Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Petasis-Type gem-Difluoroallylation Reactions Assisted by the Neighboring Hydroxyl Group in Amines Xing Yang,† Ze-Hun Cao,† Yang Zhou, Feng Cheng, Zi-Wei Lin, Zhi Ou, Ye Yuan, and Yi-Yong Huang* Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Wuhan 430070, P.R. China S Supporting Information *
ABSTRACT: A three-component Petasis-type gem-difluoroallylation reaction of using pinacol gem-difluoroallylboronates, aldehydes or isatin, and β-amino alcohols enabled by the neighboring hydroxyl group in amine is reported, affording various racemic and chiral gem-difluorohomoallylamine derivatives with good to excellent results. Based on the control experiment and stereochemistry of the product, a proposed reaction pathway is illustrated to clarify the origin of regio- and stereoselectivity under protic solvent conditions.
T
Scheme 1. Petasis Allylation Reactions Enabled by Hydroxyl Groups
he Petasis reaction is a Mannich-like three-component condensation reaction involving carbonyls, amines and boronic acid, boronate, or potassium trifluoroboronate reagents under metal-free conditions, providing a powerful route to access α-(hetero)aryl-, vinyl-, alkynyl-, and allyl-functionalized amine derivatives.1 Generally, the generation of more reactive trivalent boron intermediates via boronate ligand exchange2 or tetrahedron boron−ate complexes by in situ formed oxygencontaining anion species assisted by hydroxyl, acid, and amine functional groups results in regioselective adducts.3 Compared with the two-component imine nucleophilic addition with organoboron reagents, the Petasis reaction exhibits a more convenient operation and broader application for not requiring imine isolation, especially for some unstable cases. Therefore, the development of efficient Petasis reactions has drawn great attention. Among the established protocols, allylation with respect to boronate ligand exchange is less advanced, compared to other types of additions. Seminal work contributed by the Schaus group indicated that a chiral 3,3-disubstituted BINOL catalyst was essential to accelerate the asymmetric Petasis allylation reaction with 1,2-propanediol boronates under microwave conditions (Scheme 1a)2a as well as the traceless catalytic asymmetric Petasis allylation.2b,c In addition, the Zhang group discovered that the adjacent hydroxyl group in chiral amino alcohols promoted the diastereoselective Petasis allylation with allylboronic acids and isatins (Scheme 1b).2d Notably, Petasis allylation of using pinacol allylboronates has been less developed.4 The relatively lower reactivity of pinacol allylboronates stemming from more difficult boronate ligand exchange appears to be a bottleneck in such metal-free conversions.5 In this context, the research on new Petasistype allylations with pinacol allylboronates is still significant and desirable. As readily synthesized, nontoxic, and bench-stable organoboron reagents, pinacol gem-difluoroallylboronates6 have already been used in the gem-difluoroallylation of carbonyl compounds.7 However, the gem-difluoroallylboration of imines © XXXX American Chemical Society
