5 mins/15 mins

Carbene Additions to Alkenes

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C H H CC + :CX2 CC H H H H H H Singlet carbenes add to alkenes to yield cyclopropanes. Since a singlet carbene possesses both a high-energy filled molecular orbital in the plane of the molecule, and a low-energy, out-of-plane unfilled molecular orbital, this reaction presents an interesting dilemma. Clearly it would be more advantageous for the low-lying vacant orbital on the carbene, and not the high-lying filled orbital, to interact with the olefin π system during its approach.

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H H CC H H However, this leads to a product with an incorrect geometry. The carbene must “twist” by 90° during the course of reaction.

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CC CC H CC H H H H H H H H H H H In this tutorial, you will use the Hartree-Fock 3-21G model to find the transition state and to analyze the motion of the fragments. 1. Bring up the organic model kit and build ethylene. 2. Select from the model kit. Click on the Insert button at the bottom right of the screen (or hold down the Insert key) and then click anywhere on screen. Next, select from the model kit and click on two of the free valences on the sp3 carbon. Next,

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Chapter 6 df 103 7/20/06, 2:57 PM click on and, one after the other, click on the remaining two free valences on the sp3 carbon. Finally, click on . You are left with two fragments, ethylene and difluorocarbene. Orient the two as to be poised for reaction.*

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CC

Translations and rotations normally refer to the complete set of fragments, but if you click on a fragment (not on a free valence) to select it, and then hold down the Ctrl key they will refer to an individual fragment.

3. Click on . Click on the carbon on the CF2 fragment and then,

while holding down the Shift key, click first on the CF2 carbon and then on one of the carbons on the ethylene fragment. A dotted line between the two carbons that are to be bonded will be drawn and an arrow will be drawn from the methylene carbon to the center of this line.

F F C

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Next, click on the CC double bond and then, while holding down the Shift key, click on the other ethylene carbon and then on the

CF2 carbon. A second dotted line and second arrow will be added to the structure.

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* Proper orientation of the two fragments is not crucial in this case, but is primarily to allow you to associate the arrows with the intended reaction. Proper orientation is, however, essential where different stereochemical outcomes are possible.

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Chapter 6 df 104 7/20/06, 2:57 PM Click on at the bottom right of the screen. Your structure will be replaced by a guess at the transition state. 4. Enter the Calculations dialog, and specify calculation of a transition-state geometry using the 3-21G Hartree-Fock model. Check IR to the right of Compute, and click on Submit. Name the job difluorocarbene+ethylene. 5. When the job is complete, examine the geometry of the transition state. In light of the previous discussion, would you describe your structure as corresponding to an early or late transition state? Animate the vibration corresponding to the reaction coordinate. Bring up the IR dialog (Spectra under the Display menu and click on the IR tab). Click on the imaginary frequency at the top of the list of frequencies. Does the animation show that the carbene reorients as it approaches the double bond? Turn “off” the animation by again clicking on the imaginary frequency. 6. Select Properties (Display menu) and, in turn, click on each of the four hydrogens in the transition state. Change the value in the Mass Number menu in the Atom Properties dialog from 1 to 2 Deuterium. Resubmit the job. (No additional quantum chemical calculations are involved, but the vibrational analysis needs to be repeated to account for change in atomic masses.) When complete, examine the new set of vibrational frequencies. Note that they are uniformly smaller than those for the undeuterated system, and that the largest changes are for vibrational motions where hydrogens are involved. 7 to 10 optional 7. Make a copy of difluorocarbene+ethylene name it difluoro- carbene+cyclohexene. 8. Bring up the organic model kit with the copy (click on ). Select Freeze Center from the Geometry menu (or click on the icon at the top of the screen). One after the other, click on all atoms and free valences in the transition state except two of the free valences

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Chapter 6 df 105 7/20/06, 2:57 PM on one side of the ethylene fragment.* Each of the selected atoms will be given a magenta colored marker, indicating that it is to be frozen (not moved during optimization).** Using sp3 carbon from the model kit and then , connect the two free valences that you have not frozen by a chain of four methylene groups to make a cyclohexene ring. Click on . Note that the only four methylene groups move and not the underlying transition state. You have produced an excellent guess at the transition state for difluorocarbene addition to cyclohexene based on your previous transition state for the simpler process.*** 9. Enter the Calculations dialog. It should already specify calculation of transition-state geometry using the 3-21G Hartree- Fock model. Remove the checkmark on IR (to the right of Compute). Click on Submit. 10. When completed, compare the geometries of the two transition states (corresponding to addition of difluorocarbene to ethylene and cyclohexene, respectively). To do this, you need to put the two transition states into the same group. Select Append Molecule(s)... from the File menu and double click on difluorocarbene+ethylene in the browser that results. Then bring up the spreadsheet (Spreadsheet from the Display menu) and check each of the boxes to the right of the molecule names. Click on , select structure from the Align by menu at the bottom right of the screen and, one after another, click on the three carbons which make up the cyclopropane ring. Finally, click on Align by at the bottom right of the screen, and then click on . Are the two transition states very similar as expected? 11. Remove all molecules and dialogs from the screen.

* You could do this more quickly by firstclicking on , then drawing a selection box around all atoms to be frozen, and then clicking inside this box. ** You can remove the frozen atom markers from the model by bringing up the Molecule Properties dialog (Display menu), clicking on at the bottom right of the dialog and then removing the checkmark from Frozens in the Molecule Utilities dialog that results. *** It is not necessary to replace the two deuteriums by hydrogens. You will not request an IR spectrum and calculated geometry does not depend on atomic mass.

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