Chemical consequences of intramolecular Te ← N coordination in tellurium-containing aromatic azomethine derivatives

A tellura-Baeyer-Villiger oxidation: one-step transformation of tellurophene into chiral tellurinate lactone

Baeyer-Villiger (BV) oxidation is a fundamental organic reaction, whereas the hetero-BV oxidation is uncharted. Herein, a tellura-BV oxidation is discovered. By oxidizing a tellurophene-embedded and electron-rich polycycle (1) with mCPBA or Oxone, an oxygen atom is inserted into the Te-C bond of the tellurophene to form tellurinate lactone mono-2. This reaction proceeds as follows: (i) 1 is oxidized to the tellurophene Te-oxide form (IM-1); (ii) IM-1 undergoes tellura-BV oxidation to give mono-2. Moreover, the hybrid trichalcogenasumanenes 7 and 8 are, respectively, converted to tellurinate lactones mono-9 and mono-10 under the same conditions, indicating that tellura-BV oxidation shows high chemoselectivity. Due to the strong secondary bonding interactions between the Te[double bond, length as m-dash]O groups on tellurinate lactones, mono-2, mono-9, and mono-10 are dimerized to form U-shaped polycycles 2, 9, and 10, respectively. Notably, mono-2, mono-9, mono-10, and their dimers show chirality. This work enables one-step transformation of tellurophene into tellurinate lactone and construction of intricate polycycles.

Kinetic and Donor Stabilization of Organotellurenyl Iodides and Azides

The first tellurium compounds containing the extremely bulky tris(phenyldimethylsilyl)methyl (Tpsi) and 2,6-bis(2,4,6-triisopropylphenyl)phenyl (2,6-Trip(2)C(6)H(3)) moieties have been synthesized and isolated. Careful oxidation of the tellurolate TpsiTeLi (1) resulted in the formation of the crowded ditellane (TpsiTe)(2) (2), and iodination of 2 gave the alkanetellurenyl iodide TpsiTeI (3). In a similar fashion, the terphenyl-substituted ditellane (2,6-Trip(2)C(6)H(3)Te)(2) (9) and the arenetellurenyl iodide 2,6-Trip(2)C(6)H(3)TeI (10) were prepared. Reaction of the iodides TpsiTeI (3) and 2,6-Trip(2)C(6)H(3)TeI (10), as well as TripTeI, MesTeI (Trip = 2,4,6-triisopropylphenyl, Mes = 2,4,6-tri-tert-butylphenyl), and the donor-stabilized 2-Me(2)NCH(2)C(6)H(4)TeI, with AgN(3) resulted in the formation and isolation of the corresponding tellurenyl azides TpsiTeN(3) (4), TripTeN(3) (7), MesTeN(3) (8), 2,6-Trip(2)C(6)H(3)TeN(3) (11), and 2-Me(2)NCH(2)C(6)H(4)TeN(3) (12). Furthermore, the corresponding tris(ethyldimethylsilyl)methyl-containing (Tesi) tellurium compounds (TesiTe)(2), TesiTeI (5), and TesiTeN(3) (6) have been prepared but could not be isolated in pure form. The crystal structures of TpsiTeLi (1), (TpsiTe)(2) (2), TpsiTeN(3) (4), 2,6-Trip(2)C(6)H(3)TeI (10), 2,6-Trip(2)C(6)H(3)TeN(3) (11), and 2-Me(2)NCH(2)C(6)H(4)TeN(3) (12) have been determined by X-ray diffraction. Additionally, computational studies of the molecules for which experimental structural data were available were performed.

Organotellurium(VI) Azides and Halides

The reaction of azide with organotellurium(VI) halides Ph(5)TeBr and cis-(biphen)(2)TeF(2) (biphen = 2,2'-biphenyldiyl) resulted in the formation and isolation of Ph(5)TeN(3) (1) and cis-(biphen)(2)Te(N(3))(2) (2), which are the first tellurium(VI)-azide species. In addition to spectroscopic data, both crystal structures have been determined. Furthermore, the stability of possible Te(VI) species with higher azide contents Ph(x)()Te(N(3))(6)(-)(x)() and Me(x)()Te(N(3))(6)(-)(x)() as well as the syntheses and properties of their Ph/Me(x)()TeF(y)() precursors was investigated, including the crystal structure determination of trans-Ph(2)TeF(4) (3). Ab initio and density functional studies of all molecules regarding the structures and electronic populations were performed.

