Newly synthesized optically active rare earth complexes made possible a catalytic asymmetric nitroaldol reaction for the first time, giving nitroaldols in up to 95% ee. The catalyst complexes were prepared from rare earth (La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er, Yb, and Y) trichloride, dilithium (S) -binaphthoxide, NaOH, and H2O. The presence of LiCl and H2O appeared to be essential to obtain nitroaldols of high optical purity. The application to catalytic asymmetric syntheses of β-blockers, (S) -metoprolol and (S) -propranolol, has been also achieved. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) of rare earth metal BINOL complexes has suggested that all complexes are possessing one rare earth atom, three BINOL units and three Li atoms. Finally, X-ray crystallographic analyses of the Li free (Na containing) rare earth BINOL complexes (Pr, Nd and Eu) have been performed and the results were very much in agreement with the spectroscopic and analytical data. Although every Li free catalyst was not effective for asymmetric nitroaldol reactions, an addition of three molar equivalents of LiCl caused complete exchange of Na with Li resulting in the formation of an effective asymmetric catalyst.
Regio- and stereoselective allylation reactions of organocopper and organozinc reagents have been developed, investigated by NMR for the reagent composition, and examined for their synthetic potential with respect to 1, 2-asymmetric induction. The SN2′-selective reaction of organocopper and organozinc reagents with allylic chlorides having a chiral center at the δ-position proceeds with up to 100% diastereoselectivity. The observed 1, 2-asymmetric induction conforms to pure steric control (Cram-Felkin-Anh model) even in cases where conventional chelation control may seem to operate, and the level of the selectivity was found to be much higher than those found for the additions of organometallics to structurally comparable α-substituted carbonyl compounds. Some examples of 1, 4-asymmetric induction are also described.
New and effective aerobic oxygenation reactions of olefins catalyzed by transition-metal complexes are described. (1) By the combined use of an atmospheric pressure of molecular oxygen and secondary alcohols, various kinds of olefins are converted into the corresponding alcohols in good to high yields catalyzed by a cobalt (II) complex. (2) Effective epoxidation of a wide variety of olefins involving styrenes and α, β-unsaturated carboxamide are achieved by the combination use of aldehyde and molecular oxygen catalyzed by nickel (II), iron (III), or oxovanadium (IV) complexes under mild reaction conditions. (3) Enantioselective aerobic epoxidations of unfuctionalized olefins are demonstrated by using optically active manganese (III) complexes as catalysts. (4) In the presence of acetal of aldehyde, cobalt (II) complex catalyzed aerobic epoxidation of olefins proceeds in high yield under neutral and mild conditions.
Diastereoselective carbon-carbon bond formation is accomplished using activated imines with organocuprates-BF3·OEt2 complexes or allylic organometallic compounds such as allyl-9-BBN and allyltributylstannane-TiCl4. Very high 1, 2- and 1, 3-asymmetric induction is achieved in the reaction of allyl-9-BBN, allylmagnesium chloride and allylaluminum ate complexes with certain chiral imines. Iminium ion intermediates generated from α-alkoxycarbamates are detected by NMR at low temperatures. Significantly high diastereoselectivities are observed in the reactions of iminium ions with organoleads, allylic stannanes and organocuprate-BF3 reagents. It is proposed that ZnCl2·OEt2 works as a radical initiator as well as a chelating agent in the reaction of the ZnCl2 mediated allylation of α-bromoglycine derivatives with allylic stannanes. Transition metal catalyzed C-C bond formation takes place successively in the reaction of activated imines with activated nucleophiles such as malononitrile and α-alkoxy malononitrile. Lithium N -benzyltrimethylsilylamide (LSA) regioselectively adds to α, β-unsaturated esters in a 1, 4-manner. Higher order cyano cuprate type analogues of LSA react regioseletively with α, β, γ, δ-unsaturated esters in a 1, 4-manner to give the corresponding β-amino esters. This cuprate amide reagent is applied to the highly stereoselective synthesis of β-lactam frameworks. Asymmetric carbocyclization is achieved via the tandem conjugate addition of metal amides to certain dienoates.
Recent developments in homogeneous asymmetric hydrogenation by use of manufactured transition metal catalysts are remarkable. Their efficiencies are now often comparable or even superior to those of biocatalysts, and an increasing number of transition metal catalyzed asymmetric hydrogenations have been applied to industrial production of optically active compounds. This report deals with asymmetric hydrogenation catalyzed by homogeneous transition metal complexes by putting the emphasis on recent advances in this research field. Especially, asymmetric hydrogenation catalyzed by BINAP-metal complexes has been discussed.
Organometal complexes play an important role in modern electroorganic synthesis. This article reviews recent advances on electroorganic reactions assisted by organometal complexes involving the VIII, IVB, VIB, and VIIB group transition metals.
