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The direct dimetallation of vinylacetylene (1-buten-3-yne) at the 2- and 4-positions can be achieved by the butyllithium-potassium tert-butoxide reagent, but not when butyllithium is used alone or in the presence of TMEDA. Supporting experimental and calculational (ab initio and MNDO) results suggest that the potassium acetylide, KC≡C-CH = CH2, is formed first. Further metallation occurs at the activated C-2 vinyl position. Preferential substitution at the C-2 position in t-BuC≡CCH = CHt-Bu also was demonstrated. Neither dianion nor dilithium models account for the dimetallation results; in both cases, 1,4-arrangements are indicated to be better than 2,4. The 2,4-dimetallated vinylacetylene is a useful intermediate allowing the synthesis of derivatives inaccessible otherwise.

The structures and energies of lithiated cyclopropenyl cations and their acyclic isomers (C3H3-nLin+, n = 0−3) have been calculated employing ab initio MO (HF/6-31G*) and density functional theory (DFT, Becke3LYP/6-311+G*) methods. The cyclic isomers (4, 6, 10, and 14) are always favored, but when lithium is substituted sequentially along the C3H3+, C3H2Li+, C3HLi2+, and C3Li3+ series, the acyclic forms (5, 7, 11, 16) become progressively less competitive energetically. A triply bridged c-C3(μ-Li)3+ geometry, 14, was preferred over the classical form 3 by 8.7 kcal/mol. A single lithium substituent results in a very large (67 kcal/mol) stabilization of the cyclopropenyl cation. The favorable effects of further lithium substitution are attenuated but are still large: 48.2 and 40.5 kcal/mol for the second and third replacements, respectively. Comparison with polyamino-substituted cyclopropenyl cations suggest c-C3Li3+ (3 and 14) to be a good candidate for the thermodynamically most stable carbenium ion. The...

The crystal structures of three 1,5-doubly lithium bridged heteroatom-linked diaryl systems with sulfur (4), oxygen (5), and nitrogen (6) as flexible centers have been determined. All are monomers in the crystals. Each lithium bridges both deprotonated ortho carbons symmetrically. The coordination sphere of the metal is completed by the bidentate ligand tetramethylethylenediamine (TMEDA). The π-system is coplanar in o,o'-bis(lithio-TMEDA)diphenyl thioether (4b), but the rings are tilted markedly in o,o'-bis(lithio-TMEDA)diphenyl ether (5b) and in 1,2'-bis-(lithio-TMEDA)-N-phenylcarbazole (6b). The distance between the metalated carbons is responsible for the degree of twisting of the phenyl rings

Lithiation of biphenyl with 2.4 mol of n-butyllithium in the presence of TMEDA led directly to the 2,2′-dilithio derivative (I) in modest, but preparatively useful yields. I, in turn, was converted to a variety of products. The activation of the 2′-position of 2-lithiobiphenyl was shown directly by a separate experiment. MNDO calculations indicate stabilization in I by double bridging and in 2-lithiobiphenyl by intramolecular π interaction of Li with the o-phenyl group. Similar interactions in substitution transition states rationalize the specificity of the reactions observed.

The structures (4-31G optimization) and energies (6-31G*) of vinyl lithium (5) and of the 1,1 (3 and 4), cis-1,2 (6), trans-1,2 (7) and doubly bridged (8) dilithioethylenes have been calculated using standard ab initio methods. Vinyl lithium (5), indicated to have a classical geometry, is about 11 kcal/mole more stable than ethyl lithium in an hydride exhange reaction (Eq. (2)). Singlet trans-1,2-dilithioethylene, the most stable C2H2Li2 isomer (see also addendum), has a highly distorted, partially bridged structure (7a) with < CCLi = 87°. Symmetrically bridged 8 and the cis-isomer 6 are considerably less stable, 54 and 19 kcal/mole, respectively. The partial Li-H bonding in 7a contributes significantly to this behavior. 1,1-Dilithioethylene is found to prefer a perpendicular (3) over the usual planar (4) geometry both in singlet and in triplet states. Triplet symmetrically bridged 8 and triplet cis dilithioethylene (6) are indicated to be local minima, but are less stable than singlet trans 7a. Cis-dilithioethylene (6) is indicated to be thermodynamically unstable towards dissociation into Li2 and acetylene. This factor, and the low lying triplet state, may contribute to the experimental difficulties in preparing 6. The nature of the carbon–lithium bonding in these molecules is discussed. We find that an electrostatic model cannot account for the structural details or for the relative energies of the C2H2Li2 isomers. The importance of the multicenter covalent nature of bonding to lithium is emphasized.

