Principles and Applications of Enantioselective Hydroformylation of

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Principles and Applications of Enantioselective Hydroformylation of Terminal Disubstituted Alkenes Yuchao Deng,†,‡ Hui Wang,† Yuhan Sun,† and Xiao Wang*,†,§ †

CAS Key Lab of Low-Carbon Conversion Science and Engineering, Shanghai Advanced Research Institute, Chinese Academy of Sciences, 100 Haike Road, Pudong, Shanghai 201210, People’s Republic of China ‡ School of Physical Science and Technology, Shanghai Tech University, 100 Haike Road, Pudong, Shanghai, 201210, People’s Republic of China § Harvard NeuroDiscovery Center, Harvard Medical School and Brigham & Women’s Hospital, 65 Landsdowne Street, Cambridge, Massachusetts 02139, United States ABSTRACT: Asymmetric hydroformylation has been primarily focused on monosubstituted and 1,2-disubstituted alkenes. In this Perspective, progress in enantioselective hydroformylation of terminal disubstituted alkenes is outlined on the basis of different categories of substrates. The origins of the commonly observed low degree of enantioselectivity are discussed. Examples with unconventional regioselectivity are also rationalized.

KEYWORDS: hydroformylation, homogeneous catalysis, oxo products, terminal disubstituted alkene, enantioselectivity, chiral pocket, linear aldehyde, regioselectivity

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INTRODUCTION Hydroformylation, also known as the oxo process, first developed by Otto Roelen in the 1930s,1 is one of the most important homogeneously catalyzed industrial processes. Catalyzed by a transition metal, most often cobalt and rhodium, alkenes react with carbon monoxide and hydrogen gas to produce aldehydes.2 Since the early 2010s, there has been an annual production of more than 12 million tons of oxo chemicals.3,4 Unlike alkene hydrogenation, which removes a functional group (e.g., CC), hydroformylation creates a new one (e.g., aldehyde) that serves as an extremely versatile intermediate for manufacturing a diverse range of fine chemicals. In the asymmetric version of hydroformylation (AHF), chiral aldehydes are produced.4−8 Publication data reveal that AHF is still an underdeveloped field as compared with other asymmetric transformations of alkenes, such as hydrogenation,9 hydroboration,10 epoxidation,11 and dihydroxylation.12 There have been only 10 or less publications annually in the area of AHF.4 The number has remained largely unchanged during the past 20 years, with exceptions in the mid-1990s and mid-2000s owing to the discovery of the milestone chiral ligands Binaphos and Bis-diazophos developed by Nozaki13 and Landis,14 whose extraordinary contributions have revived and pushed forward this research topic.15 However, limited substrate scope is still the major bottleneck for the AHF research. Ligand design holds the key to success in AHF. However, it has been extremely difficult to find a ligand that can cover different substrates. For each kind of olefin, a specific ligand © XXXX American Chemical Society

must be developed. For instance, Binaphos works best for styrene,13 Ph-BPE is optimal for allyl cyanide,16a,b and Bobphos is the ligand of choice for 1-alkenes.16c The versatile Bisdiazaphos displays a broader utility for styrene, vinyl acetate,14a,b and 1,2-substituted olefins14c (Scheme 1). There is no direct structural similarity between these optimal ligands, making the ligand design of AHF less unified. So far, the study of AHF has mainly focused on monosubstituted and 1,2-disubstituted olefins, in which ialdehydes (aka branched aldehydes) were produced exclusively. In these cases, more than 90% ee could be obtained. The most intensively investigated substrates include styrene,13 vinyl acetate,14a,b diphenylethylene,14c allyl cyanide,16a,b 1-hexene,16c and dihydrofurans.17a−c Excellent in-depth reviews published previously have well covered these aspects.4−8 Regioselectivity is a critical problem in the field of AHF that asymmetric hydrogenation does not face; however, novel approaches such as a supramolecular ligand17d and catalytic directing group17e,f have made it more controllable. In contrast, AHF of 1,1-disubstituted, trisubstituted, and tetrasubstituted alkenes has been much less intensively studied. The hydroformylation of the latter two are extremely difficult because of their inherent lack of reactivity. Hydroformylation of terminal disubstituted (1,1-disubstituted) olefins is a unique reaction that normally produces n-aldehydes (aka linear Received: June 21, 2015 Revised: August 12, 2015

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DOI: 10.1021/acscatal.5b01300 ACS Catal. 2015, 5, 6828−6837

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has different regioselectivities for different substrates (Scheme 3).

