Substituenteneffekte auf die C–C-Bindungsstärke, 8. Standardbildungsenthalpien und Spannungsenthalpien von 1,1,2,2-Tetraethylethylenglykol-dimethylether und D,L-1,2-Dimethyl-1,2-diphenylethylenglykol-dimethylether

Effects of Substituents on the Strength of C - C Bonds, 8¹. - Heats of Formation and Strain of 1,1,2,2-Tetraethylethylene Glycol Dimethyl Ether and D,L .-1,2-Dimethyl-l,2-diphenylethylene Glycol Dimethyl Ether The heats of combustion of the title compounds 1 and 2 were measured calorimetrically with the result (kcal mol ^-1, s. d. in parentheses) ΔH°_c = − 1880.1 (± 0.6) and − 2373.3 (± 1.4). The heat of vaporisation of 1 ΔH^v = 14.3 (± 0.3) and the heat of sublimation of 2 ΔH^sub = 27.2 (± 0.5) were derived from their temperature dependance of the vapor pressure. The latter were determined between 30 and 80°C using a flow method. The resulting standard heats of formation ΔH°_t(g) = −122.4 (± 0.7) and −43.8 (±1.5) for 1 and 2 correspond to a strain enthalpy (_s) of 15.9 and 8.0 kcal mol^-1, respectively. The steric strain of the dimethoxyethanes 1 and 2 is about one fourth lower than the strain of the corresponding dimethylethanes 3 and 4 bearing the same substituents. Thus, a methoxy group causes less steric stress than a methyl group.     

Carbenoid Complexes of Electron-Deficient Transition Metals—Syntheses of and with Short-Lived Building Blocks

The relation of thermodynamic stability and kinetic lability of σ-organometallic compounds of transition metals, together with an improved understanding of the subtle interactions between central metal, ligands, and substrates, has increased the chemist's ability to plan organometallic syntheses. This article presents new results on intermediary and isolable synthetic building blocks incorporating metal–ligand multiple bonds of electron-deficient transition metals; the main emphasis will be placed on compounds with titanium–carbon double bonds. This particular class of compounds is mainly generated by H-transfer reactions starting from readily accessible alkyl and alkenyl derivatives. The preparative use of [L₂Ti(CHR₂)R′] derivatives as sources for [L₂Ti?CR₂] intermediates will be discussed, as well as the nature of these intermediates. Application of the same approach to vinyltitanium compounds [L₂Ti(CH?CH₂)R] opens up an access to a short-lived metallaallene derivative [L₂Ti?C?CH₂] of an electron-deficient transition metal. The reactivity of these synthetic building blocks is mainly characterized by the nucleophilic properties of the α-C atoms as well as by the spatial orientation of the π-bonding planes. Numerous cycloaddition products with unsaturated substrates could be isolated and characterized for the first time by using [L₂Ti?C?CH₂] intermediates. Hence it is possible to compare the properties of a multitude of metallacyclic ring systems with those obtained from “Tebbe–Grubbs chemistry”, and in this context, the dependence of the properties of metallacyclic four-membered rings on the substitution pattern is discussed. This class of compounds includes the metallaoxetanes, which have been obtained for the first time by the cycloaddition of the [CpTi?C?CH₂] intermediate with cumulenes and metal carbonyls. The differing cycloreversion behavior of these metallaoxetanes enables the differentiation of species exhibiting classical and nonclassical reactivity. The number and position of the exocyclic double bonds are the determining factors of the reactivity of the formed metallacycles. The discussion of the products obtained from titanium methylene and vinylidene building blocks is an up-to-date report on the formation and applications of carbene complexes and carbene intermediates of group 4 metals.     

Substituent Effects on the C-C Bond Strength, 15[1] Geminal Substituent Effects, 7[2]. Thermochemistry and Thermal Decomposition of Alkyl-substituted Tricyanomethyl Compounds

The thermolysis reactions of the tricyanomethyl compounds 10a-c were studied in solution. 2,2-Dicyano-3-methyl-3-phenylbutyronitrile ( 10a ) and 2,2-dicyano-3-methyl-3-(4-nitrophenyl)butyronitrile ( 10b ) decomposed heterolytically into carbenium ions and (CN)₃C⁻ anions, while 9-methyl-9-(tricyanomethyl)fluorene ( 10c ) underwent about 11% homolytic C-C bond cleavage into 9-methyl-9-fluorenyl- and tricyanomethyl radicals. The rates of the homolysis were determined by a radical scavenger procedure under conditions of pseudozero order kinetics. From the temperature effect on the rate constants the activation parameters were determined [ΔH ( 10c ) = 155· 2 kJ mol⁻¹, ΔS ( 10c ) = 58· 5 J mol⁻¹ K⁻¹]. Standard enthalpies of formation ΔH (g) were determined for 2,2-dicyanopropionitrile ( 2 ) (422.45 kJ mol⁻¹), 2,2-dicyanohexanenitrile ( 3 ) (349.74 kJ mol⁻¹), 2,2-dicyano-3-phenylpropionitrile ( 4 ) (540.75 kJ mol⁻¹), 2-butyl-2-methylhexanentrile ( 5 ) (-133.20 kJ mol⁻¹), 2,2-dimethylpentanenitrile ( 6 ) (-45.78 kJ mol⁻¹), and 2-methylbutyronitrile ( 7 ) (2.44 kJ mol⁻¹) from the enthalpies of combustion and enthalpies of sublimation/vaporization. From these data and known Δ (g) values for alkanenitriles and -dinitriles, thermochemical increments for ΔH (g) were derived for alkyl groups with one, two, or three cyano groups attached. The comparison of these increments with those of alkanes reveals a strong geminal destabilization, which is interpreted by dipolar repulsions between the cyano groups. - From ΔH (g) of 10c and ΔH of its homolytic decomposition the radical stabilization enthalpy for the tricyanomethyl radical 1 RSE ( 1 ) = -18 kJ mol⁻¹ was determined. Thus, 1 is destabilized, in comparison with the RSEs of tertiary α-cyanalkyl (23 kJ mol⁻¹) and α,α-dicyanoalkyl (27 kJ mol⁻¹) radicals, which were recalculated from bond homolysis measurements^[4] and the new thermochemical data. This change of RSE on increasing the number of α-cyano groups is discussed as the result of the additive contributions by resonance stabilization and increasing destabilization by dipolar repulsion. The amount of the dipolar energies was estimated by molecular mechanics (MM2).     

