The Experts below are selected from a list of 90 Experts worldwide ranked by ideXlab platform
Ken'ichi Takeuchi - One of the best experts on this subject based on the ideXlab platform.
-
Structural modification and solvent interactions in solvolytic reactions of open-chain compounds
Pure and Applied Chemistry, 1998Co-Authors: Ken'ichi TakeuchiAbstract:The Grunwald-Winstein type analyses with respect to Ycl scale on the specific rates of solvolysis of extremely crowded tertiary alkyl chlorides having a Neopentyl or a (1-adamanty1)methyl Group at the reaction center show that they behave as k, substrates. The Neopentyl Group is more effective in backside shielding than a tertiary butyl Group. There are now many unprecedented examples that show downward dispersions of aqueous ethanol and aqueous acetone data points relative to fluorinated solvents in the Grunwald-Winstein type relations. The principal cause for such behavior is attributed to diminished rates in water relative to those in nonaqueous solvents. In the solvolyses of (t-BuCHz)Et2CCl and (t-BuCH2)2MeCCl (9), the deviated aqueous ethanol data exhibit a downward convex, enhancing the downward deviations. Secondary mesylates, (t-BuCH2)2CHOMs and (t-BuCH2)( 1 AdCH2)CHOMs, and even l-bromo-3,5,7-triisopropyladamantane behave similarly to congested tertiary open-chain alkyl chlorides. In the solvolysis of 9 in aqueous ethanol, the product selectivity with respect to ethanol is considerably increased compared with the case of tert-butyl chloride. Steric and hydrophobic perturbation to solvation in the ground state and intermediate would provide a reasonable explanation.
-
Structural effects of the Grunwald–Winstein correlations in the solvolysis of some simple tertiary alkyl chlorides
Journal of Physical Organic Chemistry, 1997Co-Authors: Ken'ichi Takeuchi, Yasushi Ohga, Takuhiro Ushino, Masaaki TakasukaAbstract:The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald–Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The Neopentyl Group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl Group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl Groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl Groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl Group. © 1997 John Wiley & Sons, Ltd.
-
The Grunwald‐Winstein type correlation for 2‐chloro‐2,4,4‐trimethylpentane: A simple tertiary alkyl chloride that shows essentially limiting behaviour in solvolysis
Journal of Physical Organic Chemistry, 1996Co-Authors: Ken'ichi Takeuchi, Yasushi Ohga, Takuhiro Ushino, Masaaki TakasukaAbstract:Evaluation of the rates of solvolysis of 2-chloro-2,4,4-trimethylpentane (4) in 17 solvents on the basis of the Grunwald-Winstein type equation [log(k/k0) = lNT + mYCl + c] gives an excellent correlation with l = −0·01 ± 0·02 and m = 0·74 ± 0·01. The Neopentyl Group in 4 more effectively shields the rear side of the reaction center than the tert-butyl Group in 2-chloro-2,3,3-trimethylbutane. The rate ratio between 4 and 2-chloro-2-methylpropane (3) at 25°C is 275 in trifluoroethanol and predicted to increase to 950 in trifluoroacetic acid. The previous 4:3 rate ratio of 22 in 80% ethanol evidently underestimated the B-strain effect on the solvolysis rate of 4 by a factor of at least 40.
Jennifer Seifert - One of the best experts on this subject based on the ideXlab platform.
-
Steric hindrance classified: treatment of isothiocyanatoallene with secondary amines bearing bulky substituents to generate 2-aminothiazoles
Organic Chemistry Frontiers, 2019Co-Authors: Klaus Banert, Jennifer SeifertAbstract:Twenty secondary amines, bearing two secondary, two tertiary, or a secondary and a tertiary alkyl Group or a Neopentyl Group, were treated with allenyl isothiocyanate to prepare the corresponding 2-dialkylamino-5-methylthiazoles. The relative rates of these reactions, which were strongly dependent on the steric demand of the N-alkyl Groups, were measured by using competition experiments and analysis by 1H NMR spectroscopy. Not only steric, but also some electronic effects on the nucleophilicity of the dialkylamines were discussed.
