Leaving Group Assisted Strategy for Photoinduced Fluoroalkylations Using N-Hydroxybenzimidoyl Chloride Esters

Article abstract of DOI:10.1002/anie.201812192

Redox-active esters (RAEs) as alkyl radical precursors have been extensively developed for C?C bond formations. However, the analogous transformations of fluoroalkyl radicals from the corresponding acid or ester precursors remain challenging because of the high oxidation potential of the fluoroalkyl carboxylate anions. The newly developed N-hydroxybenzimidoylchloride (NHBC) ester provides a general leaving group assisted strategy to generate a portfolio of fluoroalkyl radicals, and can be successfully applied in photoinduced decarboxylative hydrofluoroalkylation and heteroarylation of unactivated olefins. In addition, DFT calculations revealed that the NHBC ester proceeds by the fluorocarbon radical pathway, whereas other well-known RAEs proceed by the nitrogen radical pathway.

Full text of DOI:10.1002/anie.201812192

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Accepted Article
Title: A General Leaving Group-Assisted Strategy for Photoinduced
Fluoroalkylations using N-Hydroxybenzimidoyl Chloride Esters
Authors: Weigang Zhang, Zhenlei Zou, Yuanheng Wang, Yi Wang,
Yong Liang, Zhengguang Wu, Youxuan Zheng, and Yi Pan
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201812192
Angew. Chem. 10.1002/ange.201812192
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10.1002/anie.201812192
Angewandte Chemie International Edition
COMMUNICATION
A General Leaving Group-Assisted Strategy for Photoinduced
Fluoroalkylations using N-Hydroxybenzimidoyl Chloride Esters
Weigang Zhang, # Zhenlei Zou, # Yuanheng Wang, Yi Wang,* Yong Liang,* Zhengguang Wu,* Youxuan
Zheng and Yi Pan
Abstract: Redox-active esters (RAEs) as alkyl radical precursors
have been extensively developed for C-C bond formations. However,
the analogous transformations of fluoroalkyl radicals from the
corresponding acid or ester precursors remain challenging due to
the high oxidation potential of the fluoroalkyl carboxylate anions. The
newly developed N-hydroxybenzimidoylchloride (NHBC) ester
provides a general leaving group-assisted strategy to generate a
portfolio of fluoroalkyl radicals, which can be successfully applied in
photoinduced
decarboxylative
hydrofluoroalkylation
and
heteroarylation of unactivated olefins. In addition, DFT calculations
revealed that the NHBC ester was in favor of fluorocarbon radical
pathway while other well-known RAEs were in preference of the
nitrogen radical pathway.
The wide application of fluorinated compounds in agrochemicals,
pharmaceuticals and materials science has triggered every
endeavor to develop efficient methods for selective incorporation
of a fluoroalkyl group into organic molecules.1 Despite progress
in the past decade, the existing trifluoromethylation reagents
require multistep syntheses and costly starting materials.2-5 The
well-documented redox-active esters (RAEs) as alkyl radical
precursors appear to be feasible strategy for C-C bond formation,
some of which taking the advantage of the weak N-O bond
cleavage and decarboxylative fragmentation. However, the
analogous transformations of fluoroalkyl radicals (•CF3, •CF2H,
•C2F5, etc.) generating from the corresponding acids or esters
have confronted inevitable difficulties6-10 due to the high
oxidation potential of the fluoroalkyl carboxylate anions (e.g.
Figure 1. Origin of the Reaction Design.
CF3CO2 E1/2 > +2.4V vs SCE) (Figure 1A).6 For the mere
reason, the versatile N-hydroxyphthalimide (NHPI) esters found
unable to engage in decarboxylative radical trifluoromethylation
-
ox
radical by manipulating the nature of the electron affinity of the
oxime.15 Indeed, by simply installing a chlorine to oxime, the
stabilized electron-rich imine in harness, for surpassing the
strong electron-withdrawing effect of CF3 group in the redox-
active ester, in favor of generating TFA radical and iminyl anion.
in
a
photocatalytic manner.11 As Sanford demonstrated,
increasing the electron withdrawing character of the alkyl
substituent on NHPI ester (CF3 in particular) resulted in the
Thus, the CF3-bearing N-hydroxybenzimidoyl chloride (NHBC)
- 12
formation of PhthN• and the release of CF3CO2 . Recently,
red
ester 1a (E1/2