Scheme 1. Summary of AHF of Representative Monosubstituted and 1,2-Disubstituted Alkenes

Scheme 3. Orientation of the Substituents in the AHF of Different Alkenes

Following the quadrant rule,19 steric repulsion is minimized when the olefin approaches the catalyst where the larger substituent does not fill in the blocked quadrant. This is true for monosubstituted and 1,2-disubstituted olefins, but for 1,1disubstituted substrates, it becomes problematic. Regardless of the coordination pattern (equatorial−equatorial for, e.g., BPE or Duphos; or equatorial−axial for, e.g., Binaphos),13b the two groups at C1 must orient away from the center of the quadrants because the hydride adds to the more substituted carbon (Scheme 4). As a consequence, it is very likely that the two C1 groups are at least partially out of the chiral pocket, thus encumbering the differentiation of the two prochiral faces.

aldehydes), according to Keulemans’ empirical rule that the hydride is preferentially added to the disubstituted C1.18 The AHF of this category of olefins is a highly efficient protocol to synthesize carbonyl compounds with β-chirality, which are valuable chiral precursors for various bioactive and pharmaceutical molecules (Scheme 2). Scheme 2. AHF of Terminal Disubstituted Alkenes

Scheme 4. Quadrant Diagrams of the Rh AHF Catalyst Modified with a C2-Symmetric Ligand

Among all the challenges in asymmetric hydroformylation, the reaction of terminal disubstituted alkenes remains the most difficult in terms of enantioselectivity. 4,8 The unique regioselectivity of this AHF makes it a desirable tool for stereoselective synthesis; therefore, it deserves to be discussed separately and intensively. This perspective will summarize this particular AHF in detail by different categories of alkenes. Rationalization of Low Enantioselectivity. The origin of the insufficient stereocontrol in the AHF of terminal disubstituted alkene can be qualitatively explained. The foremost reason is that in such substrates, the size difference between the two C1 substituents is far smaller compared with that of a monosubstituted alkene. This intrinsic obstacle for stereoselectivity has also been notoriously challenging in other asymmetric transformations of terminal disubstituted alkenes, including hydrogenation,9 epoxidation,11 and dihydroxylation.12 However, compared with these analogous reactions, a more subtle factor may also complicate the case of AHF: that AHF

Advances in the AHF of Terminal Disubstituted Alkenes. To demonstrate the evolution of the AHF of 1,1disubstituted olefins, recent efforts as well as early development will be outlined. For clarity reasons, substrate-controlled diastereoselective hydroformylation20 will not be discussed in this perspective. 1. 2-Substituted Acrylates. 2-Substituted acrylates are typical substrates to examine the effectiveness of an AHF catalyst. The sp2-hybridized carbonyl oxygen is capable of binding to the metal center, which potentially renders better chiral induction. In the early studies in the 1980s, examples were reported mainly by Stille21 and Kollár.22 They worked on several specific substrates that displayed moderately good enantioselectivity. Stille utilized ((−)-BPPM)PtCl2 as the chiral catalyst in combination with SnCl2 as the cocatalyst to 6829


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Scheme 7. Alper’s i-Selective AHF of α-Methylene-γbutyrolactone

hydroformylate methyl methacrylate into the n-aldehyde with 60% ee (Scheme 5). In the meantime, Kollár published a series Scheme 5. PtCl2−SnCl2-Catalyzed AHF of 2-Substituted Acrylates

Scheme 8. Börner’s i-Selective AHF

with Rh forms the thermodynamically more favored 6membered ring, which blocks H2 cis-oxidative addition more effectively than the 5-membered ring formed by the i-acyl complex (Figure 1).24 However, we argue that the bias caused

of studies using PtCl(SnCl3)[(R,R)-DIOP], with the focus on the reactions of dimethyl itaconate. The ee under optimized conditions could reach as high as 82%, which might arise from the additional metal-binding by the β-directing group (e.g., −COOMe). Kollár also investigated the AHF of methyl 2phenyl acrylate, but the ee of the n-aldehyde product dropped sharply to 16%.22 Unfortunately, all of these Pt-catalyzed reactions suffered a large extent of hydrogenation as the major side reaction and displayed a fairly narrow substrate scope. Moreover, as a major drawback of the PtCl2−SnCl2 system, these AHF reactions require high pressure (around 200 bar) and a very long reaction time (up to 50 h). Kollár and Pino also investigated the itaconate-type substrates with [Rh(CO)2Cl]2 as precatalyst.23 Using 1:1 CO/H2, the reaction could be carried out under milder conditions (80 bar) than the PtCl2−SnCl2 system (Scheme 6). Scheme 6. Kollár and Pino’s Rh-Catalyzed AHF of Dimethyl Itaconate

