Ethers, crotyl propenyl

The Claisen rearrangement goes through a transition state in the chair conformation. This is the supported by the fact that trans, trans-crotyl propenyl ether gave more than 97% of the threo aldehyde showing a preference for the chair form while reaction in the boat gave mainly erythro aldehyde. 

One classic example that confirms the preference of Claisen rearrangement for a chairlike transition state was provided by Hansen and others. In 1968, they investigated the Claisen rearrangement of the crotyl propenyl ethers 5a and 5b to examine the stereochemistry of the rearrangement in the gas phase at 160° C10 (Scheme l.V). Both the E,E- and Z,Z-isomers rearrange to afford a syn-isomer as the major product. The stereochemical outcome of the reaction can be explained... 

In acyclic systems, Claisen rearrangements show a well-established prefoence for chair-like transition states. With crotyl propenyl ether, the chair selectivity amounts to 97-98% at 142 C, which corresponds to an approx. 3 kcal mol difference between the fiee energy of activation (AAG ) of chair and boat TS (equation 26). The preference for a chair-like geometry in the TS is even more pronounced in the Cope reaiT ement 99.7% of the 3,4-dimethylhexa-1,5-diene rearranges at 225 C via a chair-like TS, corresponding to a AAG chair-boat of -5.7 kcal mol" . - The latter result closely parallels the difference in energy of the chair and boat conformations of cyclohexane (5-6 kcal mol" ). ... 

Schmid has examined the rate and stereochemistry of rearrangement of four crotyl propenyl ethers 14a-14d [25, 26]. As a result, the E,E isomer 14a is found to rearrange an order of magnitude faster than the Z,Z isomer 14b. The E,E and Z,Z isomers rearrange through a chair-like transition state to give the xyn diastereomers 15a and 15b as the major product. Moreovei the Z,E and E,Z isomers give the anti diastereomers 15c and 15d as the predominant stereoisomer Since all... 

The isomers of crotyl 1-propenyl ether have been used to demonstrate the diastereoselec-tive course of the rearrangement54-72. The expected negative activation enthalpies (AS14 = —10 to —15 eu) have been observed in all rearrangements with activation enthalpies ranging from 25 to 27 kcal/mol. Each isomer shows a clear preference for a chairlike transition state. 

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