 "C for 2 h before lowering the temperature as desired. After the solution was stirred at the final temperature for 10 min, the substrate (2.0 mmol) was added.

Catalytic Experiments with Sa,b or 10a,b as Precursors. A three-necked thermostated glass reactor equipped as described above was charged under an argon flow with propan2-01 (50 mL). Addition of Sa,b (16.94 mg, 0.02 mmol) or 10a,b (14.80 mg, 0.02 mmol) gave after a few minutes a pale yellow solution, which was heated t o the desired reaction temperature. After 10 min the substrate (2.00 mmol) was added. NMR Studies. A three-necked 10-mL flask equipped with a reflux condenser, a rubber septum, an argon inlet, and a magnetic bar was charged under an argon flow with propanol2-01 (2 mL), the iridium dimer (19.9 mg, 0.03 mmol), and the PNP* ligand (5a;32.7 g, 0.06 mmol). The resulting pale yellow solution was heated to 60 "C for 2 h; after this time, the temperature was raised to 83 "C. Samples of about 0.4 mL of the solution were withdrawn after the initial 2 h at 60 "C, after 2 h at 83 "C, and after 6 h at 83 "C. Each of these samples were transferred to an NMR tube containing about 0.3 mL of previously degassed C&. 31P NMR spectra showed the formation of compounds Sa, 9a, loa, l l a , and 12a (vide infra) with yields calculated from the integration of the NMR signals. An alternative method was to perform the reaction in propan-2-01-&, thus allowing monitoring of the IH NMR spectra. In such a case, samples of about 0.7 mL of the reaction mixture were withdrawn, and transferred to an empty NMR tube under an inert atmosphere. The same procedure was employed for the NMR studies of the reactivity of compounds Sa and 10a in propan-2-01: in these cases the above described reactor was initially charged with propan-2-01(2 mL) (or alternatively in propan-2-01-ds)and 0.06 mmol of either Sa or loa. X-ray Data Collection and Processing. Crystals offuc(R)-[IrH2(CsH4C*H(Me)N(CH2CH2PPhz)z)l(loa)were grown from a dichloromethane/methanol solution at 25 "C under an argon atmosphere. A colorless crystal of dimensions 0.15 x 0.20 x 0.35 mm was mounted on an Enraf-Nonius CAD4 diffractometer, and lattice parameters were obtained by a least-squares refinement of 25 accurately centered reflections. A summary of the crystal data and the details of the intensity data collection and refinement are provided in Table 1. No significant change in intensities, due t o crystal decay, was noticed throughout the data collection. The structure was solved by conventional Patterson and Fourier methods. The coordinated hydrides were located from the difference Fourier map after anisotropic refinement and then isotropic refinement in the final full-matrix least-squares cycles. The positions of all the other hydrogen atoms were calculated and confirmed by the difference Fourier map. Residual peaks were found only near the heavy atoms. Scattering factors, anomalous dispersion terms, and programs were taken from the Enraf-Nonius MOLEN library.8 Final positional and equivalent thermal parameters are given in Table 2. Lists of anisotropic thermal parameters and hydrogen atom coordinates are available as supplementary material. The crystal structure of 10a consists of two similar crystallographically independent neutral complexes, as suggested by the comparison of the corresponding bond distances and angles (see Tables 3 and 4, respectively). Figure 2 is an ORTEP (8) MOLEN, An interactive Intelligent System for Crystal Structure Analysis; Enraf-Nonius: Delft, The Netherlands, 1990.

Bianchini et al. Table 1. Summarv of Crvstal Data for 10a formula fw cryst syst space group a, A b, 8, c, A P, deg

Z

v,A 3

g Cm-3 F(000),e ~ ( M Ka), o cm-' cryst dimens, mm diffractometer temp, K radiation Mo K a (graphite monochromated), 8, scan type scan speed, deg min-' scan range, deg 26' range, deg no. of rflns measd no. of total measd data abs cor no. of unique data with I > 30(0 no. of variables R' &id,

RW+

W

highest peaks in final diff map, e A-3

C36H3gITNPz 738.87 monoclinic p2 I 12.623(2) 18.265(3) 14.032(3) 92.55(1) 4 3232(1) 1.60 1552 42.4 0.15 x 0.20 x 0.35 Enraf-Nonius CAD4 294 f 1 0.710 69

52 2-10 1 0.35 tan e 4-56 fh,k,l 8342 total, 8032 unique empirical from scan 545 1 376 0.037 0.041 1 -0.9, f 1 . 2

+

drawing of one of the two independent molecules. Refinement of the other enantiomer gave R- = 0.043 and R,- = 0.049. The value R = Rw/Rw+= 1.2 confirms the correct assignment of the absolute R configuration of the C7 atom as shown in Figure 2.9

Results Synthesis of the Ligands (R)-(+)and (S)-(-)-(aMethylbenzyl)bis(2-(diphenylphosphino)ethyl)amine (5a,b). Synthesis of the PNP* ligands 5a,b was achieved by following the method illustrated in Scheme 1. Nucleophilic attack with the chiral primary amines la,b and 2.0 equiv of ethylene oxide (and consequent ring opening) in ethanoywater at 0 "C gave the corresponding chiral diols 2a,b, as clear viscous oils, in excellent yields (80434%). Subsequent transformation of these diols into their dichloroammonium salts 3a,b was achieved using thionyl chloride in dry chloroform (yields 75-82%). Addition of concentrated sodium hydroxide to these ammonium salts 3a,b gave the corresponding dichloro tertiary amines 4a,b. These dichloro tertiary amines were added immediately to stirred solutions of potassium diphenylphosphide in tetrahydrofuran maintained a t a constant temperature of 50 "C. During the course of the reaction, the initial red solution gradually changed to a creamy color. After workup, the crude PNP* ligands were purified by recrystallization from acetonelethanol t o give (I?)-(+)and ($)-( -)-(a-methylbenzyl)bis(2-(diphenylphosphino)ethy1)amine (5a,b)as white microcrystals (yields 7578%). Full details of the conditions used and results are given in the Experimental Section. The presence of the stereogenic carbon, NC*H(Me)Ph, in the PNP* ligand (5a,b)imparts asymmetry to the molecule and hence inequivalence of the two phosphorus atoms of the ligand. It therefore initially proved (9) Rogers, D. Acta Crystallogr. 1981, A37, 734.

New Chiral Aminodiphosphine Ligands

Organometallics, Vol. 14,No. 3, 1995 1493

Table 2. Positional Parameters and Their Estimated Standard Deviations for 10a atom

X

v

Z

Ir P1 P2 N1 c1 H H1 c2 c3 c4 c5 C6 c7 C8 c9 c10 c11 c12 C13 C14 C15

0.83286(3) 0.9813(2) 0.8546(2) 0.9375(7) 0.807( 1) 0.760(7) 0.74(1) 0.766( 1) 0.752( 1) 0.784(1) 0.826(1) 0.8406(9) 0.8815(9) 0.944( 1) 1.0430(9) 1.0863(9) 0.9954(9) 0.932( 1) 0.938( 1) 1.0031) 1.067(1)

0.913 0.8430(2) 0.9923(2) 0.9875(6) 0.8743(7) 0.854(5) 0.986(9) 0.8087(8) 0.7941(9) 0.841(1) 0.909( 1) 0.9251(7) 0.9996(9) 1.0428(9) 0.9530(7) 0.9065(7) 0.7609(7) 0.7022(9) 0.6387(9) 0.636( 1) 0.693( 1)

