Ct".
Rev. 1907. 87. 781-778
701
Half-Sandwich Chiral Ruthenium Complexes GIAMBATTISTA CONSIGLIO' Ek@&?skdw,
Tedmlpdw, MdLs&idB,
T e c h n l s ~ L s & S Labwaloriwn. ElUZenbbm, CK8092 ZiKich. Swltlerland
FRANC0 MORANDINI
R e " j
December 18, 1986 (Revised Manuscript ReceivedMarch 30. 1987)
Contents 1. Introdu*lon
11. Types of Chkai t!aif-Sandwlch Ruthenium
76 1 762
Complexes
111. Determination of the Stereochemistry of the
IV.
V.
VI.
VII.
Chirai Complexes A. X-ray Diffraction B. Chiropticai Methods C. Mukinuciear NMR Spectroscopy Stereochemical Course of Simple Metaliorganic Reactions A. Alkylation Reactions B. Formation of HydrMo Complexes C. Other Substitution Reactions Involving tha Halide Ligand. Formation of Cationic Complexes 1. Without Chemical Modiflcath of the Incoming Ugand 2. Wkh Chemical McdMcaHon of the Incoming Ligand D. Substitution Reactions InvoMng the Acetonitrile Complexes E. Exchange Reactions F. Insertion Reactions 0. Some Reactions Not Dkectly Involving the Ruthenium Atom H. General Remarks on the Stereochemistry Diastereomeric Equilibria for Complexes Containing Prochirai Ligands A. Olefin Complexes B. Aikyldenecarbene Complexes Conclusion References
765 766 766 766 766 768 769 769
769 770 770 771 771 772
GhmbaMsta Consiglio was oorn In Fogga. Italy. and recehred his doctoral degree n in0Lstria chem slry ai the University 01 Pisa In 1965 He IS m w h a t Cozent lor iMustfwl chemisq at tha Swiss Federal Inslnute 01 Tecnnology on Zdr cn H s research interests have developed in the area 01 homogeneous and asymmetr c tumogeneous calalyss oy transition-metal complexes and In the stereochemistry 01 organotransition-metal compodnds
772 774 774 775 776 776
I . Introduction Among the many interesting developments in t h e field of transition-metal chemistry of the past years, the
use of organometallic compounds in organic synthesis,lZ both as stoichiometric a n d catalytic reagents, is experiencing a burst of activity. Reactions involving such reagents often display unique chemoselectivities, allowing simple paths to some chemical transformations that would involve many s t e p via more classical organic chemistry?,' Furthermore, t h e control o f the stereoselectivity appears more facile when organotransitionmetal templates are used? Asymmetric catalysis by transition-metal complexes represents one o f the most appealing ways to reach t h e goal of good chemo- a n d stereocontrol! However, despite some exciting succe~es,1.~.'examples o f asymmetric catalysis still remain 0009-2665/87/0787-0761$08.50/0
Franc0 Morandini was born in Rome in 1948 and received his doctoral degree at the University of Padua in 1972. After a %year stay in the laboratories of Prof. P. Pino at the E.T.H. in Zurich, he moved to Padua where he is now research associate of inorganic chemistry. His current research interests are in the field of organometallic chemistry and asymmetric catalysis.
