Since the discovery of organic azides by Peter Griess more than 140 years ago, numerous syntheses of these energy-rich molecules have been developed. In more recent times in particular, completely new perspectives have been developed for their use in peptide chemistry, combinatorial chemistry, and heterocyclic synthesis. Organic azides have assumed an important position at the interface between chemistry, biology, medicine, and materials science. In this Review, the fundamental characteristics of azide chemistry and current developments are presented. The focus will be placed on cycloadditions (Huisgen reaction), aza ylide chemistry, and the synthesis of heterocycles. Further reactions such as the aza-Wittig reaction, the Sundberg rearrangement, the Staudinger ligation, the Boyer and Boyer-Aube rearrangements, the Curtius rearrangement, the Schmidt rearrangement, and the Hemetsberger rearrangement bear witness to the versatility of modern azide chemistry.

Organic Azides: An Exploding Diversity of a Unique Class of Compounds

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

This review is essentially an update of one entitled “πBonding and The Lone Pair Effect in Multiple Bonds Between Heavier Main Group Elements” which was published more than 10 years ago in this journal.1 The coverage of that survey was focused on the synthesis, structure, and bonding of stable compounds2 of heavier main group elements that correspond to the skeletal drawings reproduced in Tables 1 and 2. A row of numbers is listed at the bottom of each column in these tables. This refers to the number of stable complexes from each class that are currently known. The numbers in parentheses refer to the number of stable species that were known at the time of the previous review. Clearly, many of the compound classes listed have undergone considerable expansion although some remain stubbornly rare. The most significant developments for each class will be discussed in detail under the respective sections. As will be seen, there are also a limited number of multiple bonded heavier main group species that do not fit neatly in the classifications in Tables 1 and 2. However, to keep the review to a manageable length, the limits and exclusions, which parallel those used earlier, are summarized as follows: (i) discussion is mainly confined to compounds where experimental data on stable, isolated species have been obtained, (ii) stable compounds having multiple bonding between heavier main group elements and transition metals are not generally discussed, (iii) compounds in which a multiple bonded heavier main group element is incorporated within a ring are generally not covered, and (iv) hypervalent main group compounds that may incorporate faux multiple bonding are generally excluded. Such compounds are distinguished from those in Tables 1 and 2 in that they apparently require the use of more than four valence bonding orbitals at one or more of the bonded atoms. The remainder of this review is organized in a similar manner to that of the previous one wherein the compounds to be discussed are classified according to those summarized in Tables 1 and 2. The key unifying feature of almost all molecules discussed in this review is that they are generally stabilized by the use of bulky substituents which block associative or various decomposition pathways.3 Since the previous review was published in 1999, several review articles that cover parts of the subject matter have appeared.4

π-Bonding and the Lone Pair Effect in Multiple Bonds Involving Heavier Main Group Elements: Developments in the New Millennium

http://www.chemie.uni-muenchen.de/ac/kluefers/homepage/L/bac/ca_2004.pdf

Synthetic analogues relevant to the structure and function of zinc enzymes.

High oxidation state multiple metal-carbon bonds.

Das Iminoboran tBuB = NtBu (1) bildet sich in der Gasphase bei 530°C aus dem Aminoboran Cl(tBu)B – N(tBu)SiMe3 durch Abspaltung von Me3SiCl. Das Boran 1 dimerisiert langsam zum Diazadiboretidin [tBuBNtBu]2 (2). Fur Iminoborane typische Reaktionen, namlich die Ethyloborierung mit BEt3, di [2 + 3]-Cycloaddition mit PhN3 und die Azidosilierung mit Me3SiN3, fuhren auch mit 1 zu den erwarteten Produkten. Die Molekul- und Kristallstrukturen von 1 und 2 werden angegeben; die zentrale CBNC-Kette von 1 ist linear gebaut, der BN-Abstand betragt 125.8 pm; das Vierringgerust von 2 wird durch die sperrigen Ring-Liganden aus der Planaritat gedrangt. In seinen strukturellen Eigenschaften ist das Iminoboran 1 mit dem isoelektronischen Alkin tBuCCtBu (3), das Cyclodimere 2 mit dem isoelektronischen Cyclobutadien [tBuC = CtBu]2 (4) vergleichbar. Die Starke der Dreifachbindung fallt von 3 nach 1 in ahnlicher Weise ab wie beim Ubergang von N2 zu CO.



