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BRYN MAWR CHEM B534 - CHEM 534 EXAM 2

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Organometallic Chemistry Name:_______Gina Carano___________ CHEM 534 Exam 2 November 25, 2008 Prof. Malachowski 1H NMR chemical shifts 13C NMR chemical shifts2 1. The following reaction was reported in the Journal of the American Chemical Society (Michael Rubin et al., 2008, 130 (41), 13804–13809). a) Suggest a mechanism for the reaction and identify the reaction type for each step. (7 pts.) Path I below shows the proposed mechanism. The steps are as follows: oxidative addition of H2 to form the LnRh(I)-H; ligand association with the cyclopropene; 1,2-insertion of Rh-H across the alkene of cyclopropene; association of CO with Rh complex; 1,1-insertion of CO; oxidative addition of H2; and reductive elimination of H-C(O)-cyclopropane. b) One competing process involves the formation of cyclopropyl dimers as shown in the reaction below. Propose a series of steps that allow this to occur and identify the steps in the process. (3 pts.) R1R2R32R1R2R3R1R2R3Rh(I)/LH2 Path II or III above. The steps for Path II beginning with LnRh(I)-H are as follows: cyclopropene association; 1,2-insertion of Rh-H across the alkene of cyclopropene; ligand association of cyclopropene; 1,2-insertion of Rh-cyclopropyl across alkene of cyclopropene; oxidative addition of H2; reductive elimination of H-cyclopropyl-cyclopropane. _____/10 pts3 2. Gerhard Erker and co-workers recently reported in Organometallics (Article ASAP) the identification of the two complexes, 6a and 7b-d, shown below. Describe an analytical experiment that would allow you to detect the alkene coordination event in 7b-d. Be as specific as possible with your experimental details. (6 pts.) A 1H NMR experiment would show an upfield shift in the alkene protons in 7b-d upon coordination to Ru. The authors of the article actually reported a ~0.5 ppm shift from ~6.0 in 6a to ~5.5 ppm. A 13C NMR would experience a similar shift upfield for the carbon signals of the alkene in 7b-d. The authors actually found a shift from ~135 ppm in 6a to ~100 ppm in 7b-d. Other experiments that would show the coordination include X-ray crystallography which would show the alkene in close proximity to the Ru. 3. A recent article by Robert Grubbs and co-workers (Organometallics, 2008, 27 (22), 5737–5740) describes the synthesis and reactivity of 2 and it’s stereoisomer cis-2. R= -CH2CH2N(CH3)2 a) What type of carbene is compound 2? (3 pts.) With the heteroatom attached to the carbene carbon, it is a Fischer carbene. b) What is the typical polarization in the Ir=C bond for this type of carbene? (3 pts.) The polarization would have a partial positive on the C and a partial negative on the Ir:  - Ir=C + c) The authors were shocked to find that cis-2 was stable. What type of reaction might occur with cis-2 that would lead to its decomposition? (3 pts.) With the hydride’s positioned cis to each other in cis-2, they would be expected to undergo a reductive elimination reaction to release H2 and generate (PNP)Ir=CHN(CH3)R. _____/15 pts4 d) The authors explained the reluctance of cis-2 to undergo this subsequent decomposition reaction based on a consideration of resonance forms of cis-2 (or 2). Draw two resonance forms of cis-2 below. (4 pts.) (PNP)IrHHNR(PNP)IrHHNR(PNP)IrHHNR e) The authors believe that the resonance forms suggest a distribution of electron density that deters the decomposition reaction in part c. Explain their reasoning. (3 pts.) Two of the resonance forms show a negative formal charge on the Ir. Although the Ir is in the +3 oxidation state, this excess electron density on Ir might deter a reductive elimination since the Ir already finds itself with a considerable amount of electron density and the addition of more electron density from the reductive elimination of H2 might be unfavorable. 4. Choose either a metal-(3-allyl cation) complex or a metal-(-benzene) complex and answer the following questions. a) Draw a MO interaction between a * orbital of the ligand and a filled d orbital of the metal. (5 pts.) answer for M-(3-allyl cation) answer for M-(-benzene) b) Show the interaction between a  orbital of the ligand and an empty d orbital of the metal. (5 pts.) answer for M-(3-allyl cation) answer for M-(-benzene) _____/17 pts5 5. a) Draw the MO interactions and label them for the following reaction. Assume this is a concerted reaction mechanism. (6 pts.) Pd(PPh3)2 + Ar-Br  (Ph3P)2Pd(Ar)Br b) What is the name of this reaction type? (2 pts.) This is an example of an oxidative addition. c) What geometry would be expected for the Pd product? Why? (4 pts.) The (Ph3P)2Pd(Ar)Br will be square planar because it is a d8 metal and crystal field theory predicts square planar as the most stable electronic arrangement with the fewest electron repulsions. d) What is the relative position of the Br and Ph groups on the metal in the product immediately after the reaction has occurred? (2 pts.) The Br and Ph will initially be cis. 6. There has recently been considerable interest in pincer complexes such as the Pd complexes 3 and 9 shown below (the Ir complex in question 3 is also a pincer complex). These complexes were reported in a recent edition of the Journal of the American Chemical Society. (Kálmán J. Szabó et al., 2004, 126 (22), 7026–7033) One of the reasons for the interest in the pincer complexes such as 9 is the important difference in their nature and reactivity. Indeed, the allyl complex 9 contrasts with most other allyl complexes that we saw in our class discussions, such as B shown below. _____/14 pts.6 PhOAcPdCl2(dppe), Ph3PPhPdP PPhPhPhPhABOAc a) Describe the most obvious difference in the nature or structure of the allyl complex in 9 versus that in B and briefly explain why you think this difference exists. (3 pts.) The Pd complex 9 is a 1-allyl complex, while that in B is a 3-allyl complex. The stable tridentate pincer complex sterically prevents a higher hapticity coordination of the Pd with the allyl group. b) The authors confirmed the allyl complex 9 by analyzing its 1H NMR spectrum. They reported a doublet of triplets (dt) for the CH2 signal in 9. Explain this pattern by identifying all the coupling partners. Use coupling


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