Oxidative addition of .ALPHA.-bromo esters to tetrakis(isocyanide) rhodium(I) complexes. Evidence for a chain mechanism.
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概要
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The oxidative additions of chiral alkyl halides, (<I>S</I>)-(−)-ethyl α-bromopropionate [α]<SUB>D</SUB>−18.9° (<I>c</I> 3.3, CHCl<SUB>3</SUB>) and (<I>S</I>)-(+)-ethyl α-phenylbromoacetate [α]<SUB>d</SUB>+56.4° (<I>c</I> 1.6, C<SUB>2</SUB>H<SUB>5</SUB>OH) to [RH(R′NC)<SUB>4</SUB>]<SUP>+</SUP> (R′=<I>t</I>-Bu, <I>p</I>-CH<SUB>3</SUB>C<SUB>6</SUB>H<SUB>4</SUB>) produced the corresponding adducts, <I>trans</I>-[RhBr(R)(R′NC)<SUB>4</SUB>]<SUP>+</SUP> (R=CH<SUB>3</SUB>CHCO<SUB>2</SUB>Et, PhCHCO<SUB>2</SUB>Et), which were well characterized by elemental analysis, IR and <SUP>1</SUP>H NMR spectra. These adducts were found optically inactive, a fact which precludes a concerted mechanism. The reaction of C<SUB>6</SUB>H<SUB>5</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB> with [Rh(<I>t</I>-BuNC)<SUB>4</SUB>]<SUP>+</SUP> commenced immediately in the absence of light. Taking this system as a typical example, the rate was studied by the stopped-flow method to find a rate equation, <I>R</I>=<I>k</I>[Rh(I)]<SUP>2</SUP>[RX]. The relative rate of addition of <I>p</I>-XC<SUB>6</SUB>H<SUB>4</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB> to [Rh(<I>t</I>-BuNC)<SUB>4</SUB>]<SUP>+</SUP> decreases in an order of Cl>H>CH<SUB>3</SUB> for X. The addition of C<SUB>6</SUB>H<SUB>5</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB> to [Rh(<I>p</I>-CH<SUB>3</SUB>C<SUB>6</SUB>H<SUB>4</SUB>NC)<SUB>4</SUB>]<SUP>+</SUP> in the dark proceeds only slowly, but rapidly under a low energy photo-irradiation (440 nm) with a large quantum yield (φ 4.8). The addition of CH<SUB>3</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB> requires photo-initiation. The optical activity of (<I>S</I>)-(−)-CH<SUB>3</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB> was completely lost before the oxidative addition commenced. The rate of decrease in optical activity of the system (<I>S</I>)-(−)-C<SUB>6</SUB>H<SUB>5</SUB>CHBrCO<SUB>2</SUB>C<SUB>2</SUB>H<SUB>5</SUB>/[Rh(<I>t</I>-BuNC)<SUB>4</SUB>]<SUP>+</SUP> exhibited approximate second order kinetic behavior, and quantitative racemization took place prior to the completion of the oxidative addition. A chain mechanism appears to be consistent with all these results. Alternate mechanisms are also discussed in the light of the present results.
- 公益社団法人 日本化学会の論文
著者
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Otsuka Sei
Department of Chemistry, Faculty of Engineering Science, and Department of Polymer Science, Faculty of Science, Osaka University
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Ataka Kikuo
Department of Chemistry, Faculty of Engineering Science, Osaka University
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