Chiral hydroxythiols as catalysts for enantioselective borane ketone reductions

Several chiral 1,2- and 1,3-hydroxythiols derived from (R)-camphor, (1S)-(+)-10-camphorsulfonyl chloride, cysteine and cystine derivatives were prepared and evaluated as catalysts in borane ketone reductions. Under the best experimental conditions (10 mol% catalyst, THF, 35°C), a 95% yield of (R)-1-phenylethanol of 64% ee was obtained in the borane reduction of acetophenone.     

Spiroborate catalyzed reductions with N,N-diethylaniline borane

Reduction of esters, amides, and ketones by N,N-diethylaniline borane is accelerated by catalysts derived from spiroborate complexes. Esters are reduced at ambient temperature in less than 4 h with this amine borane and 5 mol % spiroborate 6. Functional group selectivity shows ketone and tertiary amide reduction is faster than ester or nitrile reduction.     

Reductive openings of acetals: explanation of regioselectivity in borane reductions by mechanistic studies

The mechanisms of regioselective reductive openings of acetals were investigated in several model systems by a combination of Hammett plots, kinetic experiments, density functional calculations, and (11)B NMR. The regioselectivity of borane reductions of cyclic acetals can be controlled by the choice of borane. Lewis acid activation of BH3 x NMe3 increases the reaction rate and renders the borane the most electrophilic species, which associates to the more electron-rich oxygen of the acetal. In contrary, without activation, the regioselectivity is instead directed by the Lewis acid, as exemplified by the reaction with BH3 x THF.     

Reagent-controlled stereoselectivity in titanocene-catalyzed epoxide openings: reductions and intermolecular additions to alpha,beta-unsaturated carbonyl compounds

The generation and addition reactions of metal bound radicals derived from normal and meso epoxides by electron transfer from titanocene(III) reagents is described. The control of enantioselectivity and diastereoselectivity of these transformations is investigated by variation of the ligands of the metal complex. The reaction can lead to unprecedented and highly selective reactions, in which synthetically useful alcohols may be prepared. The synthesis presented also circumvents the use of toxic metals. Another advantage is that there is no loss of two functional groups as usually observed in reductive radical chain reactions.     

Benzolactams. 4. Reaction of 3',4'- or 4',5'-dialkoxy-substituted 1-(2'-Bromobenzyl)-2-ethoxycarbonyl-1,2,3,4-tetrahydroisoquinolines with alkyllithium. 1,2 and 1,4 additions of alkyllithium to benzolactams

Treatment of 1-(2'-bromo-3',4'-dialkoxybenzyl)-1,2,3, 4-tetrahydroisoquinoline carbamates, 1a,c, with excess alkyllithium gave 8-oxoberbines, 2a,c, which were successively attacked in situ with another molecule of alkyllithium to give 1,2 and/or 1,4 addition products. A primary alkyllithium, such as MeLi or BuLi, gave a 1,2 addition product, 8-methyleneberbine 9a or 8-butylideneberbine 3a. t-BuLi preferred 1,4 addition, followed by elimination of the alkoxy group, to give 9-tert-butyl-8-oxoberbine 6a or 7c. s-BuLi gave a mixture of 1,2 and 1,4 addition products, 1-[2'-(2' '-methylbutyryl)benzyl]-1,2,3,4-tetrahydroisoquinoline 4a and 9-s-butyl-8-oxoberbine 5a. Similar treatments of carbamate 1b having no alkoxy group at its 3' position gave 1,2 addition products, 8-butylideneberbine 3b, 1-[2'-(2' '-methylbutyryl)benzyl]-1,2,3, 4-tetrahydroisoquinoline 4b, and 1-(2'-pivaloylbenzyl)-1,2,3, 4-tetrahydroisoquinoline 6b, in all cases. Reactions of 1a with s-BuMgCl and isoPrMgCl also gave the 1,4 adduct, 5a, and its 9-isoPr analogue, 12a. Treatment of 9a with excess NaBH(4) in AcOH gave (+/-)-coralydine (10b).     

