General Papers ARKIVOC 2005 (i) 92-97
Results and Discussion
In an effort to speed up the dehydrogenation process, the possibility of adding a strong hydrogen acceptor was explored. As a first choice, 1 equivalent of DDQ 3 (Figure 1) was added to the reaction mixture since it is known that pyrrolines can be converted to the corresponding pyrroles by DDQ.3 Analyzing our first results, no ring-closing metathesis could be observed, suggesting that the second generation Grubbs’ catalyst and DDQ are incompatible. Literature study however, revealed that a combination of ring-closing metathesis and oxidative aromatization was recently reported as a new protocol for benzoannulation.4 Although this is a one-pot reaction, the DDQ was added after metathesis, thus avoiding catalyst inactivation. In a quest for quinones which do not react with the second generation Grubbs’ catalyst, our attention turned to tetramethyl-1,4-benzoquinone 4 (duroquinone) and tetrachloro-1,4-benzoquinone 5 (chloranil) as strong hydrogen acceptors (Figure 1). These quinones have been evaluated before as hydrogen acceptor in combination with RuCl3 × H2O.5
O OO
O O O Cl Cl Cl ClCl Cl CN CN 3 45
Figure 1. The evaluated different hydrogen acceptors: DDQ (3), duroquinone (4) and chloranil (5).
Upon evaluation of 4, we were pleased to find that ring-closing metathesis was not inhibited, however, the rate of aromatization was not significantly influenced. Finally, adding tetrachloro1,4- benzoquinone together with the second generation Grubbs’ catalyst resulted in the complete conversion of the diallylamine to the corresponding pyrrole within 2 hours. It was even observed that RuCl3 × H2O is not necessary in this conversion. This might suggest that either the metathesis catalyst, or decomposition compounds thereof, catalyze the hydrogen transfer or that hydrogen is directly transferred from donor to acceptor. A possible mechanism for this reaction sequence (in accordance to the mechanism of the reaction of Hantzsch esters to pyridines6) is outlined in Scheme 2. After ring-closing metathesis, the electron lone pair on N of the intermediate 3-pyrroline 6 initiates the aromatization by expelling a hydride which immediately reacts with 5. This assumption is consistent with the observation that diallylamines with strong electronwithdrawing groups on N do not aromatize. Possibly both donor and acceptor are coordinated to the transition metal center in this step, thus facilitating the H-transfer. In the next step, the intermediate iminium-ion 7 loses another proton and aromatizes to the pyrrole 2. In a final step, a proton-shift converts 9 to hydroquinone 10.
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