New Iridacyclohexadienes and Iridabenzenes by [2+2+1] Cyclotrimerization of Alkynes and Facile Interconversion between Iridacyclohexadienes and Iridabenzenes

Abstract

Iridabenzenes [Ir[=CHCH=CHCH=C(CH2R)](CH3CN)2(PPh3)2]2+ (R=Ph 4 a, R=p-C6H4CH3 4 b) are obtained from the reactions of H+ with iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](CO)(PPh3)2]+ (R'=H 3 a, R'=CH3 3 b), which are prepared from [2+2+1] cyclotrimerization of alkynes in the reactions of [Ir(CH3CN)(CO)(PPh3)2]+ with HC[triple chemical bond]CH and HC[triple chemical bond]CR. Iridabenzenes 4 react with CO and CH3CN in the presence of NEt3 to give iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CHR)](CO)2(PPh3)2]+ (6) and [Ir[-CH=CHCH=CHC(=CHR)](CH3CN)2(PPh3)2]+ (7), respectively. Iridacyclohexadienes 6 and 7 also convert to iridabenzenes 4 by the reactions with H+ in the presence of CH3CN. Alkynyl iridacyclohexadienes [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-C[triple chemical bond]CH)(PPh3)2] (8) undergo a cleavage of C[triple chemical bond]C bond by H+/H2O to produce [Ir[-CH=CHCH=CHC(=CH-p-C6H4R')](-CH3)(CO)(PPh3)2] (10) via facile inter-conversion between iridacyclohexadienes and iridabenzenes.

Recent Advances in Metallabenzene Chemistry

“…Additional evidence for the amphiprotic nature of the benzylic carbon atom is found on treatment of 90 with HCl, which protonates this carbon atom and cleanly produces iridabenzene 94. [51] This unusual reactivity parallels that observed in the previously mentioned osmabenzofuran studies (see Section 3.4). [50] The addition of an arylacetylene to (h 2 -acetato)iridacycle 95 produced an unexpected difference in the key intermediate that nonetheless led to iridabenzene synthesis.…”

“…Reaction of 43 with ethanolic HCl results in transesterification of the free ester functionality to afford 44 while leaving the metal-bound ester unchanged. Reaction of 43 with pyridinium tribromide results in bromination at C6 to give [ 51,52] Three resonance structures account for the overall bonding picture in these complexes. Structures 43 a and 43 b (Scheme 13) account for the aromatic character of these molecules, while structure 43 c contains an osmacyclohexadiene system.…”

“…Unique to 93 is its ability to reversibly react with weak Lewis bases such as CO and Et 3 N to form iridacyclohexadiene complexes 91 and 92, respectively. [51] It is particularly unusual that MeCN (ordinarily a more labile ligand) would replace the carbonyl ligand on conversion of 91 back into 93. The facile interconversion of cyclohexadienes 91 and 92 and iridabenzene 93 supports the presumed intermediacy of iridabenzenes in the original studies.…”

“…Iridabenzene 97 could also be produced by displacement of both MeCN ligands in complex 93 when it was treated with NaOAc. [51,52] The probable mechanism for the formation of 97 from 96 involves proton attack on the b-carbon atom of the alkynyl group in 96 to give a cationic alkylidene, which is then attacked at the a-carbon atom by the terminal diene. -97, thus confirming that it is the terminal carbon atom of the diene that attacks the a-carbon atom of the alkylidene complex.…”

Formation of Iridabenzenes by Coupling of Iridacyclopentadienes and Alkenes

“…[1] Recently, formation of iridabenzenes by a [2+2+1] cyclotrimerization of alkynes via iridacyclopentadiene intermediates was demonstrated, [2] constituting a valuable addition to previous synthetic methodologies.…”