remains unexplored, probably because of the lower electrophilicity of imines, compared to carbonyls.8 Considering the importance of introducing a difluoromethylene group (CF2) into organic molecules,9 and as part of our continuous research program on unsaturated boronates and fluorinated building blocks,10 we became interested in applying pinacol gemdifluoroallylboronates in the Petasis reaction to access important gem-difluorohomoallylamines.11 However, two challenges needed to be met, including the activation of pinacol Received: March 8, 2018
A
DOI: 10.1021/acs.orglett.8b00721 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters Table 2. Scope of Carbonyl and Pinacol gemDifluoroallylboronate Substratesa
gem-difluoroallylboronates, as well as eliminating the background reaction between gem-difluoroallylboronates and carbonyls. In this regard, we herein developed an unprecedented Petasis gem-difluoroallylboration reaction by installing the neighboring hydroxyl group in amines, thus allowing boronate ligand exchange between the hydroxyl group and pinacol gem-difluoroallylboronate, providing the more reactive intermediates in an intramolecular-like allylation leading to the target products (Scheme 1c).12 Initially, benzaldehyde 1a and pinacol gem-difluoroallylboronate 3a were selected as model substrates for optimizing the reaction conditions. When p-toluenesulfonamide 2a or aniline 2b was used, the desired reaction did not occur in CH2Cl2 at rt (Table 1, entries 1 and 2). With aniline 2c containing a
aldehyde
allylboronate
product
time (h)
yieldb (%)
R3 = OTs, R4 = H (3a) 3a
4aa
5
98
4ba
5
98
3a
4ca
5
97
3a
4da
5
98
3a
4ea
5
95
3a
4fa
5
97
3a
4ga
24
96
3a
4ha
24
92
3a
4ia
8
96
3a
4ja
5
96
3a
4ka
5
95
3a
4la
3
96
3a
4ma
4
94
14 15
R1 = H, R2 = Ph (1a) R1 = H, R2 = 4MeC6H4 (1b) R1 = H, R2 = 4BrC6H4 (1c) R1 = H, R2 = 4NO2C6H4 (1d) R1 = H, R2 = 3MeOC6H4 (1e) R1 = H, R2 = 3CF3C6H4 (1f) R1 = H, R2 = 2CO2MeC6H4 (1g) R1 = H, R2 = 1naphthyl (1h) R1 = H, R2 = 2naphthyl (1i) R1 = H, R2 = 2-furyl (1j) R1 = H, R2 = 2thienyl (1k) R1 = H, R2 = (E)PhCHCH (1l) R1 = H, R2 = c-hexyl (1m) isatin (1n) 1a
4na 4ab
48 5
87 96
16
1a
4ac
12
93
17
1a
4ad
12
96
18
1a
4ae
5
95
19
1a
4af
24
95
20
1a
4ag
24
94
21
1a
3a R3 = Ph, R4 = H (3b) R3 = 4-PhC6H4, R4 = H (3c) R3 = 4CO2MeC6H4, R4 = H (3d) R3 = 3-ClC6H4, R4 = H (3e) R3 = 2-MeOC6H4, R4 = H (3f) 3 R = 1-naphthyl, R4 = H (3g) R3 = 2-benzofuryl, R4 = H (3h) R3 = Ph, R4 = Me (3i) R3 = Ph, R4 = npropyl (3j)
4ah
24
93
4ai
36
94
4aj
48
91
entry 1 2 3
Table 1. Optimization of Reaction Conditionsa
4 5 6 7
entry
amine
solvent
time (h)
yieldb (%)
1 2 3 4 5 6 7c 8 9 10 11
2a (R = Ts) 2b (R = Ph) 2c (R = o-HOC6H4) 2c 2c 2c 2c 2c 2d (R = m-HOC6H4) 2e (R = p-HOC6H4) 2f (R = o-MeOC6H4)
CH2Cl2 CH2Cl2 CH2Cl2 THF Et2O toluene THF MeOH MeOH MeOH MeOH
48 48 48 48 48 48 24 5 48 48 48
NR NR 69 78 70 62 95 99 (98) 0 0 0
8 9 10 11 12 13
a
Reaction conditions: 1a (0.2 mmol) and 2 (0.24 mmol) in anhydrous solvent (0.5 mL) was stirred at rt for 2 h in the presence of 4 Å MS (100 mg), then 3a (0.24 mmol) was added. bNMR yield by using 1,3,5-trimethoxybenzene as an internal standard, and the data in parentheses is isolated yield. cMeOH (5.0 equiv).
