The study began by determining the optimal conditions for the cycloaddition of vinyl-N-triftosylhydrazone 1a with furan using NaH as the base in the presence of varied catalyst and solvent (Table 1). The use of AgOTf as a catalyst in CHCl3 at 60 °C ensued the desired product 2 in 24% yield, with a carbene dimer as the major product (Entry 1). The product yield could be substantially improved when using weakly coordinating trispyrazolylborate silver(I),49-51 especially Tp(CF3)2Ag affording 2 in 91% yield, with only a trace amount of undesired triene byproduct (entries 2 and 3). In contrast, Rh2(OAc)414 and Cu(hfacac)2,18 the most commonly used catalysts in the [4 + 3] cycloaddition of 1,3-dienes with vinyldiazoacetates, led to low yield, whereas Pd(OAc)2 only enabled the carbene dimerization and the self-cyclization to pyrazole (entries 4-6). Further, the screening on solvents disclosed that CH2Cl2 and PhCF3 were less effective than CHCl3, whereas 1,4-dioxane was completely ineffective (entries 7-9). Vinyl-N-tosylhydrazone 1b as vinylcarbene source, instead of the easily decomposable vinyl-N-triftosylhydrazone 1a, afforded 2 in a far low yield (40%), owing to the self-cyclization to pyrazole (entry 10). The structural configuration of 2 was unambiguously established by the X-ray crystallographic analysis. 52
With the optimized conditions in hand (Table 1, entry 3), the substrate scope of this reaction with a series of furans was then investigated. As shown in Fig. 2, furans with various substituents at 2-position, including alkyl, alkenyl, alkynyl, and (hetero)aryl as well as bromo, tributylstannyl, siloxy, allyl, and benzyl ether, ester, and ketone functional groups reacted smoothly with phenyl vinyl-N-triftosylhydrazone 3 to afford the corresponding products (4-18) in 44-90% yield. We were pleased to find that the substrates bearing a carbon-carbon double/triple bond and an activated C(sp3)−H bond (15-18) did not undergo competitive [2 + 1] cycloaddition53-55 and C−H insertion,10 thus achieving high chemoselectivity. The relatively low yields observed in some cases were ascribed to the ring-opening of furans to trienes and the carbene dimerization to olefins. In comparison, 2-substituted furans resulted in trienes as the major product in rhodium-catalyzed [4 + 3] cycloaddition with alkenyl diazoacetates. [12a-c] In addition, the steric hindrance of furans had no apparent effect on the reaction, for instance, di- and tri-substituted furans reacted efficiently with 3, providing multi-substituted oxa-bridged bicycles 19-23 in 55-87% yield.
Next, we investigated the reaction scope with respect to vinyl-N-triftosylhydrazones. As summarized in Fig. 2, a broad range of β-aryl vinyl-N-triftosylhydrazones with both electron-donating groups (e.g., Me and OMe) and -withdrawing groups (e.g., halogen, CN, NO2, CO2Et, and CF3) at various positions of aryl ring afforded the corresponding oxa-bridged bicyclic products (24–35) in 68-85% yield. Similarly, vinyl-N-triftosylhydrazones bearing an indolyl, thienyl, or furyl group at β-position all efficiently reacted with furan to afford the heteroaryl-substituted products (36-38) in 72-81% yield, whereas a ferrocenyl-substituted substrate led to the product (39) in a moderate yield (45%). The tolerance of strongly coordinated heteroatoms renders this silver-based catalytic approach particularly useful for the construction of medicinally relevant heterocyclic molecules. Moreover, the reaction of N-triftosylhydrazones derived from 2,4-dienals produced 4-vinyl oxa-bridged bicycles (40-42) in good yield, and the vinyl group present in the reaction products allows further orthogonal derivatization. In addition, an ester-containing product (43, 54% yield) was readily accessed from the carbethoxy-substituted vinyl-N-triftosylhydrazone. Alkyl-substituted vinyldiazomethanes are known to be prone to spontaneous cyclization to pyrazoles within hours even stored at -20 oC.21,56 By using the strategy