Bicycle production: The low‐valent rhenium complex [ReCl(CO)5] catalyzes a tandem intramolecular cyclization reaction of ω‐acetylenic dienol silyl ethers. The alkyne undergoes a geminal carbofunctionalization in the presence of 0.5–3.0 mol % [ReCl(CO)5] under photoirradiation to give bicyclic enol silyl ethers in high yields (see scheme, TIPS=triisopropylsilyl).

We previously reported the geminal carbo-functionalization reaction of alkynes on treatment of 3-siloxy-1,3-dien-7-ynes 1 with [W(CO) 6 ] or [ReCl(CO) 5 ] under photoirradiation to give bicyclo[3.3.0]octane derivatives 2 in good yield (Scheme 1).[1]The reaction was thought to proceed through nucleophilic attack of the silyl enol ether moiety to the electrophilically activated alkyne followed by attack of the generated alkenyl metallic species A to the a,b-unsaturated silyloxonium moiety at the position b to the metal to give bicyclic carbene complex intermediate B, which gave the product through 1,2-hydrogen shift with regeneration of the catalyst.[2] We then thought of the possibility of carrying out a similar geminal carbofunctionalization reaction by employing one-carbon-atom elongated substrates (3-siloxy-1,3-dien-8-ynes 3) with the expectation that 5-exo attack of the silyl enol ether moiety followed by ring closure (geminal carbo-functionalization) would give bicyclo[3.3.0]octane derivative D having a carbene complex moiety as a substituent on the bridgehead carbon atom. As there is no a-hydrogen atom at the carbene complex moiety, 1,2-hydrogen shift is not possible and the behavior of this intermediate is intriguing (Scheme 1). We now report that 1,2-alkyl migration [3] occurred at the generated carbene moiety to give ring-expanded products by employing 3-siloxy-1,3-dien-8-yne derivatives as substrates, to stereoselectively obtain synthetically useful bicyclo[4.3.0]nonane derivatives, whose configuration was different from that of the Diels-Alder adduct.We first examined the reaction of 3-siloxy-1,3-dien-8-yne 3 a with the geometry of the silyl enol ether moiety as (Z) [4] by using various electrophilic transition-metal catalysts. Although [W(CO) 5 (thf)] or [ReCl(CO) 5 ] gave the products in reasonable yield (45 % and 78 %, respectively), [5] the cationic gold catalyst gave the best result. Thus, treatment of 3 a with 10 mol % of [AuClPPh 3 ]/AgSbF 6 [6] in the presence of molecular sieves (4 ) gave bicyclo[4.3.0]nonane derivative 4 a, a formal [4+2] cycloadduct, as a single stereoisomer in 92 % yield (Scheme 2). [7,8] Interestingly, the configuration of the product was different from that of the thermal DielsAlder adduct 5 obtained stereoselectively by heating a solution of (Z)-3 a in toluene for 1.5 h.[9] Thus, two diastereoisomers [10] of the same bicyclo[4.3.0]nonane derivative were obtained selectively by choosing Au I -catalyzed or thermal conditions.Next, we examined the reaction of 3-siloxy-1,3-dien-8-yne derivative 3 a with the geometry of the silyl enol ether moiety as (E) [4] under thermal and Au I -catalyzed conditions (Scheme 2). In this case, (E)-3 a did not undergo thermal Scheme 1. Tandem cyclization of dienol silyl ethers.Scheme 2. Au I -catalyzed and thermal cyclization of dienynes 3 a. E = CO 2 Me; TIPS = triisopropylsilyl; M.S. = molecular sieves; DCE = dichloroethane.

When omega-acetylenic dienol silyl ethers containing NMs part in the tether were treated with a catalytic amount of W(CO)6 under photoirradiation, 2-azabicyclo[3.3.0]octanes were obtained in good yield via pi-alkyne complexes. On the other hand, treatment of the same substrates with a catalytic amount of W(CO)6 in the presence of n-Bu3N under the same reaction conditions gave 3-azabicyclo[3.3.0]octanes in good yield exclusively via vinylidene complexes. Thus, the pi-alkyne and vinylidene complex pathways are easily controlled by using a catalytic amount of W(CO)5(L) and an amine.

Tungsten(0)- and rhenium(I)-catalyzed reactions of acetylenic dienol silyl ethers based on the concept of geminal carbo-functionalization of alkynes are reported. Treatment of 3-siloxy-1,3-diene-7-ynes with catalytic amounts of [W(CO)(6)] or [ReCl(CO)(5)] under photoirradiation conditions gives synthetically useful bicyclo[3.3.0]octane derivatives in good yields. Extremely high catalytic activity is noted for the rhenium(I) complex. The reaction has been extended to substrates containing a nitrogen atom in their tethers. In this case, two kinds of synthetically useful heterocyclic compounds-the 2-azabicyclo[3.3.0]octane derivatives 9 and the monocyclic dihydropyrroles 10, with allenyl substituents-are obtained, and selective preparation of either product can be achieved through the use of an appropriate combination of the nitrogen substituent and the type of the rhenium(I) catalyst. The 2-azabicyclo[3.3.0]octane derivatives 9 are obtained selectively by carrying out treatment of N-Ns derivatives in the presence of [ReCl(CO)(4)(PPh(3))], whereas the dihydropyrrole derivatives 10 are obtained by treatment of N-Mbs derivatives with [ReCl(CO)(5)]/AgSbF(6) . Finally, we have applied this geminal carbo-functionalization to one-carbon-elongated substrates containing N-Ts moieties in their tethers. Selective 5-exo cyclization is achieved in the presence of gold(I) or rhenium(I) catalysts, whereas 6-endo cyclization is observed on use of [W(CO)(6)].

Utilizing the biscarbene character of electrophilically activated alkynes, a novel tandem intramolecular cyclopropanation/Cope rearrangement of 3-siloxy-1,3,9-trien-7-ynes catalyzed by W(CO)5(L) for the stereoselective construction of bicyclo[5.3.0]decane framework is achieved. When 3-siloxy-1,3,9-trien-7-ynes were treated with a catalytic amount of W(CO)6 under photoirradiation, bicyclo[5.3.0]decanes were obtained in good yield stereoselectively. In this reaction, the Cope rearrangement of the divinylcyclopropane intermediates, generated by the intramolecular cyclopropanation of 3-siloxy-1,3,9-trien-7-ynes based on the W(CO)5(L)-catalyzed electrophilic activation of alkynes, occurs to give synthetically useful functionalized bicyclo[5.3.0]decane derivatives stereoselectively.