Three-component chemo-selective oxy-allylation of α-diazo carbonyl compounds: Access to α-ternary carboxylic esters
Graphical abstract
Introduction
Transition-metal-catalyzed allylic alkylations of α-diazo carbonyl compounds have been established as indispensable tools for accessing valuable molecules [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. A commonly exploited pathway to generate metal-carbenes A from α-diazo esters with allylic reagents, followed by allyl migratory insertion, gives rise to the alkyl-metal intermediate B [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. In most cases, substituted dienes can be easily obtained by β-hydride elimination of B (Scheme 1a, path I) [2], [3], [4], [5], [6], [7], [8], [9]. For example, Wang and co-workers pioneered the Pd(0)-catalyzed olefination of α-diazo carbonyl compounds with allylic halides, which afforded the poly-substituted 1,3-dienes [2]. However, the intermediate B has been found reluctant to react with external nucleophiles via substitution to deliver α-ternary allylated esters (Scheme 1a, path II) [10], [11], [12]. Therefore, the development of novel and efficient catalytic strategies is of great significance and in high demand.
Multicomponent reactions, involving electrophilic trapping of onium ylides, can offer substantial advantages over traditional approaches to rapidly assemble complex molecules in an atom-economical and convergent manner under mild conditions [24], [25], [26], [27], [28], [29], [30], [31], [32], [33]. Remarkably, Hu [25], [26], [28], [29], [30], [31] and other research groups [32], [33] demonstrated a series of elegant studies about cooperative Rh(II)/Lewis acid (or phosphoric acid) dual-catalyzed nucleophilic additions of protic oxonium ylides C, generated in situ from Rh(II)-catalyzed reactions of α-diazo esters and alcohols, with electron-deficient double bonds (e.g., CO, CN, and CC), instead of [1], [2]-proton transfer [34]. In contrast, the Pd(0)-catalyzed allylic alkylation of protic oxonium ylides C is less explored, probably due to the redox non-compatibility of Rh(II) and Pd(0) catalysts and catalytic deactivations.
Dual transition-metal catalysis has emerged as a powerful strategy for the development of reactivity and selectivity [35], [36], [37], [38], [39]. As a continuation of our efforts in Rh(II)/Pd(0) dual catalysis [13], [14], [15], [16] with α-diazo carbonyl compounds as substrates [11], [12], [13], [15], [16], we envisioned that electrophilic π-allyl Pd(II) intermediates [40] could be appropriate interceptive-coupling partners of protic oxonium ylides C. Herein, we disclose a three-component Rh(II)/Pd(0) dual-catalyzed oxy-allylations of alcohols, α-diazo esters, and allylic benzoates to produce α-ternary allylated carboxylic esters under redox-neutral conditions, involving the simultaneous formation of two new bonds (CO and CC) at the same carbon atom (Scheme 1b). Although possible in principle, some competitive reactions such as OH insertion [34], CO insertion [11], and cyclopropanation [41] offer formidable challenges in chemo-selectivity control.
Section snippets
Results and discussions
To validate the feasibility of the above hypothesis, the redox-compatible Rh(II)/Pd(0) dual catalytic system [13], [14], [15], [16] established by our group was employed to investigate the three-component allylic alkylations of 4-methoxybenzyl alcohol (1a), methyl α-phenyldiazoacetate (2a), and allyl benzoate (3a) as model substrates. After numerous trials (Table 1), we determined that the combination of Rh2(OAc)4, [PdCl(allyl)]2, and Xantphos gave the best results, providing the desired
Application and mechanism
The practical applicability of this approach was demonstrated in Scheme 4. Firstly, we applied the Rh(II)/Pd(0) dual-catalyzed three-component reaction of 1a, 2a, and 3a to the gram scale under the standard conditions. The acyclic α-ternary allylated carboxylic esters 4a could be isolated in a slightly lower yield (1.76 g, 87 % yield). Furthermore, the Wacker oxidation of 4a by using the Grubbs-Stoltz protocol afforded the aldehyde-selective product 9 in high yield [44]. Additionally, in the
Conclusion
In summary, we have realized a highly chemo-selective three-component oxy-allylation of α-diazo esters, alcohols, and allylic benzoates by employing Rh(II)/Pd(0) dual catalysis. The reaction occured by trapping active protic oxonium ylides with catalytic π-allyl Pd(II) intermediates. More remarkably, when alcohols were replaced with inexpensive and environmentally benign water. this transformation also proceeded well involving a different allylic migratory insertion of palladium-carbenes.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (No. 22171228) and the Youth Training Project of Northwest A&F University (No. 2452017034). We sincerely thank Dr. Shaukat Ali (Institute of Chemical Sciences, University of Peshawar) for polishing English.
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