of the merge of silver catalysis and vinyl-N-triftosylhydrazones, we were pleased to find that β-alkyl vinyl-N-triftosylhydrazones smoothly reacted with furan to give the corresponding 4-alkyl-8-oxabicyclo[3.2.1]oct-2,6-dienes (44-49, 62-85% yield), in which the common functional groups, including ester (OAc, 47), benzyloxy (OBn, 48), and dimethyltertbutylsilyloxy (OTBS, 49), were well tolerated. Furthermore, the substitution at α-position with an alkyl or halogen atom did not affect the efficiency and stereoselectivity (50-53, 67-84% yield). N-triftosylhydrazones derived from α,β-unsaturated ketones also underwent this reaction, even though an electron-withdrawing substituent at carbene carbon is required to suppress the self-cyclization to pyrazoles (54-61, 53-94% yield).2 Note that the compounds 60 and 61 could be prepared by rhodium-catalyzed method, albeit slow addition of styryldiazoacetate was required to achieve good efficiency.35 Furthermore, unsubstituted allyl aldehyde-derived N-triftosylhydrazone was examined in the current protocol; to our delight, the desired oxa-bridged bicyclic product 62 was obtained in 72% yield. This achievement clearly demonstrated the superiority of current method, as unsubstituted vinyl carbenes underwent self-condensation more easily than the substituted ones.56,57 Overall, the results summarized in Fig. 2 clearly demonstrated the superior features of silver-catalyzed [4 + 3] cycloaddition between furans and vinyl-N-triftosylhydrazones, namely the broad substrate scope, good functional group tolerance, excellent chemo-, regio- and endo-stereoselectivity, and good to high yield, thereby constituting a versatile method for the stereoselective synthesis of a wide range of endo oxa-bridged 1,4-cycloheptadienes.
Encouraged by the above achievements, we then turned our attention towards the synthesis of fused [5.n.0] bicyclic systems, a challenging target that has attracted the interest of synthetic and medicinal chemists due to their structural diversity and biological relevance (Fig. 3).1,58 First, we explored the [4 + 3] cycloaddition of furan with cyclic vinyl-N-triftosylhydrazones derived from cyclic enals, which were easily prepared by the one-carbon homologation reaction from commercially available cyclic ketones.59 A series of cyclic vinyl-N-triftosylhydrazones, having 6 to 8 membered rings, were found to be suitable in this reaction, affording the desired fused [5.4.0], [5.5.0], and [5.6.0] bicyclic compounds (63-66) in 53-78% yield. Furthermore, several commercially available 2,3-fused cyclic furans were suitable for this [4 + 3] cycloaddition, delivering the corresponding oxa-bridged [5.4.0] bicyclic compounds (67-70) in 49-80% yield. Finally, the intramolecular [4 + 3] cycloaddition of diverse vinyl-N-triftosylhydrazones bearing a tethered furan was rationally designed and applied to prepare benzannulated bicyclic compounds, which are the common structural core of several biologically active natural products.60 We were pleased to find that these intramolecular [4 + 3] cycloaddition reactions proceeded smoothly under the silver-catalyzed conditions, affording the desired benzannulated bicyclic compounds (71-74) in 60-76% yield. To the best of our knowledge, this is the first example of an intramolecular type I [4 + 3] cycloaddition between vinylcarbenes and furans.61
To demonstrate the practicality of the protocol, a gram-scale reaction between hydrazone 75 and furan was performed under standard conditions, affording the product 50 in 80% yield (1.05 g) that is comparable to that obtained on small scale (Fig. 4). Further, the silver-catalyzed [4 + 3] cycloaddition protocol was applied to the late-stage modification of drug molecules, such as Aspirin, Indomethacin, and Fenbufen bearing a furan motif. All these compounds reacted smoothly with vinyl-N-triftosylhydrazone 3 and gave the corresponding oxa-bridged [3.2.1] bicyclic compounds (76-78) in synthetically useful yield.
