Rapid Markovnikov addition of HCl to a pendant alkyne: evidence for a quinoidal cumulene

Abstract

Reaction of cis-[RuCl2(dppm)2]BF4 with TlBF4 and 1,4-diethynyl-benzenes results in the formation of the vinylidene cations trans-[Ru([double bond, length as m-dash]C[double bond, length as m-dash]CH-C6H2-2,5-R2-4-C[triple bond, length as m-dash]CH)Cl(dppm)2](+) (R = H, Me). Subsequent reaction with [N(n)Bu4]Cl results in nucleophilic attack at the coordinated organic ligand, but not at the expected metal-bound carbon atom. Instead, trans-[Ru(C[triple bond, length as m-dash]C-C6H2-2,5-R2-4-CCl[double bond, length as m-dash]CH2)Cl(dppm)2] was generated which, when coupled with DFT calculations, provides evidence for an intermediate quinoidal cumulene complex.

Further Evidence for ‘Extended’ Cumulene Complexes: Derivatives from Reactions with Halide Anions and Water

“…The structures of 2 a‐Cl and 2 a‐Br were determined by single crystal X‐ray diffraction (Figure , Table ). Structural parameters of the terminal C(X)=CH 2 moieties are consistent with those of similar vinyl‐halide analogues, such as trans ‐[Ru{C≡C‐2,5‐R 2 ‐C 6 H 2 ‐4‐C(Cl)=CH 2 }Cl(dppm) 2 ] (R=H, Me) and [Mn{C=C(H)‐1,4‐C 6 H 4 ‐C(Br)=CH 2 }(CO) 2 Cp] . The pattern of alternating long/short/long bond lengths along the Ru‐C1‐C2‐C3 chain is typical of ruthenium alkynyl complexes, but rather less pronounced in the case of 2 a‐Cl , which may be indicative of a small contribution from a more cumulated resonance structure stabilised by the electronegative Cl atom.…”

“…The isolation of 2 a‐Cl and 2 a‐Br , together with the spectroscopic observation of vinylhalide products formed from the formal Markovnikov addition of HX to the pendent ethynyl moiety in trans ‐[Ru{C=C(H)‐1,4‐C 6 H 4 ‐C≡CH}Cl(dppe) 2 ], gave confidence that the rearrangement of the ethynyl‐substituted phenylvinylidene complexes to a quinoidal cumulene ligand reported earlier could be extended to a broader range of metal‐ligand fragments and nucleophiles. To advance these studies, further members of the family of half‐sandwich ethynyl vinylidene complexes [M{C=C(H)‐1,4‐C 6 H 4 ‐C≡CH}(LL)Cp]BF 4 [M(LL)Cp’=Ru(dppe)Cp* ([ 1 b ]BF 4 ); Fe(dppe)Cp ([ 1 c ]BF 4 ); Fe(dppe)Cp* ([ 1 d ]BF 4 )] were prepared from the respective half‐sandwich chloride complexes [MCl(LL)Cp’] and isolated as the [BF 4 ] − salts in around 60 % yield in each case.…”

“…The facile reaction of trans ‐[Ru{C=C(H)‐2,5‐R 2 ‐C 6 H 2 ‐4‐C≡CH}Cl(dppm) 2 ]BF 4 (R=H, Me) with [NBu 4 ]Cl to give trans ‐[Ru{C≡C‐2,5‐R 2 ‐C 6 H 2 ‐4‐C(Cl)=CH 2 }Cl(dppm) 2 ]BF 4 (Scheme ) prompted consideration of initial, exploratory NMR‐scale reactions of the analogous dppe‐complex trans ‐[Ru{C=C(H)‐1,4‐C 6 H 4 ‐C≡CH}Cl(dppe) 2 ]BF 4 with a range of tetrabutylammonium halide salts [NBu 4 ]X (X − =Cl − , Br − , I − ; Scheme ). The formation of further examples of products formally derived from Markovnikov addition of HX to the terminal alkyne moiety was clearly evinced by the characteristic resonances from the methylene protons at 5.42 and 5.72 (X=Cl, J HH =1.8 Hz), 5.67 and 6.07 (X=Br, J HH =2.1 Hz), and 6.00 and 6.44 (X=I, J HH =1.8 Hz) ppm, with further support garnered from mass spectrometry.…”

“…In light of these various interests in cumulated carbon chains, the isolation of complexes trans ‐[Ru{C≡C‐2,5‐R 2 ‐C 6 H 2 ‐4‐C(Cl)=CH 2 }Cl(dppm) 2 ] ( C , R=H, Me; Scheme ) from reactions of the (4‐ethynylphenyl)vinylidene complexes trans ‐[Ru{C=C(H)‐2,5‐R 2 C 6 H 2 ‐4‐C≡CH}Cl(dppm) 2 ]BF 4 ( A ) with [NBu 4 ]Cl points to a further new line of investigation . The formation of these unusual Markovnikov addition products is consistent with the isomerisation of the (4‐ethynylphenyl)vinylidene ligand to a transient quinoidal cumulene B , a suggestion which was supported by DFT‐based calculations and explorations of possible mechanistic pathways (Scheme ).…”

“… The reaction of vinylidene complexes A to give Markovnikov addition products C via putative quinoidal cumulenes B (R=H, Me) …”