a Molar ratio of substrate to catalyst.b 1H NMR yield with 1,3,5-trimethoxybenzene as an internal standard. c Determined by1H NMR.d HCO2H (4 equiv).e HCO2H (12 equiv).
A number of cis -2-acyl cycloalkane-1-carboxylic acids were synthesized via either AlCl3-catalyzed Friedel-Crafts acylation19 or Ni-catalyzed cross-coupling of anhydrides with organozinc reagents,20 followed by submitting them to the optimal conditions (Table 2). The stereochemistry of products was further demonstrated by the one-dimensional NOE (Nuclear Overhauser Effects) spectra of four randomly selected representative compounds (2c ,2x , 2aa , and 2b ). We focused our substrate scope on 2-aroyl substrates (R = aryl, cis -1 ), to which previously established syn -selective reductive lactonization protocols were not applicable.10b In addition tocis -1a , other 2-aroyl substituted substrates were converted to the desired diastereopure products in 60-99% yields and 98:2 to >99:1 diastereoselectivity, strongly favoring the formation of syn -isomers (2a -2u ). The substituents, for instance, alkyls, phenyl, phenyloxyl, benzenesulfenyl, and halogen atoms, on phenyl rings, whether electron-donating or withdrawing, all uniformly gave ≥ 98:2 dr (2a -2n ), although the yields varied. Substrates bearing more complex arene rings such as 5,6,7,8-tetrahydronaphthalen-2-yl (2o ), 4-methylnaphthalen-2-yl (2p ), naphthalen-2-yl and naphthalen-1-yl (2q ), pyren-4-yl (2r ), 9H -fluoren-3-yl (2s ), and dibenzo[b ,d ]thiophen-2-yl (2u ) also transformed in >99:1 dr. The ketoacid with dibenzo[b ,d ]furan-2-yl (2t ) was reductively lactonized in ≥ 98:2 dr values. Oxidation of the sulfur atom ofsyn -2u did not reduce its diastereopurity.
We next examined the effects of backbone architectures on the stereochemial results. Introduction of an endocyclic disubtituted (2w ) or tetrasubstituted alkene moiety (2x ), being fused with a bridged bicyclic [2.2.1] or [2.2.2] scaffold (2y and 2z ), or contraction of the cyclohexane ring to cyclopentane (2aa ) or cyclobutane (2ab ), did not affect the stereochemical outcomes. All products were produced in >99:1 dr values, and syn -products were formed exclusively.
Substrates with alkanoyls (R = alkyl, cis -1 ) were also amenable to the diastereocontrol. The reactions ofcis -1b and cis -1ac both gave > 99:1 dr values and reasonable yields.
Table 2 Substrate scopea
a cis -2a , 2c2h ,2k2o , 2q, 2w, 2ab were isolated by extraction. b The regioselective ratio (rr) of ketoacid was 94:6 (para :ortho ), and it was inherited to the next step. c The rr of ketoacid and product was 50:50. d 0.1 mmol of ketoacid was used. e HCO2H (16 equiv.),C1 (S/C = 500), H2O/EtOH (1:1), 80oC, 6 h. f The rr of ketoacid and product was 96:4.g HCO2H (12 equiv.), C1(S/C = 500), H2O/EtOH (1:1), 80oC, 6 h.
Mechanistic studies were performed (Scheme 2). The isotope tracing experiments with HCO2D, DCO2D, and D2O (Scheme 2a) demonstrated that (1) the transfered hydride originated from the formyl group of formic acid, and (2) that H–D exchange, both between iridium hydride ([Ir]–H) and D2O and between iridium deteride ([Ir]–D) and H2O, occurred before the hydride or deuteride transfer.12, 15a, 21 The H–D exchange between [Ir]–D and solvent was dramatically suppressed by using DCO2D and D2O-EtOH as the respective deteride source and solvent, and thereby a 95% D-incorporation was obtained. We also measured the kinetic isotope effect (KIE), and akH /kD = 4.8 was observed (Scheme 2b). The primary KIE implied that the rate-determining step should involve a C–H cleavage event.
