Electronic Effects in Elimination Reactions. V. The E<sub>2</sub> Reaction of β-Phenylethyl Fluorides and Chlorides<sup>1,2</sup>

DePuy, Charles H.; Bishop, Charles A.

doi:10.1021/ja01495a035

Cited by 29 publications

(17 citation statements)

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“…the so-called koTs/kBr ratio. According to DePuy & Bishop [7] and Hoffmann [8] these rate ratios depend largely on the degree of charge separation between the cationic center and the nucleofuge in the transition state, high ratios, as observed for tertiary substrates [9], indicating large charge separation, low ratios, as observed for secondary and primary substrates [8], indicating small or negligible charge separation'). On the other hand Schleyer et al [lo] have reasoned that the observed increase of the koTs/kBr ratio in the order primary< secondary<< tertiary is due to a steric effect, namely ground state repulsion between the bulky OTs-group and the substituents at the cationic center which increases in the same order.…”

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confidence: 99%

Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Grob

Schaub

1982

Helvetica Chimica Acta

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SummaryThe rate constants for 3-substituted adamantyl p-toluenesulfonates 3a-3k in ethanol/water 80 : 20 correlate well with the respective inductive substituent constants up. The reaction constant p for the toluenesulfonates 3 is 10% larger than for the corresponding bromides 2, indicating somewhat more charge separation in the activation of the toluenesulfonates. Evidence is presented that stabilization of the resultant 1-adamantyl cations by induction involves graded 1,3 -bridging, which is favored when the substituent is an electrofugal group, and that stabilization by n-electron donors involves C, C-hyperconjugation. Rate ratios for the toluenesulfonates 3 and the bromides 1 exceed lo3 and are almost independent of the 3-substituents. The implications of this are discussed in the light of current hypotheses.Introduction. -As shown in recent studies [ 1-51 reaction constants p, as derived

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confidence: 99%

Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Grob

Schaub

1982

Helvetica Chimica Acta

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“…Similar results come from the ethoxidepromoted eliminations of C 6 H 5 C i HBrCH 2 Br, C 6 H 5 C i HBrCH 2 Cl and C 6 H 5 C iHBrCH 2 F: k HBr /k HCl = 35 and k DBr /k DCl = 16; k HCl /k HF = 67 and k DCl /k DF = 35 [37]. An element effect of k Cl /k F = 68 had been previously reported for the ethoxide-promoted dehydrohalogenataions of C 6 H 4 CH 2 CH 2 X [38].…”

Section: Proton Transfer Associated With Methoxide Promoted Dehydrohamentioning

confidence: 76%

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Koch

2006

Hydrogen‐Transfer Reactions

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This chapter is divided into three sections: Proton transfer from carbon acids to methoxide ion; proton ttransfer from methanol to carbanion intermediates; proton transfer associated with methoxide promoted dehydrohalogenation reactions. Proton Transfer from Carbon Acids to Methoxide IonKnowledge of equilibrium pK a values of carbon acids is important for an understanding of organic chemistry; however, they are not always reliable indicators of relative rates for proton transfer reactions. Ritchie [1] predicted in 1969 that carbon acids whose conjugate bases have localized charge will show "kinetic acidities" greater than their thermodynamic acidities. This is illustrated by the weaker acid pentafluorobenzene-t [PFB-t], pK a = 25.8 [2], with a methanolic sodium methoxide catalyzed protodetritiation rate, k = 2.57 · 10 -2 M -1 s -1 [3], that is 15 times faster at 25 C than that for 9-phenylfluorene-9-t [9-PhFl-9-t], pK a = 18.5 and k = 1.54 · 10 -3 M -1 s -1 [4]. Primary kinetic isotope effects [PKIE] can be used to determine C-H bond breaking in the rate limiting steps of any reaction mechanism. Normal hydrogen isotope effects are largely due to zero-point energy differences of the stretching frequencies of C-H vs. C-D bonds. If only stretching frequencies are considered the value for k H /k D would be 6.2 for the C-i H bond. With allowance for bending vibrations this could result in a k H /k D = 10, k H /k T = 27 and k D /k T = 2.7 at 25 C [5]. Melander [6] and Westheimer [7] suggested that lower values of k H /k D could be due to residual zero-point energy in an asymmetric transition structure. For this reason the magnitude of k H /k D was often used to assign early or late transition structures for hydron-transfer reactions. Smaller values due to asymmetric transition structures should still obey the Swain-Schaad relationship [8], k H /k T = (k H /k D ) 1.442 . The k D /k T = 1.0 associated with the exchange reactions of PFB i H suggest that the C-H bond is not broken in the rate limiting step and this differs significantly from the k D /k T of 2.54 measured from the reactions of 9-PhFl-9-i H.

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“…The bulkier the system is, the less stability is gained by having a considerable amount of double bond character in the transition state and therefore, more Hofmann products are realized. Also, it has been noted that poor leaving groups On the other hand, Sauhders (19) and DePuy (20,21) have shown that Eg rates and product ratios follow a reverse relationship to the steric size of the halogen leaving groups.…”

Section: T-butoxide-t-butanol Asmentioning

confidence: 99%

Bimolecular elimination reactions of cyclopentyl compounds

Smith

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Electronic Effects in Elimination Reactions. V. The E₂ Reaction of β-Phenylethyl Fluorides and Chlorides^1,2

Cited by 29 publications

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Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Bimolecular elimination reactions of cyclopentyl compounds

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Electronic Effects in Elimination Reactions. V. The E2 Reaction of β-Phenylethyl Fluorides and Chlorides1,2

Cited by 29 publications

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Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Polar Effects. X. Polar Substituent Effects in the Solvolysis of 3‐Substituted 1‐Adamantyl Bromides and Toluenesulfonates

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Bimolecular elimination reactions of cyclopentyl compounds

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Electronic Effects in Elimination Reactions. V. The E₂ Reaction of β-Phenylethyl Fluorides and Chlorides^1,2