Preparation, Reactions, and Structure of tert-Butyl(tert-butylimino)borane



The iminoborane tBuB = NtBu (1) is formed from the aminoborane Cl(tBu)B – N(tBu)SiMe3 by the elimination of Me3SiCl in the gas phase at 530°C. Compound 1 slowly dimerizes to the diazadiboretidine [tBuBNtBu]2 (2). In a way typical for iminoboranes, the borane 1 undergoes an ethyloboration with BEt3, a [2 + 3]-cycloaddition with PhN3, and an azidosilation with Me3SiN3. The crystal and molecular structure of 1 and 2 are described; there is a linear central CBNC-chain in 1 with a BN-bond distance of 125.8 pm; the ring-skeleton of 2 deviates from planarity by the influence of the bulky ring ligands. The structural properties of the iminoborane 1 are comparable to those of the isoelectronic alkyne tBuCCtBu (3), and the properties of the cyclodimer 2 are comparable to those of the isoelectronic cyclobutadiene [tBuC = CtBu]2 (4). The strength of the triple bond decreases from 3 to 1 in a similar manner as by going from N2 to CO.

Darstellung, Reaktionen und Struktur von tert-Butyl(tert-butylimino)boran

Bei der Gasphasenthermolyse der Aminoborane RB(Cl)-N(SiMe3)R' (la–j) bei 500–600°C erhalt man neben Me3SiCl entweder die Borazine (RBNR')3 (3a–f) oder die Diazadiboretidine (RBNR')2 (2g–h) oder das Borimid C6F5–B = N–tBu (4i) oder - im Falle R' = Mesityl – das bicyclische Produkt eines intramolekularen Ringschlusses 5. 4i dimerisiert bei Raumtemperatur ohne Losungsmittel schnell, in Losung weniger rasch zu seinem Cyclodimeren 2i. Dieses kristallisiert in der Raumgruppe C2/c. 2i erfahrt mit Ketonen im Verhaltnis 1:1 eine enolytische Ring-offnung (6i,6i). Die CO-Bindung ungesattigter Aldehyde schiebt sich in den Vierring von 2i unter Sechsringbildung (7i, 7i) ein. Mit PhHC = N(O)Me setzt sich 2i unter Abspaltung von PhHC = NtBu zum Oxadiazadiborolidin 9i um. IR- und NMR-spektroskopische Befunde sprechen sowohl fur eine BN-Bindungsordnung groser als zwei als auch fur BC-π-Wechselwirkungen in der Titelverbindung 4i. Mit Ph2C = O reagiert 4i analog zur Wittig-Reaktion zu (C6F5BO)3 und Ph2C = NtBu und dieses geht mit 4i quantitativ eine [2 + 2]-Cycloaddition zu 10i ein. Mit Cyclopentadien setzt sich 4i als Dienophil im Sinne einer Diels-Alder-Reaktion zu 11i um.



Formation, Structure, and Reactivity of (Pentafluorophenyl)boron tert-Butylimide and its Cyclodimer



Gas-phase thermolysis of the aminoboranes RB(CI)-N(SiMe3)R' (la–j) at 500–600°C gives Me3SiCl and either the borazines (RBNR')3 (3a–f) or the diazadiboretidines (RBNR')2 (2g–h) or the boron imide C6F5 – B = N – tBu (4i) or – in the case of R' = mesityl – the bicyclic product of an intramolecular ring closure, 5. At room temperature, the pure imide 4i quickly dimerizes to 2i; this process is retarded in solution. 2i crystallizes in the space group C2/c. Ring opening to 6i, 6i is achieved by the 1:1-reaction of 2i with the enolic tautomer of ketones. The CO bond of unsaturated aldehydes can be inserted into the ring skeleton of 2i to form 7i, 7i′. The oxadiazadiborolidine 9i and PhHC=NtBu are formed by the reaction of 2i with PhHC = N(O)Me. IR and NMR data indicate a BN bond order greater than two and BC-π bond interactions in 4i. A Wittig-type reaction is observed with 4i and Ph2C = O, giving (C6F5BO)3 and Ph2C = NtBu. 4i quantitatively adds to Ph2C = NtBu, forming a [2 + 2]-cycloadduct l0i. 4i also reacts as a dienophile in a Diels-Alder reaction with cyclopentadiene yielding 11i.

