Thermal and microwave-assisted [3+2] cycloadditions between differently substituted propiolamidinium tetraphenylborates 3a–d and N-methyl-C-phenylnitrone, benzyl azide, and N-(3-azidopropyl)phthalimide were studied. The activation parameters of the [3+2] cycloaddition between alkyne 3a and benzyl azide were determined. A Diels–Alder reaction between the terminal alkyne 3a and cyclopentadiene could be achieved with the aid of microwave activation. The reaction between 3a and triphenylphosphorane imine provides the β,β-bis(dimethylamino)vinylphosphonium salt 21, which might or might not have been formed through an initial [2+2] cycloaddition reaction.

Phosphirano[1,2-c][1,2,3]diazaphospholes 5, which feature a strongly pyramidalized phosphorus atom at a ring fusion position, were identified as a new type of phosphane ligands for transition metals. The derivatives 5a–c smoothly form W(CO)₅ complexes, which are thermally quite stable – in contrast to the known lability of various carbonylmetal complexes of simple phosphiranes. In the temperature range 120–150 °C, they undergo a clean decomplexation in toluene solution. Bicyclic phosphiranes 5a and 5b, but not the acceptor-substituted derivative 5c, readily react with the [(E,E)-dibenzylideneacetone]palladium(0) complex Pd₂(dba)₃·CHCl₃ to form the corresponding isolable complexes [(5a)₂(dba)Pd] and [(5b)₂(dba)Pd]. In agreement with the formation of these complexes, it was found that 5a,b as ligands effectively promote the Suzuki cross-coupling of 4-bromotoluene with benzeneboronic acid. The solid-state structure of bicyclic phosphirane 5b and of the complexes [(5a)(CO)₅W], [(5b)₂(dba)Pd] (pentane solvate) and [(5b)₂(dba)Pd] (benzene solvate) were determined by X-ray diffraction analysis.

Spiro[fluorene-9,6′-[2]thia[1]phosphabicyclo[3.1.0]hex[3]enes] 7a–c have been obtained in one step from 3,5-diaryl-1,2-thiaphospholes and 9-diazofluorene or its 2,7-dibromo derivative. The bicyclic phosphiranes are stable against water and resist attempts at sulfuration or selenation of the phosphorus atom. However, cleavage of the P–C ring fusion with hydrogen chloride followed by hydrolysis led to the monocyclic 2-(9H-fluoren-9-yl)-2,3-dihydro-1,2-thiaphosphole 2-oxides 8a–c. Phosphiranes 7a–c also react to form the hexacarbonyltungsten-(P–W) complexes 10a–c readily and in high yields. These complexes rearrange in toluene solution at 50–80 °C to form the spiro[fluorene-9,2′-[1]phospha[6]thiabicyclo[3.1.0]hex-3-ene]-W(CO)₅-(P–W) complexes 11a–c, which are the first representatives of ring-fused thiaphosphiranes. Compound 11a is readily desulfurated with tributylphosphane to form the highly oxygen-sensitive spiro[fluorene-9,2′-[2H]phosphole] 13, which is reconverted into 11a upon treatment with sulfur. Bicyclic 1,3,2-dithiaphospholanes 12a–c are formed as minor byproducts of the thermal isomerization of phosphiranes 10. Compound 10a slowly rearranges in solution to give 12a as the sole product. Compounds 12 may result from a formal [3+2] cycloaddition reaction of an α,β-unsaturated thione, formed by partial decomposition of 10, with a dipolar valence isomer of 10 or 11, featuring aW(CO)₅-complexed thiocarbonyl-phospha-ylide dipole (R₂C=S⁺–P^–R). W(CO)₅ complexation is not a prerequisite for the cycloaddition reaction: the uncomplexed phosphirano-thiaphosphole 7a reacts with thiobenzophenone in the same way to form the bicyclic 1,3,2-dithiaphospholane 18 in high yield. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Diazo compounds (R¹R²CN₂) are known as versatile and useful substrates for an array of chemical transformations and, therefore, diazo chemistry is still far from losing anything of its long-standing fascination. In addition to many studies on the subsequent chemistry of the diazo group, the inventory of methods for the preparation of diazo compounds is continuously supplemented by new methods and novel variations of established procedures. Several of these synthetic approaches take into account the lability and remarkable chemical reactivity of certain classes of diazo compounds, and environmentally more benign procedures also continue to be developed.

Aufgrund der vielseitigen und nützlichen Folgechemie von Diazoverbindungen (R¹R²CN₂) ist das Interesse an dieser Substanzklasse seit Jahren ungebrochen. Dabei wird auch das bereits vorhandene Methodenrepertoire zur Herstellung von Diazoverbindungen immer wieder um neue Verfahren oder um neuartige Varianten etablierter Synthesewege ergänzt. Nicht selten tragen die neuen Synthesevorschläge der Labilität und beachtlichen chemischen Reaktivität bestimmter Typen von Diazoverbindungen Rechnung, aber auch dem Aspekt umweltfreundlicherer Methodik wurde Beachtung geschenkt.

Aryl-substituted α-silyl α-diazo ketones are readily transformed into aryl silyl ketenes in the presence of a catalytic amount of triflic acid. Thus, a convenient method to prepare these silyl ketenes becomes available, which combines two steps, silylation of an aryl diazomethyl ketone and acid-induced Wolff rearrangement of the formed α-silyl α-diazo ketone, in a one-pot procedure. It appears that the trialkylammonium salt, which is formed in the silylation step, can also catalyze the Wolff rearrangement, but distinctly more slowly than the proton acid. The silyl ketenes react smoothly with α-silyl α-diazo ketones to form 3-silyl-1-silyloxyallenes in fairly good yields. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)