Schrock (1974)

The ability for society as a whole to build molecules tailored to a specific use has very far reaching applications. From building a stereospecific products used in the pharmaceutical industry to synthesizing polymers with special properties related to strength or conductivity through conjugated π systems, olefin metathesis is responsible for many of the products we use today. The use of transition metals for the catalysis of these reactions would become a topic of much interest in the years following this paper. The subject of this paper is one example of the synthesis of these transition metal carbine complexes. Things to think about could be what made this complex unique at the time? How stable of a compound is this relative to similar complexes of the time or complexes made with transition metals of the same group or period? How might these complexes differ from Fischer carbenes?

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7 Responses to “Schrock (1974)”

  1. Tony Says:

    Schrock probably set the framework for reagents such as Tebb’s reagent from organic chemistry. So in a way he paved the way for olefination. Is the difference between Schrock’scarbenes and Fischer’s just a difference between soft and hard acids or is that completely unrelated?

  2. Hunter Burgin Says:

    On the second page of Schrock’s paper he diagrams a 4 step reaction beginning with Ta[CH2C(CH3)3]5Cl5 and ultimately resulting in compound 1 following three rate determining steps. Schrock alludes to a compound 3 briefly in the previous paragraph, stating that if the intermediate compound is indeed present then its rate of formation (R1) must be less than that of the ensuing chemical 2 (R2). Does intermediate 3 actually take place or is it just a tentative theory according to Schrock on the off chance that the reaction does undergo an intermediate? The only identity of intermediate 3 that I can imagine would be Ta[CH2C(CH3)3]5Cl.

    • Adam Settimo Says:

      He’s trying to say that it is unlikely that there is an intermediate complex 3, and the most likely reaction is that of 2 being formed and then decomposing into 1 by alpha-hydrogen abstraction forming neopentane and complex 1.

  3. Kevin Greenwood Says:

    As I understand this paper, hydrogen abstraction from a coordinated neopentyl ligand takes place at the carbon attached to the tantalum complex. The high oxidation state of tantalum is pulling electron density from the attached carbon, reducing the energy of the CH bond, allowing the proton to be removed by the alkyl lithium or alkyl magnesium compound.

    Does transmetalation occur to any extent? This would attach the incoming organometallic carbon to tantalum, temporarily giving the complex a negative charge before proton transfer and kicking out of the neopentane. The reason I ask is because of the yield differences when using neopentyl lithium compared to the neopentyl Grignard reagent.

    Yield is quantitative when using the alkyl lithium, but only 50% when using the Grignard reagent, and to get that they had to increase the temperature. Magnesium’s coordination sphere is bigger than lithium’s, allowing the alkyl lithium easier access to the center of the molecule where this reaction is taking place, but tantalum is big (for an atom anyways). Considering that, does magnesium’s increased size relative to lithium account for the reduced yield, or is there something else that could account for this?

  4. Porter Marsh Says:

    The paper says that the neopentylidene complex reacts violently with oxygen and water. Tantalum is number 73 while Oxygen is only 8 so Ta is a very soft acid while O is a very hard base. Why do they react so violently? Ta is used in laboratory equipment because of its inertness and is highly corrosive resistant. Why does it react at all, let alone violently, with oxygen when it’s part of a neopentylidene complex? The neopentylidene complex is mostly hydrocarbons, which aren’t particularly reactive with oxygen, and Ta, which is even more unreactive.

  5. Daniel Begay Says:

    When it comes to them hypothesizing about the rate constants, I’m curious to see how well their hypothesis is accurate to future studies that they will do. I’m curious for reasoning why even though the formation with diethyl ether was faster, the isotopic distribution of the deuterium was basically unchanged. Does the oxygen in the diethyl ether play a huge role in this reaction? Can you explain this? Or has it been explained in future studies?

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