The key intermediate during the reductive lactonization was observed by1H NMR of the crude reaction mixture (Scheme 2c). It turned out to be an alcohol (3a ), which was generated from the diastereospecific reduction of the ketone moiety. Treatment of this alcohol under acidic conditions (AcOH) yielded lactonesyn -2a with >99:1 syn :antiratio. We also found that the reduction of 4-ketoestercis -1m’ did not occur (Scheme 2d), which indicated the important role of the carboxylic acid group in facilitating the reduction. Similar carboxylic acid effect was also observed by Rovis and coworkers.10b
Scheme 2 Isotope labelling, KIE, and other mechanistic expecriments.
Scheme 3 Proposed mechanism and diastereocontrol models
On the basis of the mechanistic studies, a plausible mechanism was proposed (Scheme 3a). In light of the KIE studies, the generation of iridium hydride B via β-hydride elimination was suggested to be the rate-determining step.22 Subsequent hydride transfer to the carbonyl group, which was activated by the carboxylic group, served as the diastereo-determining step. Considering the experimental results in Scheme 2d, we proposed that the hydrogen bonding between the carboxylic acid and ketone moiety accounted for the activation. In detail, protonation of the C=O bond of carboxylic acid1a increased its acidity, and an intramolecular hydrogen bonding between O–H and carbonyl C=O occurred, as designated by the structure C . Although C could resonate withD , it was suggested as the major contributor for explaining the carboxylic effect. On the other hand, the intramolecular hydrogen bonding, to some extent, fixed the configuration by connecting the ketone and carboxylic acid moieties. The fixed and somewhat rigid configuration of the molecule rendered the hydride transfer more stereoselective. In this regard, it is the intramolecular hydrogen bonding that holds responsibility for the excellent diastereocontrol in the hydride transfer step. Once diastereospecifically formed,3a immediately undergoes intramolecular esterification, following classic acid-catalyzed addition-elimination mechanism, to form lactone syn -2a , without eroding the stereochemistry of the newly generated stereocenter.
To better understand the diastereocontrol of the hydride transfer, we performed the DFT calculations. The calculated fixed configurations and Bürgi-Dunitz angles of attack for hydride delivery to carbonyl were shown in Scheme 3b and 3c.23 In the case that the carboxylic assumes axial position and benzoyl equatorial position (Scheme 3b), the frontside attack will encounter severe steric repulsion against two axial hydrogen atoms and one axial carboxylic group. However, the backside attack faces very little steric retardation. Consequently, the hydride delivery in this direction kinetically preferentially gave syn -2a . Similar stereocontrol is also applicable in the other configuration bearing an equatorial carboxylic and an axial benzoyl (Scheme 3c). In one word, it is the steric repulsion between sterically bulky iridium hydride and the cycloalkyl ring that governs the diastereoselectivity.
To explain the backbone architecture effects in Table 2 and also to predict the stereochemical outcomes of the potential reactions of substrates with similar stereochemistry, we proposed a rule of thumb (Scheme 3d). The hypothesis that the hydrogen bond form an additional ring (ring B) leads to a cis -fused bicyclic system, and thereby the hydride reduction of the “pseudo-endocyclic” carbonyl was endowed with facial selectivity—the hydride can attack from either convex or concave face.24 Owing to the large steric hindrance of the iridium hydride, it would preferentially donate its hydride from the sterically much more accessible convex face, in this way generating a configuration with all the hydrogen atoms on the three tertiary stereocenters residing in the same convex face.