Based on related precedents,8,13,30,32 this transformation may proceed via a tandem cyclopropanation / Cope rearrangement sequence. Therefore, our initial mechanistic studies aimed to distinguish a direct [4 + 3] cycloaddition pathway and a tandem [2 + 1] cycloaddition followed by a divinylcyclopropane [3,3]-rearrangement. To address this, we first tried to capture the furanocyclopropane intermediate by shortening the reaction time or performing the reaction at a low temperature (0 °C), but all failed (for details, see Fig. S1). Fortunately, the use of vinyl-N-triftosylhydrazone 79 in the reaction with furan produced a mixture of vinylcyclopropane 80 (which would correspond to the putative intermediates of the stepwise process)13,32 and triene 81, along with a trace amount of 1,4-cycloheptadiene 82. When the isolated compound 80 was heated at 160 °C for 12 hours, only a triene 81 was isolated in 90% yield by the ring-opening of furanocyclopropane intermediate, whereas no rearrangement product 82 was observed (Fig. 5a). This result is different from the Rh-catalyzed [4 + 3] cycloaddition of 1,3-dienes with vinylcarbenoids.13,30,32 Finally, we got a circumstantial evidence from the analysis of regioselectivity in the reaction of 3-phenylfuran with vinyl-N-triftosylhydrazone 3, which afforded a 1:1 mixture of 83 to 83'. However, this is not consistent with the stereospecificity resulting from a tandem cyclopropanation / Cope rearrangement, where the initial cyclopropanation should selectively occur at the double bond with less sterical hindrance.13,30,32 Hence, the involvement of a concerted [4 + 3] cycloaddition is more possible than a step-wise tandem cyclopropanation / Cope rearrangement.
To support our hypothesis concerning the involvement of a concerted [4 + 3] cycloaddition, in which silver vinylcarbenes act as a delocalized 2π system,62 density functional theory (DFT) calculations were carried out for a model reaction, namely the silver-catalyzed reaction of 2-methylfuran with vinyl-N-triftosylhydrazone 3 (for computational details see Fig. S2-S5 in supporting information). First, the diazo compound Int1, generated in situ from the decomposition of 3, coordinates to Tp(CF3)2Ag and then releases a N2 molecule to generate a silver carbene Int2 with an energy barrier of 11.3 kcal/mol (Fig. S3). Subsequently, a concerted [4 + 3] cycloaddition of Int2 with 2-methylfuran occurs affording 1,4-cycloheptadiene products 4, 4', and 4'', respectively, due to the approach of carbene intermediate Int2 to 2-methylfuran from different orientations (Fig. 5b and Fig. S4). The energy barrier for the generation of the desired product 4 from Int3-B1 via TS2-4 is only 1.8 kcal/mol, which is 12.0 and 23.4 kcal/mol lower than those for the formation of regioisomer 4' via TS2-4' from Int3-B2 and stereoisomer 4'' via TS2-4'' from Int3-C. To investigate the origin of the regio- and stereoselectivity, NCI and frontier molecular orbital analyses of transition states were performed (Fig. 5c and Fig. S2).63,64 A secondary orbital interaction exists in the boat form of transition states TS2-4 and TS2-4', and remarkably lowers the energy barriers for the generation of endo products and determines the stereoselectivity of this silver-catalyzed [4 + 3] cycloaddition. Furthermore, due to the C–H···π interactions between methyl and phenyl groups in TS2-4 and the steric exclusion between the methyl group and the bulky Tp(CF3)2 ligand of carbene moiety in TS2-4' (Figure 2c, for details see Figure S2), the secondary orbital interaction in TS2-4' is weaker than that in TS2-4. This result provides a suitable rationale for high regioselectivity observed in the experiment. By comparison, the DFT calculations for the tandem cyclopropanation / Cope rearrangement pathway have been carried out, but this process entails an energy barrier (14.1 kal/mol) that is higher than that for concerted [4 + 3] cycloaddition pathway (1.8 kal/mol) (see Fig. S5). At this moment, we cannot rule out completely that a tandem cyclopropanation / Cope rearrangement pathway is involved, but the experimental results and theoretical calculations indicate that a concerted [4 + 3] cycloaddition pathway is more favorable.