Klaus Banert - One of the best experts on this subject based on the ideXlab platform.
-
Steric hindrance classified: treatment of isothiocyanatoallene with secondary amines bearing bulky substituents to generate 2-aminothiazoles
Organic Chemistry Frontiers, 2019Co-Authors: Klaus Banert, Jennifer SeifertAbstract:Twenty secondary amines, bearing two secondary, two tertiary, or a secondary and a tertiary alkyl Group or a Neopentyl Group, were treated with allenyl isothiocyanate to prepare the corresponding 2-dialkylamino-5-methylthiazoles. The relative rates of these reactions, which were strongly dependent on the steric demand of the N-alkyl Groups, were measured by using competition experiments and analysis by 1H NMR spectroscopy. Not only steric, but also some electronic effects on the nucleophilicity of the dialkylamines were discussed.
Kevin H. Shaughnessy - One of the best experts on this subject based on the ideXlab platform.
-
Mechanistic Study of the Role of Substrate Steric Effects and Aniline Inhibition on the Bis(triNeopentylphosphine)palladium(0)-Catalyzed Arylation of Aniline Derivatives
2017Co-Authors: Deidra L. Gerlach, Kevin H. ShaughnessyAbstract:The mechanism of the bis(triNeopentylphosphine)palladium(0) (Pd(PNp3)2)-catalyzed coupling of aryl halides and aniline derivatives was studied in an effort to understand the role of substrate steric effects on the reaction. Prior studies had shown that the rate of Pd/PNp3-catalyzed coupling of aryl bromides and aniline derivatives was largely unaffected by substrate steric demand. The oxidative addition of aryl bromides to Pd(PNp3)2 is found to follow first-order kinetics with a rate that is independent of both ligand and aryl halide concentration. Thus, the rate limiting step for oxidative addition of aryl bromides is irreversible ligand dissociation. In the case of aryl chlorides, the oxidative addition rate has a first-order dependence on [ArCl] and an inverse dependence on [PNp3], indicating a mechanism involving reversible dissociation of the ligand followed by rate limiting oxidative addition. This difference in aryl halide effect was also found for the catalytic coupling reaction. Aryl bromide steric demand does not affect the coupling rate with hindered anilines, whereas the coupling rate of aryl chlorides is negatively affected by substrate steric demand. These results suggest that oxidative addition is rate limiting in the catalytic reaction for aryl chlorides but that oxidative addition is not rate limiting for aryl bromides. Aniline was found to give coupling rates significantly slower than those of 2,6-diisopropylaniline for both aryl bromides and chlorides. Aniline promotes the decomposition of the [(PNp3)Pd(Ar)(μ-X)]2 catalytic intermediate to a catalytically inactive palladacycle ([(κ2-P,C-Np2PCH2C(Me2)CH2)Pd(μ-X)]2) through C–H activation of a Neopentyl Group and elimination of arene. These studies show that the ability of the Pd/PNp3 catalyst system to tolerate steric demand in aryl bromides stems from the fact that the rate limiting step of the catalytic cycle is independent of the concentration and steric demand of aryl bromides. A catalyst deactivation pathway involving ligand metalation was identified that is promoted by unhindered aniline derivatives
-
Bulky Alkylphosphines with Neopentyl Substituents as Ligands in the Amination of Aryl Bromides and Chlorides
The Journal of organic chemistry, 2006Co-Authors: Lensey L. Hill, Lucas Moore, Rongcai Huang, Raluca Craciun, Andrew J. Vincent, David A. Dixon, Joe Chou, Christopher J. Woltermann, Kevin H. ShaughnessyAbstract:Di(tert-butyl)Neopentylphosphine (DTBNpP) in combination with palladium sources provided catalysts with comparable or better activity for the Hartwig-Buchwald amination of aryl bromides than tri(tert-butyl)phosphine (TTBP) under mild conditions. DTBNpP also provided effective catalysts for amination reactions of aryl chlorides at elevated temperatures. Further replacement of tert-butyl Groups with Neopentyl substituents resulted in less effective ligands for amination reactions. Computationally derived cone angles showed that replacement of a tert-butyl Group with a Neopentyl Group significantly increased the cone angle of the phosphine. The larger cone angle of DTBNpP than TTBP appears to correlate with the higher activity of catalysts derived from DTBNpP in the amination of aryl bromides. TTBP is a stronger electron donor than DTBNpP, which may explain the higher activity for TTBP-derived catalysts toward aryl chlorides.