0.11033(3) 0.097 l(2) -0.0133(2) 0.1939(6) 0.2448(9) 0.046(7) 0.14(1) 0.2731) 0.372( 1) 0.439(1) 0.413(1) 0.3190(8) 0.2872(9) 0.363(1) 0.2193(9) 0.1396(8) 0.1716(9) 0.149(1) 0.203(1) 0.280( 1) 0.306( 1)

Ir P1 P2 N1 c1 H H1 c2 c3 c4 c5 C6 c7 C8 c9 c10 c11 c12 C13 C 14 C15

0.33382(4) 0.4626(3) 0.3612(3) 0.4584(7) 0.312(1) 0.25 l(8) 0.22( 1) 0.256( 1) 0.249( 1) 0.292( 1) 0.352( 1) 0.362( 1) 0.414( 1) 0.493( 1) 0.564( 1) O m ( 1) 0.437( 1) 0.343(2) 0.3 18(2) 0.388(2) 0.484(2)

0.99434(4) 1.0827(2) 0.9376(2) 0.9226(7) 1.0144(7) 0.923(7) 1.046(8) 1.0679(9) 1.066(1) 1.014(1) 0.959( 1) 0.9571(8) 0.8958(7) 0.852( 1) 0.9602(7) 1.0413(8) 1.1729(9) 1.207(1) 1.276(1) 1.305(2) 1.277(2)

0.32972(3) 0.3315(3) 0.4725(3) 0.2782(6) 0.1858(9) 0.304(7) 0.38(1) 0.136(1) 0.034( 1) -0.016( 1) 0.030(1) 0.130(1) 0.1808(9) 0.126(1) 0.27 12(9) 0.2513(9) 0.282(1) 0.300(1) 0.259(2) 0.205(2) 0.181(2)

B

(A2)"

atom

B

(A2In

X

Y

Z

Molecule A 3.070(7) C16 3.35(6) C17 3.55(6) C18 3.4(2) C19 3.5(2)* c20 2(2)* c 21 8(4)* c22 4.4(3)* C23 5.3(3)* C24 5.9(4)* C25 5.1(3)* C26 3.9(2)* C27 4.3(2)* C28 5.8(3)* C29 3.7(2)* C30 3.7(2)* C3 1 3.6(2)* C32 5.1(3)* c33 5.3(3)* c34 5.9(4)* c35 C36 6.5(4)*

1.063(1) 1.027(1) 1.132(1) 1.16l(2) 1.088(1) 0.984(2) 0.951(1) 0.947( 1) 0.960( 1) 0.899( 1) 0.824( 1) 0.855( 1) 0.956(1) 1.033(1) 1.005(1) 0.7474(9) 0.649( 1) 0.568(1) 0.590(1) 0.687( 1) 0.767( 1)

0.7567(9) 0.8052(7) 0.7971(9) 0.765( 1) 0.745( 1) 0.753( 1) 0.7846(9) 1.0596(7) 1.0513(8) 0.9707(7) 0.9633(9) 0.943( 1) 0.928( 1) 0.933 1) 0.9570(9) 1.0577(7) 1.0465(8) 1.097(1) 1.159(1) 1.1715(9) 1.1207(8)

0.253(1) -0.0152(9) -0.032( 1) -0.116(1) -0.183( 1) -0.172( 1) -0.085( 1) 0.1461(9) 0.040(1) -0.1324(9) -0.206( 1) -0.298( 1) -0.31 l(1) -0.243(1) -0.150(1) -0.0412(9) -0.0090(9) -0.027( 1) -0.079( 1) -0.1 12(1) -0.094( 1)

4.9(3)* 3.9(2)* 5.4(3)* 7.9(5)* 6.7(4)* 7.4(5)* 4.9(3)* 3.8(2)* 4.5(3)* 4.2(3)* 5.6(3)* 7.2(4)* 7.3(4)* 6.9(4)* 5.2(3)* 3.5 (2)* 4.1(3)* 6.1(4)* 6.5(4)* 5.6(3)* 4.8(3)*

Molecule B 3.672(8) C16 4.24(7) C17 4.62(8) C18 3.7(2) C19 4.1(2)* c20 5(3)* c 21 8(4)* c22 5.5(3)* C23 6.6(4)* C24 6.4(4)* C25 6.6(4)* C26 4.4(3)* C27 4.2(3)* C28 6.3(4)* C29 4.2(3)* C30 4.1(2)* C3 1 5.1(3)* C32 8.0(5)* c33 9.5(6)* c34 11.9(8)* C35 11.3(8)* C36

0.508(2) 0.541( 1) 0.643( 1) 0.698(1) 0.646(2) 0.550(1) 0.497( 1) 0.474( 1) 0.480( 1) 0.385( 1) 0.469(2) 0.47 l(2) 0.404(2) 0.3 19(2) 0.310(2) 0.26% 1) 0.297(1) 0.223(2) 0.120(1) 0.090(1) 0.162(1)

1.207(1) 1.1067(9) 1.078(1) 1.094(1) 1.136(1) 1.162(1) 1.1484(8) 0.8584(8) 0.8843(9) 0.9832(9) 0.968( 1) 1.009(1) 1.060(1) 1.075(1) 1.038(1) 0.8668(8) 0.809( 1) 0.758( 1) 0.761( 1) 0.8 184(9) 0.8699(9)

0.226(2) 0.440( 1) 0.459( 1) 0.547( 1) 0.613(1) 0.595( 1) 0.509(1) 0.347( 1) 0.45 1(1) 0.587(1) 0.651(1) 0.740(1) 0.757(2) 0.695(2) 0.609(1) 0.506(1) 0.570( 1) 0.590( 1) 0.555( 1) 0.496( 1) 0.472( 1)

9.0(6)* 4.9(3)* 6.0(3)* 6.7(4)* 7.2(4)* 6.4(4)* 5.2(3)* 4.7(3)* 6.0(4)* 5.5(3)* 8.1(5)* 8.3(5)* 8.5( 5 )* 8.7(5)* 7.6(5)* 4.9(3)* 6.7(4)* 7.3(4)* 6.7(4)* 5.3(3)* 4.9(3)*

a Starred temperature factors were refined isotropically. Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as 4/3[a2Bll b2B22 &E33 ab(cos y)B12 ac(cos P)B,3 bd(cos a)B23].

+

+

+

+

puzzling t o find only one peak in the 31PNMR spectrum (and not two) for these compounds. Rapid inversion at nitrogen is a well-known and documented dynamic process of amino compounds with only a small energy barrier separating the two epimers.loa Protonation of the ligand (at nitrogen) effectively "blocks" the epimerization process while not altering the integral makeup of the molecule. Such a protonation experiment was performed on the (R)-(f)-CsH&*H(Me)N(CH2CH2PPh& ligand (5a)to give the ammonium hydrochloride salt 6a. Protonation of the chiral tertiary amine 5a was achieved by addition of concentrated hydrochloric acid. Addition of water to the acidic solution induced precipitation of the ammonium salt. Filtration of this precipitate gave 6a (90%). Blocking of epimerization at nitrogen gave the expected two singlets in the 31PNMR spectrum of the ammonium hydrochloride salt, at -19.7 and -20.0 ppm. By simply using the three-step process for preparation of chiral PNP* ligands described in this paper and an appropriately chosen starting chiral primary amine, it (10) (a) March, J.Advanced Organic Chemistry; Wiley: New York, 1985; p 86. (b) Ibid., p 124.

