Recent Methodologies That Exploit C–C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

Abstract

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Synthesis of Polyethylene with In‐Chain α,β‐Unsaturated Ketone and Isolated Ketone Units: Pd‐Catalyzed Ring‐Opening Copolymerization of Cyclopropenone with Ethylene

“…[6] The strained ring can be opened by subsequent b-carbon elimination to form an a,b-unsaturated ketone unit. [8,9] Although the chemistry of cyclopropenones has been extensively investigated in synthetic methodology,t heir application as polar monomers in olefin polymerization remains rather unexplored. [7] Ther ing-opening coupling reactions of cyclopropenones with olefins were also reported to form fivemembered cycloaddition products by as train-driven oxidative addition of the C À Cbond to atransition metal as the key step.…”

Radical Reactions for the Synthesis of 3‐Substituted Chroman‐4‐ones

“…Cyclopropanols, a class of three‐membered cyclic compounds, are useful synthetic intermediates in organic synthesis. For example, a variety of transition‐metal‐catalyzed ring‐opening cross‐coupling reactions have been developed . In the past years, cyclopropanols have been reported as radical precursors through C–C bond cleavage giving β‐carbonyl alkyl radicals for the ring‐opening/cascade radical coupling strategy .…”

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

“…At this stage,t he keys for straightforward synthesis of 3 with broad scope were attributed mainly to optimal insertion reactivity of 1 and 2 by both their steric interactions with L4/ L6Ni II and the high rearrangement reactivity of vinylcyclopropane assisted by (NHC)Ni II ,u nder mild reaction conditions.O ne of the possible reaction sequences might involve a( NHC)Ni II directed selective cross-hydroalkenylation of 1 with 2.Here,achemoselective insertion of 1 occurs (favored by ring-strain release), [8] followed by afast regioselective insertion of 2,rather than either ah omo-insertion of 1 (by steric interaction of 1 with bulky NHCs as depicted in Table 1) or premature ring-opening in Pd. After a( NHC)Ni II assisted vinylcyclopropane rearrangement at just 30 8 8C, as compared to Scheme 1c…”

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Ac ross-hydroalkenylation/rearrangement cascade (HARC), using ac yclopropene and alkyne as substrate pairs, was achieved for the first time by using new [(NHC)Ni-(allyl)]BAr F catalysts (NHC = N-heterocyclic carbenes). [8] Thei ntermediates,s uch as the reactive vinyl metal carbenoids and the cyclopropane ions,are often trapped by either nearby functional groups on the skeleton or external nucleophiles (Scheme 1a), providing ag eneral route to substituted furans,p yrans,a nd acyclic products.T ransition-metal hydride catalysts or their equivalents [9] could potentially be one of the most ideal approaches for cyclopropene functionalization since it could insert into the olefin and allow subsequent insertions of various reaction partners.However, their use in selective transformation of 1 is rarely reported. The mild reaction conditions employed may serve as an entry for exploring (NHC)Ni II -assisted vinylcyclopropane rearrangement reactivity.

Cyclopropenes (1)are important building blocks in organic synthesis,a nd significant advances for their synthesis have been made recently by using new transition-metal catalysts.

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