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Rhenium‐Catalyzed Dehydrogenative Olefination of C(sp3)−H Bonds with Hypervalent Iodine(III) Reagents Received 00th January 20xx, Accepted 00th January 20xx
Haidong Gu and Congyang Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
A dehydrogenative olefination of C(sp3)−H bonds is disclosed here by merging rhenium catalysis with an Alanine‐derived hypervalent iodine(III) reagent. Thus, cyclic and acyclic ethers, toluene derivatives, cycloalkanes, and nitriles are all successfully alkenylated in a regio‐ and stereoselective manner. Hypervalent iodine(III) reagents (HIRs) have wide applications in organic synthesis because of their ready availability, easy handling, varied reactivities, and benign environmental character.1 As a prominent instance, HIRs have received considerable attention in alkene difunctionalization reactions, which are of great importance for chemical elaborations of complex molecules from simple olefin feedstock.2 In general, HIRs play a dual role in metal‐free alkene difunctionalization: as electrophiles to activate olefins thus affording three‐membered iodonium ion intermediates and then as good leaving groups towards attack of other nucleophiles. Though successful, this mechanistic profile accounts for the limited reaction types of alkene difunctionalizations promoted solely by HIRs.3 Since 2005, the merging of transition metal catalysis with HIRs has evoked surging investigations on alkene difunctionalizations due to the power and versatility of transition metal catalysts.4‐9 In these events, HIRs are imparted a new function, that is, oxidation of transition metal species from low oxidation states to high ones, which is essential to achieve efficient catalytic turnovers for varied alkene difunctionalization reactions. So far, the successful HIR‐enabled redox couples in these transformations include PdII/PdIV,4 AuI/AuIII,5 CuI/CuIII,6 RuII/RuIII,7 IrIII/IrIV,7 ReI/ReII,8 and so on.9 As a result, the control of reaction regio‐ and stereoselectivity as well as the diversity of reaction categories has been largely improved. In comparison with alkene difunctionalization, alkene monofunctionalization, maintaining the olefin double bonds, has been less studied through the combinatorial use of transition metal catalysts and HIRs. Kang,10a‐b Ma,10c and others10d‐f described the
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E‐mail: [email protected]. † Electronic Supplementary Information (ESI) available: Experimental details, characterization data and NMR spectra for all new compounds. See DOI: 10.1039/x0xx00000x
direct arylation of olefins by using Pd‐catalysts and diaryliodonium salts, while Gaunt et al.10g demonstrated the Cu‐variant of these reactions with wider substrate scopes (Scheme 1, a). Szabó and co‐ workers11 disclosed the Pd‐catalyzed acyloxylation of alkenes with carboxylic acid‐derived HIRs giving formally allylic C‐H bond functionalized products (Scheme 1, b). Meanwhile, Buchwald,12a Wang,12b Sodeoka,12c and Xiao et al.12d developed the Cu‐catalyzed alkene trifluoromethylation reactions by employing Togni reagents (Scheme 1, c). Afterwards, Li,13a Glorius,13b and Loh et al.13c reported the Rh‐ and Ir‐catalyzed alkene alkynylation reactions with the aid of alkynyl‐HIRs (Scheme 1, d). We have recently developed an oxyalkylation reaction of olefins by merging rhenium catalysis with carboxylic acid‐derived HIRs.8 As our continuous interests in the use of Re‐catalysis14,15 and HIRs in organic synthesis, herein we disclose a dehydrogenative olefination of C(sp3)‐H bonds by merging Re‐ catalysis with an Alanine‐derived HIR (Scheme 1, e).
Scheme 1 Transition‐Metal‐Catalyzed Alkene Mono‐Functionalization Reactions with HIRs.
