Addition of phosphorus-stabilized carbanions to cyclic enones and further transformations of the reaction products

a-Lithiated diethyl alkylphosphonates react with cyclic enones according to a 1,2-addition, yielding the corresponding allylic alcohols (β-hydroxyalkylphosphonates). The alcohols undergo acid-catalyzed dehydration with preferential exocyclic location of the new olefinic bond; in some cases, allylic rearrangement to a 2° alcohol was observed. In pure methanol, allylic rearrangement is accompanied by the formation of an allyl methyl ether. Lithiated prop-2-enylphosphonate adds to the β-carbon (1,4-addition) via its γ-carbon; the only exception is 3-methylcyclohexenone, in which the methyl group directs the nucleophile toward the carbonyl center. © 1996 John Wiley & Sons, Inc.     

Perfluoroalkylation of carbanions

A successful perfluoroalkylation of various carbanions by the use of per$\text{f}$luoroalkylphenyl$\text{i}$odonium $\text{t}$rifluoromethane$\text{s}$ulfonate (FITS) was described. The reactivity of other perfluoroalkyliodonium salts was also examined.     

Free three-dimensional carborane carbanions

Carbon atom functionalization via generation of carbanions is the cornerstone of carborane chemistry. In this work, we report the synthesis and structural characterization of free ortho-carboranyl [C₂B₁₀H₁₁]⁻, a three-dimensional inorganic analog of the elusive phenyl anion that features a “naked” carbanion center. The first example of a stable, discrete C(H)-deprotonated carborane anion was isolated as a completely separated ion pair with a crown ether-encapsulated potassium cation. An analogous approach led to the isolation and structural characterization of a doubly deprotonated 1,1′-bis(o-carborane) anion [C₂B₁₀H₁₀]₂²⁻, which is the first example of a discrete molecular dicarbanion. These reactive carbanions are key intermediates in carbon vertex chemistry of carborane clusters.

Free three-dimensional carborane carbanions, which are inorganic siblings of deprotonated aryls with the “naked” anionic carbon atom are reported.     

Concurrent cyclopropanation by carbenes and carbanions

The generation of phenylhalocarbenes in the presence of varying quantities of halide ions and electron-poor alkenes affords concurrent cyclopropanation of the alkenes by an equilibrating mixture of phenylhalocarbenes and phenylhalomethide carbanions, which permits the smooth modulation of selectivity between electron-poor alkenes and electron-rich alkenes, a feature of potential synthetic utility.     

Nucleophilic reactivities of benzenesulfonyl-substituted carbanions

Kinetics of the reactions of four benzenesulfonyl-stabilized carbanions (1a-d)- with reference electrophiles (quinone methides 2 and diarylcarbenium ions 3) have been determined in dimethyl sulfoxide solution at 20 degrees C in order to derive the reactivity parameters N and s according to the linear free-energy relationship logk(20 degrees C) = s(N + E) (eqn (1)). The additions of (1a-d)- to ordinary Michael acceptors (e.g., benzylidene Meldrum's acid 4a, benzylidenebarbituric acids 5a-c, and benzylidene-indan-1,3-diones 6a-d) were also studied kinetically and found to be 5-24 times slower than predicted by eqn (1).     

Infrared spectra and structure of carbanions: XVIII. A semiempirical study of carbanions of cyanoacetic acid derivatives

The IR spectra of cyanoacetic acid, ethyl cyanoacetate and cyanoacetamide as well as those of related mono- and, whenever possible, dianions have been studied in dimethyl-sulphoxide (DMSO) and DMSO-d₆.The observed nitrile and carbonyl absorption frequencies correlate linearly with the corresponding Wiberg bond indices given by CNDO/2 calculations with full geometry optimization. These calculations predict carbanionic structures throughout except in the case of the dinegative ion of cyanoacetamide, which could be considered as originating from the aminoacetylenic tautomer of NCCH₂CONH₂. Parallel MINDO/3 calculations, however, predict that the latter dianion is again a carbanion. This result is in reasonable agreement with normal coordinate calculations and the experimental isotopic shifts of vibrational frequencies of the dianion ¹⁵NCCH^?CONH^?.     

