Is Carbon Dioxide Able to Activate Halogen/Lithium Exchange?

Abstract

The unexpected effect of carbon dioxide on halogen–lithium exchange (HLE) reactions of selected haloarenes with tBuLi was investigated. In an aliphatic hydrocarbon solvent (pentane), the HLE does not occur at ca. –70 °C but, surprisingly, pouring the mixture of reactants onto dry ice and subsequent aqueous acidic hydrolysis gave carboxylic acids resulting from the quench of the first‐formed aryllithiums with carbon dioxide. This suggests that CO2 acts as a promoter of the HLE and, subsequently, serves as an electrophile to trap the aryllithium intermediates that are generated in situ. Theoretical DFT calculations were used to develop a plausible mechanism for the reaction, which indicates that CO2 is a much weaker donor than tetrahydrofuran (THF) so the cleavage of inert tBuLi cubic tetramers into more reactive solvated dimeric species (tBuLi)2(CO2)4 is disfavored by 42.8 kJ per mol of (tBuLi)4. It is possible that this deaggregation process occurs to some extent when a large excess of CO2 is used.

Bifunctional Indenyl‐Derived Receptors for Fluoride Chelation and Detection

“…Thus, phosphine-borane receptors 7g and 9g weree xposed to cyanide or fluoride in dry THF or chloroform, but only on addition of cyanide was ab inding event detected. For both isomers, the 11 19 FNMR spectrum. [3] The absence of ap eak envelope for the fluoridea dduct detectedb yn egativeion ESI-MS further suggestst hat fluoride is not bound by either 7g or 9g,e ven in dry aprotics olvents.…”

“…[5f] The long contacts between phosphorus and the cyanide guest (3.102 f or [7gÀCN] À , 3.240 f or [9gÀCN] À ), together with the marginal shifts in the respective 31 PNMR spectra on CN À binding, argue however against any interaction betweent he phosphorus atom and the bound cyanide. (2)1 .575 (4) 1.584 (3)1 .599 (14) 1.562(4) 1.571 (2)1 .581(5) 1.575 (3)1 .587 (15) 1.577 (4) 1.569 (2)1 .572(4) 1.578 (3) 1.550 (15) 1.585 (4) (2) 114.8 (1) 126.0 (7)1 24.4(2) aB-C-C 123.2 (1)1 29.3(3)1 24.2(2) 131.0 (9)1 [3] and, additionally,b road new signals in the corresponding 19 FNMR spectra are also measured (at d F = À149.0 and À145.1 ppm for 7hÀFa nd 9hÀF, respectively). Evidence for the chelation of the fluoride ion between borane and phosphonium Lewisa cids is providedb y 31 PNMR spectroscopy.…”

“…[18] MeI, MeSO 3 CF 3 and Ph 2 PCl were used as supplied (Sigma Aldrich). 1 Ha nd 13 CNMR spectra were measured on aB ruker AVII 500 FT-NMR or Varian Mercury VX-300 spectrometer; 1 Ha nd 13 CNMR spectra were calibrated using the residual proton or natural abundance 13 Cr esonances of the solvent; 11 Ba nd 19 Fs pectra were referenced with respect to Et 2 O·BF 3 and CFCl 3 ,r espectively. Mass spectra were measured by the EPSRC National Mass Spectrometry Service, Swansea University,a nd elemental microanalysis by London Metropolitan University.…”

“…Despite the relatively straightforward syntheses of both 3 and 4,a ttempts to introducet he boryl function by the use of al ithiation/Mes 2 BF electrophilic quenchm et with no success. [17] Previous reports on the generation of benzyne species from ortho-dichlorobenzenes, suggest that the elimination of lithium chloride from (lithio)chlorobenzenes is extremely facile (even at very low temperatures), [19] and this maya ccount for the lack of success in borylatinge ither 3 or 4.W ith this in mind, and given the greater thermal robustness reported for ortho-(lithio)bromobenzenes, [20] attention was shiftedt ot he corresponding dibromoindenyl precursors (Schemes 1,2,and 3).…”

Asymmetric Cycloaddition Reactions of Aryne Intermediates with a Chiral Carbon–Carbon Axis: Syntheses of Axially Chiral Biaryl Compounds

“…14 The reaction course through intermediate A would not proceed because the simultaneous generation of a dianion intermediate, which leads to intermediate A, appears to be extremely difficult. 15 We expected that asymmetric synthesis via an axially chiral arylaryne would be achieved through intermediates B and C. We attempted a [4 + 2]-cycloaddition reaction of an aryne intermediate, which was generated from precursor 1 16 through a metal−halogen exchange reaction, followed by the elimination of triflate, with furan (4a) using n-butyllithium (n-BuLi) as an alkylmetal reagent. Treatment of precursor 1 (88% ee) with 2.0 equiv of n-BuLi in the presence of 4.4 equiv of 4 at −78 °C in tetrahydrofuran (THF) for 15 min gave a 1:1 diastereomeric mixture of 2a in 80% yield (Table 1, entry 1).…”