Figure 1. Blocking effect of the intramolecular 6-membered ring structure.

by this effect must be amplified with a lactone structure because the regiochemical control is much less absolute in the AHF of open-chain α,β-unsaturated esters with a noncoordinating βsubstituent. We believe the coordination to Rh by the carbonyl group of a lactone is more stable than by an open-chain ester because the more rigid lactone structure suffers less entropy loss during the process of coordination. In Börner’s case, the acetamide nitrogen replaces the ester carbonyl oxygen to occupy the equatorial position because of its stronger metalbinding ability. Similarly, the less stable i-acyl complex with 5membered ring leads to the predominant branched product. A general method for Rh-catalyzed AHF of this substrate class with high ee had not been available, until Buchwald and Wang discovered that using P-chirogenic ligands BenzP* and QuinoxP* significantly improves the stereoinduction.26 In their approach, 2-alkylacylates react with CO/H2 (1:5, 10 bar)27 in dodecane at 100 °C for 4−8 h, and chiral n-aldehydes could be obtained in 81−94% ee (Scheme 9). Side products such as ialdehyde, isomerized olefin and hydrogenated product were also observed, but could be minimized by fine-tuning the reaction conditions. According to the authors, other prominent ligands for the AHF of monosubstituted olefins gave only negligible ee. The much enhanced stereoinduction by the structurally rigid P-chirogenic ligands may result from their

Large amounts of side products, mostly i-aldehyde and hydrogenated products, were inevitably formed. It is worth noting that although the same chiral ligand (R,R)-DIOP was employed as in the Pt-catalyzed reaction, nearly racemic product was obtained with the Rh catalyst. An unconventional AHF of α-methylene-γ-butyrolactone was demonstrated by the Alper group in 1995, in which the ialdehyde was formed as the only product, especially when the BINAP/Rh ratio was >6 (Scheme 7).24 No olefin isomer, hydrogenation product, or n-aldehyde was observed. The ee of the i-aldehyde was 36% and seemed inert to temperature change. The strong preference for i-aldehyde was also observed by the Börner group in the AHF of methyl 2-(acetamidomethyl)acrylate.25 The branched aldehyde was obtained as the major product with an ee of 33% using the optimal ligand (S,S)-DIOP (Scheme 8). The reaction was carried out slightly above ambient temperature (30 °C). Alper attributed the exclusive i-selectivity to the H2-blocking effect by the lactone carbonyl group in the acyl-Rh complex. In the n-acyl complex, coordination of the lactone carbonyl group 6830


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explored.22 The yield was satisfactory (85%); however, the ee was rather poor (10%) (Scheme 11).

Scheme 9. Buchwald’s AHF of 2-Alkylacylates

Scheme 11. Kollár’s AHF of Methyl 3-methylbut-3-enoate

Allylic ethers and alcohols are more common substrates to be investigated. Nozaki’s AHF of isobutyl alcohol using (R,S)Binaphos gave a set of diastereomers of lactols derived from the n-aldehyde, with 54% conversion of isobutyl alcohol (Scheme 12).29 No side products, such as i-aldehyde and the Scheme 12. Nozaki’s AHF of Isobutyl Alcohol ability to bring chiral information closer to the reaction site. In addition, the low molecular weight of BenzP* makes it soluble in most organic solvents. The enantioselectivity of the n-aldehyde seems to be dependent on the size of the 2-substituent. The best results were obtained when the substituent was a secondary alkyl group, such as isopropyl and cyclohexyl. The corresponding naldehyde products were obtained in good yields with more than 90% ee, which can serve as building blocks for many important pharmaceutical compounds in different therapeutic areas (Scheme 10).28a,b Traditionally, this structure has been

corresponding lactols, were observed. It is a bit surprising to us that no hydrogenolysis of the allylic alcohol took place. The lactols were subsequently oxidized using Ag2CO3 on Celite to afford the corresponding lactone in 37% isolated yield with only 12% ee. It was expected that the hydroxyl group could intramolecularly coordinate to Rh, thus promoting the stereoinduction, although this was not confirmed. In fact, by comparing the results with 1,2-disubstituted allylic alcohols and the structurally similar internal aliphatic olefins under the same AHF conditions using the Rh-Binaphos catalyst,30 we may consider that the presence of the hydroxyl group actually has a negative effect on stereochemical control (Scheme 13).

