Rh^I‐Catalyzed CC Bond Activation: Seven‐Membered Ring Synthesis by a [6+1] Carbonylative Ring‐Expansion Reaction of Allenylcyclobutanes

Abstract

The transition-metal-catalyzed activation of carbon-carbon (CÀC) sigma bonds represents an important and fundamental challenge in organometallic chemistry that has the potential to broaden how we approach problems in organic synthesis. [1] We have previously shown that transition metals can be used to catalyze the strain-assisted CÀC bond activation of vinylcyclopropanes (VCPs) to produce organometallic intermediates that can be intercepted with various trapping agents to afford new two-, three-, and four-component cycloadditions. Representative examples include the [5+2] cycloaddition of VCPs and p systems (Scheme 1a);[2] the [5+2+1] cycloaddition of VCPs, alkynes (and allenes), and CO; [2c, 3] and the [5+1+2+1] cycloaddition of VCPs, alkynes, and CO, [4] thus providing new routes to seven-, eight-, and nine-membered rings and their derivatives.[5] Whereas the extension of this five-carbon VCP chemistry to simple six-carbon vinylcyclobutanes has failed thus far (Scheme 1 c), the metal-catalyzed activation of the CÀC bond of vinylcyclobutanones has been successful, thus leading to a new [6+2] cycloaddition for the formation of eight-membered rings (Scheme 1 b). [6,7] To explore whether vinylcyclobutanes could be activated for use as six-carbon components in metal-catalyzed reactions, we examined the reaction of 1-siloxy-1-vinylcyclobu-

Rhodium‐Catalyzed [4+2+1] Cycloaddition of In Situ Generated Ene/Yne‐Ene‐Allenes and CO

“…In recent years, transition‐metal‐catalyzed cycloadditions such as [5+2], [6+1], [4+3], [3+2+2], etc. have emerged as powerful tools to synthesize various seven‐membered carbocycles.…”

Reactivity and Chemoselectivity of Allenes in Rh(I)-Catalyzed Intermolecular (5 + 2) Cycloadditions with Vinylcyclopropanes: Allene-Mediated Rhodacycle Formation Can Poison Rh(I)-Catalyzed Cycloadditions

“…Murakami and co-workers have also observed a similar trend in Rh(I)-catalyzed (4 + 1) cycloadditions between allene-enes and carbon monoxide, where terminally mono- and disubstituted allene-enes underwent the desired cycloaddition, and terminally unsubstituted allene-enes formed an unreactive rhodium complex with the catalyst. 18 Similarly, in cases of Rh(I)-catalyzed carbonylative rearrangements of allenyl ethers, 19 (6 + 1) cycloadditions of allenylcyclobutanes, 20 and even iridium-catalyzed (5 + 1) cycloadditions of allenylcyclopropanes, 21 terminally unsubstituted or monosubstituted allene substrates gave lower yields than their disubstituted counterparts or were altogether unreactive. This enigmatic methyl effect limits the intermolecular use of allenes.…”

Cyclobutane based “overbred intermediates” and their exploration in organic synthesis

“…The rhodacycle then successively undergoes carbonylative ring expansion and reductive removal of Rh afforded the desired compound. 47 Cramer and co-workers reported Rh( i ) catalyzed desymmetrization of allenyl- tert -cyclobutanols ( 154 , having the cb overbred skeleton) to furnish enantioenriched 5,5-disubstituted cyclohexenones as shown in Scheme 24. 48 As a mechanistic proposal, the initial coordination of the Rh( i ) center to both the hydroxy group and the proximal double bond of the allene in the allenyl cyclobutanol precursor is shown.…”