Solutions_Manual_for_Organic_Chemistry_6th_Ed 333

Solutions_Manual_for_Organic_Chemistry_6th_Ed 333 - 1 5 1 7...

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Unformatted text preview: 1 5- 1 7 These structures show the alignment of diene and dienophi le i n the Diels-Alder transition state, l eading to 1 ,4-orientation in (a) and 1 ,2-orientation in (b). ( b) ( a) :0: t his left structure is a VE RY m inor resonance contributor; however, it explains the orientation for the diene as carbon- l is more negative and carbon-2, a 3°C, is s lightly more positi ve because of methyl group stab i lization 1 5 - 1 8 For clarity, the bonds formed in the Diels-Alder reaction are shown in bold. COOCH3 (b) 1 5 - 1 9 For a photochemically a llowed process, one molecule must use an excited state in which an e lec tron has been promoted to the first anti bonding orbital . All orbital interactions between the excited molecule's HOMO* and the other molecule's LUMO must be bonding for the interaction to be al lowed; otherwise, i t is a forbidden process. HOMO * excited state o f diene HOMO bonding interaction ( constructive overlap J � LUMO of dienophi le anti bonding interaction (destructive overlap) In the Diels-Alder cycloaddition, the LUMO of the dienophile and the excited state of the HOMO o f t he diene ( l abeled HOMO * ) produce one bonding interaction and one anti bondin g i nteraction. Thus, this is a p hotochemically forbidden process. 3 26 ...
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This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

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