Ab-initio-Berechnungen unter Heranziehung isodesmischer Reaktionen zeigen, das σ-Donor- und π-Akzeptorgruppen X in H3P = CHX wie z. B. X. = Li, BeH und BH2 Phosphoniumylide stabilisieren. Der anionische Charakter (π-Elektronendichte) des ylidischen C-Atoms liegt zwischen dem von Olefinen und substituierten Carbanionen. π-Akzeptoren delokalisieren die Ladung. Die Polarisation durch die PH3-Gruppe ermoglicht eine Stabilisierung des Anions H3P = CH− und von Yliden mit elektropositiven Gruppen X wie z. B. Li, die ein planares ylidisches C-Atom bevorzugen, wahrend die Stammverbindung (X = H) nicht planar ist, jedoch eine kleine Inversionsbarriere von 0.8 kcal/mol besitzt. Letztere wird in H3P = CHF auf 2.8 kcal/mol erhoht. Gute π-Donatoren wie OH und NH2, die Singulett-Carbene stabilisieren, destabilisieren Phosphoniumylide, die dadurch eine erhohte Dissoziationstendenz zeigen. Solche Ylide konnen als Komplexe zwischen Phosphanen und Carbenen aufgefast werden. Es wird zwischen Onium- und At- Yliden unterschieden. In letzteren sind die aufgefuhrten Stabilitatseinflusse durch Heterosubstituenten invers. Phosphine Alkylenes, 47 Ab-initio calculations, evaluated by means of isodesmic equations, show that σ-donor and π-acceptor groups, e.g. X = Li, BeH, and BH2, in H3P = CHX stabilize phosphonium ylides. The anionic character (π-density) of the ylidic carbon is intermediate between that of olefins and of substituted carbanions, and π-delocalization by π-acceptor substituents is effective. Polarization of the PH3 group also leads to stabilization of the anion H3P = CH and of corresponding ylides with electropositive substituents, e. g. Li. These favor planar carbon centers; the parent ylide has a nonplanar PCH2 group with a small (0.8 kcal/mol) inversion barrier. This barrier is enhanced somewhat (to ca. 2.8 kcal/mol) in H3PCHF. Since good π donor substituents like OH and NH2, stabilize singlet carbenes but not the ylides, such ylides have an enhanced tendency to dissociate, in agreement with experimental observations. Indeed, such phosphonium ylides can be considered to be carbene complexes of PR3. One can differentiate between onium and ate ylides; the latter are characterized by inverted substituent stabilization effects.

Due to electrostatic interactions and a contribution of multicenter bonding involving lithium 2s and 2p valence orbital contributions to cyclic 6π delocalization, the doubly bridged cis structure 1 for the title compound is more stable than other alternatives.

The energetic and structural relationships among various C2H2NM isomers (M = Li, Na, MgH, and the anion) are explored by theoretical calculations. The cyanomethyl anion, CH2CN-, is predicted to have a pyramidal structure but with a very low inversion barrier of 0.3 kcal/mol (MP2/6-3l+G*//6-3l+G*). At the same level of ab initio theory, a significantly higher barrier of 2.8 kcal/mol is found for the isocyanomethyl anion, CH2NC-. r-Delocalization in CH2CN- results in a large stabilization relative to CH3- (calculated 45.3, experimental 44.4 kcal/mol). In contrast, the CH2NC- anion is stabilized (by 36.3 kcal/mol relative to CH3-) largely inductively by the sp-hybridized nitrogen. Extensive ab initio examination of the lithium, sodium, and magnesyl (MgH) derivatives of these anions (as well as those of isomeric structures) found lithium-bridged and, to a somewhat lesser extent, sodium-bridged geometries 7 and 12 to be favored. The magnesyl derivatives, 1 and 12, have more covalent character and nearly classical structures. The metallated ketenimine forms, M-N=C=CH2 (2, M = Li, Na, MgH), are next lowest in energy. The metallated nitrile ylides, M-C=N=CH2, are substantially higher in energy than the bridged forms but also show the geometrical variations associated with increasing ionic character. Other C2H2NM isomers, including metallated aminoacetylenes and carbene complexes, are energetically unfavorable. The energy orderings of the acyclic C2HlNM isomers also reflect the relative stabilities of the parent systems (in kcal/mol), e.g., CH3CN (O.O), CH3NC (+20.8), H2C=C=NH (+35.3), HCC-NH2 (+51.4), HC=N==CH2 (+69.0), at the 6-31G*//6-31G* level. More revealing are the stabilization energies relative to CH3M or to NH2M, depending on the principal site of metal attachment of the metal derivatives based on the parent C2H3N structures. Thus, the amino (vs. NH2M) stabilization energies, 40.9 kcal/mol for Na-N=C=CH2 and 32.4 kcal/mol for Li-N=C=CH2 (3-21G//3-21G), are 70% and 56% those of the free anion; the value for M=MgH (23.3 kcal/mol) is decreased even further. Likewise, the methyl stabilization energies (vs. CH3M) of the CH,(M)CN forms (Li, 21.5; Na, 23.2; and MgH, 8.7 kcal/mol) are all substantially less than that of the free anion, 45.3 kcal/mol (CHICN- vs. CH3-). Similar attenuation is found for the CH2(M)NC species vs. the CHINC- anion. Aggregation and solvation effects were probed by MNDO calculations. The most stable unsolvated dimers of both LiCH2CN and LiCH2NC are indicated to prefer eight-membered-ring structures, but solvation favors the alternative four-membered-ring N-lithiated ketenimine dimer form of the former, which may be the structure in the solid state.