+

is possible t o easily and routinely prepare a large range of chiral PNP* ligands. For example, we have, in addition t o the preparation of the PNP* ligands (5a,b) bearing phenyl and methyl substituents, also prepared mixed-donor chiral ligands with methyyethy1 and methyynaphthyl substituents. The investigation of the catalytic activity and organometallic chemistry of these new chiral ligands in association with a variety of platinum-group metals will be the subject of some forthcoming publications. Synthesis and Characterization of (R)and (S)[I~H(COD){C~H&*H(M~)N(CH~CH~PP~Z)~}I (8a,b) [IrH(COD){CsH~C*H(Me)N(CH2CH2PPhz)z)l(8a,b) was prepared routinely, in good yields (70-80%), via addition of 2 equiv of the PNP* ligand (Sa,b)t o the iridium methoxy dimer [Ir(COD)(OMe)lzin dichloromethane at room temperature. (S)-[IrH(COD){CsH5C*H(Me)N(CH2CH2PPh&I (8b) was found to have the following diagnostic spectroscopic properties: a strong Ir-H stretch in the IR spectrum at 2099 cm-l, an upfield triplet ( 2 J ~ , p22.0 Hz) in the NMR (C&, 20 "c) spectrum corresponding to the terminal hydride proton, and an AM system in the phosphorus spectrum cor-

Bianchini et al.

1494 Organometallics, Vol. 14, No. 3, 1995 Scheme 1

Cl

Cl

JPPh2

PxKPPhZ /THF (4)

R'-N

I

HCI (5)

reflux -KCI

-

JPPhZ

f R'-N*-H CI- \

(6)

\

PPh2

HPh

Ci,o,m,p

1

ye

'RNH2 =

yo

(R)-(+)H2N-CAH

(S)-(-)HZN-6"

0 Ci,o,m,p Hph

responding to the two inequivalent phosphorus atoms of the PNP* ligand. In the aliphatic region of the IH NMR spectrum signals due to coordinated cyclooctadiene are observed at 3.80 ppm (CH) and between 2.10 and 1.90 ppm (CH2), while two downfield signals at 6 47.9 and 79.2 ppm in the 13C{lH)NMR spectrum, which do not disappear in a DEPT9O experiment, are illustrative of the four olefinic carbons of the COD ligand.6 Both [IrH(COD){C6H&*H(Me)N(CH2CH2PPh2)2}1 stereoisomers (8a,b) can be described structurally as trigonal-bipyramidal species, with the PNP* ligands (Sa,b) and the COD moiety being q2-bound t o the iridium(1) and the hydride adopting an apical position. This structural assignment precludes the bonding of the nitrogen donor and is confirmed by a recent X-ray crystal structurell of [IrH(COD){C2HhC*H(Me)N(CH2CHzPPh&}l (bearing methyyethy1 groups on the asymmetric carbon) conducted in conjunction with our studies of the PNP* ligands. A view of the molecular geometry of the complex [IrH(COD){C2H&*H(Me)N(CH&H2PPh2)2}] is shown in Figure 1.

8a (11)Bianchini, C.; et al. X-ray Crystal Structure of (R)-[IrH(COD){ C ~ H & * H ( M ~ ) N ( C H Z C H ~ P PTo ~ ~be ) ~ }submitted ]. for publication. Crystal data: space group P1 (No. 2); a = 9.852(2) A, b = 12.689(2) A,

c = 15.776(3) A, a = 68.66(3)", p = 75.27(3)", y = 78.95(4)", V = 1765.8 1.503 g ~ m - R~ = ; 0.036; R, = 0.041.

A 3 ; dealc,j =

C40

c39

Figure 1. View of the molecular geometry of (R)-[IrH-

(COD){ C2H5C*H(Me)N(CH2CH~PPh2h}1. The AM spin system in the 31PNMR room temperature spectrum of [IrH(COD)(PNP*)I(8a,b)is due to the inequivalence of the two phosphorus atoms attached to iridium. This inequivalence is believed to be the result of the presence of the asymmetric center NC*H(Me)Ph in the molecule. A thorough investigation, by means of variable-temperature (VT) NMR experiments, was performed in order t o exhaustively study and explain this phenomenon. The VT NMR experiment involved collection of lH and 31P spectra over the temperature range +75 to -100 "C. Heating a sample of [IrH(COD)(PNP*)I(8a,b)in C6D5CD3 to a final temperature of 75 "c showed no physical change of the 31Por IH NMR spectra. The 31P NMR maintained its AM structure (6-2.5 and 1.2) and the IH NMR showed the illustrative triplet (6-13.5) of the cis hydride, with all signals in both spectra showing


Organometallics, Vol. 14,No. 3, 1995 1495

Scheme 2 a

kcoalesc= 2.22Av "Boat"

"Chair

For clarity only the skeleton of the eight-membered ring has been illustrated. a

an anticipated increase in resolution. In light of these high-temperature spectra having the same physical makeup of the lH and 31PNMR spectra acquired at room temperature, it follows that the trigonal-bipyramidal structure described above is maintained at 75 "C. Lowering the temperature of [IrH(COD)(PNP*)I(8a,b) in CsD5CD3 to a final temperature of -100 "C produced some very interesting spectra. The lH NMR spectrum showed an anticipated loss of resolution with temperature, as the tumbling motion of the complex molecules in solution is slowed and the viscosity of the solution increased. The upfield hydride (6 -13.5 at 25 "C) maintained its chemical shift position but during the course of the NMR experiment lost its fine structure such that at -100 "C only a broad lump was visualized. Similarly, the signals of the COD system showed no physical change during the course of the NMR VT experiment to -100 "C. In light of these results it is apparent that the structural integrity of the hydride and the COD ligand do not undergo any thermodynamically induced change during the course of the VT experiment and the trigonal-bipyramidal system is maintained. Analysis of the 31P NMR VT spectra to -100 "C proved most interesting, with the initial AM spin system coalescing at -45 "C and finally at -100 "C giving four broad signals (6 13.2,5.4,1.9,and -11.2) with a number of smaller peaks (6 14.6, -5.5, -6.2, -10.8, and -14.5). The four major peaks appear as two AM spin systems corresponding t o four unique environments for the phosphorus atoms. Considering the physical rigidity displayed by the complex during the course of the lH VT NMR experiment, it proved puzzling that such a dramatic change in the 31P NMR would occur. We suggest that this intriguing result may be the consequence of conformational isomerization between two minimum-energy conformations, for example a "boat" and a "chair", depicted in Scheme 2. Such a hypothesis explains the occurrence of four distinct environments (and hence spin systems) for the two phosphorus atoms. Interconversion of cyclic (nonaromatic) systems consisting of six or more atoms between two minimum-energy conformations, for example, chair and boat conformations, is a well-documented process in organic chemistry.lob Conformational changes in large rings ( > c8)proceed through various "twisted" conformers of the ring, which may explain the occurrence of the minor peaks in the 31PNMR at -100 "C. Typical values of AG* for the interconversion of cyclic c8 ring systems lie in the range 8.1-10.5 kcal molW1.l2At the coalescence temperature of -45 "C a rough evaluation of AG* associated with the overall dynamic process exhibited by 8a,b can be made with the use of eq l . 1 3 Values (12) Anet, F.; Anet, R. Dynamic Nuclear Magnetic Resonance Spectroscopy; Academic Press: New York, 1975; pp 567-572. (13) Gunther, H. NMR Spectroscopy: A n Introduction; Wiley: New York, 1980; pp 242-243.

varying from 9.4 to 9.3 kcal mol-l can be calculated for Av between 1800 and 2780 Hz, which are the frequency differences observed for the two major AM systems in the slow-exchange regime. Synthesis and Characterization of (R)-and

