To identify conditions that would deliver homoallylic alcohol 3 in favor of 1,2-disubstiuted alkenes (VII, VIII, Figure 1c) or alcohols (IX, X, Figure 1c), we chose the reaction involving allylbenzene 1a and benzaldehyde 2a (Figure 2). We soon found that unlike reactions with CO2 and ketones,47–49 branched homoallylic alcohol 3a was furnished exclusively, indicating that h1-Co(I)–allyl complex V reacts preferentially and the rate of isomerization of IV to V is more rapid than that of aldehyde addition of h1-Co(I)–allyl complex IV to afford 4a (Figure 1c). Further investigation of a variety of chiral phosphine ligands led us to find that few chiral Co complexes could promote the reaction. Reaction of allylbenzene 1a with benzaldehyde 2a in the presence of Co complex derived from 6b furnished homoallylic alcohol 3a in trace amount (16% yield). Co complex formed from 6f provided significant higher efficiency (52% yield) albeit low enantioselectivity (56:44 er). Follow-up studies revealed that reaction involving phosphine 6j produced homoallylic alcohol 3a in 69% yield, 90:10 dr and 93.5:6.5 er. Control experiment showed that in the absence of the allylbenzene 1a, the Me-addition product 5a was generated in 21% yield, whereas 5a was not detected in the reaction of allylbenzene 1a and benzaldehyde 2a. It is worth mentioning that although most ligands did not deliver the homoallylic alcohol 3a, b-methyl-styrene and b-ethyl-styrene (trace amount) formed from reductive elimination of the allyl and Me/H (complex III, Figure 1c) was observed in most cases, illustrating that most ligands promoted the C–H bond cleavage but not the aldehyde addition.
Further optimization of reaction temperature indicated that transformation performed at 65 oC furnished the desired product 3a with improved diastereo- and enantioselectivity (92:8 dr and 95:5 er) without significant erosion of yield (68%). The approach can be utilized to prepare a wide range of enantioenriched homoallylic alcohols (Figure 3). The requisite Co complex is derived from commercially available Co salt and chiral phosphine ligand 4j. Aldehydes bearing halogens are tolerant in the reaction conditions (3b–d, 3h–j), although it is known that phosphine–Co(I) complex can undergo oxidative addition to carbon–halogen bond.51 Aldehydes that contain electron-donating (3e–g), electron-withdrawing (3k–m) and sterically demanding (3n–o) aryl groups are suitable substrates. Reactions of heteroaryl aldehydes afforded the homoallylic alcohols (3p–q, 3s–v) in 51–67% yield, 83:17–>95:5 dr and 95:5–98:2 er. Furyl, thienyl and 3-indoyl are not tolerate in the elevated temperature, but the reactions proceeded smoothly at room temperature (3s–v). Aliphatic aldehydes were transformed to the desired products as a single diastereomer albeit with diminished enantioselectivity (3r, 3w). Allylbenzenes substituted with electron-donating groups were transformed to the homoallylic alcohols (7a–c, 7m–n) in 49–67% yield, 88:12–91:9 dr and 90:10–95:5 er at 65 oC. We found that reactions with allylbenzenes bearing electron-withdrawing groups can proceed in high efficiency even at room temperature with high diastereo- and enantioselectivity (7d–l, 7o–r). Particularly, the aldehyde moiety in the substrates that contain ketone (7i–j, 7q) or cyano groups (7g, 7p) reacted chemoselectively. Allylbenzenes that contain sterically congested aryl (7s–v) and various heteroaryl groups (7w–aa) are suitable substrates.
1,4-Dienes, upon undergoing the enantioselective allylic C–H functionalization, will produce homoallylic alcohols bearing a 1,4-diene unit, which widely exist in natural products52 and are intermediates that are commonly used in the synthesis of biological active molecule53 and small molecule probe54 (see Supplementary Information for more references). It is more challenging to control the chemo- and site selectivity compared with allybenzenes, as two different olefins are present in the substrate. However, very few enantioselective methods have been developed for addition of a 1,4-diene unit to carbonyls and the diversity of 1,4-diene groups that can be introduced is very limited.55,56 We next applied the reaction conditions to the transformations of 1,4-dienes (Figure 4). Unexpectedly, we found that a trisubstituted alkene moiety is required for high efficiency and stereoselectivity. 1,4-Dienes containing a (E)- or (Z)-trisubstituted alkene moiety can be transformed to the desired homoallylic alcohols in 41–67% yield, 91:9 dr and 94:6–95:5 er (8a–b) at room temperature. Further studies suggested that 1,4-dienes bearing a range of aryl (8c–m) and heteroaryl (8n–q) alkenes are suitable substrates. The reactions proceeded in high efficiency and stereoselectivity without the need of elevated temperature, whereas transformations of 1,4-dienes that contain trisubstituted alkenes of other substitution patterns (8s–t) or without an aryl group (8r, 8t) furnished the homoallylic alcohols in 38–42% yield, 87:13–>95:5 dr and 90:10–91:9 er. The limitation of this method for the allyl precursors is that 1,4-enynes and simple alkyl-substituted alkenes are not reactive.