The title compounds have been studied by using both molecular mechanics (MMI and MM2) and molecular orbital (MINDO/3, MNDO, and ab initio (3-21G basis)) calculations. The optimum structure is found to be planar in each case, although the energy required for nonplanar distortion is small and decreases regularly with the degree of benzannulation. The large puckering amplitudes to be expected explain the experimentally observed behavior. 4 figures, 1 table.

Durch Addition von Lithium an 2,2,7,7-Tetramethyl-3,5-octadiin (9) in Tetrahydrofuran (THF) wurde 3,6-Dilithio-2,2,7,7-tetramethyl-3,4,5-octatrien (10) als Bis-THF-Addukt kristallin erhalten. Das Dilithiobutatrien 10 erwies sich bei der Rontgenstrukturanalyse als Dimeres des cis-Isomeren mit zwei unterschiedlichen Sorten von Lithiumatomen und echter Butatriengeometrie (19). MNDO-Berechungen an Modellen ohne und mit Solvensmolekulen zeigen eine bemerkenswerte Ubereinstimmung mit dem Experiment und geben Aufschlus daruber, warum die beobachtete Struktur vor anderen Geometriealternativen bevorzugt ist. Preparation of Cumulenes by Lithium Metal Addition to 1,3-Diynes. X-Ray Structure of [3,6-Di(lithio-THF)-2,2,7,7-tetramethyl-3,4,5-octatriene]2, a 1,4-Dilithiobutariene Dimer Addition of lithium metal to 2,2,7,7-tetramethyl-3,5-octadiyne (9) in tetrahydrofuran (THF) gave 3,6-dilithio-2,2,7,7-tetramethyl-3,4,5-octatriene (10), as a crystalline bis-THF adduct. X-ray analysis revealed the dilithiobutatriene 10 to have a dimeric cis-structure with two different kinds of lithium atoms and a true butatriene geometry (19). Model MNDO calculations (both with and without solvation) agree remarkably well with experiment and help reveal why the observed structure is preferred over alternative geometrical arrangements.

Etude de la stabilite et de la structure des α,ω-dilithioalcanes par des methodes MNDO et ab initio (theorie de Hartree-Fock); l'instabilite des dilithio-1,2 ethane et dilithio-1,3 propane est attribuee a de faciles eliminations-transpositions intermoleculaires au profit de complexes LiH plus stable thermodynamiquement

Three C 2 Li 6 isomers characterized by triple (III), double (VII), and single (VI) CC bonds are revealed by minimal basis set ab initio calculations to be favorable minima on the singlet potential energy surface.