Reaction of allene cyclodimers with alkyllithium compounds

Conclusions 1,3-Dimethylenecyclobutane and 1-methyl-3-methylenecyclobutene are metallated in the presence of THF by alkyllithium compounds under mild conditions with the formation of one and the same organolithium compound of the -pentadienyl type. Under the same conditions, 1,2-dimethylenecyclobutane adds alkyllithium compounds, to form a -alkenyl complex.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1996–1999, September, 1988.The authors wish to express their gratitude to E. R. Dolinskaya and her fellow workers for recording the spectra on the Bruker HX-270 spectrometer.     

Metallation and reaction of 4,5-dicyanoimidazoles with alkyllithium reagents

The conditions required for selective lithiation at the 2-position of 4,5-dicyanoimidazoles are described in detail. The course of the reaction of alkyllithium reagents with 4,5-dicyanoimidazoles depends on the temperature, the number of equivalents of alkyllithium and whether the 1-nitrogen is protected. Under certain circumstances, the alkyllithium reagent adds to one of the cyano groups giving cyanoimidazole ketones.     

Cyclization via carbolithiation of alpha-amino alkyllithium reagents

We report a new route to tertiary alpha-amino stereocenters by sequential alkylation of alpha-amino nitriles followed by reductive lithiation of the nitrile and cyclization onto an alkene. Reductive lithiation of alpha-amino nitriles using lithium 4,4'-di-tert-butylbiphenylide (LiDBB) and subsequent intramolecular carbolithiation proceeded with modest to high diastereoselectivity to deliver cyclic or spirocyclic ring systems. The stereoselectivity of these intramolecular carbolithiations was examined using density function calculations to evaluate plausible transition state models.     

Temperature and concentration effects on polar-modified alkyllithium polymerizations and copolymerizations

Addition of polar modifiers to alkyllithium-initiated homopolymerizations of butadiene causes substantial changes in the microstructure of the polymers produced. These changes are shown to depend not only on the concentration of modifier, but also on the polymerization temperature. The combined effects of modifier concentration and reaction temperature have been considered, and a method is presented for quickly determining the proper conditions for preparation of a polybutadiene of any 1,2-microstructure within a range of 10–80%. It is also shown that in anionic polar-modified copolymerizations of butadiene–styrene, the reaction temperature is again critical. Within a certain concentration range of modifier, the temperature will influence the rate of styrene incorporation or the randomness of styrene units in the resulting copolymers.     

Polymerization of vinyl chloride by alkyllithium compounds

The polymerization of vinyl chloride initiated by alkyllithium compounds was investigated. The effect of temperature, initiator concentration, and monomer concentration on the conversion and the properties of the resulting polymers were studied. The optimum temperature in the investigated range (between ?20°C and +20°C) was +5°C. The conversion is directly proportional to the concentration of both the initiator and the monomer. The molecular weight is inversely proportional to the initiator concentration and directly proportional to the monomer concentration. Under optimum conditions the molecular weight of the polymers is as high as 140,000. These results differ by an order from hitherto published data on the nonradical polymerization of vinyl chloride. The proportion of isotactic and syndiotactic structures resulting from the presence of tert-butyllithium does not differ from that obtained by radical polymerization, but the occurrence of anomalous structures is reduced to a minimum. The stability of the macromolecules is higher. A mechanism of the polymerization is suggested.     

Vinylgallium and alkyllithium compounds: transmetalation and generation of oligolithium cages

Vinylgallium compounds [C(6)H(6-n){(H)C=C(SiR(2) R')-GaR'(2)}(n ] (3, R=Ph, Me; R'=Ph, Me; R'=tBu, Et; n=1, 2) are easily accessible by hydrogallation of the corresponding alkynylbenzene derivatives with H-GaCl(2) and subsequent reaction with alkyllithium derivatives. Treatment of 3 with an excess amount of tert-butyl- or ethyllithium yielded by transmetalation and ortho-deprotonation of the aromatic rings the unprecedented solvent-free oligolithium cluster compounds [{(C(6)H(4)Li)HC=C(SiPh(3))Li}(2)(tBuLi)(2)] (4), [{(C(6)H(4)Li)HC=C(SiPh(2)Me)Li}(4)] (5) and [{(C(6)H(3)Li){HC=C(SiMe(3))Li}(2)}(3)] (6) in moderate yields. Their solid-state structures revealed the presence of unique molecular lithium clusters with 6, 8, or 9 lithium atoms that may be derived from two edge-sharing Li(4) tetrahedra (4), three Li(4) tetrahedra in a chain joined by two common edges (5) or a tricapped trigonal prism of lithium atoms (6).     

