The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the – naturally subjective – “reactive”-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

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Reactive p-block cations stabilized by weakly coordinating anions

Advances in organic tellurium chemistry

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.

Cyclic aromatic systems with hypervalent centers.

Le compose 1 cristallise dans le groupe P2 1 /c (Affinement jusqu'a R=0,022), le second dans P1 (R=0,042) et le troisieme dans P1 (R=0,037). Les trois composes ont une geometrie pseudo bipyramidale trigonale; ils forment des liaisons secondaires pour donner des systemes plan pentagonaux autour de Te et de I

Secondary bonding. XIII: Aryl-tellurium(IV) and -iodine(III) acetates and trifluoroacetates: the crystal and molecular structures of bis-(p-methoxyphenyl)tellurium diacetate, μ-oxo-bis[diphenyltrifluoroacetoxytellurium] hydrate, and [bis(trifluoroacetoxy)iodo]benzene

Copper (I)-Catalyzed Efficient and Stereoselective Synthesis of (E)-Vinyl Selenides and Tellurides by the Reaction of Potassium Vinyltrifluoroborates with Diphenyl Dichalcogenides

Synthesis and structure of β-tellurovinylcarbonyl compounds

Data on the synthesis, reactions and structures of stable aromatic tellurols and sterically hindered tellurols containing E–TeH bonds (E = C, Si or Ge) are systematised and generalised Methods for the preparation of various metal derivatives of tellurols as well as applications of these derivatives in the low-temperature synthesis of metal tellurides are considered The bibliography includes 90 references

http://iopscience.iop.org/article/10.1070/RC1999v068n11ABEH000544/pdf

Stable tellurols and their metal derivatives

Literature data concerning the synthesis and reactions of diorganyl derivatives of tellurium(IV) (σ-telluranes of R2TeX2 type) are systematized and generalized in this review.

Diorganyl Derivatives of Tellurium(IV) (σ-Telluranes of R2TeX2 Type)

Data on the synthesis, reactions and structures of monoorganyl derivatives of tellurium(IV) (σ-telluranes) RTeX3 are systematised and generalised. The use of these compounds in preparative organic chemistry is considered. The bibliography includes 238 references.

Monoorganyl derivatives of tellurium(IV)

The published data on the synthesis, structures and properties of triorganyltelluronium salts are generalised and systematised. Special attention is given to the applications of these compounds in preparative organic synthesis.

A. V. Zakharov

Papers

Synthesis and structure of β-tellurovinylcarbonyl compounds

Stable tellurols and their metal derivatives

Diorganyl Derivatives of Tellurium(IV) (σ-Telluranes of R2TeX2 Type)

Monoorganyl derivatives of tellurium(IV)

Synthesis, reactions and structures of telluronium salts