| Abstract: | Methodology involving the use of vinylboranes as Diels-Alder dienophiles was developed. Experimental evidence revealed that vinylboranes are exceptional Diels-Alder dienophiles displaying outstanding reactivity and remarkable regio- and stereoselectivity. An assessment of the activation of various boron activating groups for vinylboranes and their effects on selectivity was made. The studies found that the activation provided by appropriately substituted boron functionalities in vinylboranes exceeds that of most traditional Diels-Alder dienophiles. While there were a number of boron substituted dienophiles known before this work began, they displayed low reactivity, and consequently, limited utility in organic synthesis. Vinyl-9-BBN exhibited exceptional reactivity. With acyclic dienes, vinyl-9-BBN displayed outstanding regio- and stereoselectivity. These remarkable features were due mainly to unique steric effects invoked by the very bulky BBN group. Electronic effects normally induced by electronically different dienes were absent in the reactions of these dienes with vinyl-9-BBN. However, substituent effects due to the dienes were significant. Overall, yields were high. Vinyl-DMB Diels-Alder reactions were slower. However, owing to its added stability, its reactions with less reactive dienes yielded cleaner products and slightly improved yields. Vinyl-DMB exhibited lower regioselectivity than vinyl-9-BBN, but an increase in stereoselectivity was observed especially with cyclopentadiene. Although vinyl-DMB appears to be indefinitely stable at 25 °C, it decomposes with prolonged heating at 65 °C. Although the reactivity and selectivity of vinylboranes was highly susceptible to the varying structure of the substituents on the boron, extreme effects due to substitution on the vinyl group were also observed. In the event, a highly reactive and remarkably selective silyl substituted vinylborane (TMS-VBBN) was found. The reactivity of TMS-VBBN was only slightly less than the reactivity of the unsubstituted vinyl-9-BBN and in some instances exceeded it. TMS-VBBN proved to be one of the most reactive and efficient acetylene equivalents. The application of vinylboranes to Diels-Alder chemistry was simplified by performing their reactions in situ. This is crucial, since it would allow application of vinylboranes which would otherwise be very difficult to isolate, allowing potential pathways into a variety of complex molecules. The structures for all of the compounds have been carefully deduced from 1H and 13C NMR studies. In one case, an X-ray analysis was utilized. |
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