Steric Effects on Reaction Rates. Part IX. Force-Field Parameters for Bridgehead and Rigid Tertiary Carbenium Ions

Molecular-mechanics calculations for strain of carbenium ions are tested using Bentley's unified reactivity scale for bridgehead solvolysis as reference. Excellent correlations are obtained for solvolytic bridgehead reactivity with the calculated steric-energy difference (ΔE_st) between substrate (R? H or R? OH) and cation (R⁺). After adjustment of appropriate force-field parameters, the approach is successfully extended to the rigid, but planar cations derived from structures 15 – 20 ; however, the general set of parameters cannot be applied to highly strained systems such as the cation formed from 17 . With all of the 18 sets of parameters tested, the 2-endo-norbornyl derivative 16 , is adequately correlated, while the exo isomer 15 exhibits enhanced reactivity by a factor of ca. 10² to 10³.     

Probing the anomeric effect. The diaminomethylene group: Calculations of N?C?N-containing molecular systems1

A study of systems containing the title moiety is described, with special reference to the anomeric effect. We have calculated ab initio, using Gaussian-80 with the 3-21G basis set, all basic conformations of methylene-diamine (H₂N? CH₂? NH₂) and its N-methyl derivative with full geometry optimization of energy minima and barriers. The structural data thus obtained, were then employed to parameterize Allinger's MM2-80 force field in a procedure similar to that described for oxygen derivatives, including hydrogen-bonding effects and C? N bond shortening in tertiary amines. This modification, termed MM2-AE was then used to calculate larger molecules, including N,N′-di- and N,N,N′,N′-tetramethyl-methylenediamine, various 1,3-diazane systems, and 1,4,5,8-tetraazadecalin derivatives of established (x-ray) structures. The results are discussed in light of their verificative and predictive power and appear to validate MM2-AE as a useful computational procedure.     

Steric gamma effect on 29Si shielding in (CH3)3Si?O?C?R groups

It is shown that a γ-methyl group causes approximately a 3 ppm upfield shift of the silicon resonance in (CH₃)₃Si? O? C? CH₃ fragments. It is estimated that polar effects contribute about 0.8 ppm to this shift leaving a net steric γ upfield shift of 2 ppm. Variations around these average values are interpreted by different conformer populations, the importance of which are illustrated by the shifts in trimethylsiloxy derivatives of adamantane.     

Steric effects on reaction rates. XI. Solvolysis of tertiary carbon substrates rationalized by molecular mechanics calculations

The steric energy difference (ΔE_st) between tertiary carbenium ions (R⁺) and the corresponding alcohols has been calculated by MM2 for a series of tertiary nonbridgehead substrates and correlated with their rate of solvolytic reactivity. Satisfactory correlation is obtained, except for p-nitrobenzoates of highly congested substrates. The slope and intercept of the correlations remain almost unchanged if bridge-head substrates are included in the plot. However, the quality of the fit is better for bridgehead substrates alone.     

In search of 1,3-disila/germa/stannabicyclo[1.1.1]pentanes with short bridgehead-bridgehead distances and low ring strain energies

The strain energies and through-space distances between the two bridgehead E atoms of a selection of 1,3-dimethyl-1,3-ditetrelbicyclo[1.1.1]pentanes (tetrel E = Si, Ge or Sn) were examined by quantum chemical calculations at MP2 and B3LYP levels. The aim is to identify which bridges lead to short through-space E,E distances, and simultaneously, to as low strain as possible. A short E,E distance should improve through-space interaction, and a low strain should promote the thermal stability and possibly also facilitate their synthesis. The bridges examined included CH₂, CMe₂, CtBu₂, C(CH₂)n (n = 2–4), O, NMe, S, PMe, SiMe₂, GeMe₂, and SnMe₂. The calculations indicate that the phospha bridge is a good compromise providing reasonably low strain as well as E,E through-space distances which are only longer than normal E–E single bonds by factors of 1.06–1.10. This paper is dedicated to Professor Mitsuo Kira in recognition of his stimulating Si chemistry and his 2005 Wacker Award.     

