Synthesis, Structures, and Reactivity of Cyclometalated Complexes Formed by Insertion of Alkynes into M–C (M = Ir and Rh) Bonds

Vinylidene rearrangements of internal borylalkynes via 1,2-boryl migration

Vinylidene rearrangement of alkynes is a well-established and powerful method for alkyne transformations, while use of borylalkynes has remained largely unexplored. This paper describes vinylidene rearrangements of internal borylalkynes using a cationic ruthenium complex. This rearrangement is applicable to alkynes with both tri-(B(pin), B(dan)) and tetracoordinate (B(mida)) boryl groups, and the reaction rate is dramatically affected by the Lewis acidity of the boryl group. Mechanistic study revealed that the rearrangement proceeds via 1,2-boryl migration regardless of the coordination number of the boron center. The migration mode was elucidated by theoretical calculations to indicate that the migration of the tricoordinate boryl groups is an electrophilic process in contrast to the previous vinylidene rearrangements of internal alkynes with two carbon substituents.

(P,C)-cyclometalated complexes derived from naphthyl phosphines: versatile and powerful tools in organometallic chemistry

The chemistry of (P,C)-cyclometalated complexes derived from naphthyl phosphines [^Np(P,C)M] is presented and analysed in this review. The three main synthetic approaches, namely P-chelation assisted C-H activation, oxidative addition and transmetalation, are described and compared. If a naphthyl framework inherently predisposes a phosphorus atom and transition metal to interact, a rigid metallacycle may induce some strain and distortion, as apparent from the survey of the single-crystal X-ray diffraction structures deposited in the Cambridge Structural Database (77 entries with metals from groups 7 to 11). Generally, the ^Np(P,C)-cyclometalation imparts high thermal and chemical robustness to the complexes, and a variety of stoichiometric reactions have been reported. In most cases, the metalacyclic structure is retained, but protodecyclometalation and ring-expansion have been sparingly observed. [^Np(P,C)M] complexes have also proved to be competent and actually competitive catalysts in several transformations, and they act as key intermediates in some others. In addition, interesting phosphorescence properties have been occasionally pointed out.

Iridium-catalyzed regioselective B(3,6)-dialkenylation or B(4)-alkenylation of o-carboranes via B-H activation and 1,2-carbon migration of alkynes

An efficient Ir-catalyzed cage boron alkenylation of 1-(2'-picolyl)-o-carboranes with diarylacetylenes has been developed, leading to a wide variety of B-H geminal addition products via 1,2-carbon migration of alkynes. The steric effect of cage carbon substituents has a great impact on the regioselectivity of such alkenylation reactions.

Rh(iii)-Catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes

We reported a Rh(iii)-catalyzed switchable C–H functionalization of 2-(1H-pyrazol-1-yl)pyridine with internal alkynes, which provided diversiform functionalized N,N-bidentate chelating compounds.

Isolation of a C3-metalated indolizine complex and a phosphonium ring-fused bicyclic metallafuran from the osmium-induced transformation of pyridine-tethered alkynes

The first C3-metalated indolizine complex was prepared from the reaction between cis-[Os(dppm)2Cl2] (dppm = 1,1-bis(diphenylphosphino)methane) and propargylic pyridine, HC[triple bond, length as m-dash]CC(OH)(2-py)2. A phosphonium ring-fused bicyclic osmafuran complex was also prepared from the reaction between cis-[Os(dppm)2Cl2] and pyridyl ynone, HC[triple bond, length as m-dash]C(C[double bond, length as m-dash]O)(2-py). The formation of these two products revealed the intermediacy of metal-vinylidene species regarding [Os(dppm)2Cl]+-mediated alkyne transformations. Comparison of the d6 transition-metal precursors employed to activate HC[triple bond, length as m-dash]CC(OH)(2-py)2 suggests that precursors with higher π-basicity favor the vinylidene-involving pathway.

Annulating thiazolium cations via a direct double C–H activation strategy: Rh–N,S-heterocyclic carbene is the key

Metal–N,S-heterocyclic carbene intermediates are conveniently generated and utilized for the first time to construct N,S doubly-doped cationic tricyclic organic molecules which exhibit easily-tuneable emission.

Rhenium-Promoted C-C Bond-Cleavage Reactions of Internal Propargyl Alcohols

The first examples of C-C bond cleavage reactions of internal propargyl alcohols to give vinylidene complexes are described. Treatment of [Re(dppm)₃ ]I with RC≡CC(OH)R'R'' (R=aryl, alkyl; C(OH)R'R''=C(OH)Ph_2, C(OH)Me₂ , C(OH)HPh, C(OH)H₂ ) produced the vinylidene complexes ReI(=C=CHR)(dppm)₂ with the elimination of C(O)R'R''. Computational studies support that the reactions proceed through a β-alkynyl elimination of alkoxide intermediates Re{OC(R')(R'')C≡CR}(dppm)₂ .