Intrigued by synthetic transformations of cheap feedstock (such as tetrahydrofuran and 1,4‐dioxane),16 we selected alkene 1d and 1,4‐dioxane 2a as model substrates to screen the reaction conditions (Table 1).17 In the presence of 2.5 mol% Re2(CO)10, HIR 3a did promote the reaction to give the dehydrogenative
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olefination product 4da, albeit only in 12% yield with contaminant formation of the oxymethylation product 5da (entry 1).8 The use of HIR 3b, however, completely inhibited the expected reaction (entry 2).17 While no reaction occurred with N‐unprotected HIR 3c (entry 3), N‐methyl and –ethyl variants (3d and 3e) substantially increased the yields of 4da (entries 4‐5). Further enhancement of the bulkiness of N‐substituents (Bn, t‐Bu) gave comparable results (entries 6‐7). Interestingly, HIR 3h, 3i, and 3j, derived from the simple amino acid Glycine, β‐Alanine, and Alanine respectively,18 showcased promising reactivities with 3j being the best (entries 8‐ 10). Presumably, the subtly tuned steric and electronic properties of the N‐substituents might attribute to the stability and reactivity of the N‐radical species (vide infra). Of note, the use of other oxidants like tBuOOH and (tBuO)2 instead of HIRs gave much worse results under otherwise the same conditions.17 The control experiment demonstrated that only a low yield of 4da was obtained in the absence of Re2(CO)10 (entry 11). Remarkably, inferior results were got by using other Re‐catalysts, Mn‐catalysts, and other transition metal like Pd‐, Cu‐, and Fe‐catalysts as well as traditional Lewis acid catalysts (entries 12‐17),17 which underlined the unique reactivity of Re2(CO)10 in this reaction. Finally, 4da could be isolated in 66% yield as a sole E‐isomer under the optimized reaction conditions (entry 18). Of note, 3j was reduced to 2‐iodobenzamide 6j in 96% isolated yield, which can be easily oxidized by peracetic acid to regenerate HIR 3j.17
a
o Reaction conditions: 1d (0.2 mmol), 3 (0.4 mmol), 1,4‐dioxane 2a (0.1 M), 120 C, View Article Online b 3 h. Determined by GC‐MS using mesitylene (0.1 mmol) as an internal standard. DOI: 10.1039/C5OB00619H c E‐isomer only. d No catalyst. e 3j (0.8 mmol), 1,4‐dioxane (0.05 M), 4 h. f Isolated yield on 1.0 mmol scale. g 6j was isolated in 96 % yield.
Then, we commenced to investigate the scope of olefins (Scheme 2). It was shown that styrene derivatives bearing both electro‐donating and –withdrawing groups were tolerant in the reaction (4aa‐ia). 2‐Vinylnaphthalene also delivered the corresponding product 4ja smoothly. Meta‐ and ortho‐substituents had no obvious effect on the reaction outcome (4ka‐ma). Furthermore, 1,1‐diphenylethylene was also amenable to this protocol giving the expected product 4na in good yield. It should be pointed out that 1,2‐disubstituted olefins showed low reactivity in this reaction presumably due to the steric hindrance in the step of C‐C bond formation.17 Interestingly, the exocyclic olefin 1o reacted with 1,4‐dioxane under the modified reaction conditions affording the double‐bond migrated product 4oa and its oxidatively aromatized product 4oa' (Scheme 3).
Table 1 Screening of Reaction Parametersa
b
entry
cat. (mol%)
HIRs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 e 18
Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) d ‐ Re(CO)5Br (5) Mn2(CO)10 (5) Pd(OAc)2 (5) CuBr (5) ZnCl2 (5) Sc(OTf)3 (5) Re2(CO)10 (2.5)
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3j 3j 3j 3j 3j 3j 3j 3j
OCOCF3 OAc I I OCOCF3 Ph OAc Ph 3a
3b
OAc I N O
CO2Me
3i
yields (%) c 4da 5da 12 19 0 0 0 0 15 0 53 0 46 0 55 0 48 0 64 0 73 0 24 0 18 0 10 0 0 0 14 0 0 0 19 0 f,g 84(66) 0
OAc R' = H, 3c I R' = Me, 3d N R' R' = Et, 3e R' = Bn, 3f R' = t-Bu, 3g O OAc CO2Me I N O 3j
OAc CO2Me I N O 3h I H N O
CO2Me 6j
Scheme 2 Scope of Olefins. Reaction conditions: 1 (1.0 mmol), 3j (4.0 mmol), Re2(CO)10 (2.5 mol%), 1,4‐dioxane (0.05 M), 120 oC, 4 h. Isolated yields of products 4 (%) are shown. a Re2(CO)10 (5 mmol%), 1,4‐dioxane (0.03 M), 150 oC.