(S)-[(a-Methylbenzyl)bis(2-(diphenylphosphino)ethy1)amineliridiumDihydride (10a,b). Refluxing [IrH(COD){C6H&*H(Me)N(CH2CH2PPh2)2)1(8a,b) under nitrogen in propan-2-01for 9 h liberates, on workup, an off-white precipitate of fuc-[IrH2{C6H4C*H(Me)N(CHzCH2PPhz)z)l (10a,b; yield 80-90%) with the following illustrative spectroscopic properties; two strong bands in the IR spectrum at 1975 and 2079 cm-l indicative of two terminal hydrides,14two upfield signals in the proton NMR spectrum corresponding to the two hydride hydrogens, a doublet of doublet of doublets ('JH hydride,Pt,,, 139.2 Hz, 'JH hydride,P,,, 22.0 Hz, 2 J ~hydride,H hydride 4.9 HZ) corresponding t o the hydride trans to phosphorus, and a triplet of doublets ( 2 J hy~ drideP,,,(,) 2 J ~hydride,Pt,,(b, 11.8 Hz, 2 J ~hydride,H hydride 4.9 Hz) further upfield corresponding to the cis hydride,14 and an AM system in the 31P NMR spectrum corresponding to the two inequivalent phosphorus atoms of the PNP* ligand. The lH-coupled 31PNMR spectrum shows that the signal at 6 17.72 corresponds to the phosphorus atom trans to the hydride. The 13C(lH} NMR spectrum (C6D6, 20 "C) shows in the low-field region a doublet of doublets attributable to the carbon atom of an ortho-metalated phenyl groupl5 (6 154.9, Jc,pt,, 82.5 Hz, Jc,P,,, 10.3 Hz). For the unambiguous identification of compound loa, an X-ray crystal structure of a single crystal grown from dichloromethanel methanol was solved. The crystal structure of 10a consists of neutral monomeric molecules of the complex without interposed solvent molecules in the lattice. In the monoclinic unit cell there are two crystallographically independent, but practically equivalent, molecules. The structure of one of the independent molecules is depicted in Figure 2 together with the atomic numbering scheme. For the sake of clarity, only the first carbon atom of the phenyl groups on the phosphorus atoms is labeled. Selected bond distances and angles are given in Tables 3 and 4, respectively. The complex has a distorted-octahedral configuration in which the iridium center is surrounded by the two phosphorus and nitrogen atoms of a 5a ligand, by an ortho-metalated phenyl ring which lies trans to one of the phosphorus donors, and by two hydrides in mutually cis positions. The hydrides have been located clearly in the Fourier difference map in the expected positions. They exhibit almost identical distances from the metal in the two independent complexes (&-H = 1.64(9) and 1.8(2)A in complex A &-H = 1.8(2)and 1.7(1) A in complex B). The Ir-H bond lengths are in excellent agreement with the values determined by X-ray methods for several hydride complexes of iridium.16 The Ir-N (2.19(1) A) (14)Barbaro, P.; Bianchini, C.; Meli, A,; Peruzzini, M.; Vacca, A.; Vizza, F. Organometallics 1991,10,2227. (15)Bianchini, C . ; Masi, D.; Meli, A,; Peruzzini, M.; Zanobini, F. J . Am. Chem. SOC.1988,110,6411. (16) See for example: Milstein, D.; Calabrese, J. C.; Williams, I. D. J.Am. Chem. SOC.1986, 108, 6387.

Bianchini et al.


Table 4. Selected Bond Angles (deg) for loaa

ci7

c3

Figure 2. ORTEP plot of one of the two crystallographically independent molecules of fac-exo-(R)-[IrHz(CsH4C*H(Me)N(CH2CH2PPh2)2]1(104. Table 3. Selected Bond Distances (A)for loaa Ir-P1 Ir-P2 Ir-N1 Ir-C1 Ir-H Ir-H1 PI-c10 P1-c11 P1-C17 P2-C24 P2-C25 P2-C31 N1 -C7 N1-C9 Nl-C23 Cl-C6 C23-C24 C6-C7 C7-C8 C7-H7 C9-C10

molecule A

molecule B

2.287(3) 2.282(3) 2.19(1) 2.06(1) 1.64(9) 1.8(2) 1.84(1) 1.84(1) 1.84(1) 1.84(1) 1.83(1) 1.84(1) 1.52(2) 1.50(2) 1.48(2) 1.44(2) 1.51(2) 1.53(2) 1.52(2) 0.95(1) 1.52(2)

2.290(4) 2.267(4) 2.19(1) 2.06(1) 1.8(2) 1.7(1) 1.82(1) 1.8l(2) 1.83(1) 1.82(2) 1.82(2) 1.84(2) 1.53(2) 1.51(2) 1.53(2) 1.47(2) 1.53(2) 1.47(2) 1.52(2) 0.95(1) 1.5l(2)

a Numbers in parentheses are estimated standard deviations in the least significant digits.

and Ir-P distances (2.267(4)-2.290(4) A) fall within the ranges reported for iridium-amine and iridium-phosphine complexes17and are quite comparable with those reported for the iridium complex [(PNPPr)Ir(a,y2C8H13)], for which a distorted-trigonal-bipyramidalgeometry about iridium has been determined by X-ray methodsa6 The C,N-coordination from the cyclometalated aryl group gives rise t o a five-membered C(sp2)Ir-N(sp3) ring in which the C(1)-Ir-N angle is acute (80.8(4)' in complex A and 81.3(4)' in complex B). The Ir-C(l) bond distance of 2.06(1) A in both independent molecules matches well the values reported for several = 2.070 &.17J8 (a-arylliridium complexes (dIr-ccaryl,(av) Compound 10a is therefore identified as the dihydride fuc-[IrHz{C ~ H ~ C H ( M ~ ) N ( C H Z C H ~(see PP~ Scheme ~)~)] 31, formed via loss of cyclooctadiene from 8a and sub(17)Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, 0.;Watson, D. G.; Taylor, R. J. Chem. SOC.,Dalton Trans. 1989, S1. (18)(a) van der Zeijden, A. H.; van Koten, G.; Luijk, R.; Nordeman, R. A.; Spek, A. L. Organometallics 1988, 7,1549. (b) Albeniz, A. C.; Schulte, G.; Crabtree, R. H. Organometallics 1992,11, 242. (c) Manotti Lanfredi, A. M.; Tiripicchio, A,; Ugozzoli, F.; Ghedini, F.; Neve, F. J. Chem. Soc., Dalton Trans. 1988, 651. (d) Neve, F.;Ghedini, M.; Tiripicchio, A,; Ugozzoli, F. Inorg. Chem. 1989,28, 3084.