to be exploited, due to the lack of criteria t h a t would permit generalization and extrapolation o f results? T h e chiral transition-metal complexes acting as catalysts d o interact within the catalytic cycle with a 'prochiral" (or sometimes w i t h a chiral) substrate, resulting in dia-
0 1987 American Chemical Society
762 Chemical Reviews, 1987, Vol. 87, No. 4
stereomeric intermediates; these, in turn, will form through diastereomeric transition states. The factors influencing energy differences between diastereomeric intermediates and diastereomeric transition states have scarcely been investigated. For instance, although chiral platinum(I1)-olefin complexes (containing either chiral amines or chiral olefin ligands) have been studied for some years8l9 as models for catalytic sites in stereospecific'O and stereoelectivell olefin polymerization, it is now widely accepted1°J2J3that ~tereogenic'~ metal centers are responsible for the stereospecific polymerization of olefins over Ziegler-Natta catalysts. The remarkable stereoselectivities obtained in the polymerization of a-olefins using chiral homogeneous Ziegler-Natta ~atalysts'~J~ provide strong support for this proposal. It is likely that, due to the geometric requirements of transition-metal complexes, stereogenic metal atoms could be a common feature in asymmetric c a t a l y s i ~ . ' ~During - ~ ~ catalysis, ligands coordinated to a metal atom can participate in a number of stereochemically significant transformations, including substitution, migration, and insertion reactions (among others). Investigation of the stereochemical course of simple reactions at transition-metal centers provides detailed information regarding the identification and definition of catalytic cycles.21i22Despite the wealth of information provided by such studies, this approach has not been fully exploited until For example, in the past few years diastereomeric ruthenium complexes (the subject of the present review) and chiral enantiomeric rhenium complexesz4have been extensively investigated. In addition, experiments on the transfer of a methylcarbene moiety to styrene by diastereomeric cyclopentadienyl iron complexesz5 containing a stereogenic metal atom and a chirality center on the phosphino ligand ( [(q-C5H5)Fe(CO){PPh2CH2CH(CH3)C2H5)(CHCH3)]OSO2CF3) revealed that the chirality at the metal center was primarily (if not exclusively) responsible for the asymmetric synthesis of cyclopropanes. In spite of these studies, the role of the stereogenic metal atom in addition to that of the chiral ligand in asymmetric induction phenomena in model compounds remains poorly understood, particularly with regard to chiral ligands that are actually used in asymmetric synthesis. The recent development of the organometallic chemistry of ruthenium,26especially the preparation of chiral ruthenium complexes, has provided a unique opportunity to address these important stereochemical questions. This review summarizes studies of the stereochemical course of selected metallorganic reactions and the diastereomeric equilibria of compounds containing prochiral ligands carried out by using monometallic half-sandwich chiral ruthenium complexes. Some references to related results obtained with different metallic systems are also presented when appropriate.
I I . Types of Chlral Half-Sandwich Ruthenium Complexes
A number of monometallic ruthenium half-sandwich complexes have been synthesized; these will be discussed according to the number of chirality elements present. For complexes containing only one chiral element, chirality is due to the presence of a chirality center
Consiglio and Morandini
either at the metal or at the ligand(s). In addition, chirality can arise from the presence of a coordinated prochiral ligand (such as an alkylidenecarbene, an olefin, or an allyl). The first ruthenium complexes (la,b) containing a chiral metal center were synthesized long ag026,27 but only as racemates. The fact that la was isolated as a
L=CO
la
L = PPh3
1b
R=H
2a
R =CH)
2b
he
3
red oil was attributed to the presence of optical isom e r ~ Attempts .~~ to resolve this racemic mixture did not succeed. Moreover, the stereochemical stability at the metal center was not investigated. The question of stereochemical stability was first studied by using complexes 2b and 3, in which two chiral centers are present, one a t the ruthenium atom and one at the ligand^.^^^^^ Stereochemical stability at a stereogenic metal resulted for complex 430(and the hexamethylbenzene analogue31) and complex 532containing the $-benzene ligand. It was shown that the two methyl
5
U
6
groups on the phenyldimethylphosphine ligand in complex 4 and in its hexamethylbenzene analogue give two sets of doublets in the 'H NMR spectrum. This anisochrony is due to diastere~topicity~~ of the two methyl groups and suggests some optical stability at the metal, at least on the NMR time scale. Analogously, the two methyl groups of the isopropylamine ligand of complex 5 are diastereotopic, as shown by 13C NMR. Complexes containing chirality center(s) on the polyhapto ligand 6%and 7%3 or on one of the two-electron ligand^^'.^^ 8 and 9 were recently synthesized. ComMe2CI
Chemical Revlews, 1987, Vol. 87, No. 4 763
Half-Sandwich Chiral Ruthenium Complexes
SCHEME 1 plexes 7-9 were used as starting materials for the synthesis of compounds having more than one chiral element (vide infra). L-