Scheme 3 Reaction of 1‐Methylene‐1,2,3,4‐tetrahydronaphthalene and 1,4‐Dioxane.
Next, the compatibility of substrates bearing various C(sp3)‐H bonds was tested with olefin 1n as the reaction partner (Table 2). Alkyl ethers, no matter cyclic or acyclic, could deliver the expected products successfully (entries 1‐2). Toluene, xylenes, and mesitylene containing benzylic C‐H bonds were also smoothly alkenylated to afford the corresponding products in good yields (entries 3‐6). Functionalities like cyano and iodo groups were well tolerant under the reaction conditions (entries 7‐8). Cyclic alkanes
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such as cyclopentane, ‐hexane, and –ocatane were all suitable substrates giving the olefination products in good to excellent yields (entries 9‐11). Furthermore, allylic nitrile 4nm could be easily obtained from the reaction of olefin 1n and acetonitrile in synthetically useful yield with the concurrent formation of N‐ olefination product 7j, originating from the reaction of 1n and Ala‐ HIR 3j (entry 12).
to form benzylic cation 13 and regenerate the catalytically View Articleactive Online DOI: 10.1039/C5OB00619H Re(n) species. It should be noted that 12 might also be oxidized by HIR 3j, which constitutes an alternative but less competent pathway particularly when the rhenium catalyst is absent in the reaction (Table 1, entry 11). Finally, deprotonation of 13 leads to the final formation of product 4. a) Trapping the C-radical formed via H-abstraction
Table 2 Scope of Substrates Bearing Various C(sp3)‐H Bondsa O
5 mol% Re2(CO) 10 Ala-HIR 3j
+
N O
H
N O
THF, 150 oC, 4 h
O
Ph TEMPO
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2b
entry
1
yield b (%)
entry
4nb: 88
7
4nc: 50
8
product 4
c
2
4nd: 62
3 d
4
product 4
5
4ni: 61
d
4nj: 91
4nk: 95
e
4ne: 64
10
4nf: 67
11
4ng: 66
12
e
d
4nh: 69
d
9
e
6
yield b (%)
f
8: 31%
(1n)
Ph
no 1n
8: 25%
b) Reaction of 1-phenylvinylcyclopropanewith 1,4-dioxane O 5 mol% Re2(CO) 10 O Ala-HIR 3j O H + H
1p
O
1,4-dioxane 120 oC, 4 h
O
O
+
34% (4oa:4oa' = 6.3:1)
2a radical addition
O
O
O
[O], - H
O
O
O rearrangement
cyclization H
Scheme 4 Mechanistic Experiments.
4nl: 71 4nm: 58
a Reaction conditions: 1n (1.0 mmol), 3j (4.0 mmol), Re2(CO)10 (5 mol%), 2 (0.03 M), 150 oC, 4 h. b Isolated yields of products 4. c Re2(CO)10 (2.5 mmol%), Et2O (0.05 M), 120 oC. d 2 (0.2 M). e 2 (0.1 M), 12 h. f 7j was isolated in 11% yield.
