Tandem Cyclization of Enynes Containing a Thioether or Ether Linkage via Ruthenium Allenylidene and Vinylidene Complexes

Abstract

The two aromatic S-enynes HCCCH(OH)(C6H4)SCH2C(R)CH2 (1a, R = Me; 1b, R = H) containing olefinic groups with and without an internal methyl substitutent and the O-enyne HCCCH(OH)CMe2CH2OCH2C(Me)CH2 (1c) also with an internal methyl substituent on the olefinic group but with no aromatic group have been prepared. In the [Ru]Cl-induced ([Ru] = Cp(PPh)3Ru) reactions of 1a,c, the presence of the methyl group promotes cyclization reactions and their tandem cyclizations are further induced by MeOH. The reaction of 1a in CH2Cl2 gives the three products 2a–4a. Complex 2a, with a seven-membered thio ring bonded at Cβ of the vinylidene ligand, is formed via a C–C bond formation between two unsaturated groups in moderate yield. Complex 3a is formed via migration of PPh3 from the metal onto the terminal carbon of the alkynyl group followed by coordination of the S atom. The carbene complex 4a is formed by S addition to the internal carbon of the alkynyl group accompanied by migration of the allylic group from sulfur to the newly formed thiophene ring. Tandem cyclization of 1a in MeOH generates the organic product 8a via 2a. In the reaction, the vinylidene complex 7a, a formal methanol addition product of 2a, is also formed as a side product. Deprotonation of 7a gives the acetylide complex 9a. The reaction of 1c affords the vinylidene complex 2c in CH2Cl2 via a similar cyclization process with no other side product. Deprotonation of 2c followed by allylation gave the disubstituted vinylidene complex 10c. Tandem cyclization of 1c in MeOH also gives the organic product 8c. In the reaction of [Ru]Cl with 1b containing no methyl group in the olefinic part, no C–C bond formation was observed. The reactions of [Ru]NCCH3 + with 1a,b each gave only 4a,b, respectively, with no side product. All of these reaction products are characterized by spectroscopic methods as well as elemental analysis. In addition, the structures of three complexes 5a, 9a, and 10c have been confirmed by X-ray diffraction analysis.

Ruthenium‐Induced Cyclization of Heteroatom‐Functionalized Alkynes: Progress, Challenges and Perspectives

“… Isolation of, and proposed formation mechanisms for 3 a , 3 b and 3 c . The synthesis of 3 a’ – 3 c’ is also depicted for reference …”

“…Later,t he same group reported the isolation of am ixture of Ru-benzothiophenyl-carbene (3a), -alkenyl (3b)a nd -vinylidene (3c)c omplexesf rom the reactions betweenp ropargylic thioether S3 and [CpRu(PPh 3 ) 2 Cl] (Scheme 19). [29,30] Complexes 3a and 3b were rationalized to be derived from" non-vinylidene-involving" pathway whereas 3c were from "vinylidene-involving" pathways. The analogous primedd erivatives 3a'-3c' were obtained by replacing S3 by S3'.…”

Reactions of Ruthenium Complexes Containing Pentatetraenylidene Ligand

“…Various transition metals, such as titanium, vanadium, chromium, manganese, and iron, have all been utilized to form carbene complexes or that with even higher unsaturated carbon chains, the structure characteristics and chemical reactivities of the resulted complexes have been studied as well. Complexes of the second period transition metals, such as ruthenium carbenes, vinylidenes,, and allenylidienes, were also synthesized and characterized in recent years. Different kinds of metal cumulenes were synthesized by modified organic precursors, particularly alkyne and enyne compounds.…”