Organometallics 1995, 14, 4977-4979
4977
Diastereoselectivity and Regioselectivity in the Intramolecular Phosphine Attack on a Coordinated Alkyne Ligand in Co2(CO)4(bmf)@-PhC=CH).Formation of the Chiral Hydrocarbyl Complex
Kaiyuan Yang, Simon G. Bott,* and Michael G. Richmond* Center for Organometallic Research and Education, Department of Chemistry, University of North Texas, Denton, Texas 76203 Received June 16, 1995@ Summary: The reaction between the alkyne-bridged compound Co2(CO)dp-PhCWH)and the diphosphine ligand 3,4-bis(diphenylphosphino)-5-methoxy-none (bmf3 affords the chelating diphosphine complex Coz(CO)kbmfl(p-PhC=CH) (l),which exists as a mixture of four diastereomers. Thermolysis of 1 leads to the formation of a single diastereomer of the zwitterionic hydrocarbyl-bridged compound Coz(CO)$p-+,+,ql,ql-
-
PhC=C(H)PPh2C=C(PPh2)C(O)OCH(OMell(3)as a result of the diastereoselective and regioselective attack of the PPh2 moiety that is conjugated with the keto moiety of the bmf ligand. The use of transition-metal-complexed alkynes for the regioselective and stereoselective construction of new C-X (where X = C, N, 0)bonds in a variety of acyclic and cyclic compounds of natural products continues t o attract strong interest in both the organometallic and organic c0mmunities.l Undoubtedly, the Pausonn a n d 2 and Nicholas3a4reactions represent the two bestknown reactions involving the metal-mediated functionalization of alkynes. The former cyclization reaction leads to the coupling of an alkyne, alkene, and CO, with the synthesis of a larger, more complex molecule, often with impressive regiochemical and stereochemical control, while the latter reaction affords propargylium complexes capable of conversion to propargylated compounds without the troublesome complications that plague the traditional organic sequences. The activation of a coordinated alkyne to give a p q 2 : +hydrocarby1 moiety has only recently been demon~ t r a t e d ,and ~ , ~examples of diastereoselectivity in the formation of a chiral p-v2:y1-hydrocarbylmoiety are, to Abstract published in Advance ACS Abstracts, October 1, 1995. (1)(a) Harrington, P. J . Transition Metals in Total Synthesis; Wiley: New York, 1990. (b) Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, CA, 1987. (2) (a) Pauson, P. L. Tetrahedron 1985,41, 5855. (b) Krafft, M. E.; Juliano, C. A,; Scott, I. L.; Wright, C.; McEachin, M. D. J.Am. Chem. SOC.1991, 113, 1693. (b) Krafft, M. A.; Scott, I. L.; Romero, R. H.; 1993,115,7199.(c) Feibelmann, S.; Van Pelt, C. E. J.Am. Chem. SOC. Jamison, T. F.; Shambayati, S.; Crowe, W. E.; Schreiber, S. L. J.Am. Chem. SOC.1994, 116, 5505. (d) Jeong, N.; Hwang, S. H.; Lee, Y.; Chung, Y . K. J.Am. Chem. SOC.1994, 116, 3159. (3) (a) Nicholas, K. M. Acc. Chem. Res. 1987,20, 207. (b) Bradley, D. H.; Khan, M. A.; Nicholas, K. M. Organometallics 1992, 11, 2598. (c) C a m , A. J. M.; Nicholas, K. M. J.Am. Chem. SOC.1993,115,6438. (4) (a) Schreiber, S. L.; Sammakia, T.; Crowe, W. E. J.Am. Chem. SOC.1986, 108, 3128. (b) Schreiber, S. L.; Klimas, M. T.; Sammakia, T. J. Am. Chem. SOC.1987,109,5749. @
