Carbenoid chain reactions through proton, deuteron, or bromine transfer from unactivated 1-bromo-1-alkenes to organolithium compounds

The deceptively simple vinylic substitution reactions Alk2C=CA-Br + RLi --> Alk2C=CA-R + LiBr (A = H, D, or Br) occur via an alkylidenecarbenoid chain mechanism (three steps) without transition metal catalysis. 2-(Bromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CH-Br, 2a) is deprotonated (step 1) by phenyllithium (PhLi) to give the Br,Li-alkylidenecarbenoid Alk2C=CLi-Br (3). In the ensuing chain cycle, 3 and PhLi (step 2) form the observable alkenyllithium intermediate Alk2C=CLi-Ph that characterizes the carbenoid mechanism in Et2O and is able to propagate the chain (step 3) through deprotonation of 2a, furnishing carbenoid 3 and the product Alk2C=CH-Ph. The related 2-(dibromomethylidene)-1,1,3,3-tetramethylindan (Alk2C=CBr2, 2c) and methyllithium (MeLi) generate carbenoid 3 (step 1), which incorporates MeLi (step 2) to give Alk2C=CLi-CH3, which reacts with 2c by bromine transfer producing Alk2C=CBr-CH3 and carbenoid 3 (step 3). PhCCLi cannot carry out step 1, but MeLi can initiate (step 1) the carbenoid chain cycle (steps 2 and 3) of 2c with PhC[triple bond]CLi leading to Alk2C=CBr-C[triple bond]C-Ph. Reagent 2a may perform both proton and bromine transfer toward Alk2C=CLi-CH3, feeding two coupled carbenoid chain processes in a ratio that depends on the solvent and on a primary kinetic H/D isotope effect.     

Carbonation of polymeric organolithium compounds: Effects of chain end structure

The carbonation of the adduct of poly(styryl)lithium with 1,1-diphenylethylene with gaseous carbon dioxide in benzene proceeds in high yield (>99%) compared to the inefficient functionalizations for poly(styryl)lithium (66%), poly(isoprenyl)lithium (59%), and polybutadienyl)lithium (27%). The quantitative carboxylation of poly(isoprenyl)lithium and poly(butadienyl)lithium requires the presence of larger amounts of Lewis bases such as THF ([THF]/[Li] = 300) or TMEDA ([TMEDA]/[Li] = 46) compared to poly(styryl)lithium in which a ratio of [TMEDA]/[Li] = 12 is effective. © 1992 John Wiley & Sons, Inc.     

Selective isomerization of 1-alkenes by binary metal carbonyl compounds

An efficient and selective isomerization of 1-alkenes to their corresponding 2-alkenes is achieved by using binary metal carbonyls such as Ru₃(CO)₁₂ as catalysts. Possible mechanisms are discussed. Substituents on the 1-alkene have a significant effect on the isomerization.     

γ-手性顺式的共轭烯醛与有机锂化合物的立体选择性反应

本文研究了在手性的顺式-γ-氨基-α,β-不饱和醛(7)与有机锂化合物的亲核加成反应中的1,4-不对称诱导效应.结果表明: 7与α-烷氧基乙烯醇锂类化合物的反应导致了优良的非对映立体选择性(d.e. : 60～90%), 而与烷基锂试剂的反应也给出了高度的立体选择性(d.e.: 74～86%). 这说明在这些反应中 , 存在着一种1,4-不对称诱导反应 .     

Carbenoid transfer in competing reactions catalyzed by ruthenium complexes

Aiming at improving catalyst activity, ten ruthenium promoters have been investigated in carbenoid transfer from ethyl diazoacetate to styrene as a model substrate. Optimal selectivity in cyclopropanation has been attained with the new NHC–Ru complex 10 , as well as with the Fischer carbene 7 . The surprising non‐metathetical behavior of the Grubbs’ first‐generation catalyst in this multifaceted process is highlighted. Copyright © 2014 John Wiley & Sons, Ltd.     

