Aggregation and Alkylation of the Cesium Enolate of 2-<i>p</i>-Biphenylylcyclohexanone<sup>1</sup>

Streitwieser, Andrew; Wang, Daniel Zerong; Stratakis, Manolis

doi:10.1021/jo990251u

Cited by 17 publications

(19 citation statements)

References 18 publications

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“…These numbers suggest that entropy dominates the equilibrium, probably by loss of solvation on dimerization, and fits with other 1:1 lithium salts in THF whose dimerization is also entropy driven. 18 These numbers are also comparable to those found for the corresponding cesium enolate, CsBPCH, K 1,2 ) 1.9 × 10 3 M -1 , ∆H°) 0.7 kcal mol -1 , and ∆S°) 17.5 eu, 9 and for LiPCH, K 1,2 ) 2.8 × 10 3 M -1 . 13…”

Section: Resultssupporting

confidence: 55%

“…This explanation finds support in the relative pK values; CsPhIBP has a pK 9.3 units higher than that of CsBPCH compared to a difference of only 3.5 units for the lithium enolates. 7,9 Moreover, in line with the relative basicities of the cesium enolates, CsPhIBP monomer is more reactive (by about an order of magnitude) toward methyl tosylate and p-tert-butylbenzyl chloride than is CsBPCH, the opposite reactivity order of the lithium enolates.…”

Section: Resultsmentioning

confidence: 97%

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Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Streitwieser

Wang

1999

J. Am. Chem. Soc.

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Deprotonation of biphenylylcyclohexanone (BPCH) with a lithium base in THF occurs preferentially at the secondary enolate position to give the unconjugated lithium enolate which is gradually converted to the more stable conjugated enolate by further proton transfer from the tertiary enolate position of the ketone. The unconjugated lithium enolate is present dominantly as the tetramer, but it is the monomer that reacts with ketone to give the conjugated enolate, LiBPCH. The conjugated enolate is present as a monomer-dimer mixture with K 1,2 ) 4300 M -1 . The equilibrium constant changes only slightly at lower temperatures, indicating that dimerization is primarily entropy-controlled. The ion pair pK of LiBPCH, 12.3, is 6.0 units less than that of the corresponding cesium enolate, CsBPCH. Alkylation of LiBPCH with methyl brosylate or benzylic bromides occurs at the tertiary carbon, and k M . k D . These reactions are faster than those of LiPhIBP, although LiPhIBP has a higher ion pair pK value. The ion pair displacement reaction has a relatively low activation energy but a normal entropy of activation.

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Section: Resultssupporting

confidence: 55%

Section: Resultsmentioning

confidence: 97%

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Streitwieser

Wang

1999

J. Am. Chem. Soc.

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“…In contrast, the apparent order with respect to 1BT is close to unit both in MCH at 258 K ( x = 0.90 ± 0.05) and TOL at 223 K ( x = 0.97 ± 0.13, Figure , bottom). According to the literature, the apparent reaction order with respect to a reactant undergoing self-aggregation is related to its thermodynamic average aggregation number ( N ) by eq − where n k represents the average aggregation number of species that are actively involved in the reaction. Because N ≥ 1 and values of x are found close to unit or bigger, n k is ≥1, meaning that the contribution of aggregates larger than single ion pairs to the total reaction rate must be significant in all cases investigated here.…”