our knowledge, unknown. The novelty of a chiral hydrocarbyl moiety, coupled with its potential synthetic use, prompts us t o report our results on the diastereoselectivity and regioselectivity associated with the formation of the title dicobalt compound. Treatment of C O ~ ( C O ) ~ ( ~ - P ~with C~C the H oxidative ) decarbonylation reagent Me3NO in the presence of the diphosphine ligand 3,4-bis(diphenylphosphino)-5-methoxy2(5H)-furanone,' hereafter called bmf, affords the chelating diphosphine complex Co~(CO)4(bmf)(u-PhC= CH) (1). On the basis of the IR and 31PNMR data, binuclear 1 exists as a mixture of four diastereomers, all of which contain a chelating bmf ligand.8 Moreover, binuclear 1 displays spectral properties nearly identical with those of related 2,3-bis(diphenylphosphino)maleic anhydride (bma) substituted complexes already prepared in our l a b o r a t ~ r i e s . ~While ,~ many different chelating diastereomers of 1 may be envisioned, the relative stereochemistries of the four most likely ones are10-12
(5) Takats, J.; Washington, J.; Santarsiero, B. D. Organometallics 1994,13, 1078. (6)(a)Yang, K.; Bott, S. G.; Richmond, M. G. Organometallics 1994, 13,3767. (b) Submitted for publication in J. Organomet. Chem. 1995. (7)Fenske, D.; Becher, H. J. Chem. Ber. 1975, 108, 2115. (8) Synthesis and spectroscopic data for 1: In a Schlenk tube containing 0.30 g (0.77mmol) of C@(CO)&-PhC=CH) and 0.37 g (0.77 mmol) of the diphosphine ligand bmf in 50 mL of THF was added 0.12 g (1.61 mmol) of Me3NO. The reaction mixture was stirred at room temperature for 1.5 h and then examined by TLC and IR analyses, which revealed the presence of the desired product. Solvent removal under vacuum, followed by chromatography over silica gel using CHZCldpetroleum ether (2:1),afforded the crude product. Recrystallization of 1 from CHzClheptane (8:2)at 0 "C gave 0.35 g (56%yield) of greenblack 1. IR (CH&12,24 "C): Y 2044 (s), 1985 (vs), 1770 (m, bmf C-0) cm-l. lH NMR (CDC13, 24 "C): 6 3.25, 3.33, 3.38, 3.40 (MeO, all singlets), 4.94 (dd, =CH, J = 8.40 and 3.25 Hz) 5.10 (s, broad, =CHI, 5.14(s, broad, =CH), 5.27 (dd, W H , J = 10.2 and 2.12 Hz), 5.52,5.62, 5.64, 5.70 (H, furanone ring, all singlets), 6.60-8.00 (aromatic multiplet). 31P{1H} NMR (THF, -97 "C): 6 44.05 and 49.50, 52.89 and 56.94, 61.21 and 61.89, 66.91 and 67.61. Anal. Calcd (found) for C41H3oCoz07Pz: C, 60.46 (60.38);H, 3.71 (4.04). (9)Yang, K.; Bott, S. G.; Richmond, M. G. Organometallics 1994, 13, 3788. (10)As is customary with this genre of alkyne compounds, we show phosphine coordination at the pseudo-axial and pseudo-equatorial sites in 1 (see ref 11 for terminology and related structures). These diastereomers minimize unfavorable intramolecular contacts between the Ph group of the alkyne and the bmf ligand. Moreover, preliminary X-ray diffraction data on a weak crystal of 1 have already confirmed the presence of the diastereomers labeled as C and D. (11) (a) Thorn, D. L.; Hoffmann, R. Inorg. Chem. 1978,17, 126. (b) Cunninghame, R. G.; Hanton, L. R.; Jensen, S. D.; Robinson, B. H.; Simpson, J. Organometallics 1987,6, 1470. (c) Sappa, E.; Predieri, G.; Marko, L. Inorg. Chim. Acta 1995,228, 147. (12) All compounds shown are racemic, with only one enantiomer shown for clarity.