We assumed that the initial C(sp3)‐H activation occur through a radical H‐abstraction pathway. In order to verify this hypothesis, the radical‐trapping reagent TEMPO was subjected to the reaction conditions of olefin 1n and tetrahydrofuran 2b (Scheme 4, a). To our delight, the α‐THF radical trapped product 8 was isolated in 31% yield and the olefination reaction was completely prohibited, which confirmed our assumption. Also, compound 8 could be obtained in comparable yield in the absence of 1n. Next, the reaction of 1‐ phenylvinylcyclopropane 1p, susceptible to radical rearrangements, and 1,4‐dioxane 2a was carried out (Scheme 4, b). It turned out that the ring expansion product 4oa and its aromatized product 4oa' were formed in 34% yield. This result supported a reaction sequence of radical addition/rearrangement/cyclization/oxidative deprotonation.19 Based on the above observations, a tentative reaction mechanism was proposed in Scheme 5. Heterolytic cleavage of the I‐O bond in HIR 3j gives rise to the ionic species 9, which undergoes a single electron transfer process with the Re(n) catalyst affording N‐radical species 10. The H‐atom α to oxygen in 1,4‐dioxane is abstracted by 10 giving rise to the α‐C radical species 11 and 2‐ iodobenzamide 6j. The ensuing radical addition of 11 to olefin affords benzylic radical 12, which is oxidized by the Re(n+1) species
Scheme 5 A Tentative Reaction Mechanism.
In conclusion, the dehydrogenative olefination of C(sp3)‐H bonds was developed through the combination of rhenium‐ catalysis and an Alanine‐derived hypervalent iodine(III) reagents, which demonstrates a reactivity quite distinct from known combinations of transition metal catalysts and hypervalent iodine(III) reagents. Also, this reaction represents a rare example in organic synthesis where amino acid‐derived HIRs play a unique role since they were synthesized by Zhdankin et al.18 Thus, cheap and easily available feedstock such as cyclic and acyclic ethers, toluene derivatives, cycloalkanes, and nitriles could all be successfully alkenylated in a regio‐ and stereoselective way.20 Mechanistic studies revealed a radical‐initiated pathway operating in the reaction.
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Further exploitations on merging rhenium‐catalysis with hypervalent iodine(III) reagents in organic synthesis are underway in our laboratory. Financial support from the National Basic Research Program of China (973 Program) (No. 2011CB808600) and the National Natural Science Foundation of China (21322203, 21272238, 21472194) are gratefully acknowledged.
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Rhenium‐Catalyzed Dehydrogenative Olefination of C(sp3)−H Bonds with Hypervalent Iodine(III) Reagents Received 00th January 20xx, Accepted 00th January 20xx
Haidong Gu and Congyang Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
A dehydrogenative olefination of C(sp3)−H bonds is disclosed here by merging rhenium catalysis with an Alanine‐derived hypervalent iodine(III) reagent. Thus, cyclic and acyclic ethers, toluene derivatives, cycloalkanes, and nitriles are all successfully alkenylated in a regio‐ and stereoselective manner. Hypervalent iodine(III) reagents (HIRs) have wide applications in organic synthesis because of their ready availability, easy handling, varied reactivities, and benign environmental character.1 As a prominent instance, HIRs have received considerable attention in alkene difunctionalization reactions, which are of great importance for chemical elaborations of complex molecules from simple olefin feedstock.2 In general, HIRs play a dual role in metal‐free alkene difunctionalization: as electrophiles to activate olefins thus affording three‐membered iodonium ion intermediates and then as good leaving groups towards attack of other nucleophiles. Though successful, this mechanistic profile accounts for the limited reaction types of alkene difunctionalizations promoted solely by HIRs.3 Since 2005, the merging of transition metal catalysis with HIRs has evoked surging investigations on alkene difunctionalizations due to the power and versatility of transition metal catalysts.4‐9 In these events, HIRs are imparted a new function, that is, oxidation of transition metal species from low oxidation states to high ones, which is essential to achieve efficient catalytic turnovers for varied alkene difunctionalization reactions. So far, the successful HIR‐enabled redox couples in these transformations include PdII/PdIV,4 AuI/AuIII,5 CuI/CuIII,6 RuII/RuIII,7 IrIII/IrIV,7 ReI/ReII,8 and so on.9 As a result, the control of reaction regio‐ and stereoselectivity as well as the diversity of reaction categories has been largely improved. In comparison with alkene difunctionalization, alkene monofunctionalization, maintaining the olefin double bonds, has been less studied through the combinatorial use of transition metal catalysts and HIRs. Kang,10a‐b Ma,10c and others10d‐f described the