Scheme 10. Pharmaceutical Compounds with 2-Alkyl-1,4dicarbonyl Moieties and the Traditional Synthesis via Asymmetric Hydrogenation

Scheme 13. Comparing the Enantiomeric Excess in the AHF of Allylic Alcohols and Internal Aliphatic Alkenes

prepared by an enantioselective hydrogenation with a multistep sequence from the 3-isopropylenylsuccinic acid monoester.28c,d The asymmetric hydrogenation, however, works only with the corresponding ammonium salt. The time-consuming salt formation step and an extra step of salt-break further lower the overall efficiency. In contrast, AHF requires no additional carboxylate as the directing group and delivers the aldehyde as a new functional group in a more direct manner. 2. Substituted Allylic Compounds. Substituted allylic alcohols, ethers, and amines, etc., are another substrate class of interest in AHF. When Kollár discovered PtCl(SnCl3)[(R,R)-DIOP] as an efficacious catalyst for itaconate hydroformylation, the AHF of methyl 3-methylbut-3-enoate was also

In 2009, Müller developed a tandem reaction to prepare hydroxy-functionalized bicyclic imidazoles from N-(βmethallyl)imidazole via hydroformylation.31 At 120 °C, by using the strong σ-donating phosphabarrelene ligand, naldehyde was synthesized in a short reaction time of 2 h, which spontaneously cyclized to give the bicyclic product in 80% yield (Scheme 14). Weakly σ-donating, strong π-acceptor ligands, such as P(OPh-o-tBu)3, could hardly convert any starting olefin, possibly because they failed to bind with rhodium in the presence of the stoichiometric donor-function6831


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ACS Catalysis Scheme 14. Müller’s Synthesis of Hydroxy-Functionalized Bicyclic Imidazoles

Figure 2. γ3-Amino acids in bioactive compounds.

3. 1-Substituted Vinyl Esters, Amides and Imides. AHF of 1-substituted vinyl compounds was much less studied than that of vinyl acetate. For 1-substituted vinyl esters and amides, the regioselectivity strongly depends on the electronic nature of the substituent (Scheme 16). In the case of an electron-withalized substrate. Chiral HPLC analysis showed four stereoisomers, with a syn/anti ratio of 2:1. Unfortunately, the reaction was barely enantioselective, which might be due to the fast rotation of the ligand Cα−Cβ bond at high temperature, eroding the axial chirality.32 Systematic study of AHF of allylic compounds with a wide substrate scope was first reported by the Zhang group.33 Rhcatalyzed enantioselective hydroformylation of 1,1-disubstituted allylphthalimides with good ee was achieved using the Ph-BPE ligand (Scheme 15). Similar to Buchwald’s study,26 the authors

Scheme 16. Rules of Regioselectivity for the AHF of 1Substituted Vinyl Esters/Amides

drawing substituent, the Keulemans’ rule18 may be overridden, and regioselectivity would be reversed to give predominantly ialdehydes with a chiral quaternary carbon center. Naturally, in the AHF of for example, vinyl acetate, the i-alkyl-Rh complex is more favored in the hydride addition step because the ester oxygen helps delocalize the negative charge.14a Therefore, when the 1-R group is also electron-withdrawing and not too bulky, the i-selectivity remains predominant. One of the early examples was Gladiali’s work on AHF of dehydroamino acid derivatives in the 1990s.35 With the (R,R)DIOP-modified HRh(CO)(Ph3P)3 catalyst and high pressure syngas (100 bar) enriched with H2, methyl N-acetamidoacrylate was transformed exclusively to the i-aldehyde in high yield (Scheme 17). The authors observed an obvious negative correlation between reaction temperature and stereoselectivity. At 30 °C, the optimal ee of 59% was achieved.

Scheme 15. Zhang’s AHF of Allylphthalimides

Scheme 17. Gladiali’s AHF of Dehydroamino Acid Derivatives

screened different solvents and found the nonpolar heptane to be optimal. By varying syngas composition and the L/Rh ratio, the authors identified that using syngas composed of 1:5 CO/ H2 (20 bar) and an L/Rh ratio of 4:1 assures the best balance of conversion, chemoselectivity, and enantioselectivity. When the substituent is an ethyl, isopropyl, or cyclohexyl group, >90% ee could be obtained; however, the isobutyl, benzyl, and cyclopentyl analogs unexpectedly resulted in products with ee’s of only 50−60%. These products can be derivatized into chiral γ3-amino acids by oxidation of the aldehyde and hydrazinolysis of the imide. In particular, the isopropyl analog can serve as the building block of important bioactive compounds, such as the agonists of the thrombopoietin (TPO) receptor (Figure 2).34