Complexes of P-stabilised carbanions with s- and p-elements

The chemistry of complexes of the main group metals (Groups 1, 2, 13, 14, and 15, including Si and As) with P-stabilised carbanions is described. Such ligands, in which a carbanion centre is immediately adjacent to a P(III) or P(V) centre are remarkably versatile and adopt a wide range of coordination modes, depending on the nature of the metal centre, the presence of co-ligands and the ligand substituents; certain main group derivatives have also found applications in organic synthesis, especially for the olefination of carbonyl compounds (Horner, or Horner-Wadsworth-Emmons reagents). The focus of the review is on complexes which have been structurally characterised, either by crystallographic means or by unambiguous spectroscopic evidence.     

Photoinduced decomposition of the polybromostyryl carbanions

The photoinduced decay in THF of the sodium salt of the polybromostyryl carbanions (PS_B?) was investigated in vacuum at ?78°C. Maximal rates of decay were obtained when the wavelength of the incident light corresponded to the absorption maximum of the PS_B? at 495 nm. The quantum yield of the process was found to be 13 eq./Einstein. Products of the photostimulated decompositions at ?78°C were found to be different from those obtained as result of the “dark” thermal decomposition at –20°C. Decomposition of the PS_B? carbanions was accompanied by evolution of Br^? anions and by formation of polyaryl carbanions absorbing in the 270- to 400-nm range. The mechanism of the photoinduced decomposition of PS_B?, though basically analogous to the S_RN1 reaction mechanism, was found to differ significantly from the latter. The intramolecular dissociative electron transfer was assumed to represent its initiation step. Its propagation was attributed to the addition of PS_B? to the initially formed biradicals, ejection of Br^? from the resulting radical anions, and intramolecular electron transfer yielding polybromostyryl radicals. Products of the photoinduced decomposition of PS_B? remained soluble in THF when the intensity of the incident light was relatively weak, though their molecular weight distribution broadened. Insoluble, highly crosslinked polymers were obtained when the intensity of the incident light was very high.     

Stable perfluorinated cyclic carbanions and their NMR spectra

Conclusions Perfluoro-1-alkylcyclocarbanions were synthesized from perfluoro-1-ethylcyclobutene, perfluoro-1-methylcyclopentene, perfluoro-1-ethylcyclohexene and CsF in DMF and were identified by¹⁹F and¹³C NMR spectra. Perfluoro-1-alkylcyclocarbanions add chlorine to form 1-chloro-1-perfluoroalkylcycloalkanes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1325–1331, June, 1986.     

On the inversion barriers of pyramidal carbanions

A simple MO model is used to explain the trends exhibited by inversion barriers of pyramidal carbanions upon variation of its substituents. Free carbanions as well as carbanions solvated by H₂O molecules are considered.     

Alkylation of carbanions from isoxazolin-5-ones

Methyl acetoacetate and other β-keto esters can be converted into the corresponding isoxazolin-5-ones which can be metalated α to C-3. These anions are trapped by a range of electrophiles to produce 3-substituted isoxazolin-5-ones.     

Condensations of boron-stabilized carbanions with carbonyl compounds

The interactions of carbonyl compounds with boron-stabilized carbanions can be controlled to give synthetically valuable syntheses ofE-alkenes,Z-alkenes,erythro-1,2-diols, ketones and allenes.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1177–1185, July, 1993.The work on aromatic aldehydes was carried out in collaboration with D. Buss and E. Colcough, and on aliphatic aldehydes with K. Smith, M. Rowlands and S. Elgendy. The production o f allenes was carded out in collaboration with K. Smith and K. D. Jones. I thank them all, and also the SERC for support o f this work.     

Negative hyperconjugation in phosphorus stabilized carbanions

The electronic Fukui function is used to give qualitative electronic proof on the existence of back-bonding from the carbon lone pair toward the sigma* P-Y and P-O orbitals in phosphorus stabilized carbanions. NBO analyses are used to investigate the energetic, electronic, and structural impacts of this negative hyperconjugation interaction. The observed energetic stabilization can indeed be attributed to the electronic delocalization of the lone pair toward the antibonding orbitals. This delocalization is furthermore responsible for the shorter P-C bonds, longer P-Y (P-O) bonds, and wider Y-P-Y angles observed for the anionic compounds compared to their neutral counterparts. From the electronic NBO analysis it becomes clear that phosphorus containing functional groups are best described as sigma donor/pi acceptors.     

