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Chemical Science
Volume 7 Number 1 January 2016 Pages 1–812
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DOI: 10.1039/C8SC01882K
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(Salen)Mn(III)-catalyzed chemoselective acylazidation of olefins Liang Zhang, Shuya Liu, Zhiguo Zhao, Hongmei Su, Jingcheng Hao, and Yao Wang* Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
We describe a (salen)Mn(III)-catalyzed three-component reaction of aldehydes, olefins, and sodium azide for the installation of two useful groups (C=O and N3) into the double bond. Traditionally, (salen)Mn(III) in conjunction with iodosobenzene is a classical catalysis system for epoxidation of olefins. Owing to the highly competitive oxygenation approaches, it is a true challenge to establish a distinct strategy for the exploration of new olefin transformations based on this (salen)Mn(III) catalysis system. Herein, the key to this (salen)Mn(III)-catalyzed acylazidation of olefins was the rational application of different reactivity of oxomanganese(V) species which is capable of abstracting a hydrogen atom from a substrate C-H bond. This chemoselective reaction occurred in a precisely designed reaction sequence and tolerates complex molecular structures.
Introduction Catalytic transformation of olefins is one of the fundamental approaches for the generation of molecular complexity and diversity. A range of elegant and powerful strategies have been established for the installation of a diverse array of useful functional groups into the double bonds, which essentially 1 advanced the art and practical use of organic synthesis. Amongst various catalysts for olefin transformations, manganese Schiff-base complexes have been well-established as powerful catalysts for epoxidation of olefins in the last three 2-5 decades . It has been a general notion that the in situ generated oxomanganese(V) species through oxidation of (salen)Mn(III) complex is an efficient oxygen-transfer species 6-7 for epoxidation of various olefins . Owing to the fast and 5-7 strong oxygenative background reactions , it remains a substantial challenge to develop a distinct strategy from a fresh vision that enables the discovery of previously unknown olefin transformation based on this oxidative (salen)Mn(III) catalysis system. (Figure 1A). The reactive oxomanganese(V) species can readily abstract a hydrogen atom from even inert C-H bonds such as alkanes to generate a substrate-derived radical and a hydroxomanganese(IV) intermediate under mild reaction 8-9 conditions . This distinct reactivity of oxomanganese(V) species provides a basis point and opens up new opportunities for the development of a useful strategy enabling transformation of olefins from a fresh perspective. As outlined
School of Chemistry and Chemical Engineering & Key Laboratory of the Colloid and Interface Chemistry, Shandong University 27 Shanda Nanlu, Jinan 250100, Shandong, China E-mail:
[email protected]. Electronic Supplementary Information (ESI) available. See DOI: 10.1039/x0xx00000x
A. (salen)Mn(III)-catalyzed transformation of olefins.
(salen)Mn(III) Oxidant (PhIO, etc.)
R1 O R2
R3 classical approach
R1
R3
competitive reactants (salen)Mn(III) RH + R'M Oxidant (PhIO, etc.)
R2
unknown transformations
R' R2 R1
R
R' R2 R1
R
R3
B. New str ategy: rational application of different reactivity of oxomanganese(V) species R3
R1
R1 O Mn (Mn(III)+PhIO) selective C-H abstraction
R2
+ R-H + R'M
epoxidation 1
R R2
O
R + OH Mn
R
3
selective radical addition
radical rebound R-OH
R3
R2
R2 R1
R R3
R' R'M + OH Mn Mn selective radical rebound
OH OH rebound Mn HO R2 R1
R R3
R3
R' rebound
suppressed competitive outcomes
Figure 1 (Salen)Mn(III)-catalyzed competitive transformation of olefins. in Figure 1B, we envisioned that the addition of a carefully selected reactant bearing a weak C-H bond to the oxidative (salen)Mn(III) catalysis system would lead to a preferential C-H abstraction, thus shutting down the highly competitive epoxidation pathway. Considering the fast radical rebound 10-11 pathway , it is essential that the in situ generated radical must be capable of immediately reacting with the ‘spectator’ olefin to start the precisely designed reaction sequence towards the desirable outcome. Finally, a selective radical rebound reaction would finish the whole reaction sequence. A standing challenge for implementing the designed strategy has been the suppression of these consecutively competitive 12-13 approaches .
