Rhodium-Catalyzed Selective Partial Hydrogenation of Alkynes

The cationic rhodium complex [Rh­(P^<i>c</i>Pr₃)₂(η⁶-PhF)]⁺­[B­{3,5-(CF₃)₂C₆H₃}₄]⁻ (P^<i>c</i>Pr₃ = triscyclopropylphosphine, PhF = fluorobenzene) was used as a catalyst for the hydrogenation of the charge-tagged alkyne [Ph₃P­(CH₂)₄C₂H]⁺[PF₆]⁻. Pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS) was used to monitor reaction progress. Experiments revealed that the reaction is first order in catalyst and first order in hydrogen, so under conditions of excess hydrogen the reaction is pseudo-zero order. Alkyne hydrogenation was 40 times faster than alkene hydrogenation. The turnover-limiting step is proposed to be oxidative addition of hydrogen to the alkyne (or alkene)-bound complex. Addition of triethylamine caused a dramatic reduction in rate, suggesting a deprotonation pathway was not operative

Asymmetric [2+2+1] cyclopentannulation of olefins. Ring expansion of 2-N-methyl-N-tosyl-cyclobutanone

alpha-N-Methyl-N-tosyl cyclobutanones 2 which had been previously prepared in good yields and high enantiomeric excesses from olefins and chiral keteniminium salts have been converted into the corresponding oxiranes 3 by reaction with dimethylsulfonium methylid. The stereochemistry of this reaction was found to be dependent on several factors which have been analyzed. Treatment of these oxiranes with a stoichiometric amount of lithium iodide in refluxing tetrahydrofuran gave excellent yields of monocyclic or fused cyclopentenones 4 resulting from a P-elimination of N-methyl-N-tosylamide from a primarily formed cyclopentanone. The ring-expansion was totally selective but for oxiranes attached to a bicyclo[4.2.0]octanone system. In all cases, the enantiomeric purities of the starting cyclobutanones were preserved throughout the sequence which thus represents a useful [2+2+1] strategy for the cyclopentannulation of olefins. (C) 2002 Elsevier Science Ltd. All rights reserved