The consequences of negative (anionic) hyperconjugation (interactions of orbitals of π-symmetry of saturated groups with filled p orbitals) upon geometries, energies, and charge distributions are calculated for anions and related neutral systems using the ab initio split valence basis sets, 3−21+G and 4−31+G, which are augmented by a set of diffuse s and p functions on the non-hydrogen atoms. The β-fluorethyl anion, which has often been used for analysis, is not a minimum on the potential energy surface. Essentially complete transfer of negative charge to fluorine leads to rupture of the C-F bond and formation of a hydrogen bonded F− ethylene complex, 10, during geometry optimisation. The effects of fluorine negative hyperconjugation, assessed by using the trifluorethyl anion, are in accord with the implications of the “no-bond” formalism for negative hyperconjugation: the C-C bond is shortened by 0.10 A and the C-F bond antiparallel to the carbanion lone pair is elongated by 0.13 A compared to trifluoroethane. Similarly large geometrical differences are found when β-amino-and β-hydroxyethyl anion are compared with ethylamine and ethanol. The total anion stabilisation energies are quite large. The inductive contributors, evaluated using conformations in which the carbanion lone pair cannot interact hyperconjugationally with the C-X bond, are 11.3, 10.3 and 5.3 kcal mol-1 for F, OH and NH2 in β-ethyl anions. The hyperconjugative contributions to the total effect are of similar magnitude. The potential rotational energy surfaces of fluoromethyl amine and aminomethanol also demonstrate the importance of negative hyperconjugation in neutral systems (anomeric effect). Unsaturated olefinic systems, e.g. the 2-fluorovinyl and 2,2-difluorovinyl anions, also show the expected C-F bond lengthening and CC bond shortening. The planar inversion barrier of the 2,2-difluorovinyl anion (11.9 kcal mol-1; 3−21+G//3−21+G) is lowered from that of the vinyl anion due to hyperconjugative stabilisation of the transition state. Fluorine negative hyperconjugation is also illustrated in the stabilisation of the rotational transition states of species like F2BCH2- and F2AlCH2-. Negative (anionic) hyperconjugation and the anomeric effect are the same phenomena from a qualitative molecular orbital viewpoint. Both are well established, important, and uncontroversial. We conclude that negative (anionic) hyperconjugation is of great significance in organic chemistry. Energetically, hyperconjugation is generally as important as the inductive effects of β-electronegative substituents. Rather large hyperconjugative stabilisations, on the order of 10kcalmol-1 for an optimally aligned substituent, are indicated. In such alignments, charge transfer from a carbanion lone pair to an electronegative group can play a significant role; polarisation may be operative in addition to or in place of charge transfer. There is a concerted set of geometrical changes which correspond to expectations based on “no bond resonance” formulations (2). Anomeric effects in neutral molecules, e.g. NH2CH2F, are similar: energetic stabilisation, conformational dependence, geometrical differences, and charge transfer. Negative hyperconjugation is not controversial; it is firmly established. We underscore Bingham's conclusion,31 “anionic hyperconjugation is, indeed, a very real, general, and important concept”.

Calculs ab initio sur les composes MCH 2 X avec M=Li ou Na et X=CH 3 , NH 2 , OH, F, SiH 3 , PH 2 , SH ou Cl, avec corrections de correlation electronique. Comparaison avec especes anioniques correspondantes

Gas-Phase Oxidation (“Charge Stripping”) of Pagodane Radical Cations and Related Cage Hydrocarbons. — A Theoretical-Experimental Search into Tetramethylene Dications A theoretical screening of the family of tetramethylene dications furnished the open C2htrans configuration as the global minimum. The pagodane dications must owe their stability to the rigidity of their molecular framework. By means of charge-strip-Ping mass spectrometry stable dications could be generated for [1.1.1.1]pagodane, its diene valence isomer, and several analogues (including „half-pagodanes”). Ionization energies for the gas-phase oxidation of the radical cations were determined. A distinction whether common or isomeric radical cations serve as precursors in the charge stripping reactions could not be achieved due to either the relatively large uncertainties in the determination of the Qmin values or due the fact that in these particular cases charge stripping does not discriminate between structurally closely related hydrocarbon isomers.

Ab initio and MNDO molecular orbital calculations support experimental evidence that 2-metallo-substituted cyclopropyl radicals prefer cis- while 2-halogenocyclopropyl radicals prefer trans-configurations.

Two symmetrically doubly lithium-bridged structures for dilithiated propene,CH2 CHCHLi2(3) and (4), are indicated to be comparably stable by ab initio(3-21G//3-21G) calculations.

Hyperconjugation and inductive effects, rather than homoaromaticity, are responsible for the stabilization of the title anion in the gas phase; interaction of the double bond with the Li+ gegenion in the endo geometry contributes additionally in solution.

While the methyl stabilisation energies of fifteen C1–C3 organolithium compounds, RLi, reflect (attentuation factor = 0·71) the methyl stabilisation energies of the corresponding anions, R–, deviations are found when the degree of charge localisation or delocalisation is changed significantly by association with lithium.

Symmetrical (C2v) double bridging, commonly exhibited by 1,4-dilithium compounds e.g. in (1) and (2), is not favoured in (4) and (5) since charge deloclization in the dianion, hybridization, and orbital orientation effects are more important than quadrupole-like electrostatic interactions (3).

An exceptionally large 1,2-hydride shift barrier, over 40 kcal/mol, is indicated by high-level theoretical calculations for the phenyl cations.

Les effets des substituants de la premiere ligne Li, BeH, BH 2 , CH 3 , NH 2 , OH et F sur les structures et la stabilite des boranes, borirenes et borirannes sont etudiees par les methodes ab initio

Ab initio and semi-empirical MNDO calculations indicate that monomeric dilithiated and bis-magnesyl derivatives of carboxylic acids (Ivanoff reagents) favour the 1,3-1,3′ doubly bridged structures generally found in metallated Y-conjugated dianion systems.

La forme chirale avec conjugaison en forme de Y est la plus stable des 5 structures calculees