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China. E‐mail: [email protected]. † Electronic Supplementary Information (ESI) available: Experimental details, characterization data and NMR spectra for all new compounds. See DOI: 10.1039/x0xx00000x
direct arylation of olefins by using Pd‐catalysts and diaryliodonium salts, while Gaunt et al.10g demonstrated the Cu‐variant of these reactions with wider substrate scopes (Scheme 1, a). Szabó and co‐ workers11 disclosed the Pd‐catalyzed acyloxylation of alkenes with carboxylic acid‐derived HIRs giving formally allylic C‐H bond functionalized products (Scheme 1, b). Meanwhile, Buchwald,12a Wang,12b Sodeoka,12c and Xiao et al.12d developed the Cu‐catalyzed alkene trifluoromethylation reactions by employing Togni reagents (Scheme 1, c). Afterwards, Li,13a Glorius,13b and Loh et al.13c reported the Rh‐ and Ir‐catalyzed alkene alkynylation reactions with the aid of alkynyl‐HIRs (Scheme 1, d). We have recently developed an oxyalkylation reaction of olefins by merging rhenium catalysis with carboxylic acid‐derived HIRs.8 As our continuous interests in the use of Re‐catalysis14,15 and HIRs in organic synthesis, herein we disclose a dehydrogenative olefination of C(sp3)‐H bonds by merging Re‐ catalysis with an Alanine‐derived HIR (Scheme 1, e).
Scheme 1 Transition‐Metal‐Catalyzed Alkene Mono‐Functionalization Reactions with HIRs.
Intrigued by synthetic transformations of cheap feedstock (such as tetrahydrofuran and 1,4‐dioxane),16 we selected alkene 1d and 1,4‐dioxane 2a as model substrates to screen the reaction conditions (Table 1).17 In the presence of 2.5 mol% Re2(CO)10, HIR 3a did promote the reaction to give the dehydrogenative
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olefination product 4da, albeit only in 12% yield with contaminant formation of the oxymethylation product 5da (entry 1).8 The use of HIR 3b, however, completely inhibited the expected reaction (entry 2).17 While no reaction occurred with N‐unprotected HIR 3c (entry 3), N‐methyl and –ethyl variants (3d and 3e) substantially increased the yields of 4da (entries 4‐5). Further enhancement of the bulkiness of N‐substituents (Bn, t‐Bu) gave comparable results (entries 6‐7). Interestingly, HIR 3h, 3i, and 3j, derived from the simple amino acid Glycine, β‐Alanine, and Alanine respectively,18 showcased promising reactivities with 3j being the best (entries 8‐ 10). Presumably, the subtly tuned steric and electronic properties of the N‐substituents might attribute to the stability and reactivity of the N‐radical species (vide infra). Of note, the use of other oxidants like tBuOOH and (tBuO)2 instead of HIRs gave much worse results under otherwise the same conditions.17 The control experiment demonstrated that only a low yield of 4da was obtained in the absence of Re2(CO)10 (entry 11). Remarkably, inferior results were got by using other Re‐catalysts, Mn‐catalysts, and other transition metal like Pd‐, Cu‐, and Fe‐catalysts as well as traditional Lewis acid catalysts (entries 12‐17),17 which underlined the unique reactivity of Re2(CO)10 in this reaction. Finally, 4da could be isolated in 66% yield as a sole E‐isomer under the optimized reaction conditions (entry 18). Of note, 3j was reduced to 2‐iodobenzamide 6j in 96% isolated yield, which can be easily oxidized by peracetic acid to regenerate HIR 3j.17
a
o Reaction conditions: 1d (0.2 mmol), 3 (0.4 mmol), 1,4‐dioxane 2a (0.1 M), 120 C, View Article Online b 3 h. Determined by GC‐MS using mesitylene (0.1 mmol) as an internal standard. DOI: 10.1039/C5OB00619H c E‐isomer only. d No catalyst. e 3j (0.8 mmol), 1,4‐dioxane (0.05 M), 4 h. f Isolated yield on 1.0 mmol scale. g 6j was isolated in 96 % yield.
