pK a Measurements in tetrahydrofuran

Fraser, Robert R.; Bresse, Monique; Mansour, Tarek S.

doi:10.1039/c39830000620

Cited by 63 publications

(49 citation statements)

References 2 publications

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“…change in ApK over this 90" temperature range ( I I). We earlier observed no pK change for a 5-fold change in concentration (7).…”

Section: Methods Of Pk Measurementmentioning

confidence: 99%

Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

Fraser¹,

Mansour²,

Savard³

1985

Can. J. Chem.

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The pKa's of 13 heterocyclic aromatic compounds have been measured in tetrahydrofuran (THF) using 13C nmr spectroscopy. The acidifying effect of a nitrogen heteroatom is seen to be quite large (4–6 pK units versus C—H) on an adjacent C—H bond but small on a more remote hydrogen. Most of these aromatic heterocycles are sufficiently acidic, pKa < 34, to be completely deprotonated by lithium tetramethylpiperidide in THF. Benzoxazole appears to be an extremely strong carbon acid, its ring-opened isomer having a pKa of less than 15.7.

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“…change in ApK over this 90" temperature range ( I I). We earlier observed no pK change for a 5-fold change in concentration (7).…”

Section: Methods Of Pk Measurementmentioning

confidence: 99%

Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

Fraser¹,

Mansour²,

Savard³

1985

Can. J. Chem.

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“…This provides a value for K for the equilibrium [ l ] where K = K,/K2 and thus yields a ApK = -log K = pKlpK,.j Using this technique we have determined an increasing order of acidities for tetramethylpiperidine, diisopropylamine, and cis-2,6-dimethylpiperidine (20) (relative pK,'s being 2.1, 0.5, and 0). We have subsequently utilized the lithio derivatives of these amines to measure the relative pK's of fifteen weak carbon acids (21). One of the most notable facts emerging from these preliminary studies is a very different order of acidities in THF compared to cyclohexylamine.…”

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confidence: 99%

Acidity measurements in THF. III. Aldimines and ketimines

Fraser¹,

Bresse²,

Chuaqui-Offermanns³

et al. 1983

Can. J. Chem.

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. Can. J. Chem. 61, 2729 (1983).The relative acidities of fiteen aldimines and ketirnines have been measured in THF using either ' " nrnr spectroscopy or trapping experiments with methyl iodide. The pK,'s are found to vary over a range of more than four pK units. The effects of structure on acidity indicate the acid-weakening effect of alkyl substitution in ketirnines to be due to steric effects. This interpretation is supported by the results of ab ir~itio calculations. The data also permit a quantitative estimation of the preference for the syn configuration of lithiated irnines to be at least four kcal/rnol. The anti selectivity previously reported for the lithiation of endocyclic irnines can be accounted for by angular constraints, as indicated by both ab initio calculations and pK, data on this class of irnine.ROBERT R. FRASER, MONIQUE BRESSE, NOEMI CHUAQUI-OFFERMANNS, K. N. HOUK et N. G. RONDAN. Can. J. Chern. 61, (1983).On a rnesurC les aciditCs relatives de quinze aldirnines et cCtirnines dans le THF en faisant appel soit ii la spectroscopie rrnn du "C ou i des expkriences de trappage avec I'iodure de rnCthyle. On a trouvC que les pK, varient sur plus de quatre unites de pK. Les effets de structure sur I'aciditC indiquent que l'abaissernent de I'aciditC lors de la substitution alkyle dans les cCtCrnines serait dC i des effets stCriques. Cette interprktation est en accord avec les rCsultats de calculs ab initio. Les donnCes perrnettent aussi d'Cvaluer quantitativement, i au rnoins 4 kcal/rnol, la prCfCrence pour la configuration sytz des irnines IithiCs. On peut expliquer la sClectivitC anti rapportCe anterieurernent lors de la lithiation d'irnines endocycliques en faisant appel i des contraintes angulaires qui sont suggCrCes i la fois par les calculs ab initio et les donnCes de pK, sur cette classe de cornposCs.[Traduit par le journal]During the course of our investigations on the chemistry of lithiated aldimines (1) and ketimines (2) a number of unusual results were encountered for which satisfactory explanations required further experimental support. For example, with but two exceptions the lithiation of aldimines and ketimines gave exclusively anions having the syn configuration, the main exception being the endocyclic imines 9 to 12 which formed anti lithio derivatives only (3).2 A second striking feature of the lithiation of ketimines concerns the regioselectivity of the reaction. Just as had been observed earlier for ketone oximes (4), oxime ethers ( 3 , and hydrazones (6), lithiation is markedly favored at a primary vs. secondary carbon atom (7) or at a secondary vs. a tertiary one (2,8). This is anomalous too, in the sense that the stability differences at primary, secondary, and tertiary carbons are normally small and in the reverse order when a change of sp3 to sp2 hybridization is involved (9, 10).As a potential aid to our understanding these results, we set out to obtain quantitative information on the deprotonation reaction, that is, to measure the relative acidities of a variety of imines o...

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“…Most ortho ‐lithiations are carried out using n BuLi or even more basic alkyllithiums. ortho ‐H's of many aryl substrates possess p K A 's between 37 and 41 , . As the basicity/nucleophilic nature of an aryllithium is significantly lower than that of n BuLi (p K A of the conjugate acid, butane, is ca.…”

Section: Resultsmentioning

confidence: 99%

Focused ortho‐Lithiation and Functionalization of p‐Bromo‐ and p‐Iodoanisole

et al. 2018

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By use of the strategy for ortho‐lithiation (DoM) that conceptually diminishes the pKA difference between the ortho‐H of the substrate and the conjugate acid of the metalating agent, effective metalation of bromine and iodine bearing aryl substrates can be carried out. As a set of prototypes, p‐bromoanisole (p‐BrA) and p‐iodoanisole (p‐IA) have been successfully ortho‐metalated in promoted hydrocarbon media using ortho‐lithiodimethylbenzylamine (o‐LiDMBA) as the metalating agent. No hint of exchange of either halogen was noted using these conditions. These studies add to the emerging evidence of a hitherto unidentified acidifying effect on the proton(s) ortho‐ to a directing metalation group (DMG) by both a p‐iodo and a p‐bromo substituent.

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pK a Measurements in tetrahydrofuran

Cited by 63 publications

References 2 publications

Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

Acidity measurements in THF. V. Heteroaromatic compounds containing 5-membered rings

Acidity measurements in THF. III. Aldimines and ketimines

Focused ortho‐Lithiation and Functionalization of p‐Bromo‐ and p‐Iodoanisole

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