Planarized Phenyldithienylboranes: Effects of the Bridging Moieties and π-Extension on the Photophysical Properties and Lewis Acidity

Two kinds of planarized phenyldithienylboranes, which contain (CH₃)₂C- or CH₂-bridging moieties, were synthesized. The difference of the bridging moieties affects their packing structures and photophysical properties. In particular, the (CH₃)₂C-bridged derivative exhibits a large Stokes shift, unusual for such planarized compounds, that results from a large structural relaxation in the excited state. A series of π-extended derivatives was synthesized, among which a p-(diphenylamino)phenyl-substituted derivative shows large solvatochromism in the fluorescence spectra, while maintaining high quantum yields even in polar solvents. The Lewis acidity of the phenyldithienylborane derivatives was also assessed by titration with pyridine. The Lewis acidity of the boron center is affected not only by the difference in the steric bulk of the bridging moieties, but also by the electronic effect of the substituents introduced at remote positions relative to the boron atom. These results demonstrate the characteristic features of planarized phenyldithienylboranes as building blocks for boron-based π-electron materials.     

A quantum-mechanical study of the inductive and steric effects for the example of <Emphasis Type="Italic">tert</Emphasis>-butylalkanes

A relation between the inductive and steric effects (concepts of the classical theory of molecular structure) and electron density distribution parameters was revealed by quantum chemistry methods. The propagation of the inductive and steric influence of the (CH₃)₃C and CH₃ groups along carbon chains in tert-butylalkanes was analyzed. The steric effect of the (CH₃)₃C group was shown to extend to two CH₂ groups, and the inductive effect, to one CH₂ group. Definitions of the standard (CH₃)₃CCH₂ and (CH₃)₃C(CH₂)₂ groups were given. Configurations of transferable groups decreasing additive approach errors were suggested. Additive procedures for calculating the properties of molecules containing (CH₃)₃C were refined.     

Symmetrical and lopsided zirconocene pro-catalysts

Me₂C[1-Cp-9-Flu]ZrCl₂ derivatives with H-, CH₃-and (CH₃)₃C- substituents β to the Cp bridgehead carbon atom as well as complexes having mono- or di-substituted fluorenyl ligands (R = CH₃-, (CH₃)₃C-, CH₃O-, CH₃OCH₂-, (CH₃)₃CC≡C-, (CH₃)₂N-, F-, and Cl-) have been investigated as propylene polymerization pro-catalysts. Steric effects from the β-Cp substituents determine the iso-, hemi-iso-, and syndio-specificities of the catalysts. The relative stereospecificities of Me₂Si[1-Cp-3t-Bu-9-Flu]ZrCl₂ and rac-Me₂Si[1-Ind-3-t-Bu]₂ZrCl₂ are in accord with molecular models. The more un-symmetrical the catalysts are the more m and mm stereosequences there are in s-PP. The m dyads also increase with the concentration of Me₂C[1-Cp-9-Flu-2-OMe]ZrCl₂.     

Steric Effects on Reaction Rates. IV. Evaluation of the Ketone Model for the Solvolysis Transition State of Secondary p-Toluenesulfonates

The rates of solvolysis of secondary p-toluenesulfonates in acetic acid or 97% trifluoroethanol are interpreted in terms of strain changes between substrate and the corresponding ketone. Such strain changes are obtained from force-field calculations (ΔE_st) and from equilibration of alcohols and ketones (ΔG_ox). This simple model reproduces the behaviour of substrates reacting by kc-pathways to afford unstrained carbenium ions. Anchimeric assistance and leaving group hindrance in the transition state are recognized in clear-cut cases by deviations from the expected reactivity. However, the model breaks down when highly strained carbenium ions of the cyclobutyl or 7-norbornyl type are involved.     