J. Name., 2013, 00, 1-3 | 1
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DOI: 10.1039/C8SC01882K
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In contrast to alkanes, aldehydes have weaker (O=)C-H 14bond which can be feasibly cleaved to generate acyl radicals 15 . Hydroacylation of olefins using aldehydes as an acyl source represents a simple and efficient method for the preparation 15-18 of ketones . Under substantially different reaction pathways, aldehydes have been applied in acylation of highly 19-22 23-28 electron-deficient olefins or unactivated olefins . Despite the important achievements in radical acylation of olefins, there are considerable limitations that remain to be 19-22 resolved with regard to scope of aldehydes and olefins . At room temperature, acyl radicals are generally reactive towards addition of highly electron-deficient olefins while they show very poor or no reactivity towards less electron-deficient olefins. Aldehydes were generally restricted to specific class to avoid the decarbonylation problem and to enable the desirable acyl radical addition of olefins to take place. Furthermore, for an intermolecular hydroacylation approach, the installation of a functional group instead of a hydrogen atom through trapping of the acylative intermediate remains a largely elusive problem. We envisioned that the rationally designed strategy could provide a promising solution to these synthetic problems. Herein, we present our findings on the establishment and application of this strategy in the development of a chemoselective acylazidation of olefins.
Results and discussion Extensive investigation on numerous possible combinations of competitive reactants was carried out. Encouragingly, we were able to identify that the addition of a combination of aldehyde and sodium azide to the (salen)Mn(III) catalysis system could suppress oxygenation approaches, resulting in the chemoselective formation of an acylazidation product. Organic azides can participate in a range of important reactions 29 30 including Staudinger ligation , 1,3-dipolar cycloaddition , and 31 the aza-Wittig reaction , making them highly attractive 32 targets for organic synthesis . A range of elegant methods 33 have been developed for azidation of olefins and C-H 34 azidation . To optimize the reaction condition, styrene was used as a model substrate while n-butylaldehyde and sodium azide were employed as a combination of competitors. As shown in Table 1, initially, several control experiments were conducted. No product was observed in the absence of either (salen)Mn(III) catalyst or PhIO (entries 1-2). In the absence of n-butylaldehyde and sodium azide, as previously reported by 12 Kochi and co-workers , a fast consumption of styrene was observed (95% conv. of styrene, 30% epoxide, entry 3). Acylazidation product was formed in 57% yield catalyzed by C1 in CH3CN (entry 4). Solvent optimization showed that a mixed solvent of EtOAc and H2O is an optimal choice, which afforded the desired product in 75% yield (entries 5-9). The evaluation of different (salen)Mn(III) complexes revealed that catalyst C1 is a promising candidate (entries 10-14). Upon using 1 mol% of catalyst C1, the reaction yield was decreased (entry 15). Further optimization of the amount of aldehyde and iodosobenzene did not improve the reaction yield (entries 1618). The optimized reaction condition did not provide an observable quantity of epoxidation product. In contrast, 41%
Table 1 Optimization of reaction condition
Ph 1a
entry
+ n-PrCHO + NaN 3 2a
cat.
a
(salen)Mn(III) C (2 mol%) PhIO (3.0 equiv) solvent, rt, N 2
solvent
time (h)
N3 Ph
O
O n-Pr
3a
epoxide (%)b
+ Ph
yield b (3a %)
1 − CH3CN 12 n.r. n.r. 2c C1 CH3CN 12 n.r. n.r. d 3 C1 CH3CN