P 1-Ir-P2 P1-Ir-N1 P1-Ir-C1 P1-Ir-H P1-Ir-H1 P2-Ir-N 1 P2-Ir-C1 P2-Ir-H P2-Ir-H1 Nl-Ir-C1 N1-Ir-H N1-Ir-HI C1-Ir-H C1-Ir-H1 H -1r-H 1 Ir-P1-C10 Ir-P 1-C 11 Ir-P1 -C 17 C10-P1-C11 c10-PI-c17 C11-PI-C17 Ir-P2-C24 Ir-P2-C25 Ir-P2-C31 C24-P2-C25 C24-P2-C3 1 C25-P2-C31 Ir-Nl-C7 Ir-Nl-C9 Ir-Nl-C23 C7-N1 -C9 C7-N1 -C23 C9-N1 -C23 Ir-C 1-C6 Cl-C6-C7 Nl-C9-ClO Pl-ClO-C9 N 1-C23-C24 P2-C24-C23

molecule A

molecule B

99.6(1) 85.1(3) 92.5(4) 92(3) 164(5) 85.8(3) 161.2(4) 9x3) 81(5) 80.8(4) 177(3) 79(4) 99(3) 84(4) 104(6) 101.4(4) 118.2(4) 124.9(4) 106.2(6) 105.4(6) 99.1(6) 99.8(5) 127.5(5) 117.5(4) 104.6(6) 102.5(6) 101.6(6) 104.8(7) 111.9(8) 111.3(7) 107.0(9) 108(1) 113.0(9) 112.7(9) 117(1) 114(1) 108.1(8) 112(1) 110.7(9)

103.4(1) 84.5(3) 87.1(4) 99(5) 167(4) 86.3(3) 162.9(4) 91G) 85(4) 81.3(4) 176(5) 86(4) lOl(5) 83(4) 9 1(6) 100.2(5) 121.4(5) 122.3(5) 104.6(7) 106.0(6) 100.4(7) 101.1(6) 125.5(6) 117.9(5) 106.6(7) 102.6(8) lOOS(7) 104.2(7) 113.5(8) 108.7(7) 112.2(9) 111(1) 107.7(9) 111.1(9) 118(1) 114(1) 109.6(9) 111(1) 108(1)

a Numbers in parentheses are estimated standard deviations in the least significant digits.

sequent cyclometalation of the phenyl group bound to the chiral carbon atom. The loss of the diene occurs by reduction to cyclooctene effected by propan-2-01 and subsequent decoordination of the olefin, as evidenced by quantitative formation of free cyclooctene detected in the propan-2-01 solution when 8a is completely transformed into loa. Isomerization of fuc-exo-(R)-EIrH2{CsH4C*H( M ~ ) N ( C H ~ C H ~ P P ~ Z (loa) ) Z } Ito fac-endo-(R)-[IrH2{C6&C*H(Me)N(CH2CH2PPhdz)l (lla). Serendipitously it was noted that prolonged heating of [IrH(COD){C6H&*H(Me)N(CH2CHzPPhz)z)l (8a,b)under nitrogen in propan-2-01 to form the dihydride fuc-exo(R)-[I~H~{C~H~C*H(M~)N(CH~CH~PP~Z,)~}~ (loa) also gave small quantities (2- 10%) of fuc-endo-(R)-[IrHz(C6H4C*H(Me)N(CH2CH2PPh2)2}1 (1la). A controlled experiment to examine this isomerization process involved the refluxing of 10a in propan-2-01. After 2 h of refluxing 15% conversion t o the endo isomer l l a was determined by 31PNMR spectroscopy. Further refluxing, for up to 9 h, failed t o convert more of the fuc-exo(R)isomer (loa) into the isomer (lla),which suggests the occurrence of a steady state. In a NOEDIFF experiment performed on a sample containing both 10a and l l a , the effect of irradiating either the methyl or the hydrogen bound to the asymmetric carbon on the intensity of the hydride multiplet was measured. When the NCHMe group of compound

New Chira2 Aminodiphosphine Ligands

Organometallics, Vol. 14, No. 3, 1995 1497

Scheme 3

H

k

11

10

10a was irradiated, the intensity of the hydride signal was increased, whereas irradiation of NCHMe has no effect on the hydride multiplet. The opposite result was observed for compound l l a , as irradiation of NCHMe caused an increase in the intensity of the hydride signal. We can therefore conclude that complexes 10a and l l a are both formulated as fac-[IrHz(C6H&H(Me)N(CHzCHZPP~Z)~}], their difference lying in the reciprocal orientation (ex0 vs endo) of the hydride and the NCHMe hydrogen atom.

H

1Oa

I

H

positioned cis to two phosphorus atoms (VH hydride,P,,, 26.9 Hz). The 31PNMR spectrum displayed a broad singlet at room temperature with the I3C NMR displaying the carbonyl carbon as a downfield triplet at 185.7 ppm ('Jc earbony1,P 10.6 Hz). Experimental evidence for the incorporation of only 1 equiv of carbon monoxide, in 14a, was obtained by dissolving 14a in pyridine and adding an excess of iodine, using a closed gas buret.lg On the basis of these results, it has been concluded that the carbonyl species 14a is best described as trigonal bipyramidal. In such a species the r3 (R)-PNP* ligand adopts a fic orientation (the phosphorus atoms being meridional and the nitrogen donor being diagonally opposed and apical to the hydride). Such a conformation allows the addition of the carbonyl moiety in the phosphorus-phosphorus plane to participate in n-n bonding interactions (back-bonding) with iridium.20

lla

Synthesis and Characterization of (R)and ( S ) [(a-Methylbenzyl)bis(2-(diphenylphosphino)ethy1)amineliridiumCarbonyl Hydride (14a,b). A 1O:l mixture of fuc-exo- and fac-endo-[IrH~{C6H&H(Me)N(CHzCHzPPhz)z}](10a,b)in toluene was placed in an autoclave and charged with carbon monoxide to 5.0 atm. The autoclave was then heated to an internal temperature of 80-90 "C for 3 h. Discharging the autoclave and removing the solvent gave the monocarbonyl compound [I~H(CO)(C~H&H(M~)N(CHZCHZPP~Z)Z}I (14a,b),in quantitative yield. The following spectroscopic observations were noteworthy for 14a. Two bands in the IR spectrum (1971 and 1914 cm-'1 were observed in both the solid and liquid phases. The lH NMR spectrum contained an upfield triplet diagnostic of a hydride

li 14a

Reduction of Benzylideneacetone and Other Prochiral Ketones Catalyzed by the Ir/PNP* System. The catalytic reactions were initially performed under the same experimental conditions used previously for the achiral ligand (PNPPr"-).6 The catalyst was prepared in situ by starting with [Ir(COD)(OMe)l2and ~~~~

~

(19) Thanks are due to Prof. F. Calderazzo, University of Pisa, Pisa,

Italy, for his assistance in performing this experiment. (20)Rossi, A. R.; Hoffmann, R. Inorg. Chen. 1975,14, 365.

Bianchini et al.

1498 Organometallics,Vol. 14,No. 3, 1995

Table 6. Reduction of Ketones Catalyzed by [Ir(COD)(OMe)lz i(R)-PW

Table 5. Reduction of PhCH=CHCOMe Catalyzed by [Ir(COD)(OMe)]z -I- PNP* entry no.

PNP'

1 2 3 4 5 6 7 8 9

(R)-PNP (S)-PNP (R)-PNP (S)-PNP (R)-PNP (S)-PNP (R)-PNP (R)-PNP (5')-PNP

10 11 12

(R)-PNP (R)-PNP (R)-PNP

temp ("C)

conversn

selectivity

(9%)

(%)d

eeb

Donor: Propan-2-01 15 93 15 93 45 83 45 90 150 85 150 86 240 84 300 65 360 64

90 90 92 91 95 92 95 97 97

22 17 33 27 41 32 50 54 42

Hydrogen Donor: Cyclopentanol 60 84 60 240 82 40' 25' 900 93

80 92 89

21 44 40

Hydrogen 83 83 60 60 40' 40' 30' 25' 25'

t(min)

M; [Sub]/[Ir] = 100; solvent a Experimental conditions: [Ir] = 4 x hydrogen donor. With (R)-PNP the main product is the R allylic alcohol, and with (S)-PNP the main product is the S allylic alcohol. The catalytic system was heated to 60 "C for 2 h prior to lowering the temperature as indicated and adding the substrate. Selectivity 8 = ((a unsaturated alcohol)/(% yield)) x 100.