Masaaki Takasuka - One of the best experts on this subject based on the ideXlab platform.
-
Structural effects of the Grunwald–Winstein correlations in the solvolysis of some simple tertiary alkyl chlorides
Journal of Physical Organic Chemistry, 1997Co-Authors: Ken'ichi Takeuchi, Yasushi Ohga, Takuhiro Ushino, Masaaki TakasukaAbstract:The rates of solvolysis in various solvents at 25 °C were determined for five tertiary alkyl chlorides: 2-chloro-2,4,4-trimethylpentane (4), 2-chloro-2,4-dimethylpentane, 2-chloro-2-methylpentane, 1-chloro-1,3,3-trimethylcyclopentane (7) and 1-chloro-1-methylcyclopentane. The rate data were analysed on the basis of the original and extended Grunwald–Winstein-type equation [log(k/k0)=mYCl+c and log(k/k0)=lNT+mYCl+c] and the results were compared with those reported for 2-chloro-2-methylpropane (1) and 2-chloro-2,3,3-trimethylbutane (3). The rate data for 4 in 18 solvents give an excellent correlation with l=0·00±0·02 and m=0·74±0·01. The Neopentyl Group in 4 more effectively shields the rear-side of the reaction center than the tert-butyl Group in 3 that is correlated by l=0·10±0·04 and m=0·81±0·04. The rate ratio between 4 and 1 at 25 °C is 275 in TFE and predicted to increase to 950 in TFA. The previous 4/1 rate ratio of 21 in 80% ethanol evidently underestimates the B-strain effect on the solvolysis rate of 4 by a factor of at least 40. The remote methyl Groups in 7 works to increase rear-side shielding without increasing B-strain. The marked difference in the effect of the remote methyl Groups between 4 and 7 suggests that the leaving chloride ion in 4 takes a locus that is nearly antiperiplanar to the tert-butyl Group. © 1997 John Wiley & Sons, Ltd.
-
The Grunwald‐Winstein type correlation for 2‐chloro‐2,4,4‐trimethylpentane: A simple tertiary alkyl chloride that shows essentially limiting behaviour in solvolysis
Journal of Physical Organic Chemistry, 1996Co-Authors: Ken'ichi Takeuchi, Yasushi Ohga, Takuhiro Ushino, Masaaki TakasukaAbstract:Evaluation of the rates of solvolysis of 2-chloro-2,4,4-trimethylpentane (4) in 17 solvents on the basis of the Grunwald-Winstein type equation [log(k/k0) = lNT + mYCl + c] gives an excellent correlation with l = −0·01 ± 0·02 and m = 0·74 ± 0·01. The Neopentyl Group in 4 more effectively shields the rear side of the reaction center than the tert-butyl Group in 2-chloro-2,3,3-trimethylbutane. The rate ratio between 4 and 2-chloro-2-methylpropane (3) at 25°C is 275 in trifluoroethanol and predicted to increase to 950 in trifluoroacetic acid. The previous 4:3 rate ratio of 22 in 80% ethanol evidently underestimated the B-strain effect on the solvolysis rate of 4 by a factor of at least 40.