Activating effects of unsaturated and cyclopropyl groups in gas phase pyrolysis of acetates. Paper XVIII

The activating effects of a number of unsaturated groups and a cyclopropyl group have been evaluated in a solvent free system by determining the absolute rate constants, and energies and entropies of activation in the vapor phase pyrolysis of secondary and tertiary esters of the type RC(R′CH₃) OAc where R′ = H or CH₃ and R = c-Pr, i-Pr, CH₃, CH₂?CH, CH₂?CHCH₂, C₆H₅; the cyclopropyl showed only a moderate activating effect. The results are in contrast to the very significant activating effect of a cyclopropyl group in solvolysis of cyclopropylcarbinyl derivatives. Apparently marked activation by this group occurs only when a highly developed positive center forms adjacent to it. The lack of marked activation by the cyclopropyl group supports a mechanism for ester pyrolysis which involves a modest, but detectable, charge separation in the transition state [2] but questions a mechanism in which an intimate ion-pair was proposed [3].     

Steric Effects on Reaction Rates. VI. Application of a New MM2 Force-Field for Carbenium Ions to Solvolysis Rates of Secondary p-Toluenesulfonates

An empirical force-field for carbenium ions has been incorporated in Allinger's MM2 programme. Structural parameters of secondary carbenium ions calculated by this method are compared with those obtained with Schleyer's BIGSTRN calculations. The strain changes occurring upon solvolysis of secondary p-toluenesulfonates are evaluated by means of this force-field and correlated with the rate constants for solvolysis. The equation for correlation of acetolysis, relative to cyclohexyl p-toluenesulfonate, of 28 k_c substrates is ΔG = 0.67 ΔE_st - 0.20 (r = 0.958).     

Synthesis of 3-alkyl-6-phenyl-4(3H)-pteridinones and their 8-oxides. Potential substrates of xanthine oxidase

Synthetic routes for the preparation of 3-alkyl-6-phenyl-4(3H)-pteridinones 6 and their corresponding 8-oxides 5 (R = CH₃, C₂H₅, (CH₂)₂CH₃, (CH₂)₃CH₃, CH(CH₃)C₂H₅, CH(CH₃)₂ and CH(C₂H₅)CH₂OCH(OC₂H₅)₂ are described and their reactivities towards xanthine oxidase from Arthrobacter M-4 are determined. Only the 3-methyl derivative of 6-phenyl-4(3H)-pteridinone and its 8-oxide i. e. 6a and 5a are found to be substrates although their reactivities are still very low. Oxidation takes place at C-2 of the pteridinone nucleus. All the 3-alkyl derivatives are less tightly bound to the enzyme than 6-phenyl-4(3H)-pteridinone. Introduction of the N-oxide at N-8 considerably lowers the binding of the substrates. Inhibition studies have revealed that 3-methyl-6-phenyl-4(3H)-pteridinone ( 6a ) is a non-competitive inhibitor with a Ki-value of 47 μM and the 3-ethyl derivative ( 6b ) an uncompetitive one with a Ki-value of 19.6 μM.     

Recherches sur la formation et la transformation des esters LXXIX [1]. Sur la réaction de l′isothiocyanate d′o-méthoxycarbonyl-phényle avec divers acides et l′acide sérinephosphorique

Amino acids devoid of «leaving groups» on their β carbon atom (neither ? OH: serine, nor ? SH: cysteine) react with o-methoxycarbonyl-phenyl isothiocyanate (I) in the presence of one equivalent of NaOH, in water-dioxane or in ethanol, to yield the corresponding hydroquinazolinone derivatives II which were isolated as free acids. When treated with CH₃COOH + conc. HCl the hydroquinazolinone resulting from the reaction of L -serine with I undergoes a nucleophilic attack of the carbon bearing the leaving group ? OH by a lone pair of electrons of S; this attack produces the formation of an additional thiazolidine ring, yielding the thiazoloquinazolinone derivative (?)-III. In an analogous reaction DL -serine phosphoric acid treated with I at pH 8–9 yields the corresponding substituted hydroquinazolinone which, boiled in 1N hydrochloric acid, undergoes ring closure to (±)-III by the same mechanism as the serine derivative (leaving group: ? OPO₃H₂). L -Cysteine reacted with 2 equiv. of I and then treated with CH₃COOH + conc. HCl gives two products: (?)-III produced by the same mechanism as for serine (leaving group: ? SH), and the quinazolinone derivative IV where the ? SH group is also thiocarbamoylated.     

