1. Aromatic systems are generally inert to nucleophilic addition/substitution. How would you alter the electronic nature of aromatic rings by metal complexation such that they would undergo nucleophilic addition/substitution? Explain briefly with an appropriate example.
Halogenated aromatic compounds are reluctant to undergo nucleophilic substitution. Reasons for this non reactivity is that neither an SN1 nor an SN2 pathway for nucleophilic displacement is available. One way to overcome this problem is to introduce strong electron-withdrawing functional groups onto halogenated aromatics and carry out an addition elimination sequence (see scheme below). The nitro groups help stabilize the negative charge after nucleophilic addition. Higher number of electron withdrawing groups facilitate the nucleophilic displacement.
An alternate solution is to introduce metal carbonyls which help in stabilization of the negative charge. Chromium, tungsten, and molybdenum hexacarbonyls react with aromatic compounds to form aryl metal tricarbonyl complexes. These complexes are stable compounds and they undergo nucleophilic substitutions readily.
Chromium tricarbonyl complexes of haloarenes undergo nucleophilic substitution in good yields. The reactivity order for the halogens is F>Cl>Br>I.
A variety of nucleophiles (even some stable ones) can be used in this reaction. However, very reactive carbanions are not suitable. With these nucleophiles, cyclohexadienyl complexes are formed. The rearrangement shown below is required for ipso substitution (Nu is any nucleophile).
2. State the 18 electron rule and explain it with the help of an example.
3. Define oxidative addition and reductive elimination. Provide an example for each.
Prof. Rob Toreki, University of Kentucky, has excellent information on basic concepts in Organometallic chemistry. The following links are to his web page.
4. Discuss in detail the utility of a palladium reagent in organic synthesis. (Hydrogenations should be excluded).
5. Discuss briefly an recent article you have read which utilizes organometallic chemistry in organic synthesis. Hand in a copy of the article to Rose on Monday.
Palladium functions as a very useful catalyst in a variety of carbon-carbon bond forming reactions. The Heck, Stille, and Suzuki cross-coupling reactions in particular have been extensively investigated in the last 15 years. A few leading references to these reactions are given below.
Fine Feathers Make Fine Birds: The Heck Reaction in Modern Garb. de Meijere, A.; Meyer, F. E. Angewandte Chemie 1995, 33 , 2379.
Recent Developments And New Perspectives In The Heck Reaction Acc. Chem. Res. 1995, 28, 2.
Application Of Intramolecular Heck Reactions For Forming Congested Quaternary Carbon Centers In Complex Molecule Total Synthesis. A Review. Overman, L. E.; Pure Appl. Chem. 1994, 66, 1423.
Review. Palladium Catalyzed Reactions Of Organotin Compounds Synthesis 1992, 803.
Work by NDSU
faculty in Palladium chemistryPalladium Mediated
Biomimetic Synthesis of Narwedine.
Holton R. A.; Sibi, M. P.; Murphy, W. J. J.
Am. Chem. Soc. 1988,
110,
314.
Other publications by Dr. Sibi
An Asymmetric Synthesis
of (-)-Epibatidine. Trost, B. M.;
Cook, G. R.
Tetrahedron Lett. 1996, 37, 7485.
Other publications by Dr. Cook