Cyclic aromatic systems with hypervalent centers

Strong attractive intramolecular X<--Y(S, NR') interactions of 1,5-type occurring between terminal electron-rich main-group 15-17 centers (X = pnictogen, chalcogen, or halogen) and carbonyl, thione, or imine groups (Y = O, S, NR') incorporated into the conjugated -X-CH=CH-CH=Y fragments result in significant stabilization of cis-s-cis structure of these molecules, which may be viewed as the five-membered pseudo-heterocycles with the hypervalent arrangements across the pnictogen, chalcogen, or halogen centers. In derivatives of beta-chalcovinylaldehydes and isoelectronic chloronium cations, one of two lone electron pairs at X = S, Se, Te, Cl+ possesses pure p character and is involved in conjugation with the pi-system of the rest of the molecule, which leads to an appreciable contribution of the aromatic stabilization of the 6 pi-electron ring closed by the X<--Y bond. The aromaticity of these structures evaluated through calculation of the homodesmotic stabilization energies (HSE) increases in parallel with an increase in the strength of the intramolecular coordination bonds, i.e., in the order X = S, Se, Te and with an increase in electronegativity of a substituent attached to the chalcogen atom. Clear manifestation of the aromatic character of the cis-s-cis isomers of beta-chalcovinylaldehydes, their imines, as well as congeneric thiones and chloronium cations comes from the theoretical and experimental evidence for the pronounced equalization (as compared with the open trans-s-trans isomers) of the CC bond lengths. The cooperative effects of the hypervalent bonding and aromaticity are most distinct in the bicyclic structures of 1,6,6a lambda 4-trichalcapentalenes, their 1,6-dioxa(aza) analogues, and 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes, in which two conjugated -X-CH=CH-CH=Y fragments are fused together via the symmetric -Y-X-Y- triad. The assessments based on the HSE values indicate that the aromaticity of these 10 pi-electron compounds is estimated as about 30-50% of the aromatic character of the most aromatic bicyclic structure of naphthalene. As shown by extensive ab initio calculations, substantial hypervalent bonding effects also operate in the case of beta-pnictovinylaldehydes formed by second and lower row pnictogen atoms (X = P, As, Sb, Bi), providing for the enhanced stability of their cis-s-cis configuration with respect to the free-of-strain trans-s-trans structures. The bicyclic 1,6,6a lambda 5-dioxapnictapentalene structure is also the most energy favorable form of these compounds. However, by contrast with the chalcogen and halonium analogues, the pseudo-cyclic structure of beta-pnictovinylaldehydes and the bicyclic structure of 1,6,6a lambda 5-dioxapnictapentalenes are maintained through primarily the occurrence of the hypervalent bonding across the pnictogen centers, whereas pi-delocalization is not (or weakly) operative in these compounds. A possible explanation for this finding is the low p character of the single lone electron pair at the pnictogen and orientation of the axis of its orbital, which is unfavorable for the optimal overlap with the pi-system of the rest of a molecule. As first proposed by Arduengo and Dixon, configurational isomerizations at the second and lower row tricoordinate pnictogen centers may proceed through the T-shaped hypervalent structures. Such a structure is strongly stabilized in the aromatic 1,6-dioxa-3a-aza-6a lambda 4-pnictapentalenes. It was found by calculations that the aromatic stabilization of the T-shaped hypervalent arrangement at the tricoordinate pnictogen center in 2H-1,3,2-dioxaphosphole and arsole arising due to withdrawal of two electrons from the pi-system of the ring onto the hypervalent electron-excessive center facilitates the rearrangement and directs it along the reaction path with the aromatic transition-state structure. Investigation of the influence of cooperative effects of hypervalent bonding and aromaticity on stability and mechanisms of reactions of main-group 14-17 compounds not considered in this review is the subject of our continuing research.

Isolation of stable enantiomerically pure telluroxides and their stereochemistry

Optical resolution of kinetically and thermodynamically stabilized diaryl telluroxides possessing bulky substituents (rac-1a-d) and amino group (rac-2a-c), respectively, by liquid chromatography using optically active columns yielded stable enantiomerically pure telluroxides. The absolute configurations of the optically active telluroxides were determined by comparing their specific rotations and CD spectra with those of sulfur or selenium analogues. The kinetics for the racemization of optically active telluroxides in solution was studied, and it was found that kinetic and thermodynamic stabilization were very effective preventing the racemization of telluroxides. The stabilization energy of telluroxides by intramolecular coordination of the amino group to the tellurium atom was estimated to be ca. 5 kcal mol-1 by variable temperature 1H NMR measurement. The mechanism for the racemization of optically active telluroxides was studied by an isotope experiment using H2(18)O, and the results indicated that optically active telluroxides underwent racemization via an achiral tetracoordinated hydrate.