Surprisingly slow reaction of dimethylsilylene with dimethylgermane: time-resolved kinetic studies and related quantum chemical calculations

Time-resolved studies of silylene, SiH2, and dimethylsilylene, SiMe2, generated by the 193 nm laser flash photolysis of appropriate precursor molecules have been carried out to obtain rate constants for their bimolecular reactions with dimethylgermane, Me2GeH2, in the gas phase. SiMe2 + Me2GeH2 was studied at five temperatures in the range 299-555 K. Problems of substrate UV absorption at 193 nm at temperatures above 400 K meant that only three temperatures could be used reliably for rate constant measurement. These rate constants gave the Arrhenius parameters log(A/cm(3) molecule(-1) s(-1)) = -13.25 +/- 0.16 and E-a = -(5.01 +/- 1.01) kJ mol(-1). Only room temperature studies of SiH2 were carried out. These gave values of (4.05 +/- 0.06) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + Me2GeH2 at 295 K) and also (4.41 +/- 0.07) x 10(-10) cm(3) molecule(-1) s(-1) (SiH2 + MeGeH3 at 296 K). Rate constant comparisons show the surprising result that SiMe2 reacts 12.5 times slower with Me2GeH2 than with Me2SiH2. Quantum chemical calculations (G2(MP2,SVP)//B3LYP level) of the model Si-H and Ge-H insertion processes of SiMe2 with SiH4/MeSiH3 and GeH4/MeGeH3 support these findings and show that the lower reactivity of SiMe2 with Ge-H bonds is caused by a higher secondary barrier for rearrangement of the initially formed complexes. Full details of the structures of intermediate complexes and the discussion of their stabilities are given in the paper. Other, related, comparisons of silylene reactivity are also presented.

Ring-rearrangement metathesis of substituted 2-aminonorbornenes

In this report we describe the ring-rearrangement metathesis of 2-aminonorbornene derivatives. Ail efficient ruthenium-catalysed metathesis reaction Occurs with a wide range of pendent alkenes and alkynes to generate bicyclic amines and amides.

First efficient and general copper-catalyzed [2,3]-rearrangement of tetrahydropyridinium ylids

The factors affecting the copper-catalyzed rearrangement of ammonium ylids derived from tetrahydropyriclines and diazoesters; have been examined,and the first examples of high-yielding metal-catalyzed [2,3]-sigmatropic rearrangements of a wide range of such ylids are reported. The nature of the alpha-substituent in the diazo component of the reaction has a dramatic effect upon the yields of the reaction, with electron-withdrawing substituents enhancing the yield of the reaction.

A low-temperature ammonium ylid rearrangement: enhanced reactivity engendered by rigidity

Sigmatropic rearrangement of tetrahydropyridine-derived ammonium is a valuable method for the preparation of substituted prolines. These reaction normally require elevated temperatures to proceed, but bicyclic tetrahydropyridine-like ylid I undergoes rearrangement at -15 degrees C; the extra rigidity of the azabicyclo[3.3.0]octene system preorganizes the transition state and lowers the activation energy for rearrangement.

A dramatic isotope effect in the reaction of ClSiH with trimethylsilane-1-d: experimental evidence for intermediate complexes in silylene Si-H(D) insertion reactions

A kinetic isotope effect (kD/kH) of 7.4 has been found for the reaction of chlorosilylene with trimethysilane (Me3SiD vs Me3SiH). Such a value can be accounted for by theoretical modelling, but only if an internal rearrangement of the initially form complex is included in the mechanism. This provides the first concrete evidence for such complexes.