The structure

Organic Chemistry By Dr. I. L. Finar. Vol. 2: Stereochemistry and the Chemistry of Natural Products. Pp. xi + 733. (London and New York: Longmans, Green and Co., Ltd., 1956.) 40s. net.

Advanced Organic Chemistry

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

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

Ketenes and Other Cumulenes as Reactive Intermediates

(Me3Si)2CNN isomerizes upon addition of traces of [Me3Si]+ ions to give (Me3Si)2NNC, which then undergoes an unusual trimerization reaction to give exclusively 4-diazenyl-3-hydrazinylpyrazole. As catalyst the isonitrilium ion, [(Me3Si)2NNC(SiMe3)]+, was identified and fully characterized. Experiments and computations indicate a three-step reaction including isomerization of diazomethane, a C–C or N–C coupling, and a formal cycloaddition reaction. The kinetics and thermodynamics are discussed on the basis of DFT calculations.

Catalytic Trimerization of Bis-silylated Diazomethane

The reaction of m-terphenyl amine (2,6-bis-(2,4,6-trimethylphenyl)aniline, m-Ter = m-terphenyl = 2,6-bis-(2,4,6-trimethylphenyl)) and ECl3 (E = P, As) in the presence of different bases (Et3N, n-BuLi, LDA, DBU) and under different reaction conditions was studied. The reaction with excess Et3N yielded m-Ter–N(H)–AsCl2 for E = As, while for E = P m-Ter–N(PCl2)2 was formed. m-Ter–N(H)–ECl2 was obtained in the reaction of m-terphenyl amine with n-BuLi and ECl3 for E = As and P. Further treatment of m-Ter–N(H)–PCl2 with Et3N led to the formation of 1,3-dichloro-cyclo-1,3-diphospha-2,4-diazane, a synthesis protocol which cannot be applied to the analogous arsenic species. 1,3-dichloro-cyclo-1,3-diarsa-2,4-diazane was isolated when DBU was added to m-Ter–N(H)–AsCl2 at low temperature (−80 °C).

Synthesis of sterically encumbered 2,4-bis-m-terphenyl-1,3-dichloro-2,4-cyclo-dipnictadiazanes [m-TerNPnCl]2, (Pn = P, As)

A new synthetic approach enabled the generation of highly labile thionylimide, H-NSO, which was trapped by adduct formation with the bulky Lewis acid B(C6 F5 )3 and fully characterized. For comparison, a series of different Me3 Si-NSO Lewis acid adducts were studied. Treatment of Me3 Si-NSO with the silylium ion [Me3 Si](+) led to the formation of the hitherto unknown iminosulfonium ion [Me3 Si-N=S-O-SiMe3 ](+) , which could be isolated and fully characterized as a salt in the presence of weakly coordinating carborate anions.

Isolation of Labile Pseudohalogen NSO Species.

Synthesis of the First Persilylated Ammonium Ion, [(Me3Si)3NSi(H)Me2]+, by Silylium-Catalyzed Methyl/Hydrogen Exchange Reactions

The dinuclear zirconocene chloride complex 1 is a highly active precatalyst for the dehydropolymerisation of methylamine borane. Comparison with mononuclear Zr chlorides and related dinuclear complexes suggests that the nature of the bridging motif is essential for the unique reactivity of 1.

Fabian Reiß

Papers

Catalytic Trimerization of Bis-silylated Diazomethane

Synthesis of sterically encumbered 2,4-bis-m-terphenyl-1,3-dichloro-2,4-cyclo-dipnictadiazanes [m-TerNPnCl]2, (Pn = P, As)

Isolation of Labile Pseudohalogen NSO Species.

Synthesis of the First Persilylated Ammonium Ion, [(Me3Si)3NSi(H)Me2]+, by Silylium-Catalyzed Methyl/Hydrogen Exchange Reactions

Dehydropolymerisation of methylamine borane using a dinuclear 1,3-allenediyl bridged zirconocene complex.