Low-Temperature I3C-NMR. Spectroscopy of Organolithium Derivatives. -I3C, 6Li-Coupling, a Powerful Structural Information SummaryThe 13C-NMR. spectra of thirteen lithiated hydrocarbons (lc-13 c, Table 2) and of eighteen a-halo-lithium carbenoids (14 c-31c, Table 3) have been recorded in donor solvent (R20, R3N) mixtures at temperatures down to -150". The organolithium species were generated from singly or doubly 13C-labelled precursors by H/6Li-or Br/6Li-exchange. -13C, 6Li-Coupling was observed of all species but those which supposedly contain contact ion pair C, Li-bonds (benzylic and acetylenic derivatives). The multiplicities of the signals are correlated with the degree of aggregation in solution: the triplets of the halocarbenoids must arise from monomers or heteroatom-bridged oligomers, the quintuplets of butyl-, cyclopropyl-, bicyclo [ 1.1 .O]butyl-, vinyl-, and phenyllithium from dimers with planar arrangement of two Li-and two C-atoms, as known from crystal structures (Scheme 3). All 13C, 6Li-couplings are temperature-dependant, dynamic processes cause them to disappear above ca. -70" (Fig. 1-4). -Types of organolithium compounds are categorized according to the change of chemical shift AS (H, Li) upon H/Lisubstitution, according to the 13C, 6Li-coupling constants ranging from 0 to 17 Hz, and according to the multiplicities which indicate the aggregation: type A are Li-derivatives of alkanes and cycloalkanes, type B are 0-bonded vinyl, aryl, and alinyl derivatives, type C are a-heterosubstituted ( R S , hetero = halogen) organolithium compounds, and type D are n-bonded allylic and benzylic systems ( Table 5). The C , Li-distances in the crystal structures of representatives of all four classes are within the small range of 2.18-2.28 A (cj Scheme 3). -Some surprising observations and their interpretations and consequences are: a) butyllithium solutions in THF, THF/TMEDA, and dimethyl ether contain increasing amounts of dimer upon cooling, the equilibrium (tetramer . 4 THF) + 4 THF + 2 (dirner . 4 THF) ') Teil der geplanten Dissertationen von R. H. und J. G., ETH Zurich. HFLVFTICA C I~I M I C A ACTA -Vo1.66, F a x . 1 (1983) -Nr.27 309being shifted to the right (Fig. 1 and Scheme 4 ) ; thus, more of a different species is present at low temperatures, with the accompanying changes in reactivity; b) mixed higher aggregates are formed upon addition of butyllithium to bicyclobutyllithium; these are broken up to dimers upon addition of TMEDA (Tetramethylethylenediamine) (Fig. 2 and Scheme 5); c) the solid state, the calculated gas-phase and the solution species of phenyllithium all have dimeric structures, and so do vinyl and cyclopropyl lithium derivatives; the 13C-deshielding observed upon replacement of H by Li on sp2-and sp-C-atoms is related to a polarization of the 7c-electrons (Table 3, Fig. 3 and Scheme 6); d) the spectra of halo-lithium carbenoids show three striking features as compared to the C, H-compound: deshielding of up to 280 ppm ( Table 3), strong decrease of the coupling constant with 'Hand ...

The 13C‐NMR spectra of 19 different, singly, doubly, and triple 13C‐labelled α‐sulfur‐ and α‐selenium‐substituted 6Li‐derivatives generated from methyl and phenyl thioethers, thioacetals, trithio‐orthoesters and from their selenium analogues have been recorded in ethereal solutions (tetrahydrofuran (THF), 2‐methyltetrahydrofuran (MTHF) at temperatures between −30° and −110°. The effects of H/Li‐exchange upon chemical shifts and coupling constants, as well as the values and multiplicities of Li, 13C‐coupling are interpreted in vie wof crystal structures of some of the same compounds. In two thirds of the cases studied, the H‐decoupled 13C‐NMR signals observed below −80° were triplets, proving that the C‐atoms are bonded to single 6Li‐atoms. This is compatible either with monomeric or with dimeric, heteroatom‐bridged structures. The direct 1H, 13C‐ and 13C, 13C‐coupling constants (1J) decrease, the 13C, 77Se‐coupling constants increase upon lithiation. More striking is that the geminal coupling 13C‐S‐13C (2J) is too small to be observed in the non‐metalated species, while it ranges from 3.7 to 7.5 Hz in the lithiated derivatives. These observations may be interpreted as resulting from delocalization of electron density from the carbonionic center towards the adjacent heteroatom.