Microstructure effect in the coupling reactions of polymeric organolithium compounds using chlorosilanes

The most important variable affecting the yield in the coupling reactions of polymeric organolithium compounds with chlorosilane compounds has been investigated through size-exclusion chromatographic (SEC) analysis. The coupling reaction of poly(styryl)lithium with dichlorodimethylsilane as a silane-coupling agent provided 44 wt % of the coupling yield. The coupling yield, depending on the chain end reactivity of active polymers, was not greatly affected. The addition of a Lewis base such as N,N,N′,N′-tetramethylethylenediamine (TMEDA) even after complete polymerization of the dienes in hydrocarbon seems to affect the coupling reaction, resulting in decreasing the yield. The 1,2- or 3,4-enchain contents in the polydiene backbones affected the reduction of the linking efficiency in the coupling of the poly(dienyl)lithiums with chlorosilanes as the linking agent. The linking yields of the active polymers including over 75 mol % of 1,2- or 3,4-enchainment on the polydiene segment were below 20 wt %. The linking yields exhibited a dependence not only on the steric requirement of the chain end, but also the microstructure of the polydiene segment. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1743–1753, 1998     

Practical ways from aldehydes to 2-chloro-1,1,1-trifluoro-2-alkenes and 2-chloro-1,1-difluoro-1-alken-3-ols

A reagent system CF₃CCl₃/Zn(>2 mol)/Ac₂O transformed aldehydes to 2-chloro-1,1,1-trifluoro-2-alkenes exclusively, whereas CF₃CCl₃/Zn(>2 mol)/AlCl₃(cat.) converted aldehydes into 2-chloro-1,1-difluoro-1-alken-3-ols.     

Fibre optic ATR-IR spectroscopy at cryogenic temperatures: in-line reaction monitoring on organolithium compounds

A reliable methodology, utilising an ATR-IR fibre probe, for in-line monitoring of low temperature reactions is presented. The application of this convenient set-up enables a fast and safe exploration of highly reactive substrates. Hence, in situ monitoring of lithiation reactions is realised and the potential to investigate sensitive intermediates is being demonstrated.     

New synthesis of vinyl selenides nucleophilic substitutions of unactivated vinyl halides by selenide anions

Alkyl and aryl selenide anions react with unactivated vinyl halides, in dipolar aprotic solvents, to give alkyl or aryl vinyl selenides in good yields. These reactions are stereospecific and occur with retention of configuration.     

C-Nitroso compounds—XXXV : Reaction of organometallic compounds with 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane

Reaction of Grignard reagents and organolithium compounds (RM) with the congested 1-chloro-1-nitroso-2,2,6,6-tetramethylcyclohexane 1 leads to the formation of significant amounts of the reduction product 2,2,6,6-tetramethylcyclohexanone oxime 3 (61–90%) together with the corresponding oxime O-R ether 4 (0–11%). Attack on nitrogen is unimportant as shown by very low yields of nitrone. Formation of the products is rationalised with a pathway involving transfer of an electron from RM to 1. This leads—after separation of MCI—to a radical pair consisting of R and the relatively stable iminoxy radical 2 (Schemes 1 and 2). Combination of these radicals explains formation of oxime ether 4 and nitrone 5, while reaction of iminoxy radical 2 with excess of RM can give oxime 3. Reactive radicals R (i.e. Me, Ph, and to a minor extent n-Bu) are furthermore capable of abstracting hydrogen from the solvent (diethyl ether, toluene, or cumene), and the solvent derived radicals can also combine with 2 on oxygen, under formation of oxime ether (26% of 6a). The corresponding benzyl- and cumyl ethers 6b and 6c are only formed in trace amounts because dimerisation of benzyl radicals (7%) and cumyl radicals (22%) is favoured.     

