Zimmerman-Traxler Model Created by Alexis Johnson Edited by Margaret Hilton Honors Organic Chemistry CHEM 2321 (Sigman), 2013

The Zimmerman Traxler Model Overview: The Zimmerman-Traxler model is invoked to rationalize the unexpected stereochemical outcomes of certain aldol reactions, such as the Reformatsky and Ivanov reactions. It models the stereochemistry of the products, based on the steric hindrance in the possible six-membered transition states in the aldol condensation reactions. Enolate Formation Wiki Page: Aldol Reaction- Traxler Model 1. Zimmerman and Traxler, JACS, 1957 79 (8)

The Zimmerman Traxler Model Overview: The Zimmerman-Traxler model is invoked to rationalize the unexpected stereochemical outcomes of certain aldol reactions, such as the Reformatsky and Ivanov reactions. It models the stereochemistry of the products, based on the steric hindrance in the possible six-membered transition states in the aldol condensation reactions. Possible Transition States E-enolate Z-enolate Wiki Page: Aldol Reaction- Traxler Model 1. Zimmerman and Traxler, JACS, 1957 79 (8)

Examples Ivanov Reaction Reformatsky Reaction It was noted that these reactions produced products with very high enantioselectivity, however, the rationale for this was not determined until the work of Zimmerman and Traxler. For this to be useful, however, there must be some control over the enolate formation, as this is the determining factor for the stereochemical induction. This knowledge proves to be useful in synthesis where enantioselectivity is necessary. Zimmerman and Traxler, JACS, 1957 79 (8) Wikipedia pages: Reformatsky; Ivanov

Experimental Observation Chair-like vs. Boat-like intermediates: It was recognized prior that the metal enolate, as well as the stereochemistry of the alkene, had an effect on product selectivity. With this information and more, it was deduced that the stereoselectivity of the products indicated the chair-like intermediate. Lui et. Al. have shown that the Zimmerman-Traxler model for the intermediate of the selective reaction prevails over boat-like intermediates with a ratio of 50 to 1 respectively. Lui et. al. JACS 2005, 127 (16), pp 5770–5771

Problems 1. Draw a Zimmerman-Traxler transition state model for the following reactions. 2. Draw the major product for the reactions above. 3. Repeat questions 1 and 2 for the following Reformatsky type reaction. 1 & 2. Reiser, O. Chem. Rev. 1999, 1191 3. Choppin et. al. Chem. Soc. Rev., 2013,42, 937-949

Contributed by: Created by Alexis Johnson (Undergraduate) Edited by Margaret Hilton Honors Organic Chemistry CHEM 2321 (Sigman), 2013 University of Utah