hydroxyl group at the ortho position, the Petasis borono− Mannich adduct of gem-difluorohomoallylamine 4aa was obtained in 69% NMR yield (entry 3). Encouraged by this result, further examination of solvent effect revealed that an increased yield of 78% was achieved in THF (entry 4). The addition of 5.0 equiv of MeOH improved the yield dramatically (entry 7). When the reaction was carried out in only MeOH solvent, the best result (99% NMR yield and 98% isolated yield) was obtained within a shorter reaction time (5 h, entry 8). It is noteworthy that m-hydroxyl-, p-hydroxyl-, and omethoxyaniline did not trigger this reaction (entries 9−11). Having established the optimized reaction conditions, we next investigated the scope of substrates. A variety of carbonyl compounds and pinacol gem-difluoroallylboronates were employed in this reaction, providing the Petasis products in high yields (87−98% yield). Benzaldehydes with electrondonating or electron-withdrawing groups at the 4- and 3positions underwent the Petasis reaction efficiently, delivering the target products in 95−98% yield after 5 h (Table 2, entries 1−6). Introducing an ester group at the 2-position of benzaldehyde required a longer reaction time to gain a satisfactory yield (24 h, entry 7). 1-Naphthaldehyde exhibited lower reactivity than 2-naphthaldehyde, probably due to negative steric hindrance effect (entry 8 vs entry 9). Excellent
c
22
1a
23c
1a
a
Reaction conditions: 1 (0.2 mmol) and 2c (0.24 mmol) in MeOH (0.5 mL) was stirred at rt for 2 h in the presence of 4 Å MS (100 mg), then 3 (0.24 mmol) was added. bIsolated yield. cReaction was performed at 60 °C.
results were observed for heteroaromatic, cinnamyl, and aliphatic aldehydes (entries 10−13). In the case of isatin 1n, good yield was achieved by prolonging the reaction time (entry 14). The reaction also proceeded smoothly when altering the OTs group (R3) to aryl groups in the boronate reagents (entries 15−21). gem-Difluoroallylboronates with substituents at the α- and β-position were also tolerated in this reaction, B
DOI: 10.1021/acs.orglett.8b00721 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters albeit requiring heating and extending reaction time (entries 22 and 23). From a synthetic point of view, the development of a method for the preparation of optically pure gem-difluorohomoallylamines is much more significant but also more challenging. Therefore, we started the diastereoselective Petasis reaction mediated by chiral amino alcohols. Under the above standard conditions, amino alcohol 5a engendered the desired product in 52% yield with 89:11 dr (Table 3, entry 1). When the
Scheme 2. Scope of Substrates in the Diastereoselective Petasis gem-Difluoroallylborationsa
Table 3. Optimization of the Reaction Conditions for the Diastereoselective Petasis gem-Difluoroallylborationsa
entry
aminol alcohol
T (°C)
time (h)
yieldb (%)
drc
1 2 3 4 5 6
5a 5a 5b 5c 5c 5d
rt 60 60 60 rt 60
72 72 72 24 48 48
52 82 83 95 94 (92) 91 (88)
89:11 87:13 80:20 81:19 87:13 94:6
a
Reaction conditions: 1 (0.2 mmol) and 5 (0.24 mmol) in MeOH (0.5 mL) was stirred at rt for 2 h in the presence of 4 Å MS (100 mg), then 3 (0.24 mmol) was added.
was first performed (Scheme 3). Chiral gem-difluorohomoallylamine 6a (1.19 g) was accessed with comparable results to the
a
Reaction conditions: 1a (0.2 mmol) and 5 (0.24 mmol) in MeOH (0.5 mL) was stirred at rt for 2 h in the presence of 4 Å MS (100 mg), then 3a (0.24 mmol) was added. bNMR yield by using 1,3,5trimethoxybenzene as an internal standard, and the data in parentheses are isolated yields. cDetermined by 1H NMR analysis of the crude reaction mixture.