An mndo study of structures and stabilities of representative 1-chloro-1-alkenes and of their beryllium hydride derivatives

MNDO calculations were applied in order to investigate the inherent stabilities of 1-chloro-1-alkenes and their beryllium hydride derivatives. The calculated geometries and atomic charges are compared with applicable experimental findings. The calculations also explain the observed relative stabilities of metallated 1,2-dichloroalkenes. The two factors which increase the stabilization of compounds in this series are: the absence of a chlorine substituent trans to the metal and the degree of polarization of the C-Cl bond being broken.     

Stereospecific synthesis of divinyl selenides nucleophilic substitutions of unactivated vinyl halides by vinyl selenide anions

Vinyl alkyl selenides can be dealkylated by nucleophilic substitution or by electron transfer to give vinyl selenide anions which retain the configuration of the starting products. The same anions are also produced by electron transfer from vinyl acetyl selenides. The vinyl selenide anions react with vinyl halides, in DMF or DMA, to give divinyl selenides. These reactions occur with retention of configuration and represent a new example of nucleophilic substitutions of unactivated vinyl halides.     

Synthesis of γ-butyrolactones via the ene reaction of ethyl α-chloro-α-phenylthioacetate and 1-alkenes

Ethyl α-chloro-α-phenylthioacetate reacted with 1-alkenes in CH₂Cl₂ at -78–0°C in the presence of SnCl₄ to afford the ene reaction products (80–90%), which were readily converted into the corresponding γ-butyrolactones.     

Stereospecific epoxide-opening reactions of 1,1-dibromo-3,4-epoxy-1-alkenes with carbon nucleophiles

The stereospecific epoxide-opening reactions of 1,1-dibromo-3,4-epoxy-1-alkenes with allyltributylstannane and with ketene silyl acetals in the presence of a Lewis acid are described. Both the reactions occurred regioselectively at the allylic position via an S_N2 process giving rise to a single product, respectively. Treatment of the products by the latter reaction with p-TsOH afforded various 3,4-anti- and 3,4-syn-disubstituted γ-lactones in a highly stereoselective manner and high yields.     

Aryl N- and O-trimethylsilyl substituents: reactivity toward organolithium compounds

The efficacy of using the trimethylsilyl group for protection of aryl amino and hydroxy substituents during reactions involving organolithium reagents has been assessed. Bromotrimethylsiloxy-benzenes or -pyrimidines reacted with $\text{n}$-butyl lithium by initial metal-halogen exchange followed by intra- or intermolecular rearrangement of trimethylsilyl from oxygen to the carbanionic center. Trimethysilyl groups on aryl nitrogen are stable to conditions of organolithium coupling reactions. It is suggested that for aryl 0- and aryl N-trimethylsilanes, the observed differences in reactivity at silicon are owing to the marked differences in basicity of the respective leaving groups (PhOH ? Ph0-, pK_A - 10, pHNH₂ ? PhNH^?, pK_A ? 25).     

Nitroallylation of highly reactive organolithium compounds by 2-nitro-3-pivaloyloxy-1-propene (NPP)

The nitroolefin $\text{1}$ (NPP) efficiently transfers a 2-nitroallyl group even to the most reactive nucleophilic centres of organolithium reagents to give the products of type $\text{2}$ listed in the table.     

Carbenoid reactions of trifluoromethylelement compounds. Part 4. Reactions of trifluoromethylzinc bromide with enamines and methylene bases

The reactions of ZnBr(CF₃)·2CH₃CN with 1-morpholinocyclo-pentene and -hexene have been investigated and found to yield mixtures of two isomers of 1-morpholino-1-cyanomethyl-2-difluoromethyl-cyclopentane and -hexane, respectively. The interaction with methylene bases of nitrogen heterocycles leads to the formation of trimethinecyanine dyes. This reaction sequence includes the formation of two carbon-carbon bonds in a one-pot operation. The mechanism of the reactions is discussed.