Then, we commenced to investigate the scope of olefins (Scheme 2). It was shown that styrene derivatives bearing both electro‐donating and –withdrawing groups were tolerant in the reaction (4aa‐ia). 2‐Vinylnaphthalene also delivered the corresponding product 4ja smoothly. Meta‐ and ortho‐substituents had no obvious effect on the reaction outcome (4ka‐ma). Furthermore, 1,1‐diphenylethylene was also amenable to this protocol giving the expected product 4na in good yield. It should be pointed out that 1,2‐disubstituted olefins showed low reactivity in this reaction presumably due to the steric hindrance in the step of C‐C bond formation.17 Interestingly, the exocyclic olefin 1o reacted with 1,4‐dioxane under the modified reaction conditions affording the double‐bond migrated product 4oa and its oxidatively aromatized product 4oa' (Scheme 3).
Table 1 Screening of Reaction Parametersa
b
entry
cat. (mol%)
HIRs
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 e 18
Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) Re2(CO)10 (2.5) d ‐ Re(CO)5Br (5) Mn2(CO)10 (5) Pd(OAc)2 (5) CuBr (5) ZnCl2 (5) Sc(OTf)3 (5) Re2(CO)10 (2.5)
3a 3b 3c 3d 3e 3f 3g 3h 3i 3j 3j 3j 3j 3j 3j 3j 3j 3j
OCOCF3 OAc I I OCOCF3 Ph OAc Ph 3a
3b
OAc I N O
CO2Me
3i
yields (%) c 4da 5da 12 19 0 0 0 0 15 0 53 0 46 0 55 0 48 0 64 0 73 0 24 0 18 0 10 0 0 0 14 0 0 0 19 0 f,g 84(66) 0
OAc R' = H, 3c I R' = Me, 3d N R' R' = Et, 3e R' = Bn, 3f R' = t-Bu, 3g O OAc CO2Me I N O 3j
OAc CO2Me I N O 3h I H N O
CO2Me 6j
Scheme 2 Scope of Olefins. Reaction conditions: 1 (1.0 mmol), 3j (4.0 mmol), Re2(CO)10 (2.5 mol%), 1,4‐dioxane (0.05 M), 120 oC, 4 h. Isolated yields of products 4 (%) are shown. a Re2(CO)10 (5 mmol%), 1,4‐dioxane (0.03 M), 150 oC.
Scheme 3 Reaction of 1‐Methylene‐1,2,3,4‐tetrahydronaphthalene and 1,4‐Dioxane.
Next, the compatibility of substrates bearing various C(sp3)‐H bonds was tested with olefin 1n as the reaction partner (Table 2). Alkyl ethers, no matter cyclic or acyclic, could deliver the expected products successfully (entries 1‐2). Toluene, xylenes, and mesitylene containing benzylic C‐H bonds were also smoothly alkenylated to afford the corresponding products in good yields (entries 3‐6). Functionalities like cyano and iodo groups were well tolerant under the reaction conditions (entries 7‐8). Cyclic alkanes
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such as cyclopentane, ‐hexane, and –ocatane were all suitable substrates giving the olefination products in good to excellent yields (entries 9‐11). Furthermore, allylic nitrile 4nm could be easily obtained from the reaction of olefin 1n and acetonitrile in synthetically useful yield with the concurrent formation of N‐ olefination product 7j, originating from the reaction of 1n and Ala‐ HIR 3j (entry 12).