Synthesis, Structures, and Reactivity of Single and Double Cyclometalated Complexes Formed by Reactions of [Cp*MCl2]2 (M = Ir and Rh) with Dinaphthyl Phosphines

Reactions of two dinaphthyl phosphines with [Cp*IrCl₂]₂ have been carried out. In the case of di(α-naphthyl)phenylphosphine (1a), a simple P-coordinated neutral adduct 2a is obtained. However, tert-butyldi(α-naphthyl)phenylphosphine (1b) is cyclometalated to form [Cp*IrCl(P^C)] (3b). Complexes 2a and 3a undergo further cyclometalation to give the corresponding double cyclometalated complexes [Cp*Ir(C^P^C)] (4a,b) upon heating. In the presence of sodium acetate, reactions of 1a,b with [Cp*IrCl₂]₂ directly afford the final double cyclometalated complexes (4a,b). In the absence of acetate, [Cp*RhCl₂]₂ shows no reaction with 1a,b, whereas with acetate the reactions form the corresponding single cyclometalated complexes [Cp*RhCl(P^C)] (5a,b), which react with ^t BuOK to form the corresponding rhodium hydride complexes (6a,b). Treatment of 4a with CuCl₂ or I₂ leads to opening of two Ir-C σ bonds to yield the corresponding P-coordinated iridium dihalide (7 or 8) by means of an intramolecular C-C coupling reaction. A new chiral phosphine (11) is formed by the ligand-exchange reaction of 8 with PMe₃. Reactions of the single cycloiridated complex 3b with terminal aromatic alkynes result in the corresponding five- and six-membered doubly cycloiridated complex 12 and/or η²-alkene coordinated complexes 13-15; the latter discloses that the electronic effect of terminal alkynes affects the regioselectivity. While the single cyclorhodated complex 5b reacts with terminal aromatic alkynes to form the corresponding six-membered cyclometalated complexes 16a-c by vinylidene rearrangement/1,1-insertion. Plausible pathways for formation of insertion products 13-16 were proposed. Molecular structures of twelve new complexes were determined by X-ray diffraction.

Aromatic alkyne insertion into six-membered cyclometalated pyridine complexes of iridium: different insertion modes and structurally novel products

Reactions of 2-benzoylpyridine or 2-benzylpyridine with [Cp*MCl₂]₂ (M = Ir, Rh) have been carried out in the presence of NaOAc in refluxing methanol, which form the corresponding six-membered cyclometalated products (1–3) except for the reaction of 2-benzylpyridine with [Cp*RhCl₂]₂. Insertion reactions of two six-membered cyclometalated pyridine iridium complexes (1 and 2) with terminal or internal aromatic alkynes were studied. Terminal alkynes p-XC₆H₄C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH (X = H, MeO, and F) with 1 give the corresponding five- and seven-membered doubly cycloiridated complexes 4a–c, internal alkynes p-XC₆H₄CCC₆H₄X-p (X = H, MeO, and Br) form the similar five- and seven-membered doubly cycloiridated complexes (5a,b) and/or di-insertion products (6a,c), whereas the acyl alkyne PhCCCOPh affords the novel spiro-metalated complex 7. For complex 2, internal alkynes p-XC₆H₄CCC₆H₄X-p (X = H, MeO, and Br) form similar five- and seven-membered doubly cycloiridated complexes (8a–c). However, in the case of PhCCCOPh, the reaction gives the novel four-membered cyclometalated complex 9. These results suggest that the products formed by alkyne insertion reactions of the six-membered cycloiridated pyridine complexes are very diverse. Plausible pathways for the formation of these novel insertion products were proposed. Molecular structures of seven cyclometalated complexes were determined by X-ray diffraction.

Insertion reactions of aromatic alkynes into the Ir–C bond of six-membered cycloiridated complexes formed various novel products via different insertion modes.

C–H activation-annulation on the N-heterocyclic carbene platform

This review highlights the initial development of a new C–H activation–annulation chemistry accessible on the metal–N-heterocyclic carbene platform.

Sequential Reactions of Alkynes on an Iridium(III) Single Site

Sequential insertion of terminal alkynes on Ir^III cyclometalated complexes allow the formation of novel metallapolycycles in a controlled and efficient manner. ortho-Methylarylethynyl derivatives led to an unprecedented cascade combination of three fundamental processes (C-C bond formation, C(sp³ )-H activation, and reductive coupling) on a single Ir^III center, in a process compatible with functionalized biomolecules and photoactive substrates. The reaction with tert-butylacetylene led to a [6,5,4]-polycycle that incorporates an iridacyclobutenylidene in its structure. The sequence is a multicomponent reaction in which the metal not only promotes the different steps but also determines their stereoselectivity. This is an elegant example of the synergy between a metal-promoting reaction and a symmetry-defined stereochemistry.