Sequential Chemical Transformations of a Tungsten Carbyne Ligand: Protonation, Alkyne Insertion, Vinylcarbene Formation, and Metallacyclopropene/Metallafuran Isomerization

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Abstract

Proton addition and phenylacetylene insertion convert carbyne complexes of the type Tp‘(CO)2W⋮C−OAr (Ar = Ph, p-C6H4Me, p-C6H4OMe; Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) into tungsten η3-vinylcarbene complexes of the form [Tp‘(CO)2W(η3-(Cα,Cβ,Cγ)CPh−CHCH(O(p-C6H4R)))][X] (R = H, X = BF4, 6a; R = Me, X = BF4, 6b; R = OMe, X = BF4, 6c; R = H, X = BAr‘4, 6a‘). The intermediate agostic carbene complex [Tp‘(CO)2WC(H)O(p-C6H4OMe)][BAr‘4] (7‘) was characterized by IR, 1H NMR, and 13C NMR spectroscopy. Addition of base to the cationic vinylcarbene complex 6 forms the metallafuran complex Tp‘(CO)2W(κ2-(Cα,O)CPh−CHCH−O) (9) as the major product. Although the net reaction corresponds to loss of C6H5+, mechanistically it is attractive to postulate addition of OH- to the electron-deficient Cγ site followed by proton removal by base coupled with loss of OPh- to generate the metallafuran stoichiometry. Isolation of an intermediate metallacyclopropene complex (10), possessing an aldehyde substituent on the three-membered ring, which isomerizes to the thermodynamically favored metallafuran complex, is consistent with this mechanism. The vinylcarbyne complex Tp‘(CO)2W⋮C−CPhCH(OTol) (11b) was also recovered from the reaction mixture of complex 6b and base. Addition of 0.10 equiv of H[BF4] to metallacyclopropene complex 10 resulted in complete conversion of 10 to the metallafuran complex 9 at room temperature. Addition of acid to metallacyclopropene complex 10 at low temperature allowed 1H NMR characterization of the η3-vinylcarbene intermediate [Tp‘(CO)2W(η3-(Cα,Cβ,Cγ)CPh−CHCH(OH))][BF4] (17). Upon addition of Li[Et3BH] to 6, double hydride addition and loss of aryloxide occurred at Cγ of the aryloxy-substituted vinylcarbene complex 6 to form the previously reported η2-vinyl complex Tp‘(CO)2W(η2-(Cα,Cβ)CPhCH−CH3) (8). These experiments illustrate reaction routes available to C3 ligand skeletons derived from carbyne−alkyne coupling. The oxygen of the aryloxy substituent on the original carbyne carbon plays an integral role in this reaction sequence.

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