A theoretical study of some possible alkyllithium hexamers

Using ab initio SCF calculations we optimized three possible structures of hexameric methyllithium each having a near octahedral geometry of lithium atoms with capping methyl groups. The isomer in which the vacant faces of the octahedron are trans to each other is the most stable. The alternative structure with the vacant faces in apical contact is 8.8 kcal/mol (6-31G^*//4-31G) higher in energy while the edge contact isomer is 26.3 kcal/mol more energetic than the trans structure. The methyl groups were found not to be faced centered. These results are rationalized electrostatically.     

The reaction of 2,3,4-trichlorothiophene with alkyllithium reagents

2,3,4-Trichlorothiophene undergoes metal-hydrogen exchange with one equivalent of butyllithium inTHF at ?78°C to give trichloro-2-thienyllithium in 91% yield. No metal-halogen exchange is observed to have taken place. Witht-butyllithium under similar conditions, however, the metalhalogen exchange reaction occurs but only to the extent of 8%; the main reaction remains to be the metal-hydrogen exchange. 2,3,4-Trichlorothiophene reacts with two equivalents of butyllithium in ether at ?5°C to give 3,4-dichloro-2,5-dilithiothiophene in 96% yield.     

Effect of lithium alkoxide and hydroxide on polymerization initiated with alkyllithium

The effects of lithium tert-butoxide on rates of initiation, propagation, and polymerization of butyllithium-initiated polymerization of butadiene and styrene were examined. Toluene and cyclohexane were used as solvent and the experiments covered BuLi/-t-BuOLi ratios from 1:0 to 1:6. Both polymerization and propagation rates declined in the presence of alkoxide. The initiation rate in cyclohexane increased to a maximum at BuLi/t-BuOLi ratio of 1:0.5 and then decreased. A mechanism of rapid dissociation of butyllithium oligomer by alkoxide and multiple complexation of butyllithium with excess alkoxide is proposed. Lithium alkoxide also reduced the copolymerization rate but did not affect the copolymerization characteristies. Water reacted with organolithium compound rapidly to form hydroxide. It was found that the hydroxide reacted very slowly with more organolithium compound in hydrocarbon solution to form oxide. Their effects on polymerization were investigated.     

Structural and thermochemical studies of pyrrolidine borane and piperidine borane by gas electron diffraction and quantum chemical calculations

Structural Chemistry - The gaseous structures, thermochemical properties and dehydrogenation reaction energy profiles of the borane complexes of pyrrolidine and piperidine have been investigated...     

Interconversion of contact and separated ion pairs in silyl- and arylthio-substituted alkyllithium reagents

Ether-solvated contact and separated ion pairs (CIP and SIP) for two lithium reagents, tris(trimethylsilyl)methyllithium (1) and bis(3,5-bistrifluoromethylphenylthio)methyllithium (2), have been characterized and observed for the first time under conditions of slow exchange by NMR spectroscopy, and barriers to interconversion have been measured. A Saunders isotope perturbation experiment was used to support identification of the CIP and SIP species for 2.     

Stability of polyhedral borane anions and carboranes

Polyhedral borane anions and carboranes that can be constructed formally from the interaction of rings and caps will be stable with six interstitial electrons. Interstitial electron count is obtained by summing the number of π electrons of the ring and the electrons of the caps involved in ring cap binding. Thus B₇H₇ ⁻² (D_5h) has 6 interstitial electrons (none from the B₅H₅ ring, two each from the twobh caps and two negative charge),mndo calculations on isoelectronic pyramidal molecules B₆H₆ ⁻⁴ (C_5v), B₅H₅CH⁻³ (C_5v), B₅H₅ ⁻⁴ (C_4v), B₄H₄CH⁻³ (C_4v), B₄H₄ ⁻⁴ (T _d) and B₃H₃CH⁻³ (C_3v) suggests a criterion based on the out-of-plane bendings of the ring B-H bonds to select the best combination of borocycles and BH or CH caps. Three-membered borocycle prefers CH cap, five-membered borocycle prefers BH cap. The preference of four-membered ring for BH or CH cap is not as pronounced. The extra stability of B₁₂H₁₂ ⁻² arises from the geometry of the icosahedron. The relative stabilities ofnido andcloso carboranes follow from these rules.