0276-733319512314-4977$09.00/00 1995 American Chemical Society
Communications
4978 Organometallics, Vol. 14,No. 11, 1995
B
H
-
.Ph
OMe
C
H
Ph
D
U
Ph
operative in the PPhz attack on the alkyne ligand in Co~(C0)4(bma)@-PhC=CH)~ and P-C bond cleavage in C0z(C0)4(bma)@-PhCWPh).~3 was subsequently isolated and fully characterized in solution (IR and NMR) and by X-ray diffraction ana1y~is.l~ The possibility of material loss and, hence, incomplete diastereomer characterization was addressed by repeating the thermolysis reaction of 1 3 in C6D6 in a sealed NMR tube containingp-(MeO)zC& aa an internal standard. Here o d y one product was observed (>85%), which gave a lH NMR spectrum identical with that isolated from the preparative reaction. In analogy with the reaction conducted with the bma ligand, heating the diastereomeric mixture of 1 is also expected to give the transient bridging bmf isomer, followed by PPhz attack on the terminal alkyne carbon (eq 2).14J5
2 (bridging isomer)
The thermolysis reactivity of 1 was next examined in 1,2-dichloroethane, because phosphine attack in the related complex Coz(C0)4(bma)@-PhCGCH)has been shown t o proceed by a chelate-to-bridge diphosphine conversion, followed by a rapid attack of the bridging diphosphine on the terminal carbon of the coordinated alkyne ligand t o yield the corresponding zwitterionic hydrocarbyl-bridged complex (eq 1h6 Me0
1
Me0
0
Two key questions associated with the thermolysis reaction involving 1 need to be answered. First, would each chelating diastereomer react independently of the others in the P-C(a1kyne) bond-forming step, and second, would any phosphine regiochemistry be observed, given the presence of inequivalent phosphines? Apriori, we anticipated that the least basic PPhz moiety (the one in conjugation with the keto group) should be more readily released and available for attack on the alkyne carbon. A smooth transformation of 1 t o the hydrocarbyl
The structure of 3 was unequivocally established by X-ray diffraction analysis.16 Figure 1shows the ORTEP diagram of 3 and confirms the regioselective migration of the least basic (i.e., weaker bound) PPhz moiety associated with the bmf ligand to the terminal alkyne carbon of the coordinated phenylacetylene ligand. Moreover, the formation of the ,u-$:ql-hydrocarbyl ligand is accompanied by 100% diastereoselectivity in this in-
(13) Synthesis and spectroscopic data for 3: In a Schlenk tube containing 0.20 g (0.25 mmol) of the chelating isomer of 1 was added 20 mL of 1,2-dichloroethane, after which the solution was heated overnight at 75 "C. When the temperature was lowered, IR and TLC analyses revealed the presence of only the hydrocarbyl-bridged compound 3. Purification by chromatography over silica gel using CH2Cldpetroleum ether (3:l) as the eluant afforded the desired compound as a black solid. The analytical sample and single crystals suitable for X-ray diffraction analysis were grown from a CH2Clz solution containing 3 that had been layered with hexane. Yield 0.13 g (65%).IR (CH2I Clz, 24 " 0 : v ( C 0 ) 2023 (s), 1993 (vs), 1967 (81, 1955 (sh), 1735 (m, complex Coz(C0)&-q2:q2:v1:v1-(Z)-PhC=C(H)PPh2C=C-broad, C-0 bmf) cm-'. lH NMR (CDCl3,24 "C): 6 2.83 (MeO, s), 4.20 I (dd, W H , J = 38.16 and 7.23 Hz), 4.31 (H, furanone ring, s), 7.00(PPh&(O)OCH(OMe)l(3) was indeed observed, and to 8.10 (aromatic multiplet). 3IP{lH} NMR (CDC13, 24 "C): 6 4.58 (d, J = 85 Hz, Co-PPhz), 31.70 (d, J = 85 Hz, HC-P). Anal. Calcd (found) our surprise this reaction occurred with 100% diasteC, 60.48 (61.04); H, 4.62 (4.26). for C41H30C020,P2.1/2hexane.1/~THF: reoselectivity and PPhz regiochemistry. Added CO (100 (14) The other enantiomer (S,R,S) observed in the unit cell of 3 is psi) inhibits the conversion of 1 to 3, which suggests not shown. (15) No mechanistic content is implied in the transformation given that dissociative CO loss is a prerequisite for initiation by eq 2. The possibility of this reaction proceeding by a coordinatively of the required chelate-to-bridge bmf ligand reaction. unsaturated species (i.e., Coz(C0)3(bmf)Ol-PhCrCH))is under investigation. Such an isomerization scheme has been shown to be