Recently, Buchwald and Wang reported an i-selective AHF of 1-(trifluoromethyl)ethenyl acetate (Scheme 18).36 The resulting aldehyde can serve as the precursor of 2-trifluoromethyllactic acid (TFMLA), an essential derivatizing agent in medicinal chemistry,37 and a key model compound to study the different 6832


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ACS Catalysis Scheme 18. Pharmaceutical compounds bearing the 2trifluoromethyllactic amide structure

Scheme 20. Landis’ Synthesis of β3-Aminoaldehyde via Hydroformylation

sublimation behavior between a pure enantiomer and its racemic counterpart.38 To synthesize optically pure TFMLA, 1-(trifluoromethyl)ethenyl acetate was hydroformylated under mild conditions (10 bar of 1:1 syngas) to give the corresponding i-aldehyde. The exclusive i-selectivity could be well attributed to the strong electron-withdrawing ability of the 2-CF3 group. Ligand screening revealed that P-chirogenic bidentate ligands again outperformed conventional chiral ligands. QuinoxP* and Duanphos are identified as optimal ligands, affording the ialdehyde in 91% and 92% ee, respectively (Scheme 19).

Börner’s synthesis of enantiopure 3-aryl-3-phosphorylated propanals is the most recent example with n-selectivity in this substrate class.25 A xylose-based phosphite-phosphoramidite ligand was shown to be optimal (Scheme 21). For the AHF of Scheme 21. Börner’s Enantioselective Synthesis of 3-Aryl-3Phosphorylated Propanals

Scheme 19. Buchwald’s AHF of 1-CF3-Vinyl Acetate and the Enantioselective Synthesis of TFMLA

dimethyl 1-phenylvinylphosphonate, an ee of 62% of naldehyde could be achieved at 50 °C. Elevated temperature (80 °C) results in a decreased ee of 53%. Reactions of analogs with a p-halide proved to be less enantioselective. Analogs with p-tolyl, p-methoxyphenyl, and 2-naphthyl were tested, as well; however, no chiral analytical method could separate the enantiomeric products. Interestingly, the enantioselectivity of the AHF of 1substituted vinyl compounds is usually sensitive to temperature change. This effect is not so remarkable in the AHF of vinyl acetate.14a 4. 1,1-Diaryl, 1,1-Dialkyl, and 1-Aryl-1-alkyl Olefins. Achieving a highly enantioselective AHF becomes the most challenging when the two C1 substituents are unfunctionalized (or only with remote functionalities) alkyl or aryl groups. It is extremely difficult for the catalyst to differentiate the two prochiral faces because there is a lack of functional groups that can provide binding or affinity interaction with the catalyst. So far, there has not been an effective catalytic system that can provide >50% ee of the n-aldehyde in these cases.

BenzP*, the feature ligand reported by the same authors for the AHF of 2-alkyl acrylates, gave a lower ee of 88%. An oxidation− saponification−crystallization sequence afforded (R)-TFMLA as a single enantiomer in 46% overall yield. For the n-selective reactions, Landis extended the scope of the Bis-diazaphos to the AHF of some 1,1-disubstituted ene phthalimides (Scheme 20).39 The ee of the resulting β3aminoaldehyde varies with substrate. The best ee (74%) was obtained when the 2-subtitutent was the benzyloxymethyl group. The olefin isomer was identified as the major side product, but the amount was relatively small, except when a tert-butyldimethylsilyloxymethyl (TBSOCH2−) group was present. When R is equal to phenyl or n-C12H25, in which there is no ether moiety on the side chain, the reaction requires higher temperature (110 °C) to proceed. 6833


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ACS Catalysis For the PtCl2−SnCl2 system, Kollár reported an AHF of αmethylstyrene using chiral ligand (S,S)-BDPP ((2S,4S)-2,4bis(diphenylphosphino)pentane) in the 1980s.40 Disappointingly, (R)-3-phenylbutanal was obtained with only 9% ee after the reaction was run at 50 °C for 110 h (Scheme 22). At an

(Scheme 24). In general, these chiral monophosphites gave moderate to good conversion and excellent n-selectivity, but unfortunately, the ee’s were all below 15%. Scheme 24. Bayón and Pereira’s AHF of α-Methylstyrene Using Chiral Monophosphite Ligand

