Rearrangements of cyclopropenes into five-membered aromatic heterocycles: mechanistic aspect

Abstract

This review covers rearrangements of cyclopropenes into aromatic five-membered heterocyclic compounds, namely furans and pyrroles; accompanied by small ring cleavage proceeding in the presence of catalytic amounts of transition metals, Lewis acids, or under UV irradiation. Due to significant strain in the small ring, cyclopropenes are characterized by increased energy and a conformationally constrained hydrocarbon skeleton, which chiefly defines their unusually high reactivity and selectivity in reactions atypical for other unsaturated compounds. The increased electronic density of the π-system of its strained double bond makes cyclopropene a very attractive substrate for π-philic transition metals [1]]. In turn, this opens avenues for extremely rich coordination chemistry for different types of catalytic rearrangements, as well as for addition and cycloaddition reactions. The important role of small cycles in the photochemistry of furans was first demonstrated in reports by Srinivason [2, 3] and van Tamelen [4,5], who discovered that the photo-rearrangement of 2-methylfurans 1 into 3-methylfurans 2 proceeds via a valent rearrangement forming 3-acylcyclopropene intermediate 3 (Scheme 1). In addition, in the reaction of furan 1а a competing photodecarbonylation leading to product 4 was detected, which confirmed the formation of intermediate 3-formylcyclopropene 3а (Scheme 1).Scheme 1

Regioselective Synthesis of Multisubstituted Furans via Metalloradical Cyclization of Alkynes with α-Diazocarbonyls: Construction of Functionalized α-Oligofurans

“…14 Further experiments showed that treatment of preprepared cyclopropene 4aa or 4ab in the presence of [Co( P1 )], α-diazocarbonyl ( 2a/b) and acetylene 1a at 80 °C resulted in ineffective isomerization to the corresponding furan 3 , indicating direct furan formation from cyclization of phenylacetylene with the diazo reagent rather than from ring expansion of the initially formed cyclopropene. 15 These results clearly demonstrated that the Co(II)-catalyzed cyclopropenation pathway could be completely switched to furan formation through judicious use of the diazo reagent at a higher reaction temperature (Scheme 1). …”

“…In addition to diazoacetates 2a and 2b , which formed the corresponding furans through cyclization with the ester groups (entries 1 and 2), ketone groups could also effectively participate in the catalytic furan formation process as demonstrated with diazoketone 2c , producing tosyl-substitued furan 3ac in 87% yield (entry 3). More interestingly, α-ketodiazoacetates 2d and 2e could selectively cyclize with phenylacetylene to form furylesters 3ad 15c and 3ae , respectively, indicating preferential involvement of ketone carbonyls over ester carbonyls in the [Co( P1 )]-catalyzed furan synthesis (entries 4 and 5). Similar preference of aldehyde carbonyls over ester carbonyls was also observed for the metalloradical cyclization process.…”

Mechanistic Understanding of the Divergent Reactivity of Cyclopropenes in Rh(III)-Catalyzed C–H Activation/Cycloaddition Reactions of N-Phenoxyacetamide and N-Pivaloxybenzamide

“…The strained structure makes the CC bond in such small cyclic compounds much more reactive than normal olefins toward addition reactions with electrophiles . Alternatively, the ring openings of cyclopropenes are quite favorable as a result of the release of the strain energy and are commonly seen in transition metal-catalyzed transformations in which the cyclopropene could be a precursor for a carbenoid intermediate or incorporated into the final product as a three-carbon synthon . Thus, diverse reactivities of cyclopropenes were documented in the literature. − …”

Applications of metal and non-metal catalysts for the synthesis of oxygen containing five-membered polyheterocylces: a mini review

“…In contrast, previously it was reported that the rearrangement of achiral cyclopropenecarboxylates may also proceed enantioselectively in the presence of optically active rhodium(II) complexes [316]. Therefore, the absence of enantiomeric induction (Scheme 55) was not taken as unambiguous evidence for the involvement of cyclopropene intermediate and was simply a result of inefficacy of the chiral catalyst in this particular conversion [317,318].…”