The synthesis of substituted styrenes 3 and 3-vinylphenols 9 was achieved by ring-closing enyne metathesis (RCEM)/dehydration of 7 and RCEM/tautomerization of 8, respectively. Those methods provide selective access to unique aromatic compounds and solve the problem of regioisomer formation.
The deleterious effect of ethylene gas on the ring-closing olefin metathesis (RCM) for the formation of 5-to 8-membered rings was investigated. Significant rate differences caused by ethylene gas were observed among the different ring-size formation reactions. Nevertheless, the rate differences can be advantageously utilized for chemoselective RCM.Keywords: homogeneous catalysis; metathesis; ring closure; ring size-selective reaction; rutheniumThe utilization of ring-closing olefin metathesis (RCM) has seen an explosive growth in organic chemistry over the past two decades.[1] This outstanding reaction, however, is still hampered by some difficult problems. The chemoselective RCM of substrates with more than two multiple bonds available for the metathesis is one of the problems. [2,3] During the course of our study, which is aimed at the development of new methods for the synthesis of aromatic compounds, [4] we recently tried to synthesize fused bicyclic benzenes 4 and 5 by applying the ringclosing enyne metathesis (RCEM)/RCM sequence to a mixture of trienyne 1 and its isomer 2 [5] (1:2 = 10:1) using Grubbs second-generation catalyst 3 [6] (Scheme 1). As expected, under nitrogen gas, the reaction proceeded to produce 4 and 5 with double ring closing. However, under ethylene gas, [4c,7] the products detectable by 1 H NMR were only 4 and 6. Our surprise came not from the formation of 6 that should be formed from 2 by RCEM only, but from the absence of the corresponding RCEM product from 1, indicating that ring-size selectivity was observed between the six-and seven-membered ring formation reactions under ethylene gas. [8,9] On these grounds, we have come to the conclusion that the generality of this type of selectivity is worthy of study. Here we report the results of our investigation in which rate differences caused by ethylene gas among 5-to 8-membered ring formation reactions were observed and the deleterious effect of ethylene gas could be used to improve selectivity in RCM synthesis of favorable ring sizes by limiting the formation of less-favorable ring sizes.The easily prepared nitrogen-containing dienes 7a-d[10] were adopted as model substrates for this investigation (Scheme 2). While holding the pressure constant at 1 atm, we first compared the yields of RCM products 8a-d under nitrogen gas with those under ethylene gas in the range of 0 to 80 8C using 1 mol% Scheme 1. Incidentally observed ring size-selective RCM in which ethylene gas hindered only the RCM for 7-membered ring formation.
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