Formation of a C–C double bond from two aliphatic carbons. Multiple C–H activations in an iridium pincer complex

Abstract

Iridium can mediate a reversible intramolecular coupling reaction involving up to four unactivated Csp3–H bonds, to give a carbon–carbon double bond.

Reversible α‐Hydrogen and α‐Alkyl Elimination in PC(sp3)P Pincer Complexes of Iridium

“…In line with our previous studies, [2] DFT calculations showed that the isomerization of 2 to 3 begins with an aelimination of ahydrogen atom to give Ir carbene 7,followed by migratory insertion of the carbene moiety into the IrÀC bond (Scheme 5). Other possible mechanisms are prohibitively high in energy.T he iridium(I) complex resulting from C À Cr eductive elimination could not be located.…”

“…Thereverse reaction, that is,the transformation of 3 into 7,i sm ore intriguing,a si tc orresponds to au nique aelimination of an alkyl group.W eh ave previously proposed this reaction type to be involved in C(sp 3 ) À C(sp 3 )b ond cleavage in arelated system, [2] but it has not been observed for completely unstrained systems thus far, nor have any detailed studies been reported. As the transformation of 2 to 3 is reversible,w ew ere,h owever, able to obtain kinetic and thermodynamic data for the reaction in both directions.T he enthalpic barrier for a-alkyl elimination was found to be comparable to that of a-hydrogen elimination (21.5 vs. 22.0 kcal mol À1 ), and the DG°value is only approximately 1kcal mol À1 larger.T he DFT calculated barriers are approximately the same.T hese results suggest that a-alkyl elimination reactions are fully possible for this and related systems from akinetic point of view,atleast when it comes to R 3 CÀM fragments.T hermodynamically,t he conversion of 2 into 3 is almost neutral with the equilibrium being slightly shifted towards 3,the C À Cbond-formation product.…”

“…As the transformation of 2 to 3 is reversible,w ew ere,h owever, able to obtain kinetic and thermodynamic data for the reaction in both directions.T he enthalpic barrier for a-alkyl elimination was found to be comparable to that of a-hydrogen elimination (21.5 vs. 22.0 kcal mol À1 ), and the DG°value is only approximately 1kcal mol À1 larger.T he DFT calculated barriers are approximately the same.T hese results suggest that a-alkyl elimination reactions are fully possible for this and related systems from akinetic point of view,atleast when it comes to R 3 CÀM fragments.T hermodynamically,t he conversion of 2 into 3 is almost neutral with the equilibrium being slightly shifted towards 3,the C À Cbond-formation product. [2] However,itshould be noted that the CÀCagostic interaction adds little to the overall energetics as the agostic and non-agostic forms are very close in energy in solution and interconvert very rapidly. [21] This means that C À Cb ond cleavage is expected to be more unfavorable in general, and that one of the driving forces for the current reaction is the transformation of axially metalated 3 into equatorially metalated 2, which releases some repulsive interactions in the ligand that are worth approximately 5kcal mol À1 .T herefore,for C(sp 3 )À C(sp 3 )b ond cleavage to be successful, an additional driving force is required, for example,s ome degree of strain in the substrate or subsequent transformations of resulting fragments,a sw eh ave previously shown in the hydrogenation example.…”

“…[2] We were able to show that the key step responsible for C À Cb ond formation and cleavage is the formation of ac arbene complex through double CÀHa ctivation;t his intermediate is responsible for CÀCb ond formation and cleavage by migratory insertion or deinsertion of the carbene moiety into an IrÀCb ond (Scheme 1). [2] We were able to show that the key step responsible for C À Cb ond formation and cleavage is the formation of ac arbene complex through double CÀHa ctivation;t his intermediate is responsible for CÀCb ond formation and cleavage by migratory insertion or deinsertion of the carbene moiety into an IrÀCb ond (Scheme 1).…”

Iridium Pincer Complexes with an Olefin Backbone

“…As reported previously, 9 olefin complex 3 can be readily synthesized upon heating a benzene solution of hydridochloride 1 in the presence of base and tert-butylethylene as a hydrogen acceptor (Scheme 1).…”