Scheme 4 Gram-scale reactions and synthetic applications
In the two gram-scale reactions, 2a and 2m were both isolated in excellent yields and > 99:1 dr (Scheme 4a). The purification was very convenient. Column chromatography was not required. Extraction with ethyl acetate followed by drying and concentration afforded desired products in good NMR purities (see Supporting Information). The 4-bromophenyl group in 2m acted a handle for further chemical manipulations (Scheme 4b). For examples, the Suzuki coupling of 2m with three aryl boric acids carrying electron-withdrawing or electron-donating substituents delivered more functionalized products 4a -4c in 56-96% yields, and the stereochemistry of the three stereocenters was not affected.
trans -2-Benzoylcyclohexane-1-carboxylic acid (trans -1a ) did not undergo the reductive lactonization (Scheme 4c), highlighting the importance of the configurational effect of the substrate. In other words, our iridium catalysts showed excellent capability in discriminating the cis - andtrans -substrates, and highly selectively catalyzed the reactions of cis -substrates, leaving the trans -ones intact. This selectivity provides an easy procedure to separate the cis - andtrans -2-acylcycloalkane-1-carboxylic acids, which were generated as a diasteremeric mixture in some cases and were difficult to be separated due to their almost identical polarity. As shown in Scheme 4d, subjection of equimolar cis -1a andtrans -1a to standard reductive latonization conditions produced cis -2b in 94% yield andtrans -1a in 92% recovery. Due to the largely different polarities and water solubility of cis -2b andtrans -1a , they were easily separated.
Conclusions
We have developed a highly diastereoselective method for efficient synthesis of diastereopure bi- and polycyclic γ-lactones, usingcis -2,3-fused 4-oxo-butanoic acids as the starting materials. This method features the use of a [Cp*IrIIICl/PyIm]+Cl-catalyst with formic acid as the hydride source. Advantages of this method include excellent diastereoselectivity control, use of water-ethanol as solvent, broad substrate scope, and high catalyst efficiency (S/C up to 5000). The gram-scale reactions take place efficiently in excellent yields and diastereocontrol. Mechanistic studies suggested that the iridium hydride formation be the rate-determining step, and that the hydride transfer step be the diastereo-determining step. The large steric hindrance of the iridium hydride species underlies the success of diastereocontrol. The carboxylic acid group of substrates plays important roles in activating the substrates and in rendering a relatively rigid configuration to highly diastereoselectively receive the hydride. DFT calculations provide detailed insights into the nature of diastereocontrol, from the perspectives of configurational analysis and Duniz angles of attack. An empirical rule based on facial selectivity analysis for explaining and predicting the stereochemistry is also proposed. Our iridium catalysts only work on cis -2,3-fused 4-oxo-butanoic acids, showing excellent level of molecular recognization. This selectivity can be harnessed to separate the diastereomeric mixtures of cis - andtrans -2-acylcycloalkane-1-carboxylic acids. Compared with previous methods, our method shows superiority in terms of substrate scope, degree of diastereocontrol, and sustainability.
Experimental
To a 10-mL reaction tube was sequentially added cis -2-acyl cycloalkane-1-carboxylic acids1 (0.2 mmol), ethanol (0.5 mL), 1 mL of C1 solution in deionized water (0.0001 mol/L, S/C = 2000). The tube was then sealed with a rubber cap. A syringe needle was inserted into the cap to connect the inner to outer atmosphere. The mixture was stirred for 3 minutes in an 80 °C heating block, followed by addition of formic acid (60 μL, 1.6 mmol, 8 equiv) in one portion via a microsyringe. After stirring for 2 h, the reaction mixture was cooled to room temperature, diluted with saturated brine (2 mL), and extracted with ethyl acetate (2 mL × 3). The organic phase was dried with anhydrous Na2SO4 and then removed under reduced pressure. 1H NMR pure syn -2a ,2c2h , 2k2o , 2q ,2w , 2ab were obtained without further purification. Other products were purified by silica gel column chromatography.
Supporting Information
The supporting information for this article is available on the WWW under https://doi.org/10.1002/cjoc.2023xxxxx.
Acknowledgement (optional)
This research was supported by the National Key Research and Development Program of China (No.2022YFF0709803), Natural Science Foundation of Beijing Municipality (no. 2202041), and the High Performance Computing Platform of Beijing University of Chemical Technology (BUCT).
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