The influence of some steric and electron effects on the mechanism of aromatic nucleophilic substitution (SNAr) reactions in nonpolar solvent

Kinetic studies are reported for the reactions with aniline in benzene of a series of X‐phenyl 2,4,6‐trinitrophenyl ethers [X = H; 2‐, 3‐, 4‐CH₃; 2,4‐, or 2,6‐(CH₃)₂] a–f , and the results compared with those of the corresponding nitro derivatives. In the methyl series, kinetic data show that increasing substitution reduces drastically the rates of reactions indicative of the operation of some kind of steric effect. The unfavorable steric congestion at the reaction center appears to be unimportant in determining the kinetic order of the reactions. In general, the second‐order rate constants k_A depend linearly on the square of nucleophile concentration. The change in the kinetic form observed in the nitro derivatives may be largely due to the electron‐withdrawing effect of the group. With the 2,6‐dinitro derivative, however, the uncatalyzed pathway k₂ takes all the reaction flux. Steric hindrance to intermolecular proton transfer from base to the ethereal oxygen of the intermediate is sufficient to make the base‐catalyzed pathway insignificant relative to the k₂ pathway. © 2005 Wiley Periodicals, Inc. Int J Chem Kinet 37: 744–750, 2005     

Living cationic polymerization of vinyl ethers with carboxylic acid/tin tetrahalide initiating systems. II. Effects of steric hindrance of carboxylate counteranions

Effects of steric crowding of the substituent of carboxylate counteranions on living cationic polymerization of isobutyl vinyl ether (IBVE) were investigated with the use of two series of carboxylic acids with various carbonyl substituents [RCOOH; R = (aliphatic series) CH₃CH₂, (CH₃)₂CH, (CH₃)₃C; (aromatic series) C₆H₅CH₂, (C₆H₅)₂CH, (C₆H₅)₃C] in conjunction with tin tetrabromide (SnBr₄) and 1,4-dioxane (DO) in toluene at 0°C. The overall polymerization rate increased with increasing the bulkiness of the substituents R in both the series: R = CH₃ (1) ≃ CH₃CH₂ (1) < (CH₃)₂CH (1.76) < (CH₃)₃C (2.31); C₆H₅CH₂ (0.84) < (C₆H₅)₂CH (0.98) < (C₆H₅)₃C (1.74); the values in the parentheses show the relative polymerization rate. In all the polymerizations, the number-average molecular weight (M_n) of the polymers was directly proportional to monomer conversion and in good agreement with the calculated values, assuming that one RCOOH molecule forms one polymer chain. The living nature of these polymerizations was further confirmed by a linear increase in M_n of the polymers upon sequential addition of a fresh monomer feed to the almost completely polymerized reaction mixtures. In the polymerizations with sterically less hindered carboxylic acids [R = CH₃CH₂, (CH₃)₂CH, C₆H₅CH₂, (C₆H₅)₂CH], the molecular weight distribution (MWD) of the polymers was very narrow (M_w/M_n < 1.1) throughout the polymerizations. In contrast, with bulkier substituent-containing counterparts [R = (CH₃)₃C, (C₆H₅)₃C], the polymerizations led to the polymers of relatively broad MWD (M_w/M_n ≅ 1.5 at ca. 100% monomer conversion). The bulky substituents such as (CH₃)₃C and (C₆H₅)₃C may decrease the interconversion rate between a dormant and an active species and increase the time-average concentration of the active growing species. The stereoregularity of the obtained polymers was not changed much with the steric environment of the counteranion (meso: 66–69%). © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2923–2932, 1999     

Carbon Participation in the Solvolysis of Tertiary Norbornyl Derivatives Norbornanes. Part 15. 6-substituted 2-methylnorbornyl 2-exo- and 2-endo-2,4-dinitrophenyl ethers

The solvolysis rates and products of the tertiary 2-methyl-2-exo- and -2-endo-norbornyl 2,4-dinitrophenyl ethers 1 and 2 , (X = 2,4-(NO₂)₂₂C₆H₃O) have been determined. The different sensitivities of the rates of these ethers to the inductive effect of substituents at C(6) indicate that graded bridging of C(2) by C(6) occurs in the ionization of the exo-ethers 1 , not, however, in the ionization of the endo-ethers 2. In both cases hydrolysis leads to 2-methyl-2-exo-norbornanols only. Consequently, substitution takes place with retention at C(2) in the exo-series 1 and with inversion at C(2) in the endo-series 2. It is concluded that stereoelectronic and polar effects, rather than steric bulk effects, determine the high exo/endo rate ratios of the parent norbornyl derivatives 1a and 2a .     

