The chemistry of heterocyclic carbenes has experienced a rapid development over the last years. In addition to the imidazolin-2-ylidenes, a large number of cyclic diaminocarbenes with different ring sizes have been described. Aside from diaminocarbenes, P-heterocyclic carbenes, and derivatives with only one, or even no heteroatom within the carbene ring are known. New methods for the synthesis of complexes with N-heterocyclic carbene ligands such as the oxidative addition or the metal atom template controlled cyclization of β-functionalized isocyanides have been developed recently. This review summarizes the new developments regarding the synthesis of N-heterocyclic carbenes and their metal complexes.

Heterocyclic Carbenes: Synthesis and Coordination Chemistry

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

A decade ago we initiated research, the goal of which was isolation of a stable carbene. Our success has helped to catalyze a resurgence of interest in readily available and easily handled carbenes. Research on stable (nucleophilic) carbenes is again a popular theme worldwide. Efforts in the general area of stable carbenes now focus not only on chemistry of the carbenes themselves but also on their applications to other chemical systems, where their chemical properties create technical opportunities that are unavailable with other functional groups. The quest for isolable carbenes is a long saga whose origin can be traced back to the first half of the 1800s. A recently published history of this quest provides an important backdrop for the research described in this Account.1 It is the intent of this Account to delineate the events and environment that led to the report from DuPont laboratories of the first isolation of a stable crystalline carbene. Getting Started

Looking for Stable Carbenes: The Difficulty in Starting Anew

π-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements

Transition metal chemistry of cyclopropenes and cyclopropanes.

Tetra-tert-butyltetrahedrane—a useless molecule? In the first part of this progress report an attempt is made to answer this provoking question in order that the reader can evaluate why it is worthwhile to track such an esoteric molecule. An account of the unusual properties of tetra-tert-butyltetrahedrane, the only tetrahedrane unequivocally identified so far, and the most highly strained saturated hydrocarbon of all, is followed by a summary of attempts to synthesize the parent compound. The emphasis in these two sections is on our own research achievements. Since any discussion of the tetrahedranes must necessarily include the corresponding cyclobutadienes, it was found appropriate to organize the article at the same time as a final account of the cyclobutadiene problem. Masamune[8c] characterized his excellent report in 1980 in the journal Tetrahedron as a comma in the history of cyclobutadiene. Meanwhile, the significant questions have been answered. The time is now ripe to replace the comma by a full stop.

Tetrahedrane and Cyclobutadiene

The Cyclobutadiene Problem

The recent isolation of isofulminic acid has opened the door for a comparative study of all the four possible CHNO isomers: isocyanic acid, cyanic acid, fulminic acid, and isofulminic acid. Their infrared spectra have been measured in an argon matrix at 13 K and are compared with the results of ab initio calculations.

The CHNO Isomers

Tetra-tert-butyltetrahedran – ein nutzloses Molekul? Eine Antwort auf diese provozierende Frage wird im ersten Teil des vorliegenden Forschrittsberichts versucht. Der Leser soll nachvollziehen konnen, warum es sich lohnt, einem derart esoterischen Molekul nachzujagen. Zweifelsfrei identifiziert wurde bisher nur Tetra-tert-butyltetrahedran, der gespannteste gesattigte Kohlenwasserstoff uberhaupt. Der Beschreibung der ungewohnlichen Eigenschaften dieses Derivats folgt eine Zusammenfassung uber Versuche zur Synthese der Stammverbindung. Im Vordergrund dieser beiden Abschnitte stehen die Ergebnisse eigener Arbeiten. Da die Diskussion von Tetrahedranen die entsprechenden Cyclobutadiene zwangslaufig einschliesen mus, hat es sich angeboten, den Beitrag zugleich als eine Art Abschlusbericht zum Cyclobutadien-Problem zu konzipieren. Masamune hat 1980 seinen vorzuglichen Report in der Zeitschrift Tetrahedron als ein Komma in der Geschichte des Cyclobutadiens bezeichnet. Inzwischen sind die wesentlichen Fragen beantwortet. Die Zeit ist nunmehr reif, anstelle eines Kommas einen Punkt zu setzen.

Tetrahedran und Cyclobutadien

Cyclobutadien ist in den letzten Jahren erneut in den Blickpunkt des Interesses geruckt. Durch Anwendung der Matrix-Isolier-Technik ist es jetzt moglich, Cyclobutadiene als Monomere in Matrices einzufrieren. Auf diese Weise sind sowohl der unsubstituierte Grundkorper als auch alkylsubstituierte Derivate einer Untersuchung bequem zuganglich. In diesem Fortschrittsbericht wird ein Uberblick uber das Gesamtgebiet gegeben; im Vordergrund stehen jedoch die Ergebnisse eigener Arbeiten uber dieses klassische Problem der organischen Chemie.

Günther Maier

Papers

Tetrahedrane and Cyclobutadiene

The Cyclobutadiene Problem

The CHNO Isomers

Tetrahedran und Cyclobutadien

Das Cyclobutadien‐Problem