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

10. Patented Literature 2616 10.1. Esterification 2616 10.2. Ether Formation 2619 10.2.1. Etherification without Cyclization 2619 10.2.2. Etherification with Cyclization 2624 10.3. N-Alkylation 2625 10.4. Other Reactions 2627 11. Summary and Outlook 2628 12. Note Added in Proof 2628 13. Abbreviations Used in This Review 2629 14. Acknowledgments 2629 15. Supporting Information Available 2630 16. References 2630

Mitsunobu and Related Reactions: Advances and Applications

Heterocyclische Carbene – Synthese und Koordinationschemie

Visible-light-mediated photocatalytic generation of carbon-centred radicals from alkyl, alkenyl and aryl iodides, which then undergo subsequent hydrogen-atom abstraction or reductive cyclizations, is reported. The protocol is characterized by the use of inexpensive reagents, mild conditions, exceptional functional group tolerance, and good to high yields.

Engaging unactivated alkyl, alkenyl and aryl iodides in visible-light-mediated free radical reactions

Electron-transfer reductions of unactivated aryl and alkyl iodides with a neutral ground-state organic molecule are reported. The reducing agent 1 is formed in two steps from N-methylbenzimidazole using very simple chemistry, and subsequent treatment of the iodoalkane or -arene with 1 affords cyclized products (see scheme).

Highly Efficient Reduction of Unactivated Aryl and Alkyl Iodides by a Ground‐State Neutral Organic Electron Donor

Stabilized carbanions can be prepared by deprotonation of the corresponding carbon acids, but more reactive carbanions, such as aryl anions, are usually made by metal–halogen exchange or by electrochemical routes.[1] Whereas the use of metals and organometallic compounds to form aryl anions is well established, the use of totally organic electron-transfer agents, in the absence of photochemical activation, is completely unknown.[2] Herein we report the first neutral organic molecule to achieve this feat.

The Generation of Aryl Anions by Double Electron Transfer to Aryl Iodides from a Neutral Ground-State Organic Super-Electron Donor

Previous efforts to prepare tetraazafulvalenes derived from imidazolium salt precursors have met with little success (one anomalously favourable example is known), and this is in line with the predicted reactivity of these compounds. However, we now report the preparation of a series of these tetraazafulvalenes formed either by deprotonation of 1,3-dialkylimidazolium salts or by Birch reduction of biimidazolium salts. The tetraazafulvalenes are highly reactive; for example, they act as Super-Electron-Donors towards iodoarenes. The two most reactive examples are formed more efficiently by Birch reduction than by the deprotonation route. Nevertheless, even in cases where the deprotonation approach affords a low stationary concentration, the mixture of precursor salt and base still produces the same powerful reductive chemistry that is the hallmark of tetraazafulvalene electron donors.

Imidazole-derived carbenes and their elusive tetraazafulvalene dimers

Reaction of imidazolylidene-derived enetetramine 2 with aliphatic iodides and bromides (and with aryl iodides bearing alkene-containing side-chains in the ortho-position) leads to formation of aliphatic aldehydes through an unprecedented extrusion of a one-carbon unit from the enetetramine. An intermediate 2-alkylimidazoline 24 is proposed, where the alkyl group derives from the substrate; this imidazoline undergoes further reaction in situ to afford the observed aldehydes on acidic workup. Modified substrates were designed and prepared to probe the chemistry of the alkylimidazoline adducts and provided extensive information on the chemistry of the adducts.

One-carbon extrusion from a tetraazafulvalene. Isolation of aldehydes and a study of their origin.

Neutral organic electron donors, featuring pyridinylidene–imidazolylidene, pyridinylidene–benzimidazolylidene and imidazolylidene–benzimidazolylidene linkages are reported. The pyridinylidene–benzimidazolylidene and imidazolylidene–benzimidazolylidene hybrid systems were designed to be the first super electron donors to convert iodoarenes to aryl radicals at room temperature, and indeed both show evidence for significant aryl radical formation at room temperature. The stronger pyridinylidene–imidazolylidene donor converts iodoarenes to aryl anions efficiently under appropriate conditions (3 equiv of donor). The presence of excess sodium hydride base has a very important and selective effect on some of these electron-transfer reactions, and a rationale for this is proposed.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-8-112.pdf

Douglas W. Thomson

Papers

Highly Efficient Reduction of Unactivated Aryl and Alkyl Iodides by a Ground‐State Neutral Organic Electron Donor

The Generation of Aryl Anions by Double Electron Transfer to Aryl Iodides from a Neutral Ground-State Organic Super-Electron Donor

Imidazole-derived carbenes and their elusive tetraazafulvalene dimers

One-carbon extrusion from a tetraazafulvalene. Isolation of aldehydes and a study of their origin.

Hybrid super electron donors - preparation and reactivity