The cross-coupling reaction of organoboron compounds with organic halides, the catalytic hydroboration of alkenes and alkynes with catecholborane, and the thioboration reaction of alkynes with 9-organothio-9-bora-bicyclo [3.3.1] nonanes are described. The palladium-catalyzed cross-coupling reaction of organoboron compounds provides a general and convenient method for the synthesis of stereodefined alkenes, alkadienes or alkenynes, biaryls, and vinylic sulfides. The similar coupling reaction under carbon monoxide (1 atm) produces alkyl vinyl ketones, alkyl aryl ketones, alkyl alkyl ketones and biaryl ketones in exellent yields. The hydroboration of alkenes and alkynes with catecholborane can proceed under very mild conditions with the use of Rh, Pd, or Ni catalysts. It is also observed that catalysts can direct the course of the addition of borane to different regio- or stereoselectivity, compared with uncatalyzed reactions. The reaction is extended to the cis-addition of RS-B compounds to terminal alkynes with the use of Pd (PPh3)4.
Catalytic carbon-carbon bond formation is one of the most important reaction for constructing organic molecules. A two-component system containing palladium and copper effectively catalyzes a carbon-carbon coupling between sp2- and sp-carbons to give conjugated ene-yne compounds and aromatic acetylenes. This article reviews recent development of the palladium-catalyzed sp2-sp carbon-carbon coupling and its application to the synthesis of functional materials such as bioactive compounds, π-conjungated compounds, and liquid crystals.
Highly selective reactions using organo early transition metal compounds open several novel strategies for the construction of various organic compounds. Alkynylytterbium dichlorides, obtained by transmetallation from the corresponding Grignard reagents, showed reversal diastereoselectivity towards the acyl group in optically active 2-acyl-1, 3-oxathianes. Samarium diiodide-mediated Reformatsky-type reaction of α-halo esters with above 2-acyl-1, 3-oxathianes showed high diastereoselectivity and opens a new access to optically active β-hydroxy esters. Reduction of α-halo esters with SmI2 affords 4-metallo-3-oxobutanoate which reacts with aldehyde at C-4 position selectively to give 5-hydroxy-3-oxoalkanoic esters. Low valent tantalum reacts with alkynes to give alkyne-tantalum complexes. In-situ generated complexes can be used for the reaction with various electrophiles without purification. The reactivity of the complexes can be understood as a cis-ethene dianion equivalent. The complexes can be applied to the stereoselective synthesis of allylic alcohols, α, β-olefinic esters and amides, as well as 1-naphthols.
An excellent candidate as a proton substitute in man-made organic reactions is certainly a Lewis acid. Our goal is to engineer an artificial proton substitute of a special shape, which could be utilized as an effective artificial enzyme for chemical reactions, by harnessing the high reactivity of the aluminum atom toward oxygen. Such a concept was initially researched by examining the recognition ability of a specially designed organoaluminum receptors for various oxygen-containing substrates. Our initial finding on the successful discrimination between structurally very similar methyl and ethyl ethers with exceptionally bulky methylaluminum bis (2, 6-di-tert-butyl-4-methylphenoxide) (MAD) as a highly efficient Lewis acidic receptor induced us to study the more intricate question of diastereoface differentiation for the chiral ether and carbonyl oxygens. Thus, based on our conceptually new diastereoselective activation of ether and carbonyl moieties, exceptionally bulky organoaluminum reagents, MABR and MAPH have been devised in addition to MAD. These organoaluminum receptors can be successfully utilized for various regio- and stereocontrolled organic transformations. Furthermore, electronical stabilization of aldehyde functionalities has been accomplished with MAPH by the two parallel phenyl groups of aluminum ligands.
Axially chiral monodentate optically active phosphines 1, (S)-2-(diphenylphosphino)-2'-methoxy-1, 1, -binaphthyl (MOP, 1 b) and its analogs were prepared. Reaction of simple terminal alkenes 6, (RCH=CH2. R=n-C4H9, n-C6H13, n-C13H21, PhCH2CH2, cyclo-C6H11) with trichlorosilane at 40°C in the presence of 0.1 or 0.01 mol % of palladium catalyst, prepared in situ from [PdCl (π-C3H5)]2 and (S)-1b, proceeded with unusual regioselectivity (up to 94%) and with high enantioselectivity to give high yields of 2-(trichlorosilyl) alkanes 7a together with a minor amount of 1-(trichlorosilyl) alkanes 7b. Oxidation of the carbon-silicon bond gave optically active alcohols 9 (RCH(OH)CH3), with the enantiomeric purity ranging between 94% and 97% ee. The high enantioselectivity was also observed in the enantioposition-selective hydrosilylation of norbornene (19) and related meso bicyclic olefins catalyzed by the MOP/palladium, which gave exo-2-norbornanol (21) and the related alcohols with up to 96% ee.