Luisa Pisano

VIVO versus VIV complex formation by tridentate (O, Narom, O) ligands: Prediction of geometry, EPR 51V hyperfine coupling constants, and UV-Vis spectra

Systems formed using the V(IV)O(2+) ion with tridentate ligands containing a (O, N(arom), O) donor set were described. Examined ligands were 3,5-bis(2-hydroxyphenyl)-1-phenyl-1H-1,2,4-triazole (H2hyph(Ph)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzoic acid (H3hyph(C)), 4-[3,5-bis(2-hydroxyphenyl)-1H-1,2,4-triazol-1-yl]benzenesulfonic acid (H3hyph(S)), and 2,6-bis(2-hydroxyphenyl)pyridine (H2bhpp), with H3hyph(C) being an orally active iron chelator that is commercially available under the name Exjade (Novartis) for treatment of chronic iron overload arising from blood transfusions. The systems were studied using EPR, UV-Vis, and IR spectroscopies, pH potentiometry, and DFT methods. The ligands bind vanadium with the two terminal deprotonated phenol groups and the central aromatic nitrogen to give six-membered chelate rings. In aqueous solution the main species were the mono- and bis-chelated V(IV)O complexes, whereas in the solid state neutral non-oxido V(IV) compounds were formed. [V(hyph(Ph))2] and [V(bhpp)2] are hexacoordinated, with a geometry close to the octahedral and a meridional arrangement of the ligands. DFT calculations allow distinguishing V(IV)O and V(IV) species and predicting their structure, the (51)V hyperfine coupling constant tensor A, and the electronic absorption spectra. Finally, EPR spectra of several non-oxido V(IV) species were compared using relevant geometrical parameters to demonstrate that in the case of tridentate ligands the (51)V hyperfine coupling constant is related to the geometric isomerism (meridional or facial) rather than the twist angle Φ, which measures the distortion of the hexacoordinated structure toward a trigonal prism.

REDUCTIVE ELECTROPHILIC SUBSTITUTION OF PHTHALANS AND RING EXPANSION TO ISOCHROMAN DERIVATIVES

Reductive cleavage of phthalun, 1a, with Li metal in the presence of a catalytic amount of naphthalene leads to the formation of a stable aromatic dilithium compound. The adducts of the latter with CO2, aldehydes or ketones undergo ring closure to isochroman derivatives, leading to ring expansion of the original heterocycle. The reductive electrophilic substitution procedure was successfully extended to the substituted phthalans 1b and 1c, to afford the corresponding isochroman-3ones in satisfactory yields.

Harnessing the ortho‐Directing Ability of the Azetidine Ring for the Regioselective and Exhaustive Functionalization of Arenes

This work demonstrates how the directing ability of the azetidine ring could be useful for regioselective ortho-C-H functionalization of aryl compounds. Robust polar organometallic (lithiated) intermediates are involved in this synthetic strategy. The reagent n-hexyllithium emerged as a safer, yet still effective, basic reagent for the hydrogen/lithium permutation relative to the widely used reagent nBuLi. Two different reaction protocols were discovered for regioselective lithiation at the ortho positions adjacent to the azetidine ring, which served as a toolbox when other competing directing groups were installed on the aromatic ring. The coordinating ability of the azetidine nitrogen atom, as well as the involvement of dynamic phenomena related to the preferential conformations of 2-arylazetidine derivatives, were recognized to be responsible for the observed reactivity and regioselectivity. A site-selective functionalization of the aromatic ring was achieved for aryl azetidines with either coordinatively competent groups (e.g. methoxy) or inductively electron-withdrawing substituents (e.g. chlorine and fluorine). By fine-tuning the reaction conditions, regioselective introduction of several substituents on the aromatic ring could be realized. Several substitution patterns were accomplished, which included 1,2,3-trisubstitution, 1,2,3,4-tetrasubstitution, and 1,2,3,4,5-pentasubstitution, up to the exhaustive substitution of the aromatic ring.

Tuning the Reducing Properties of 1,2-Diaryl-1,2-disodiumethanes

We investigated the reducing properties of a series of 1,2-diaryl-1,2-disodiumethanes by means of equilibration reactions. The electron-donor power of these vic-diorganometals is strongly affected by the nature of substituents present either on the aromatic ring(s) or on the carbanionic centers, and it can be correlated with their ability to delocalize the arylmethyl carbanions. These findings are supported by electrochemical analysis of the reduction behavior of the parent 1,2-diarylalkene. Applications of these results to the reduction of selected substrates are described.

Metalation of arylmethyl methyl ethers and connection with their reductive electrophilic substitution

Abstract Stable α-methoxy arylmethyl carbanions can be generated by metalation of arylmethyl methyl ethers, 1 , with n -BuLi in THF at −40 °C, avoiding Wittig rearrangement to the corresponding alkoxides 2 . Reaction of these carbanions with various electrophiles afforded the expected products 3 in satisfactory yields. Connection between the metalation procedure and the reductive electrophilic substitution of arylmethyl methyl ethers allowed the transformation of compounds 1 into 2-arylpropanoic acids, 5 .

BH3-promoted stereoselective β-lithiation of N-alkyl-2-phenylaziridines.

BH(3) complexes of N-alkyl-2-phenylaziridines have been synthesized and their structure and stereochemistry proved with DFT calculations and NMR experiments. It has been demonstrated that the Lewis acid complexation is able to promote a regioselective β-lithiation in 2-phenylaziridino-borane complexes. The lithiated intermediates were configurationally stable, allowing an enantioselective preparation of cis-2,3-disubstituted aziridines.

Reductive electrophilic substitution of pyrogallol derivatives: Synthesis of 2,3-disubstituted phenols

Abstract 1-2-Dimethoxy-3-methoxymethoxybenzene, 1 , was used as the starting material for the transformation of a 1,2,3-trioxybenzene into various 1-oxy-2,3-dicarbobenzenes, via regioselective reductive electrophilic substitution of the 2-methoxy group followed by Pd-catalyzed cross-coupling reaction.

Single electron transfer reductive cleavage of the aryl-nitrogen bond in phenyl-substituted dimethylanilines

Abstract By treatment with Li metal in THF at room temperature the three isomeric N,N-dimethylaminobiphenyls and N,N-dimethyl-2,6-diphenylaniline underwent 100% regioselective reductive cleavage of the aryl-N bond, affording biphenyl and meta-terphenyl, respectively, in various yields.

Single and double reductive cleavage of CO bonds of aromatic dimethyl acetals and ketals: Generation of benzylic mono- and dicarbanions

Abstract The reductive cleavage of aromatic dimethyl acetals and ketals, 1 , with Li metal in THF at low temperature allows the generation of stable α-alkoxy-α-arylsubstituted carbanions, avoiding the Wittig rearrangement. Reaction of these carbanions with various electrophiles afforded the expected products 2 . Further in situ reaction of compounds 2 afforded the products of reductive electrophilic disubstitution, 3 .

Regio- and Stereoselective Synthesis of Sulfur-Bearing Four-Membered Heterocycles: Direct Access to 2,4-Disubstituted Thietane 1-Oxides

Starting from readily available C2-substituted thietane 1-oxides, a straightforward synthesis of new C2,C4-disubstituted thietane 1-oxides has been developed by using a lithiation/electrophilic trapping sequence. The chemical and configurational stability of lithiated C2-substituted thietane 1-oxides has been investigated as well as the stereochemical implications for this process. The results demonstrate that a stereoselective functionalization at the C2, C4 positions of a thietane is feasible, leaving intact the four-membered ring.