Polar effects. Part 12. Inductivity and carbon participation in the solvolysis of 4-substituted 2-adamantyl arenesulfonates

The rate constants (log k) for the solvolysis of 4^e-substituted 2^e- and 2^a-adamantyl p-nitrobenzenesulfonates 14 and 15 , respectively, in 80% EtOH correlate linearly with the respective inductive substituent constants σ. Therefore, relative rates are controlled by the I effect of the substituents at C(4). The derived reaction constants, or inductivities, ρ_I of −0.80 and −0.64 for the series 14 and 15 , respectively, are far smaller than those previously determined for 6-substituted 2-norbornyl and 2-bicyclo[2.2.2]octyl sulfonates, in which the partial structure containing the substituent and the leaving group is the same. The ratio of the retained and inverted adamantanols obtained upon hydrolysis of the series 14 falls from 2.85 for R = CH₃ to ca. 1 for R = CN, i.e. as the substituent at C(4) becomes more electron-attracting. In the 2^a-series 15 this ratio is uniformly higher. These findings confirm that the 2-adamantyl cation is weakly bridged and that through-space induction in carbocations involves graded bridging of the cationic center by neighboring C-atoms.     

Steric effects on reaction rates - V. An updated version of the foote-schleyer correlation based on molecular mechanics

The strain changes occurring during solvolysis of secondary tosylates are calculated by a molecular mechanics programme (MM2) using an empirical force-field for carbenium ions. The rate constants for acetolysis of k_C-substrates correlate with these strain changes, defining a straight line representative for k_C behaviour.     

Silicon‐modified surfactants and wetting: V. The spreading behaviour of trimethylsilane surfactants on energetically different solid surfaces

The spreading behaviour of defined trimethylsilane‐based surfactants of general formula (CH₃)₃Si(CH₂)₆(OCH₂CH₂) _nOCH₃, n = 2–6, on five different solid surfaces at 21 °C has been investigated. Compounds bearing short diethylene and triethylene glycol hydrophiles do not spread. For the longer‐chained tetraethylene to hexaethylene glycol derivatives, the ability to spread depends on the surface energy. Rapid spreading is restricted to the slightly polar surface of 40 mN m⁻¹ surface energy. Lower or higher surface energies considerably reduce the spreading rates. The phase behaviour of the solutions substantially influences the spreading process. The dispersed systems of the tetraethylene glycol derivative spread constantly over long time intervals. The dispersions of the pentaethylene glycol analogue are very close to the temperature for a transition into the one‐phase state. A retardation of the spreading process occurs after a few seconds. Micellar solutions of the hexaethylene glycol derivative either spread very slowly or stop spreading after a few seconds. The largest spreading areas and highest initial spreading rates were found for the 0.1 wt% solutions. Copyright © 1999 John Wiley & Sons, Ltd.     

Multiorganyltin Compounds. Designing a novel phosphate-selective carrier

A series of distannyl derivatives, 1–9 , were studied for their application as phosphate-selective carriers in polymer-based liquid membranes. A drastically different potentiometric behavior was observed depending on the distance between the tin(IV) coordinating centers and the ligands attached to them. Some of the compounds examined, i.e. (PhSnBr₂)₂CH₂( 1 ) and (Me₃SiCH₂SnCl₂CH₂)₂ ( 3 ) showed very high potentiometric selectivity towards phosphate over other lipophilic anions such as perchlorate and thiocyanate. Results indicate that (1) the optimal number of CH₂ groups between the Sn-centers are either one or three, (2) the electron-withdrawing power of the organic ligands attached to Sn-centers strongly influences the overall response towards phosphate, and (3) the steric effect of the organic substituents is important in the potentiometric selectivity observed.