Schlenk's early "free" carbanions

Nearly a century ago, Schlenk published the syntheses and isolation of two most remarkable and unstable complexes: crystalline [Ph(3)C(-)][Me(4)N(+)] and [PhCH(2) (-)][Me(4)N(+)]. The crystal structure of the first complex contains a "free" Ph(3)C(-) ion, which displays the expected planar trigonal geometry at its central carbon atom. The phenyl groups are not orientated in the typical propeller arrangement, but instead display various orientations with respect to the molecular plane. These orientations can be directly related to the extent of charge delocalization and correlate well with other structural characteristics related to charge delocalization. The crystal structure also shows a network of C-H(delta+)...C(delta-) and C-H...pi interactions. Only C-H...pi interactions to the most negative charged phenyl rings are observed. The absolute Br?nsted acidity of Me(4)N(+) is calculated by the G2(MP2) method (287.7 kcal mol(-1)) and is compared to the calculated acidity of Me(4)P(+) (268.4 kcal mol(-1)). On this basis, the pK(a) value for Me(4)N(+) is estimated at 29.6. This makes the existence, and especially Schlenk's early isolation, of the "free" carbanions [Ph(3)C(-)][Me(4)N(+)] and [PhCH(2) (-)][Me(4)N(+)] quite noteworthy.     

Silylation products of cyclic tri-aminal carbanions and their lithiation

The structure of 1,3,5-trimethyl-1,3,5-triaza-cyclohexane (TMTAC) was determined by single crystal X-ray diffraction and compared with earlier gas-phase data. It shows a preference for an aee-conformation in all phases. Lithiated TMTAC, [(RLi)(2)·(RH)] (1) (R = 2,4,6-trimethyl-2,4,6-triaza-cyclohex-1-yl), was reacted with Et(3)SiCl, Ph(3)SiCl and PhMe(2)SiCl to afford the substituted silanes Et(3)SiR (1), Ph(3)SiR (2) and PhMe(2)SiR (3) in moderate yields. They were characterised by NMR spectroscopy ((1)H, (13)C, (29)Si). 1 reacts with Me(2)SiCl(2) and Ph(2)SiCl(2) to give Me(2)SiR(2) (5) and Ph(2)SiR(2) (6) which were characterised by NMR spectroscopy. 5 was also identified by crystal structure determination. Analogous triple substitution could not be observed by employing trichlorosilanes. Quantumchemical calculations explain this by sterical overcrowding of the silicon atom. The reaction of 1 with SiCl(4) did not yield fourfold substitution but a formal insertion product of SiCl(2) into a C-N bond of the TMTAC ring (2,4,6-trimethyl-2,4,6-triaza-1,1-dichloro-1-sila-cycloheptane, 7) in very small quantities. It was identified by X-ray crystallography and shows an intramolecular Si···N dative bond. The reactions of (3) and (5) with n-butyl lithium afforded lithiation of the silicon bound methyl groups in both cases. The products, 8 and 9, were characterised by NMR spectroscopy ((1)H, (13)C, (29)Si), 8 was also characterised by X-ray crystallography.     

Carbon-13 NMR spectra of arylmethyl carbanions

Carbon-13 chemical shifts of three arylmethyl carbanions have been determined by changing solvents and counter ions. The charge distributions in the carbanions are discussed and compared with those obtained from the ¹H chemical shifts.     

Role of base in palladium-catalyzed arylation of carbanions

The arylation reaction of carbanions, derived from certain sulfones, cyanoacetic ester and malononitrile, with aryl bromides (using the catalytic system of Pd₂dba₃/3L, L=PPh₃, PtBu₃) as well as the reaction of the carbanions with one equivalent of 4-CF₃C₆H₄ Pd(PPh₃)₂Br has been studied. These reactions proceed smoothly provided that the base stronger than the initial carbanion is present in the reaction mixture. In the absence of the above type of base the reactions do not proceed at all. Taking that into account we have proposed a novel mechanism of palladium-catalyzed arylation of CH-acids. The main feature of this mechanism is the accelaration of the reductive elimination due to the deprotonation of the intermediate ArPdL₂CHXY. The correlation between the carbanion reactivity and the pK_a values for related CH-acids as well as the ligand effect are discussed in the framework of the proposed mechanism.