Section: Resultsmentioning

confidence: 99%

Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal–Carbon Bond

2019

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An integrated thermodynamic and kinetic study was carried out to clarify how the formation of ionic aggregates higher than ion pairs affects the rate of olefin insertion into the Zr–C bond. To this aim, the reaction with olefin of three-member zirconaziridium metallacycles [Cp2Zr(η2-CH2-N(R)2)]+[X]− [R = C18H37; X– = MeB(C6F5)3 – (1BN) or B(C6F5)4 – (1BT)], models for tight (t-ISIP, 1BN) and loose (l-ISIP, 1BT) inner sphere ion pair (ISIPs), was studied at different levels of concentration and thus self-aggregation. Both 1BN and 1BT undergo a regioselective insertion of a single α-olefin molecule (1-hexene or 2-Me-1-heptene) into the Zr–C bond to form the corresponding five-member metallacycles outer sphere ion pairs (2BN, 2BT, 3BN, and 3BT), with no evidence for further olefin insertions. Under comparable experimental conditions, where monomeric ion pairs dominate (aggregation number N ≈ 1), the apparent rate constant (k app) of 1-hexene insertion into 1BT is about 200 times higher than that in 1BN, in agreement with the weaker strength of cation–anion interaction in the former. For 1BT, k app is only marginally affected by the self-aggregation level in all investigated experimental conditions. For example, k app = 1.1 M–1 s–1 (N ≈ 1.1) and 1.3 (N ≈ 4.8) for the insertion reaction of 1-hexene in toluene at 223 K and range from 19 × 10–3 M–1 s–1 (N ≈ 1.5) to 10 × 10–3 (N ≈ 39) for the insertion of 2-Me-1-heptene in methylcyclohexane at 258 K. On the contrary, k app is markedly dependent on self-aggregation in the case of 1BN. For instance, when 1BN is reacted with 1-hexene in methylcyclohexane at 258 K, k app increases more than 1 order of magnitude, from 6 × 10–3 to 93 × 10–3 M–1 s–1, upon increasing the aggregation level from N ≈ 1 ([1BN] = 6 × 10–5 M) to N ≈ 4.6 ([1BN] = 12.8 × 10–3 M). Although the insertion of 1-hexene in 1BN is faster in the slightly more polar solvent toluene (k app = 39 × 10–5 M, N ≈ 1), the relative reactivity difference between ion pairs and larger ionic aggregates is similar in both solvents (about 4× and 3× increment, on passing from N ≈ 1.0 to N ≈ 1.6, in methylcyclohexane and toluene, respectively). The addition of an inert external salt with a common counterion (2BN) to solution of 1BN dramatically raises the olefin insertion rate constant: at [1BN] = 6 × 10–5 M, k app increases from 6 × 10–3 M–1 s–1, in the absence of 2BN, to 42 × 10–3, 90 × 10–3, and 166 × 10–3 M–1 s–1 in the presence of 1, 3, and 5 equiv of 2BN, respectively. All of the thermodynamic and kinetic data can be rationalized by taking into account the strength of cation–anion interaction in the two kinds of ion pairs. In t-ISIPs, the cation–anion interaction energy is relatively strong, resulting in a relatively small dipole moment and a little thermodynamic tendency to self-aggregate. However, under conditions where homo (or mixed) ion aggregates form, an additional weakening of the cation–anion interaction occurs, causing a substantial increase of the intrinsic reactivity of each t-ISIP within aggregates. On th...

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“…In recent papers, we have described the aggregation and reactivity of the lithium and cesium enolates of several ketones: p -phenylisobutyrophenone, LiPhIBP and CsPhIBP, 2-phenylcyclohexanone, LiPCH and CsPCH, 2-( p -biphenylyl)cyclohexanone, LiBPCH and CsBPCH, 2,6-diphenyl-α-tetralone, LiPhPAT and CsPhPAT, and the lithium enolates of p -phenylsulfonylisobutyrophenone, LiSIBP, 6-phenyl-α-tetralone, LiPAT, and 2-benzyl-6-phenyl-α-tetralone, LiBnPAT . In the present paper, we extend the study to the cesium enolate, CsPAT, of 6-phenyl-α-tetralone, PAT.…”

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Aggregation and Reactivity of the Cesium Enolate of 6-Phenyl-α-tetralone: Comparison with the Lithium Enolate¹

Wang

Streitwieser

2003

J. Org. Chem.

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The cesium enolate of 6-phenyl-alpha-tetralone (CsPAT) has a lambda(max) in THF at about 387 nm, but the variation with concentration is too small for application of singular value decomposition. Proton-transfer studies with several indicators show that CsPAT forms monomer-tetramer mixtures with a tetramerization equilibrium constant, K(1,4) = 2.3 x 10(11) M(-3). The pK of the monomer is 23.39 on a scale where fluorene is assigned 22.9 (per hydrogen). For comparison, the lithium enolate, LiPAT, is also a monomer-tetramer with K(1,4) = 4.7 x 10(10) M(-3) and a monomer pK = 14.22. HMPA in large amounts promotes dissociation to monomer with both enolates. Ion-pair S(N)2 initial rates were measured for CsPAT with several alkyl halides and with methyl tosylate and compared with other rates with LiPAT. In all cases, the enolate monomers are much more reactive than the aggregates. Reaction of CsPAT with alkyl halides is generally C-alkylation but HMPA promotes increasing amounts of O-alkylation. A new indicator, 11-methyl-11H-benzo[b]fluorene, has a pK on the cesium scale of 23.39.

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Aggregation and Alkylation of the Cesium Enolate of 2-p-Biphenylylcyclohexanone¹

Cited by 17 publications

References 18 publications

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal–Carbon Bond

Aggregation and Reactivity of the Cesium Enolate of 6-Phenyl-α-tetralone: Comparison with the Lithium Enolate¹

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Aggregation and Alkylation of the Cesium Enolate of 2-p-Biphenylylcyclohexanone1

Cited by 17 publications

References 18 publications

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran1

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran1

Understanding the Role of Metallocenium Ion-Pair Aggregates on the Rate of Olefin Insertion into the Metal–Carbon Bond

Aggregation and Reactivity of the Cesium Enolate of 6-Phenyl-α-tetralone: Comparison with the Lithium Enolate1

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Aggregation and Alkylation of the Cesium Enolate of 2-p-Biphenylylcyclohexanone¹

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Aggregation and Reactivity of the Lithium Enolate of 2-Biphenylylcyclohexanone in Tetrahydrofuran¹

Aggregation and Reactivity of the Cesium Enolate of 6-Phenyl-α-tetralone: Comparison with the Lithium Enolate¹