2 equiv of the PNP* ligand 5a,b, with propan-2-01used as both hydrogen donor and solvent. The reactions performed at 83 "C were fast and highly chemoselective (Table 5, entries 1 and 2). The rate of the reaction and yield, in the allylic alcohol, were both higher than those obtained for the system with no chiral auxiliary ligand.6 The enantioselectivities of the reactions were similar (approximately 20%),with the (R)(+) enantiomer of the ligand PNP", Sa, giving a higher yield of (R)-PhCH=CHCH(OH)Me,whereas the S enantiomer of the allylic alcohol is predominantly obtained with the PNP* ligand Sb. When the reaction temperature was lowered t o 60 "C, the ee increased with the expected loss in the reaction rate. In contrast, when the reaction was performed a t 40 "C, with an initial induction period of about 3 h, a marked loss in the catalytic activity was observed. However, when the reaction is performed with the catalytic system kept a t 60 "C for 2 h before the substrate is added at 40 "C, the induction period is negligible, and the reaction selectivity is improved (chemoselectivity greater than 95%, ee approximately 40%; see Table 5, entries 5 and 6). All the other reactions at temperatures lower than 60 "C were performed by following this procedure; that is, the catalyst was maintained at 60 "C for 2 h before lowering the temperature as desired, and then the substrate was added. When the reaction temperature was lowered to 25 "C, the enantiomeric excess was more than 50%. Further attempts t o raise the ee by lowering the reaction temperature were ineffective, as the catalytic activity was extremely low at 0 "C. Similar values of chemo- and enantioselectivity have been obtained by Takaya et al. using [Ir(BINAP)(COD)I+ in the presence of PhnP(o-CsH4NMez)as the catalyst in the hydrogenation of benzylideneacetone.2b However, hydrogen donors, as in the present case, have distinct advantages over the use of molecular hydrogen, in that they avoid the risks associated with the reagent, which requires the use of high-pressure apparatus. When cyclopentanolwas employed instead of propan2-01 as both solvent and hydrogen donor, the catalytic reactions have comparable selectivities but slightly

entry no.

conversn

selectivity

substrate

t(h)

(%)

(%)b

1 2

PhCH=CHCOPh CH3CH=CHCOC2H5

5 5 1

91 0 85

61

9 [S(-)I

51

12 [S(-)]

7

52

100

17 [S(-)]

8

31

96

6 [R(+)I

3 5.5

87 86

6 7

Po

PhCOCH3 CH3(CH2)5COCH3

ee

4 [R (+)I 25 [s (+)I

Experimental conditions: [Ir] = 4 x M; [Sub]/[Ir] = 100; temperature 60 "C; solvent and hydrogen donor propan-2-01, Selectivity % = ((% unsaturated alcohol)/(% yield)) x 100.

lower reaction rates. This is a consequence of the lower volatility of the ketone formed by dehydrogenation of the hydrogen donor (cyclopentanone vs acetone).21 With conjugated enones other than benzylideneacetone the catalytic system appears to be both less active and less selective (see Table 6). The reduction of chalcone, PhCH=CHCOPh (Table 6, entry 11, only gives a 60% yield of the substituted allylic alcohol, which is formed together with the saturated ketone and the saturated alcohol. The ee is less than 10%. The aliphatic enone 4-hexen-3-one (Table 6, entry 2) is not reduced by this catalytic system, which is most likely due t o the irreversible coordination of this substrate t o iridium, as suggested by the observation that reduction of benzylideneacetone does not occur in the presence of 4-hexen-3-one. Cyclic unsaturated ketones do not lead to catalyst poisoning. Such substrates are selectively reduced at the carbonyl group only when the C=C bond is sterically hindered (see Table 6, entries 4 and 5), the ee being in all cases below 20%. Prochiral saturated ketones such as acetophenone or octan-2-one are also reduced by this catalytic system (see Table 6, entries 6 and 71, but both catalytic activity and enantioselectivity are markedly lower than those observed for benzylideneacetone. The Catalytic System. Further investigations of the catalytic system prepared in situ were expected t o provide information concerning the nature of the catalytically active species and their possible intermediates. From the catalytic reactions it was apparent that the active species was formed in significant amounts from a propan-2-01solution of [Ir(COD)(OMe)l2and an equivalent amount of (R)-PNP" (Sa),which had been heated t o 60 "C for 2 h (vide supra). By repeating such reactions at higher concentration in propan-2-01,it was possible to detect the species formed by 31Pand lH NMR spectroscopy. The 31P NMR spectrum (propan-2-oU C&) recorded after 30 min at 60 "C shows three main sets of signals: (i) the AM system of Sa (-85% yield); (ii) an AB spin system (compound Ba, -5%) at C ~ A23.29 (21) Bianchini, C.; Farnetti, E.; Graziani, M.; Peruzzini, M.; Polo, A. Organometallics 1993,12, 3753.

New Chiral Aminodiphosphine Ligands and BB 15.37 (JPP 419.7 Hz); (iii) a complex set of signals between -4.00 and -11.00 ppm (-10%). No significant change in the spectrum was observed after further heating of the mixture to 60 "C. When the solution was cooled, a yellow solid was obtained. Complete precipitation of the above species cannot be effected by addition of either n-pentane or diethyl ether, as the compound appears to be soluble in both solvents. The product is then recovered by cooling in an alcoholic solution (-10 "C). NMR spectroscopy unambiguously indicates that the yellow solid is product 8a. Compound 8a was the main species formed in our catalytic system and was subsequently tested as a catalytic precursor for the reduction of benzylideneacetone. The reaction performed at 83 "C is slower than the corresponding reaction performed with the catalyst prepared in situ (90 vs 15 min for 90%conversion), whereas chemo- and enantioselectivities are very similar. When the catalytic reaction was repeated with 8a a t 60 "C, a larger difference in the catalytic activity, with the corresponding reaction in situ, was observed, as the reaction of 8a shows an induction period of about 2 h, although chemoselectivity and ee are once again very similar. The reaction of 8a at 60 "C was then repeated, but with the following modification. The propan-2-01 solution of 8a was refluxed at 83 "C for 2 h, before the temperature was lowered to 60 "C and the substrate was added. The change in the experimental procedure resulted in a higher catalytic activity (2 vs 9 h for 85%conversion), with disappearance of the induction period. From these experimental data it was concluded that 8a is not a catalytically active species, albeit it acts as a catalyst precursor. As a matter of fact, a comparison of the catalytic data obtained starting from 8a with those for the catalyst prepared in situ suggests that the same catalytically active species is involved. Further, one can infer that the induction period observed for the reaction at 60 "C is connected with the formation of the catalyst, which is faster at 83 "C. Therefore, it seemed interesting to record the 31PNMR spectrum of a solution of 8a in propan-2-01 which had been refluxed for 2 h. After this period the 31PNMR spectrum showed formation of 10a (18%) at the expense of 8a. Complete transformation of 8a occurred after 9 h of refluxing. After this time, together with the signals of loa, two new sets of signals are observed in the 31P NMR spectrum: (i)the AM spin system of the ex0 isomer lla (yield 10%);(ii)an AB spin system (compound 12a,yield 2%) with d~ 33.02 and dg 24.18 (C6D6, J p p 385.7 Hz). For the unambiguous identification of compounds lla and 12a we recorded the lH NMR spectrum of the crude reaction products of 8a in propan-2-01,after evaporating the solvent and dissolving the residue in C6D6. As for 10a and lla,12a also appears to be a dihydride, with a doublet of doublet of doublets at 6 -11.58 (hydride trans to carbon, JHhydride,p, 26.5 HZ, JHhydride,pb 14.1 HZ, and JHhydride,H hydride 4.9 HZ) and a triplet O f doublets at - 19.16 ppm (hydride trans to nitrogen, JHhydride,P 14.9 HZ, J ~ h y d r i d ehydride ,~ 4.9 HZ). From this Set O f data15,2212a appears to be an isomer of both 10a and lla in which the two inequivalent phosphorus atoms (22)(a) Pregosin, P. S.; Kunz, R. W. In 31P and I3C N M R of Transition Metal Phosphine Complexes; Diehl, P., Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: New York, 1979. (b) Garrou, P. E. Chem. Reu. 1981, 81,229.