Enantioenriched a-hydroxyl acids and their derivatives are important motifs in biologically active molecules. However, only two examples of enantioselective allyl addition to a-ketoesters have been disclosed so far and only simple allyl group can be introduced,57,58 as it is more difficult for the catalyst to provide good efficiency and differentiate the two substituents on the carbonyl effectively. We further expanded the scope of electrophile to a-ketoesters (Figure 5). Compared with aldehydes, a-ketoesters are less reactive and require higher reaction temperatures. Both allylbenzene 1a and 1,4-diene 11a can react with a variety of a-ketoesters (10a–h, 12a–h) to afford tertiary homoallylic alcohols in 38–60% yield, 75:25–>95:5 dr and 90:10–98:2 er. a-Ketoesters derived from various alcohols are suitable substrates (10f–h, 12f–h). Reactions of the a-ketoester bearing an ester group delivered g-lactones (10e, 12e), which are key intermediates to access spirocycles that are common structures in biologically active molecules.59 Surprisingly, aryl-substituted a-ketoester provided low diastereo- and enantioselectivity. It might be because that the ester group serves as a larger substituent in the six-membered transition state of the carbonyl addition.
As shown above, a wide range of enantioenriched homoallylic alcohols bearing aryl or alkenyl groups at b-position can be prepared through this approach. Such building blocks are still difficult to access in high stereoselectivity with broad scope. We further demonstrated the utility of this method through application to synthesis of biologically active molecules (Figure 6). 2,3-Dihydrobenzofuran moiety is a ubiquitous structural motif in a vast number of natural products and synthetic compounds that display a wide range of biological activity as shown in Figure 6a.60 We envisioned that enantioselective allylic C–H functionalization involving allylbenzenes bearing an ortho-phenol substituent followed by Mitsunobu cyclization would furnish the 2,3-dihydrobenzofuran core of such class of molecules.61
Lithospermic acid has been recognized as an active component in Danshen, one of the most popular traditional herbs used in the treatment of cardiovascular disorders, cerebrovascular diseases, various types of hepatitis, chronic renal failure, and dysmenorrhea (see Supplementary Information for a complete bibliography).62 Recent studies revealed that lithospermic acid has potent and non-toxic anti-HIV activity.63,64 As indicated in Figure 6b, the synthetic route commenced with the enantioselective allylic C–H functionalization process on gram scale.65 Treatment of the allylbenzene 1b (2.03 g) prepared in one step and 93% yield from commercially available o-eugenol with aldehyde 2b (1.00 g) derived from vanillin in quantitive yield in the presence of Co complex generated from 6j at 55 oC afforded homoallylic alcohol 13 (1.04 g) in 62% yield, >95:5 dr and 93:7 er. The TMS group was removed simultaneously during the work-up. Subsequent Mitsunobu cyclization followed by switching the protecting group delivered 2,3-disubstituted benzofuran moiety in 80% overall yield. Oxidative cleavage of the alkene and Pinnick oxidation of the resulting aldehyde furnished a known fragment that can be converted to lithospermic acid in 71% overall yield. This catalytic enantioselective route is two-step shorter than previously reported with similar efficiency (6 steps vs. 8 steps).66
Dihydrodehydrodiconiferylalcohol (also named 3’,4-di-O-methylcedrusin) was isolated in low yield from the red latex produced by various South American Croton species, which has the potential interest as an inhibitor of cell proliferation.67 Further studies based on this compound indicated that some derivatives inhibit the growth of a variety of cancer cells by interaction with tubulin.68 The synthesis began with transformation of allylbenzene 1c (3.19 g) prepared from eugenol in five steps and 78% overall yield with aldehyde 2b (1.00 g) promoted by Co complex derived from 6j afforded the homoallylic alcohol 16 (1.11 g) in 49% yield, >95:5 dr and 93:7 er. Mitsunobu cyclization followed by oxidative cleavage of the alkene and subsequent reduction with simultaneous global deprotection furnished dihydrodehydrodiconiferyalcohol in 60% overall yield, >95:5 dr and 93:7 er, accomplishing a much more efficient and stereoselective synthesis than previously reported (9 steps, 23% overall yield, >95:5 dr, 93:7 er vs. 12 steps, 1.2% overall yield, >95:5 dr, 62:38 er).69
To gain some preliminary insight into the reaction mechanism, a series of experiments were conducted (Figure 7). Kinetic experiments revealed that in the transformations of allylbenzenes and 1,4-dienes, allylic C–H bond cleavage might not be the rate-determining step. Allylbenzene 1d bearing a b-methyl-styrene moiety was transformed chemoselectively to afford 7ab in 70% yield, 93:7 dr and 93:7 er. To investigate whether the cleavage of the C–H bond is enantioselective, we performed the transformation with allylbenzene 1e. A roughly 1:1 mixture of 3x and 3y associated with trace amount of 3a was generated, indicating that either the C–H bond cleavage might not be enantioselective, or isomerization of two h1-allyl–Co complexes might not be stereochemically retaining.
In conclusion, we have developed an unprecedented protocol for catalytic generation of nucleophilic allyl–Co complexes through allylic C–H bond activation followed by site-, diastereo- and enantioselective addition of carbonyls to furnish a wide range of homoallylic alcohols that are otherwise difficult to access. This approach is further applied to enantioselective formal synthesis of lithospermic acid and total synthesis of dihydrodehydrodiconiferylalcohol. The advances outlined here demonstrate that simple unsaturated molecules can be directly converted to functionalized allyl nucleophiles without the need for succumbing to one-at-a-time installation of each functional group, resulting in pathways that are unnecessarily time consuming, costly and waste-generating. A single multifunctional Co-based catalyst assembled from an inexpensive Co salt and commercially available phosphine can accurately control the chemo-, site- and stereoselectivity. The possibility of using other easily available unsaturated hydrocarbons for efficient and stereoselective preparation of high value enantioenriched building blocks through C–H functionalization strategy is under investigation.