Scheme 3. Asymmetric Synthesis of Compound 8
reaction temperature was increased to 60 °C, the yield was improved to 82%, albeit with decreased diastereoselectivity (84:14 dr, entry 2). Pleasingly, further screening of other commercially available chiral amino alcohols showed that 5d gave the best results of 88% isolated yield and 94:6 dr at 60 °C (entry 6). With the optimized reaction conditions in hand, we turned our attention to the aldehyde and gem-difluoroallylboronate scope (Scheme 2). Aldehydes bearing a bromo or methyl group at the 4-position of the phenyl group gave the corresponding chiral products in 89−90% yields with 84:16 dr (6b) and 96:4 dr (6c), respectively. The phenyl group with an electrondonating group (CH3) at the 3-position delivered superior results to the substrate bearing an electron-withdrawing group (CF3) (6d vs 6e). When a fluoro group was introduced at the 2-position, product 6f was obtained with good results. In addition, heterocyclic aldehydes were converted into the corresponding products 6g and 6h in high yields with excellent diastereoselectivities. Cinnamyl aldehyde was also found to be suitable in this reaction, and converted to product 6i with good results. Interestingly, only a single diastereoisomer 6j was observed for the aliphatic cyclohexanal, together with chiral amino alcohol 5c, instead of 5d. Good results were also achieved when switching the OTs group to aryl groups of the gem-difluoroallylboronates (6k and 6l). To evaluate the practicality and synthetic utility of this Petasis gem-difluoroallylboration reaction, a scale-up synthesis
small-scale case. Subsequently, the synthetic transformation of compound 6a was carried out. Oxidative cleavage of the amino alcohol moiety was mediated with Pb(OAc)4, and the resultant imine was converted to the desired amine by using NH2OH· HCl.13 Then the protection of the amine with a benzoyl group afforded intermediate 7 with 94% ee. Finally, treatment of 7 with K2CO3 in MeOH generated optically pure α-CF2-βaminoketone 8 in 76% yield with complete retention of enantiomeric purity. The α-CF2-β-aminoketone substructure is found as a core moiety in many pharmaceutically and biologically interesting compounds.14 The absolute configuration of the stereogenic carbon center of 8 was determined as S based on the X-ray crystal structure. Control experiment of applying the isolated imine I from 1a and 5d in such gem-difluoroallylboration reaction with 3a, the same product 6a was readily obtained. Therefore, a proposed reaction pathway is illustrated in Figure 1 to illustrate the role C
DOI: 10.1021/acs.orglett.8b00721 Org. Lett. XXXX, XXX, XXX−XXX
Organic Letters
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Letter
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yi-Yong Huang: 0000-0001-6209-8304 Author Contributions †
X.Y. and Z.-H.C. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for the financial support of this investigation by the National Natural Science Foundation of China (21573169, 21772151) and Wuhan Morning Light Plan of Youth Science and Technology (2017050304010314).
Figure 1. Proposed reaction pathway.
of an adjacent hydroxyl group in the exclusive γ-regioselectivity and high diastereoselectivity. As shown in Figure 1, the ligand exchange between the neighboring hydroxyl group of the in situ formed imine I and the gem-difluoroallylboronate leads to a more reactive acyclic gem-difluoroallylboronate intermediate. The complexation between boron and imine nitrogen lone pair may exist to stabilize intermediate II. Then the addition reaction occurs in an intramolecular manner according to Zimmerman−Traxler model (III).15 Due to the stereoinduction from the chiral amino alcohol, gem-difluoroallylboration occurs through attack of the CN double bond from its Re face. Finally, the alcoholysis of intermediate IV delivers the final chiral product 6a. In conclusion, we have developed an efficient Petasis boronoMannich gem-difluoroallylation reaction with pinacol gemdifluoroallylboronates enabled by the neighboring hydroxyl group in amines. Besides racemic gem-difluorohomoallylamines, the hydroxyl group activation strategy has been applied to the diastereoselective synthesis of diverse chiral gem-difluorohomoallylamines in good to excellent yields (up to 98% yield) and diastereoselectivities (up to 99:1 dr) by using commercially available chiral β-amino alcohols. In addition, gem-difluorohomoallylamine was readily turned into an optically pure α-CF2-βaminocarbonyl compound. The outcome of this Petasis allylation and stereoselective chemistry is suggested by the formation of a more reactive gem-difluoroallylboronate intermediate through a boronate ligand-exchange process, followed by the intramolecular-like imine allylboration via a Zimmerman−Traxler transition state. Further studies on the application of this synthetic strategy in other types of organoboronate reagents are underway in our laboratory.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b00721. Experimental procedures, characterization data, and NMR spectra for all products (PDF) Accession Codes
CCDC 1588929 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
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