Organometallics, Vol. 14,No. 3, 1995 1499 of PNP* are trans to each other. Thus, 12a is formulated as mer-@)-[IrHz(C&C *H(Me)N(CH2CHzPPhz)2}1.

As 10a was the main species formed from 8a using the experimental conditions described, the obvious conclusion was that 10a might be a close precursor of the catalytically active species used in these reactions. However, the presence of 10a was not detected when the system was generated in situ at 60 "C and followed by NMR (vide supra). This indicates that the catalytic species was not formed in situ using the experimental conditions employed. In fact, when the system in situ was maintained for 2 h at 83 "C rather than 60 "C, formation of a small amount of 10a was detected. From these data the role of 10a in the catalytic reactions was not definitely established. Such a complex was tested as a catalyst precursor for the reduction of benzylideneacetone in order to get additional information. The catalytic reaction performed a t 60 "C was rather slow (80%conversion in 6 h vs 45 min of the system in situ), both chemo- and enantioselectivity being similar t o those observed with the catalytic system prepared in situ. No induction time was observed in this case, a t variance from what we observed using 8a as a catalyst precursor. When the catalytic reaction was repeated with 10a after the precursor was maintained a t 83 "C for 2 h, no difference in the catalytic activity was observed. The evolution of 10a in refluxing propan-2-01 was followed via NMR, and after 2 h the only product formed was lla (-15%). At this stage a more detailed investigation of the system formed in situ was needed in order to formulate a hypothesis regarding the nature of the catalytically active species. The system formed in situ was then monitored by 31P NMR under the following conditions: 2 h at 60 "C (see NMR data above) and then further heating to 83 "C. After 2 h at 83 "C the partial transformation of 8a into 10a and lla was observed, together with the lowering of intensity of the set of signals corresponding t o the so far unidentified compound 9a. Heating to 83 "C for a longer time, together with an expected increase in the yield of 10a and lla,shows the disappearance of the signals of 9a, and a t the same time the set of signals of 12a appears. To acquire more information on the nature of species 9a,the lH NMR spectrum of the propan-2-0148 solution of the system in situ after 2 h at 60 "C was recorded. In the hydride region of the spectrum, together with the signals of compound 8a,a new set of signals is observed (6 -9.17, JH hydride,P, 24.3 HZ, JHhydride,Pb 10.9 HZ), the intensity ratio of the two sets being comparable to that of the 31PNMR signals of 8a vs 9a. Compound 9a therefore appears to have in the coordination sphere of iridium one hydride and one (R)-(+)-PNP* ligand, with the two phosphorus atoms in a mutually trans configuration. We therefore propose that species 9a is the square-planar iridium(1) complex [IrH(PNP*)I, formed


via decoordination of cyclooctadiene from 8a.23Such a hypothesis is supported by the fact that when a C6D6 solution of 8a is heated at 80 "C no formation of 9a is detected: the presence of a hydrogen donor such as propan-2-01 is necessary to perform the reduction of cyclooctadiene to cyclooctene, which is then released from the coordination sphere of iridium.

9a

Identical results have been obtained when 5b is substituted for 5a.

Discussion

The Chemistry of Iridium with the PNP*Ligands 54b. When a propan-2-01solution of equivalent amounts of [Ir(COD)(OMe)l2and PNP" (5a,b)was heated to 60 "C, a catalyst was formed, which showed interesting features of high catalytic activity and chemoselectivity for the reduction of enones. Due to the presence of the stereogenic center on the PNP" ligands (5a,b),asymmetric reduction was also achieved. The experiments reported above have thrown light on the organometallic chemistry of the Ir-PNP" system, by allowing the identification of a number of novel iridium complexes. The initial stages of the reaction sequence consist of coordination of the PNP" ligand (Sa,b)to the metal, which causes splitting of the methoxy bridge with formation of [Ir(OMe)(COD)(PNP*)I. Such a complex undergoes ,!%hydrogen elimination t o form 8a,b.6,24 Catalytic experiments using 8a,b as a precursor have shown that such a species is not catalytically active but is capable of evolving into a catalyst. Refluxing 8a,b is a hot propan-2-01 solution results in its reduction, effected by the secondary alcohol of coordinated cyclooctadiene t o give cyclooctene, which leaves the coordination sphere of iridium. Such a reaction results in the formation of a mixture of as many as four complexes, the first one formed being 9a,b,square-planar iridium(I) species with only one hydride and a PNP" ligand in the coordination sphere. 9a,b are apparently the common precursors for the subsequent formation of three isomers of [IrH2(C6H&*H(Me)N(CHzCH2PPhz)z)l, 10a,b, lla,b, and 12a,b,which are formed via ortho metalation of the phenyl ring bound to the asymmetric carbon. The relative amounts of the three products are related to their respective stability. The major product (10a,b) was the only one formed from 8a,b after a short reaction time. Formation of lla,b and 12a,bwere detected only (23) A square-planar complex of the formula [IrH(PNP*)I has been isolated in high yield and fully characterized with the use of the ligand (R)-(+)-(C~H~)C*H(CH~)NICH~CH~CH~P(C~H~~)Z]Z, which differs from (R)(+)-5a only in the phosphorus substituents. 'H NMR (C7D8,37 "C, 200.13 MHZ; 6): -18.6, dd, 2 J hydnde,pc,, ~ 13.4 HZ, 2 J hydnde,P,, ~ 13.4 HZ, 1 x H hydride. 31P{1H)NMR (C7D8,37 "C, 81.01MHz; 6): AB system, BA 3 0 . 1 , , 6 ~25.0, VPP355.0 Hz. Bianchini, C.;et al. To be submitted for publication. (24) Bianchini, C.; Peruzzini, M.; Farnetti, E.; Kaspar, J.; Graziani, M. J. Organomet. Chem., in press.