Bildung, Struktur und Reaktivität von (Pentafluorphenyl)bor‐tert‐butylimid und seinem Cyclodimeren

Die Synthese von Pyridin-Borabenzol (8) gelingt, wenn aus 1-Methoxy-6-(trimethylsilyl)-1-bora-2,4-cyclohexadien (7) in Gegenwart von Pyridin bei 60°C Methoxytrimethylsilan abgespalten wird. Die gelbe Verbindung zeigt eine Charge-Transfer-Bande bei 472 nm. Die Rontgenstruktur-analyse sowie die Protonensignale von 8 belegen den aromatischen Charakter des Borabenzols. Pyridin- und Borabenzolring sind um 43.3° gegeneinander verdreht. Pyridin-2-Boranaphthalin (14a), dessen Herstellung durch Abspaltung von Chlortrimethylsilan in Gegenwart von Pyridin aus 2-Chlor-1,2-dihydro-1-(trimethylsilyl)-2-boranaphthalin (13) erfolgt, bildet tiefrote Kristalle. Seine Charge-Transfer-Bande bei 486 nm kommt ebenso wie diejenige in 8 durch einen Ubergang aus dem HOMO des Boraaren-Molekulteils in das LUMO des Pyridinteils zustande. Die geringe Verdrillung der beiden Ringsysteme in 14a von 8.1° erleichtert den Elektronentransfer. Die Konjugation der beiden Molekulteile in 14a fuhrt gegenuber 8 zu einer Verkurzung der B – N-Bindung von 155.8 auf 151.5 pm. Im Gegensatz zum roten 14a ist Triethylamin-2-Boranaphthalin (14b) farblos.



Synthesis and Structure Investigation of Pyridine-Borabenzene and Pyridine-2-Boranaphthalene



The synthesis of pyridine-borabenzene (8) succeeds when methoxytrimethylsilane is eliminated from 1-methoxy-6-(trimethylsilyl)-1-bora-2,4-cyclohexadiene (7) in the presence of pyridine at 60°C. The yellow compound shows a charge transfer band at 472 nm. The X-ray structure analysis as well as the proton NMR signals of 8 prove the aromatic character of the borabenzene. Pyridine and borabenzene ring are twisted by 43.3°. Pyridine-2-boranaphthalene (14a), whose synthesis occurs by elimination of chlorotrimethylsilane from 2-chloro-1,2-dihydro-1-(trimethylsilyl)-2-boranaphthalene (13) in the presence of pyridine, forms deep red crystals. Its charge transfer band at 486 nm results like that in 8 by a transition from the HOMO of the boraarene part of the molecule into the LUMO of the pyridine part. The small twist of the two ring systems in 14a of 8.1° facilitates the electron transfer. The conjugation of both parts of the molecule in 14a leads to a shortening of the B N-bond compared with 8 from 155.8 to 151.5 pm. In contrast to the red 14a, triethylamine-2-boranaphthalene (14b) is colourless.

Synthese und Strukturuntersuchung von Pyridin‐Borabenzol und Pyridin‐2‐Boranaphthalin

[Bis(trimethylsilyl) methylene] (diisopropylamino) borane: an Allene-Type N = B = C System

The title compound iPr2NBC(SiMe3)2 (4) is formed as a storable colourless liquid by the elimination of FSiMe3 from F–(iPr2N)B–C(SiMe3)3 (3) at 520°C. The compounds OCHPh and Me3SiN3 are added to the BC bond of 4 to give the [2 + 2] cycloaddition product 5 and the [2 + 3] cycloaddition product 6, respectively. The alkylideneaminoborane 4 crystallizes in the space group P21/n. Perpendicular planes through C2NB and BCSi2 define an allene-type structure with a linear NBC skeleton, bond lengths 136.3 (NB) and 139.1 pm (BC).

Peter Paetzold

Papers

Darstellung, Reaktionen und Struktur von tert-Butyl(tert-butylimino)boran

Bildung, Struktur und Reaktivität von (Pentafluorphenyl)bor‐tert‐butylimid und seinem Cyclodimeren

Synthese und Strukturuntersuchung von Pyridin‐Borabenzol und Pyridin‐2‐Boranaphthalin

[Bis(trimethylsilyl)methylen](diisopropylamino)boran: ein Allen-homologes NBC-System

New perspectives in boron-nitrogen chemistry - I