N-Alkoxyimidazolylidene Transition-Metal Complexes:  Application to [5+2] and [4+2] Cycloaddition Reactions

Abstract

Silver(I) bis[(benzyloxy)imidazol-2-ylidene] complex 6 is prepared by reaction of an alkoxyimidazolium bromide (4) with Ag2O. In situ deprotonation of the alkoxyimidazolium bromide 4 afforded the rhodium(I) and palladium(II) complexes 7 and 8, whose structures were verified by X-ray crystallography. Rhodium(I) (benzyloxy)imidazolylidene complex 7 efficiently catalyzes the [5+2] cycloaddition reaction of an yne−vinylcyclopropane (93%) and the [4+2] cycloaddition reaction of a diene−yne (99%) at room temperature in minutes.

Advances in bis(N‐heterocyclic carbene) chemistry: new classes of structurally dynamic materials

“…With this newfound access to free NHCs, robust metal complexes with excellent catalytic activity could be prepared (e.g., Grubbs, 1999; bottom right) and the same reactivity with electrophiles as the aforementioned tetraazafulvene dimers was observed (e.g., Bielawski, 2005; bottom left) . During the last half‐century, NHCs and derived molecules have shown great utility as ligands for a wide variety of transition metals, and these organometallic complexes continue to play important roles in supporting polymerization and organic transformation catalysts and as the basis of metal‐organic frameworks, phosphors, antibiotics, and chemotherapeutics …”

Recent Developments in the [5+2] Cycloaddition

“…In this context, Waymouth, Wender, and co-workers have described the synthesis of N-alkoxyimidazol-2-ylidines, their coordination to metals such as rhodium, and the catalytic behaviour of the rhodium carbene complexes in [5 + 2] cycloaddition reactions. [17] Thus, the rhodium(I) (benzyloxy)imidazolylidene complex 10 was prepared from a rhodium-cod dimer, which reacted with the alkoxyimidazol-2-ylidinium salt in situ. This complex, pretreated with 2 mol% of AgSbF 6 , was shown to be a highly effective catalyst for the [5 + 2] cycloaddition of the yne-vinylcyclopropane 11, which furnished the corresponding cycloadduct 12 at room temperature in an excellent yield of 93%, as shown in Scheme 6.…”

“…Relative to their carbon‐substituted counterparts, heteroatom‐substituted N‐heterocyclic carbenes offer a more direct means to perturb the electronic characteristics of the complex and also a proximate coordination site for directed reactivity. In this context, Waymouth, Wender, and co‐workers have described the synthesis of N ‐alkoxyimidazol‐2‐ylidines, their coordination to metals such as rhodium, and the catalytic behaviour of the rhodium carbene complexes in [5+2] cycloaddition reactions 17. Thus, the rhodium(I) (benzyloxy)imidazolylidene complex 10 was prepared from a rhodium‐cod dimer, which reacted with the alkoxyimidazol‐2‐ylidinium salt in situ .…”

A Comparison between the Cycloadditions of Allenyl- and Vinyl-Cyclopentanes Using Density Functional Theory and GRRM Program

“…Reactants containing 3-membered, 4-membered, and 5-membered rings have been reported. For example, intramolecular [5 + 2] cycloadditions of vinylcyclopropane analogs [23][24][25][26][27][28][29][30][31] and [6 + 2] cycloadditions of vinyl cyclobutanealkynes [32][33][34] catalyzed by Rh give 7-and 8-membered carbocycles, respectively.…”