Direct Detection of Free and Counterion-Bound Carbanions by Electrospray-Ionization Mass Spectrometry

We propose electrospray-ionization (ESI) mass spectrometry as a robust and powerful method for the in situ analysis of carbanions. ESI mass spectrometry selectively probes the charged components of the sampled solution and, thus, is ideally suited for the detection of free carbanions. We demonstrate the potential of this method by analyzing acetonitrile solutions of 15 different carbon acids AH, whose acidities cover a range of 11.1 ≤ pK_a(DMSO) ≤ 29.5. After treatment with KO^tBu as a strong base, all but the two least acidic compounds were successfully detected as free carbanions A^- and/or as potassium-bound aggregates [K_n-1A_n]^-. The association equilibria can be shifted toward smaller aggregates and free carbanions by the addition of the crown ether 18-crown-6, which facilitates the evaluation of the mass spectra. When KO^tBu was replaced by other bases (LiOH, LiNⁱPr₂, NaH, NaOH, KOH, NBu₄OH) or when tetrahydrofuran or methanol was used as a solvent, carbanions were also successfully observed. For further demonstrating the utility of the proposed method, we applied it to the analysis of the Michael addition of deprotonated dimedone to butenone. ESI mass spectrometry allowed us to follow the decrease of the reactant carbanion and the buildup of the product carbanion in time.

Development of Palladium-Catalyzed Decarboxylative Allylation of Electron-Deficient Sulfones and Identification of Unusual Side Products

The use of sulfones as electron-withdrawing groups in substrates for palladium-catalyzed decarboxylative allylation was explored. A previously published trifluoromethanesulfonyl-based substrate was highly reactive and selective under mild conditions, but the substrate scope was not readily expanded. Instead, 3,5-bis(trifluoromethyl)phenyl sulfones were employed, thereby simultaneously retaining most of the electron deficiency and providing facile synthetic access. Optimization of the catalytic conditions to maximize the product distribution to a synthetically useful level of the allylation product over the protonation side product proved extremely challenging, with inconsistent and irreproducible results afforded with Pd₂(dba)₃ as the palladium source. A variety of substrates were subjected to the optimized catalytic conditions of PdCp(1-cinnamyl) and Xantphos in tetrahydrofuran at 50 °C for 30 min. These conditions were applicable to all substrates with the exception of the α,α-dimethyl allyl ester, which required more forcing conditions and afforded four products: the allylation and protonation products, as expected, along with a cyclopropylation product and an unprecedented pseudodimeric product. The mechanism for the formation of these unusual side products is considered.

A mild light-induced cleavage of the S-O bond of aryl sulfonate esters enables efficient sulfonylation of vinylarenes

A new mode of S-O bond activation has been discovered, which constitutes novel reactivity of easily available and bench-stable arylsulfonate phenol esters. This protocol enables access to putative sulfonyl radical intermediates, which enable straightforward access to valuable vinyl sulfones.

Synthesis of α,β-unsaturated α'-haloketones through the chemoselective addition of halomethyllithiums to Weinreb amides

A straightforward synthesis of variously functionalized α,β-unsaturated α'-haloketones has been achieved through the chemoselective addition of halomethyllithium carbenoids to Weinreb amides at -78 °C. A comparative study employing the corresponding esters under the same reaction conditions pointed out that the instability of the tetrahedral intermediate formed from the latter is responsible for the observed formation of carbinols instead of the desired haloketones.

Total synthesis and bioactivities of two proposed structures of maresin

Maresin is a potent anti-inflammatory lipid mediator derived from docosahexaenoic acid (DHA). A highly convergent total synthesis of two proposed structures of C7-epimeric maresins from the four known fragments was achieved in 17 steps. The three key coupling reactions were the BF(3)-mediated alkyne attack on the epoxide, chiral titanium complex-promoted enantioselective alkyne addition to the aldehyde, and a Julia-Kocienski olefination. The two synthesized diastereomers were found to be comparably active in blocking neutrophil infiltration in the acute peritonitis model.

Alpha-fluorovinyl Weinreb amides and alpha-fluoroenones from a common fluorinated building block

Synthesis and reactivity of N-methoxy-N-methyl-(1,3-benzothiazol-2-ylsulfonyl)fluoroacetamide, a building block for Julia olefination, is reported. This reagent undergoes condensation reactions with aldehydes and cyclic ketones to give alpha-fluorovinyl Weinreb amides. Olefination reactions proceed under mild, DBU-mediated conditions, or in the presence of NaH. DBU-mediated condensations proceed with either E- or Z-selectivity, depending upon reaction conditions, whereas NaH-mediated reactions are > or = 98% Z-stereoselective. Conversion of the Weinreb amide moiety in N-methoxy-N-methyl-(1,3-benzothiazol-2-ylsulfanyl)fluoroacetamide to ketones, followed by oxidation, resulted in another set of olefination reagents, namely (1,3-benzothiazol-2-ylsulfonyl)fluoromethyl phenyl and propyl ketones. In the presence of DBU, these compounds react with aldehydes tested to give alpha-fluoroenones with high Z-selectivity. The use of N-methoxy-N-methyl-(1,3-benzothiazol-2-ylsulfanyl)fluoroacetamide as a common fluorinated intermediate in the synthesis of alpha-fluorovinyl Weinreb amides and alpha-fluoroenones has been demonstrated. Application of the Weinreb amide to alpha-fluoro allyl amine synthesis is also shown.