Organometallics, Vol. 14, No. 11, 1995 4979
Communications
tramolecular P-ligand attack on the coordinated alkyne moiety. The zwitterionic nature of 3 requires a negative charge on Co(2) and a positive charge on P(2);the latter center is best described as a phosphonium center, given the tetrasubstituted nature of the P(2) atom. The maleic anhydride serves as a y2-donor ligand in the product, which allows each cobalt center to achieve a coordinatively saturated state. The presence of only one diastereomer of 3 suggests that the bmf ligand is equilibrated a t some point prior to the P-C bond formation step, in a manner that is presently not clear. We believe that the stereochemistry of the methoxy group on the bmf ligand certainly assists in determining the outcome of the reaction depicted by eq 2. In the putative bridging isomer of 2 the methoxy group is situated away (exo)from the Co-Co vector and the ancillary carbonyl ligands. This particular conformation minimizes unfavorable van der Waals contacts between the methoxy group and the binuclear core of 2. Transfer of the PPhz to the terminal alkyne carbon, coupled with the breaking of the appropriate CoUalkyne) bond and coordination of the bmf alkene bond, completes this reaction. In-depth studies on the equilibration of the bmf ligand at polynuclear centers are planned, and the functionalization and the reactivity of the hydrocarbyl ligand in CO insertion reactions will be reported in due ~0urse.l~
Acknowledgment. We wish to thank Prof. Roderick Bates for helpful discussions. Financial support from (16) Crystal data for 3: C41H3&020,P2*1/~hexaneJ/2THF, MW = 892.64, monoclinic space group C2/c, a = 23.913(4)A, b = 20.844(2)A, c = 16.907(3) A, p = 102.51(1)", V = 8227(2) A3,2 = 8, D, = 1.441 g/cm3, F(000)= 3680, T = 24 "C, p(Mo Ka) = 9.32 cm-l. Diffraction final R = 0.0598 data were collected in the o-scan mode (2" < 28 44"); (R, = 0.0660) for 3042 unique reflections (with I 3dI)). The molecular structure was solved by SHEIX-86, which revealed the positions of the Co and P atoms, and the final refinement employed MolEN and SHELXL-93. All remaining non-hydrogens were located with difference Fourier maps and full-matrix least-squares refinement. With the exception of the solvent and phenyl-ring carbons and hydrogens, all atoms were refined anisotropically. (17) Facile insertion of CO into the q1 Co-C bond of Coz(CO)&-
-
Cl20
(359
C l Z l /-\l18
02
c 22 3
Figure 1. ORTEP drawing of 3. Selected bond distances (A) and angles (deg): Co(l)-C0(2) = 2.535(3),Co(l)-C(16) = 2.04(1), c0(l)-c(17) = 1.96(1), c0(2)-c(17) = 2.00(1), CO(l)-P(l) = 2.224(4),C(16)-P(2) = 1.76(1);Co(l)-C(16)P(2) = 109.5(6), C O ( ~ ) - C ( ~ ~ ) - C O=( 79.6(4). ~)
the Robert A. Welch Foundation (Grant Nos. B-1202SGB and B-1039-MGR)and the UNT Faculty Research Program is appreciated. Supporting Information Available: Tables of crystal data, atomic positions, bond lengths, bond angles, and anisotropic thermal factors and a packing diagram for 3 (13 pages). Ordering information is given on any current masthead page.
~~,~~,~~,~~-(Z)-P~C-C(H)PP~~C-C(PP~Z)C(O)OC(O)~ is promoted by OM9504617 addition of PMe3: Unpublished results.