to form benzylic cation 13 and regenerate the catalytically View Articleactive Online DOI: 10.1039/C5OB00619H Re(n) species. It should be noted that 12 might also be oxidized by HIR 3j, which constitutes an alternative but less competent pathway particularly when the rhenium catalyst is absent in the reaction (Table 1, entry 11). Finally, deprotonation of 13 leads to the final formation of product 4. a) Trapping the C-radical formed via H-abstraction
Table 2 Scope of Substrates Bearing Various C(sp3)‐H Bondsa O
5 mol% Re2(CO) 10 Ala-HIR 3j
+
N O
H
N O
THF, 150 oC, 4 h
O
Ph TEMPO
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2b
entry
1
yield b (%)
entry
4nb: 88
7
4nc: 50
8
product 4
c
2
4nd: 62
3 d
4
product 4
5
4ni: 61
d
4nj: 91
4nk: 95
e
4ne: 64
10
4nf: 67
11
4ng: 66
12
e
d
4nh: 69
d
9
e
6
yield b (%)
f
8: 31%
(1n)
Ph
no 1n
8: 25%
b) Reaction of 1-phenylvinylcyclopropanewith 1,4-dioxane O 5 mol% Re2(CO) 10 O Ala-HIR 3j O H + H
1p
O
1,4-dioxane 120 oC, 4 h
O
O
+
34% (4oa:4oa' = 6.3:1)
2a radical addition
O
O
O
[O], - H
O
O
O rearrangement
cyclization H
Scheme 4 Mechanistic Experiments.
4nl: 71 4nm: 58
a Reaction conditions: 1n (1.0 mmol), 3j (4.0 mmol), Re2(CO)10 (5 mol%), 2 (0.03 M), 150 oC, 4 h. b Isolated yields of products 4. c Re2(CO)10 (2.5 mmol%), Et2O (0.05 M), 120 oC. d 2 (0.2 M). e 2 (0.1 M), 12 h. f 7j was isolated in 11% yield.
We assumed that the initial C(sp3)‐H activation occur through a radical H‐abstraction pathway. In order to verify this hypothesis, the radical‐trapping reagent TEMPO was subjected to the reaction conditions of olefin 1n and tetrahydrofuran 2b (Scheme 4, a). To our delight, the α‐THF radical trapped product 8 was isolated in 31% yield and the olefination reaction was completely prohibited, which confirmed our assumption. Also, compound 8 could be obtained in comparable yield in the absence of 1n. Next, the reaction of 1‐ phenylvinylcyclopropane 1p, susceptible to radical rearrangements, and 1,4‐dioxane 2a was carried out (Scheme 4, b). It turned out that the ring expansion product 4oa and its aromatized product 4oa' were formed in 34% yield. This result supported a reaction sequence of radical addition/rearrangement/cyclization/oxidative deprotonation.19 Based on the above observations, a tentative reaction mechanism was proposed in Scheme 5. Heterolytic cleavage of the I‐O bond in HIR 3j gives rise to the ionic species 9, which undergoes a single electron transfer process with the Re(n) catalyst affording N‐radical species 10. The H‐atom α to oxygen in 1,4‐dioxane is abstracted by 10 giving rise to the α‐C radical species 11 and 2‐ iodobenzamide 6j. The ensuing radical addition of 11 to olefin affords benzylic radical 12, which is oxidized by the Re(n+1) species
Scheme 5 A Tentative Reaction Mechanism.
In conclusion, the dehydrogenative olefination of C(sp3)‐H bonds was developed through the combination of rhenium‐ catalysis and an Alanine‐derived hypervalent iodine(III) reagents, which demonstrates a reactivity quite distinct from known combinations of transition metal catalysts and hypervalent iodine(III) reagents. Also, this reaction represents a rare example in organic synthesis where amino acid‐derived HIRs play a unique role since they were synthesized by Zhdankin et al.18 Thus, cheap and easily available feedstock such as cyclic and acyclic ethers, toluene derivatives, cycloalkanes, and nitriles could all be successfully alkenylated in a regio‐ and stereoselective way.20 Mechanistic studies revealed a radical‐initiated pathway operating in the reaction.
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Further exploitations on merging rhenium‐catalysis with hypervalent iodine(III) reagents in organic synthesis are underway in our laboratory. Financial support from the National Basic Research Program of China (973 Program) (No. 2011CB808600) and the National Natural Science Foundation of China (21322203, 21272238, 21472194) are gratefully acknowledged.
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