Life on earth is based on compounds that have carbon frames and backbones. Today, chemists have added to the world of biomolecules and biopolymers approximately 107 different synthetic molecules and polymers, the structures of which also depend on the formation of strong, stable carbon–carbon bonds. Although the stability of carbon–carbon bonds has been recognized for more than a century, the two natural modifications graphite and diamond were, until recently, the only allotropic forms of carbon on earth that were available in macroscopic quantities and were structurally well characterized. With the synthesis of macroscopic quantities of buckminsterfullerene (C60) and the higher fullerenes (C70, C76, C78, etc.) and the exploration of the fascinating properties of these all-carbon spheres, this situation has completely changed. In the coming decades, the design, preparation, and study of novel molecular and polymeric allotropic forms of carbon will be a central topic in chemistry. Research in this area will dramatically advance the fundamental knowledge on carbon-based matter and, as already illustrated by the ongoing work on C60, generate unprecedented technological perspectives. This review surveys synthetic organic-chemical approaches toward the preparation and study of all-carbon molecules and polymers that differ from the familiar networks of graphite and diamond as well as from the fullerenes. We will also discuss the ongoing research on fullerenes with a particular focus on the synthetic approaches to these all-carbon spheres and their transition metal complexes.

Synthetic approaches toward molecular and polymeric carbon allotropes

Synthesis and Applications

C. Pericyclic Reactions 1210 1. [4+2]-Cycloadditions 1210 2. [3+2]-Cycloadditions 1211 3. [2+2]-Cycloadditions 1212 D. Sigmatropic Rearrangements 1212 1. Claisen Rearrangement 1212 2. [2,3]-Wittig Rearrangements 1213 E. Radical Reactions 1214 IV. 1,3-Induction 1215 A. Chelation-Controlled 1,3-Induction 1215 B. Nonchelation-Controlled 1,3-Induction 1216 1. Cram−Reetz Model 1216 2. Evans Model 1216 C. 1,4-Induction 1219 V. Conclusion 1219 VI. References 1221

Around and beyond Cram's Rule.

This Minireview highlights recent advances in the field of aryne and cyclohexyne chemistry that have allowed the extraordinary reactivity of these entities to be harnessed during the course of natural product syntheses. The syntheses presented rely on the use of these reactive species in chemoselective transformations and follow unprecedented synthetic strategies that are inspiring for the practitioners of synthetic organic chemistry.

Arynes and cyclohexyne in natural product synthesis.

The 351 nm photoelectron spectra of the negative ions of o-, m-, and p-benzyne (1,2-, 1,3-, and 1,4-dehydrobenzene, respectively) and their perdeuterated isotopomers have been obtained. The o-benzyne ions were generated by the reaction of benzene and benzene-d6 with O-, while the m- and p-benzyne ions were prepared by the gas-phase reaction between the corresponding 3- and 4-(trimethylsilyl)phenyl anions and molecular fluorine, F2. The photoelectron spectra of the benzyne anions each contain two features, corresponding to formation of the singlet and triplet states of the biradicals. The electron affinities of o- and p-benzyne are found to be 0.564 ± 0.007 and 1.265 ± 0.008 eV, respectively, while the electron affinities of deuterated o- and p-benzyne are found to be 8 and 5 meV lower, respectively. The electron affinity of m-benzyne could not be determined from the photoelectron spectrum because the origin peak could not be assigned unequivocally. For o- and p-benzyne, the singlet−triplet energy splittin...

Ultraviolet Photoelectron Spectroscopy of the o-, m-, and p- Benzyne Negative Ions. Electron Affinities and Singlet−Triplet Splittings for o -, m -, and p -Benzyne

Benzyne, cyclohexyne, and 3-azacyclohexyne have been obtained by flash vacuum pyrolysis of phthalic anhydride, 4-cyclopentylidene-3-methylisoxazol-5(4H)-one (34), and 3-methyl-4-(2-pyrrolidinylidene)isoxazol-5(4H)-one (37), respectively, isolated at 77 K and observed by IR spectroscopy. Trapping and fragmentation reactions are described, and the implications for the mechanism of benzyne formation from phthalic anhydride via cyclopentadienylideneketene (9) are discussed.

Benzyne, cyclohexyne, and 3-azacyclohexyne and the problem of cycloalkyne versus cycloalkylideneketene genesis

Reactive nitrogenous molecules from meldrum′s acid derivatives, pyrrole-2,3-diones, and isoxazolones

Ethynamine (HO≡CNH) has been prepared from three different precursors (7a, 7b, and 8) by flash vacuum pyrolysis and observed by low-temperature infrared spectroscopy for the first time. The collision activation mass spectra (CAMS) strongly support the assignments. The Meldrum's acid derivative 8 also gives rise to equilibrating imidoylketene (10) and aminopropadienone (9), observable by both IR and CAMS. 9 decarbonylates above 680 °C to ethynamine. Ethynamine isomerizes in part to acetonitrile in the gas phase but polymerizes in the solid state between-70 and-50 °C. Both ketene imine (HC═C═NH) and acrylonitrile are formed on FVP of 3-hydroxypropionitrile. 2-Phenylethynamine (PhC≡CNH) is readily produced by FVP of the isoxazolone 11.

Primary ethynamines (HC≡CNH2, PhC≡CNH2), aminopropadienone (H2NCH=C=C=O), and imidoylketene (HN=CHCH=C=O). Preparation and identification of molecules of cosmochemical interest

Horst Briehl

Papers

Benzyne, cyclohexyne, and 3-azacyclohexyne and the problem of cycloalkyne versus cycloalkylideneketene genesis

Reactive nitrogenous molecules from meldrum′s acid derivatives, pyrrole-2,3-diones, and isoxazolones

Primary ethynamines (HC≡CNH2, PhC≡CNH2), aminopropadienone (H2NCH=C=C=O), and imidoylketene (HN=CHCH=C=O). Preparation and identification of molecules of cosmochemical interest

A New Synthesis of Fulminic Acid

Organic fulminates, R-O-NC