Bianchini et al. after several hours at 83 "C and can occur either via isomerization of 10a,b(which has been proven to be the case for species lla,b) or directly from 8a,b. The isomerization of 10a,b to lla,b in refluxing propan-201 indicates the reversibility of the phenyl C-H oxidative addition even though the former compound appears to be thermodynamically favored. If we assume that the catalysis observed was not due to other iridium species, which were formed in small amounts and were not spectroscopically detectable, we can make the prediction that the catalytically active species was (or was formed from) one of the four complexes formed via loss of cyclooctadiene from 8a,b. In fact, complexes 10a,b, lla,b, and 12a,b appear to be all related to each other in a reversible fashion through formation of a tetracoordinate iridium(1)intermediate that would be a good candidate for the catalytically active species. In this hypothesis, complexes 10a,b, lla,b, and 12a,b might all behave as catalyst precursors proceeding through reductive elimination of the phenyl C-H group. However, it is evident that the ease of such reductive elimination will be different for each of these Ir(II1) complexes. Compound 10a,b slowly isomerizes in refluxing propan-2-01to give lla,b (see Scheme 4). Such a reaction likely proceeds via C-H reductive elimination followed by oxidative addition of the aromatic C-H group in the ortho position via a four-coordinate fac-PNP* transient species (13a,b). A fluxional movement of one or both hydrides could also accomplish the isomerization. We are inclined to disfavor this interpretation, since both 10 and 11 appear stereochemically rigid on the NMR time scale (see also the carbonylation reactions shown in Scheme 4). Species 13a,b, once formed, might rearrange to give the square-planar complexes 9a,b. However, formation of 9a,b was never observed as an intermediate in the isomerization 10a,b lla,b,which suggests that 13a,b,once formed, readily re-form either 10a,b or lla,b. Evidence for the formation of 13a,b as an transient species in the isomerization 10a,b lla,b was obtained by trapping the iridium(1)species with CO (vide infra) t o give 14a,b. On the other hand, if we consider the results obtained using 10a,bas a precursor, in the hypothesis that the catalytic activity is bound to the formation of a tetracoordinate iridium(1)species, we must infer that both 10a,b and lla,b can act equally as a catalyst precursor, which is in agreement with the observation that transformation of lOa,b into lla,b does not result in any difference in the catalytic results (see Results; compare the catalytic reactions starting from 10a,b with and without pretreatment of the catalyst at 83 "C). In comparison with the results of 10a,b as a precursor, when 8a,b was the starting species a higher catalytic activity was observed. From the NMR data we know that, 10a,b, lla,b, and a small amount of 12a,bare formed in the reaction, and we can reasonably assume that the last species is responsible for the higher catalytic activity. Such a hypothesis appears to be reasonable if we compare the structure of compounds 12a,b t o that of 10a,b (and lla,b). In complex 12 the PNP* ligand is in a mer configuration, and one of the hydrides is in a tram position with respect t o the phenyl carbon atom. Such a configuration results in the Ir-C bond being more labile, and it is consequently easier

-

-


Organometallics, Vol. 14, No. 3, 1995 1501

Scheme 4

H

H

H

10

11

I

co

85°C 5.0atm. Toluene

H 14

for C-H reductive elimination to occur.25 In contrast, in complex 10a,b (and lla,b)the phenyl carbon is in a position trans to a phosphorus atom, which has a lower trans effect in comparison to the hydride.25 Therefore, in the hypothesis that the catalytic activity is related t o the ease of C-H reductive elimination, we can expect 12a,b t o be a more efficient catalytic precursor in comparison to 10a,b (and lla,b). The catalytic cycle proposed in Scheme 5 summarizes all the considerations made so far. The iridium(II1) complexes lOa,b, lla,b, and 12a,b can all act as catalyst precursors to give via C-H reductive elimination the coordinativelyunsaturated species 13a,b,which, before rearranging to the square-planar species 9a,b, is approached by the ketone which may coordinate in #(O) fashion. Indeed, this type of coordination for benzylideneacetone was shown to be operative in a similar hydrogen-transfer reduction catalyzed by the complex [ ( P P ~ O S ( H Z ) H I Migration +.~~ of the hydride to the carbonyl carbon gives an alkoxy species which in turn is protonated by propan-2-01 and released from the coordination sphere of iridium. Finally the cycle is closed by a hydrogen ,&elimination step and loss of acetone to give the iridium hydride 13a,b. In order to obtain further support for the hypothesis of formation of the common intermediate 13a,b starting from 10a,b, lla,b, or 12a,b the reactivity of these complexes with CO was tested. When a toluene solution of 10a,b and lla,b was treated with CO at 85 "C, the only product formed was the carbonylic complex 14a,b (see Scheme 4). Similarly, when a catalytic reaction was stopped by quenching with CO (5.0 atm), the only product formed was once again species 14a,b. Such a complex can be easily formed by addition of CO to the coordinatively unsaturated species 13a,b, which appears t o be the common intermediate.

Scheme 5 rationalizes the enantioselectivity of this reaction; that is, (R)-PNP* gives predominantly the (R)-PhCH=CHCH(OH)Meenantiomer. Preceding the a-bond metathesis reaction with propan-2-01, preliminary chemical events may involve the n-x stacking of the phenyl rings on the substrate, asymmetric carbon, and phosphine groups. The methyl substituents on the ligand and the substrate can then adopt sterically less strained positions in directions opposite to each other. Such an intermediate species would have the PhCH=CHCOMe ketone in the plane of the phosphorusphosphorus atoms, thus allowing attack of the hydride onto the Si face of the ketone.26

Conclusions In this paper we have reported a study of the selective reduction of a,p-unsaturated ketones t o optically pure allylic alcohols, using a new, chiral iridium complex as catalyst and propan-2-01 as the hydrogen-transfer reagent. Indeed, the utility of delivering optically active allylic alcohols in high enantiomeric excess is highly desirable, as these synthons can then be used in the synthesis of several biologically active compounds. Chemoselective reduction of a,P-unsaturated ketones to allylic alcohols is shown only by a restricted number of transition-metal complexes.20 Enantioselective reduction is even more rare, being limited t o the present PNP*-Ir complexes and to Takaya's [Ir(BINAP)(COD)]+/ PhzP(o-CcH4NMez) system,2bwhich is fairly efficient only for benzylideneacetone. In both cases, asymmetric induction does not exceed 60%; thus, the production of optically active allylic alcohols in high yield is still a challenging task. ~

(25)Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles a n d Application of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987; Chapter X.

~

(26) (a)Zassinovich, G.; Bettella, R.; Mestroni, G.; Bresciani-Pahor, N.; Geremia, S.; Randaccio, L. J. Organomet. Chem. 1989,370,187202. (b)Zassinovich, G.; Mestroni, G.; Gladiali, S. Chem. Reu. 1992, 92,1051.


Bianchini et al.

Scheme 5. Reaction Scheme for the Chemo- and Enantioselective Reduction of Benzylideneacetone"

H 13

-

1

a

13 was formed either (i) in situ or (ii) from complexes 10-12.

In light of the results presented in this paper, we are, at present, continuing our investigations of the chemistry of the PNP* mixed-donor ligands. Studies include the use of methyllethyl- and naphthyvmethyl-substituted PNP* systems in the reduction of a variety of prochiral ketones. Also under investigation is a detailed mechanistic study of the role played by the (a) stereogenic substituent, (b) phosphorus substituents, (c)metal center, (d) nature of the ketone, and (e) solvent system employed in effecting asymmetric reduction. Current investigations center on developing a rationale for the chemical events leading up to the incorporation of the ketone with the catalyst.

Acknowledgment. We thank the Progetto Strateg i c ~"Tecnologie Chimiche Innovative", CNR, Rome, Italy, and the EC (Contract No. ERBCHRXCT930147) for financial support. L.G. thanks the Italian Ministry of Foreign AfYairs for financial support. Thanks are due to Prof. G . Pitacco (University of Trieste) for helpful discussion. Supplementary Material Available: Tables of anisotropic thermal parameters and hydrogen atom coordinates for the two independent molecules of 10a (3pages). Ordering information is given on any current masthead page. OM9409332

Synthesis of the New Chiral Aminodiphosphine Ligands (R)-and (S

Recommend Documents