Tandem Cyclization of Enynes Containing an Ether Linkage and a Cyclopropyl Group via Ruthenium Metal Complex.

We study chemical reactions of [Ru]Cl ([Ru] = Cp(PPh3)2Ru) with three 1,8-enyne compounds containing cyclopropyl group (1, 7 and 11) in which the triple bond is associated with propargylic alcohol and the olefinic group has various substituted methyl groups. The reaction of [Ru]Cl and 1, containing one methyl group added to the internal carbon of the allyl group, affords the vinylidene complex 2 with a newly formed seven-membered ring. Furthermore, tandem cyclization reaction of 1 in MeOH/CHCl3 catalyzed by [Ru]NCCH3+PF6－ at 60 oC leads to the product 6 with spiro-cyclopropyl ring, formed possibly via sequential allenylidene vinylidene cyclization followed by a nucleophilic addition of alkoxide. In CHCl3, the catalytic reaction by [Ru]NCCH3+ PF6－ gives the isolable organic cyclization intermediate enyne 5.
Treatment of the propargylic alcohol 7 containing two terminal methyl groups at the O-allylic group, with [Ru]Cl yields a mixture of two diastereomers of the vinylidene complex 8 containing a newly formed six-membered ring in a ratio of ca. 10:1. Among diastereoisomers only the anti-isomer is isolated for complex 8. The cyclization reaction is proposed to proceed via the formation of a six-membered ring boat-like transition state with bulkiest group in the pseudo-equatorial position to reduce the 1,3-diaxial interactions. The catalytic reaction by [Ru]NCCH3+PF6－ in CHCl3 at 60 oC affords anti-9 and syn-9 in a ratio of 3:1. The thermal energy increases amount of product syn-9. The subsequent cyclization carried out in a cosolvent of ROH/CHCl3 at 60 oC affords bicyclic product 10.
In the reaction of [Ru]Cl with 11, containing no methyl group, no C−C bond formation is observed. The reaction of 11with [Ru]Cl in the presence of NH4PF6 in CH2Cl2 affords a mixture of the allenylidene complex 12 and phosphonium acetylide 13. Interestingly, thermal treatment of 11 leads to a ring expansion of the cyclopropyl group, giving the vinylidene complex 16 with a five-membered ring. This ring expansion proceeds by C-C bond formation between Cβ of the cumulative double bond and a methylene group of the cyclopropyl ring.
Reaction of 1,3-diol 18 with [Ru]Cl in CH2Cl2 yields a mixture of the alkoxy carbene 17 and 19 in a ratio of 1:2. The spontaneous dehydration of 19 to 17 by elevating temperature is failed. Fortunately, complex 17 could be the only product when the reaction is carried out in MeOH. Overall, tandem cyclizations of two 1,8-enynes with methyl-substituents have been achieved. Ring-expansion of a cyclopropyl group in a 1,8-enyne with no methyl-substituent is also accomplished.