Scheme 22. AHF of α-Methylstyrene Derivatives Using PtCl2−SnCl2-Based Chiral Catalysts

The best results for this type of substrates were reported in a patent cofiled by State University of New York and Mitsubishi Chemical Corporation.45 By using [Rh(COD)2(OAc)]2 as the precatalyst and a BIPHEP-like diphosphonite ligand, αmethylstyrene was hydroformylated to afford 3-phenylbutanal in 46% ee (Scheme 25). However, the procedure did not Scheme 25. AHF of α-Methylstyrene Patented by SUNY and Mitsubishi

elevated temperature of above 100 °C, the reaction could be completed in as quickly as 2.5 h; however, the ee dropped to only 1−2%. The authors also examined PtCl(SnCl3)[(S,S)BDPP] for the AHF of methyl methacrylate and dimethyl itaconate, but the ee’s were only 14% and 39%, respectively. Later on, the Consiglio group reported a similar system using the PtCl(SnCl3)[(R,R)-DIOP] catalyst, which required very high syngas pressure (180 bar).41 A catalytic amount of hydroquinone was added as an inhibitor of polymerization. Four substrates with para-substituents were examined, but the results were all unsatisfactory (80 bar with Pt catalyst) 6834


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ACS Catalysis Scheme 26. Botteghi’s Synthesis of (+)-Tolterodine

Scheme 29. Consiglio’s AHF of 2,3-Dimethyl-1-butene

Scheme 30. Gladiali and Botteghi’s AHF of 1,1Dialkylethenes

∼10% (Scheme 27). The major side reaction was hydrogenation. Scheme 27. Marchetti’s i-Selective AHF of 1-Phenyl-1-(2pyridyl)ethene

group, the enantioselectivity was extremely low (1% ee). In the same paper, the authors reported that the AHF of isoprene under the same conditions gave 3-methylpentanoic acid in 34% ee. However, as Botteghi explained in an earlier paper,50 this enhanced stereoinduction did not arise from the hydroformylation step, but rather, from the asymmetric hydrogenation of the intermediate α,β-unsaturated aldehyde, which could not undergo further hydroformylation because of low reactivity.

We attribute this rare regioselectivity to the electronwithdrawing ability of the 2-pyridyl group. Alternatively, it could be explained by the coordination of the pyridine nitrogen to rhodium, which facilitates hydride addition to the less substituted C2 via a six-membered ring transition-state. The resulting η3 complex undergoes CO insertion and reductive elimination to form the i-aldehyde (Scheme 28).

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CONCLUSIONS AND OUTLOOK In conclusion, the enantioselective hydroformylation of terminal disubstituted alkenes remains a long-standing and formidable challenge in the realm of asymmetric catalysis. In most cases, the n-selectivity predominates, yet the unique regioselectivity requires more demanding stereocontrol to achieve high enantioselectivity. Most traditional privileged AHF ligands failed to provide satisfactory results, and the selection of the optimal ligand was very different from case to case. However, encouraging progress has been made during the past decade. The introduction of ligands such as Bis-diazaphos, Ph-BPE, and P-chirogenic ligands allows for much improved chiral induction in the AHF of many functionalized substrates and opens a door to unexplored territories of this old but highly useful transformation. We believe subsequent development of the analogous chiral ligands can significantly expand the scope of AHF to include more unfunctionalized olefins, albeit the hope of achieving a universal catalyst is still extravagant at the moment.

Scheme 28. A Possible Cause for the i-Selectivity

The AHF of the aliphatic analogs is considered even more challenging because unlike the reaction of the aryl analogs, there is no π−π stacking interaction between the substrate and the catalyst. Representative works were done mostly before the early 1990s. So far, the best example has been Consiglio’s AHF of 2,3-dimethyl-1-butene, with the PtCl2−SnCl2 catalyst modified by the (R,R)-BCO−DBP ligand.48 The ee of the naldehyde was 46%, whereas the structurally similar DIOP ligand gave only 20% ee (Scheme 29). AHF of 1,1-dialkylethenes with Rh catalysts has not been widely investigated. Gladiali and Botteghi reported the AHF of 2,3,3-trimethyl-1-butene that gave linear aldehyde exclusively (Scheme 30).49 To our surprise, regardless of the huge steric difference between the bulky tert-butyl group and the methyl

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 6835


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ACKNOWLEDGMENTS We thank Dr. Gregory D. Cuny for his help in preparing the manuscript.

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Principles and Applications of Enantioselective Hydroformylation of

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