Thermochemical study of aliphatic and phenyl-substituted geminal diacetates

The standard enthalpies of combustion _c H ^o of aliphatic diacetates1 and aromatic diacetates2 were measured calorimetrically. The enthalpies of vaporization _vap H ^o or sublimation _sub H ^o of1 and2 were obtained from the temperature function of the vapor pressure measured in a flow system. From _f H ^o(g) of1 and2 new values of group increments for the estimation of standard enthalpies of formation of these classes of compounds were derived. The geminal interaction energy between the geminal acyloxy groups shows no anomeric stabilization.Geminal Substituent Effects, Part 12, for part 11 see Ref. 7.     

Titanium-based molecular squares and rectangles: syntheses by self-assembly reactions of titanocene fragments and aromatic N-heterocycles

This paper reports on the potential of titanium compounds as building blocks for supramolecular polygons. Self-assembly reactions of low-valent titanocene units and N-heterocyclic bridging ligands lead to novel titanium-based supramolecular squares. Pyrazine (3), 4,4'-bipyridine (4), and tetrazine (5) were used as bridging ligands, and the acetylene complexes [Cp2Ti{eta2-C2(SiMe3)2}] (1) and [(tBuCp)2Ti{eta2-C2(SiMe3)2}] (2) as sources of titanocene fragments. Molecular rectangles can be synthesized by stepwise reduction of the titanocene dichlorides [Cp(2)TiCl2] and [(tBuCp)2TiCl2] and consecutive coordination of two different bridging ligands. The resulting complexes are the first examples of molecular rectangles containing bent metallocene corner units. Single-crystal X-ray analyses of the tetranuclear compounds revealed the geometric properties of the molecular polygons in the solid state. Comparison of bond lengths and angles in coordinated and free ligands reveals the reduced state of the bridging ligand in the low-valent titanium compounds. The syntheses and properties of these novel, highly air- and moisture-sensitive compounds are discussed.     

Heats of formation of [2.2]paracyclophane-1-ene and [2.2]paracyclophane-1,9-diene - an experimental study

The enthalpies of formation [Delta(g)] of tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),10(14),11-heptaene (2, 1,2-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1-ene) and tricyclo[8.2.2.2(4,7)]hexadeca-1(13),2,4(16),5,7(15),8,10(14),11-octaene (3, 1,2,9,10-dehydro[2.2]paracyclophane or [2.2]paracyclophane-1,9-diene) have been determined by measuring their heats of combustion in a microcalorimeter and their heats of sublimation by the transpiration method. Values of the strain energies (SE) [SE(2) = 34.7 kcal mol(-)(1), SE(3) = 42.0 kcal mol(-)(1)] have been derived from the gas-phase heats of formation and are compared with those from MM3 and PM3 calculations and with the corresponding value SE(1) = 30.1 kcal mol(-)(1) for the parent tricyclo[8.2.2.2(4,7)]hexadeca-1(13),4(16),5,7(15),10(14),11-hexaene (1, [2.2]paracyclophane). The higher strain energies of 2 and 3 (by 4.6 and 11.9 kcal mol(-)(1)) are in accord with the well-known increased reactivities of their aromatic rings as a consequence of their increased bending. As revealed by an X-ray crystal structure analysis, the bending in the monoene 2 corresponds to that of 1 and 3 at one of two bridging corners.     

Thermolabile kohlenwasserstoffe. XXVI. Bildungsenthalpie von 1,1,2,2-tetra-tert-butylethan

The title compound (1) was prepared in high purity by reducing 3,3-dibromo-2,2,4,4-tetra-methylpentane (2) with magnesium in the last step. The heat of combustion, ΔH⁰_c(c), of 1 was measured using an aneroid isoperibol microcalorimeter and the heat of sublimation, ΔH_sub, was obtained from the vapour pressure (35–93°C) measured in a flow system. The results: ΔH⁰_c(c) = ?2913.3(±0.9), ΔH⁰_f(c) = ?77.6(±0.9) and ΔH⁰_f(g) = ?59.9(±0.9) kcal mol^?1 lead to an outstandingly high value for the excess strain enthalpy (H_s = 66.3 kcal mol^?1) revealing strong van der Waals repulsions in this highly crowded alkane.