Low Temperature I3C-NMR. Spectra of 13C-and 6Li-Labelled Chloro-, Bromo-, and Iodo-lithiumcarbenoids Summary The I3C-NMR. spectra of tetrahydrofuran solutions of 16 chloro-, bromo-, and iodo-61ithio-and -71ithio-carbenoids with I3C-labelled methane, ethane, ethylene, and cyclopropane C-skeletons have been measured at temperatures around -100" (for examples see Fig. 1 and 2). Invariably, the exchange of hydrogen or halogen by lithium causes deshielding (AS, see Table I ) of the 13C-signal by up to 289 and 434 ppm, respectively, and decrease of 'J (IH, I3C) and IJ(I3C, I3C) couplings (see Table 2) with the C-atom of up to 104 and 30 Hz, respectively. The 'J(6Li, I3C) and 'J('Li,13C) coupling of ca. 17 and 45 Hz, respectively, obtained in ten cases (Table 1) is independant of the substitution pattern of the C-skeleton and of the particular halogen atom.Vor etwa einem Jahr berichteten wir uber die ungewohnlichen I3C-NMR.-Spektren einiger Brom-lithium-carbenoide [ 1) [2]. Trotz unzureichender Zahl von Beispielen hatten wir damals gewagt, aus Verschiebungen und Kopplungen auf ungewohnliche Strukturen zu schliessen, wie sie auch fur gasformige, monomere Carbenoide aufgrund von theoretischen Berechnungen vorhergesagt werden [3] [4]. Inzwischen ist es gelungen, zahlreiche weitere I3C-markierte Vorlaufer herzustellen') und die zugehorigen Lithiumcarbenoide NMR.-spektroskopisch zu vermessen. Vor allem liegen jetzt Daten fur andere Halogene als die Bromverbindungen vor -lediglich die Fluorcarbenoide entzogen sich bisher der Beobachtung.

The reduction of unhindered ketones, such as acetylenic ketones, with B-3-pinanyl-9-borabicyclo[3.3.1]nonane (Alpine-Borane, prepared by hydroboration of -pinene with 9-BBN) provides a simple means of forming chiral, nonracemic alcohols of known absolute configuration in high enantiomeric purity. A dehydroboration-reduction mechanism leading to racemic alcohol is believed to be responsible for erosion of the enantiomeric efficiency with more hindered ketones. The use of elevated pressures (2000-6000 atm) accelerates the asymmetric reduction mode while suppressing the undesired dehydroboration mode. The use of high pressure permits a study of the scope and utility of Alpine-Borane in the absence of the competing side reaction. The reduction of ketones containing chiral centers was studied with use of the reagent from (+)-a-pinene. d-Carvone is reduced while l-carvone is not. Methyl-substituted cyclohexanones show no discrimination between enantiomers. For example, 2methylcyclohexanone is reduced to a 1:1 mixture of cis-and írans-2-methylcyclohexanol, each of which is about 65% ee. In all cases, one may predict the absolute configuration of the product based on a simple model. The relative steric requirements of groups on the ketone may be catagorized as very small (C=CH, C=N, H, D); small (CH3, CO2CH3); medium (n-alkyl, trares-RHC=CH); medium large (CF3, t-Pr); large (Ar); and too large (fert-butyl). Effective asymmetric reductions are achieved when groups from nonadjacent categories are attached to the carbonyl.

Synthesis of (-)-Talaromycin A Summary: The spiroketal talaromycin A has been prepared in high optical and diastereomeric purity by using a [2,3]-sigmatropic (Wittig) rearrangement and a [3,3]-Claisen rearrangement as key steps in controlling absolute configuration.Sir: Talaromycins A (1) and B (2) are two toxic metabo-