Pseudo‐tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E‐Enynes

Abstract

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP )(C H )(NCMe)] (PhBP =PhB(CH PPh ) ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe , CNtBu) produced a change in the "donicity" of the alkyne from a 4e to a 2e donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP )(C H )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.

Slicing the π in Three Unequal Pieces: Iridium Complexes with Alkyne, Iminoxolene, and Dioxolene Ligands

“…The alkyne resonances of 1a (δ 76.4, 102 ppm) are most consistent with an insignificant amount of π donation. Structurally, π donation from the alkyne results in characteristically short metal-carbon distances (typically 2.02 Å for four-electron donor alkynes to iridium). , The observed distances of 2.170(3) and 2.271(3) Å in 1a are not only much too long to be consistent with significant π donation, they are on the long end of known iridium-alkyne distances even in two-electron donor alkynes, which are typically in the range of 2.08–2.14 Å. , The only similar iridium–carbon distances in bonds to simple alkynes are in the cationic Ir­(III) complex [Cp*Ir­(P^C)­(PhCCMe)]­BAr F (P^C = cyclometalated PMe­(2,6-Me 2 C 6 H 3 ) 2 ), with Ir–C distances of 2.181(5) and 2.201(5) Å . The bulk of the triisopropylsilyl group does not appear to play a major role in the long metal-alkyne bonds in 1a , as DFT calculations on the terminal alkyne analogue show similar distances.…”

“…Four-electron donor alkyne complexes of iridium are known, and, as documented in group 6 complexes, , the key spectroscopic signature of π donation is a downfield shift in the 13 C NMR . For example, in (PhB­[CH 2 PPh 2 ] 3 )­Ir­(PhCCH), the alkyne carbons of the four-electron donor alkyne resonate at δ 179.8 and 166.3 ppm, while in the PMe 3 adduct (PhB­[CH 2 PPh 2 ] 3 )­Ir­(PhCCH)­(PMe 3 ), where π donation from the alkyne is precluded, the resonances shift upfield to δ 89.0 and 83.1 ppm . The alkyne resonances of 1a (δ 76.4, 102 ppm) are most consistent with an insignificant amount of π donation.…”

Mechanistic Versatility at Ir(PSiP) Pincer Catalysts: Triflate Proton Shuttling from 2-Butyne to Diene and [3]Dendralene Motifs

“…According to its pseudo-tetrahedral geometry and the structural parameters of the alkyne moiety, this calculated product of formal triflic acid reductive elimination should be described as containing a four-electron 2-butyne ligand. 72 Remarkably, the acid can readily re-protonate this complex, directly in one of the alkyne carbons, to form alkenyl 13 calc . Despite the different mechanisms, the energy profiles leading to alkenyl complexes [10 calc ] + and 13 calc from isomers 5 calc display similar barriers.…”

“…Calculations in benzene solution (Figure ) indicate that isomer 5 calc mer can favorably rearrange (−1.3 kcal mol –1 ) into triflic acid and intermediate 12 calc . According to its pseudo-tetrahedral geometry and the structural parameters of the alkyne moiety, this calculated product of formal triflic acid reductive elimination should be described as containing a four-electron 2-butyne ligand . Remarkably, the acid can readily re-protonate this complex, directly in one of the alkyne carbons, to form alkenyl 13 calc .…”

A Stable N‐Hetero‐Rh‐Metallacyclic Silylene

“…Particularly interesting, as suggested by the spectroscopic data, is an important deviation from as quare-planar geometry in 5, which presents an approximately tetrahedral geometry around the Rh I center with al arge dihedral angle between the two five-and sixmembered rings connected at the Rh atom (q Rh = 72. [32] To gain more insight into the electronic structure of silylene 5,D FT calculations on as impler model 6 (methyl Scheme 1. In the case of precursor 4,t his deviation is significantly smaller (q = 36.98 8), which suggests that the unusual geometry of 5 is probably due to an electronic rather than asteric effect.…”