-1.09V vs SCE) should be activated by
Stephenson and co-workers have taken an in-situ activation
strategy using pyridine N-oxide to activate volatile TFAA for
radical trifluoromethylations of arenes (Figure 1B).13 The
development of practical and air-stable redox-active
fluoroalkylating agents using inexpensive fluorine sources still
represents unmet challenge and urgent demand.
photocatalyst Ir(III)* to release benzonitrile, chloride, CO2 and
generate •CF3 via N-O bond breaking followed by
decarboxylative fragmentation (Figure 1B). Herein, we report a
general strategy for photoinduced fluoroalkylation of unactivated
olefins using bench-stable redox-active NHBC esters.16
Table 1.Optimizations of the Reaction Conditions.
Drawing inspiration from the work of Leonori and Studer on
photoredox single-electron reductions of oximes to provide
iminyl radicals14, we envisioned that the cleavage of weak N-O
bond (BDE = 54 kcal·mol-1) could also generate an oxygenated
R
BF3.Et2O (2.5 equiv)
p-Me-C6H4SH (2.5 equiv)
Ir[{dF(CF3)ppy}2(dtbbpy)]PF6 (10 mol%)
O
H
N
CF3
O
CF3
Ph
2a (0.2 mmol)
+
Ph
Cl
MeCN (2.0 mL), rt, Ar, blue LED
3a
1a
( R= H, 2 equiv)
Entry
Variation from the reaction conditions
Yield (%) [a]
1
2
3
4
5
6
7
8
9
none
79
Ir(ppy)3 instead
54
18
78
73
24
81[b]
nr
Ir[(dtbbpy)(ppy)2]PF6 insted
Weigang Zhang, Zhenglei Zou, Yuanheng Wang, Dr. Zhengguang
Wu, Prof. Yi Wang*, Prof. Yong Liang* and Prof. Yi Pan
State Key Laboratory of Coordination Chemistry, Jiangsu Key
Laboratory of Advanced Organic Materials, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023, China
*E-mail: yiwang@nju.edu.cn; yongliang@nju.edu.cn
1b
(R = Me) instead
1c (R = Cl) instead
1d
(R = NO2) instead
6 mol% Ir loading
Without photocatalyst
Without BF3.Et2O
56
#These authors contribute equally to this work.
[a] Crude yields determined by 19F NMR spectroscopy using
hexafluorobenzene as an internal standard. [b] 74% isolated yield.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201812192
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COMMUNICATION
Scheme 1. Substrate Scope of the Hydrotrifluoromethylation Reaction with Condition A: 0.2 mmol of 2, 2.0 equiv of 1 (1a isolated, 1j and 1k in situ), 2.5 equiv. of
BF3·Et2O, 2.5 equiv. of p-toluenethiol, 6 mmol% of Ir[{dF(CF3)ppy}2(dtbbpy)]PF6, 2 mL of CH3CN at room temperature. [a] The reaction was conducted for 36 h. [b]
The ratios of diastereomers were determined by 1H and 19F NMR.
To determine the suitable conditions for NHBC-initiated radical
fluoroalkylation, we first studied the hydrotrifluoromethylation3,4
fluoroalkylation could go smoothly in chemoselective fashion.
Various fluoroalkyl radicals including •CF2H, •CF3, •C2F5 and
•C3F7 were susceptible to the reaction conditions to acheive
corresponding ketones in good to excellent yields (Scheme 2).
The migration of substituted benzothiazoles (5y) and thiazoles
also took place (5n, 5o, 5u and 5w). The aryl group with
different electronic and steric characters were tolerated (5a−5j
and 5p−5r). Linear or cyclic alkyl substituted alcohols could also
furnish the difunctionalized products (5k−5m, 5s and 5x).
Increasing the steric hindrance around the olefin led to slightly
decreased yield (5t).
of
butenylbenzene
(2a)
in
the
presence
of
trifluoroacetoxyarylimidoyl chloride (1a-1d,
2
equiv), 2.5
equivalents each of BF3·OEt2 and p-toluenethiol with a survey of
iridium catalysts in acetonitrile at room temperature (Table 1).
Ir[{dF(CF3)ppy}2(dtbbpy)]PF6 (PC1) was able to promote the
radical fragmentation to afford the desired trifluoromethylated
product 3a in 79% yield. When using Ir(ppy)3 (PC2) and
Ir[(dtbbpy)(ppy)2]PF6 (PC3), low to moderate yields were
obtained (entries 2-3). When switching 1a to electron-donating
group substituted NHBC esters 1b and 1c, similar yields were
achieved (entries 4-5), while electron-withdrawing substitution
showed very poor result (entry 6). Decreasing the catalyst
loading to 6 mol% did not affect the reaction (entry 7). In the
absence of BF3•Et2O, the yield decreased to 56%, and a small
amount of chlorinated byproduct was observed (entry 8).17 Thus,
the optimized conditions were selected for further investigation
of the reaction scope.
Using 6 mol% of the iridium catalyst, trifluoroacetoxybenzimidoyl
chloride (2.0 equiv), and thiol (2.5 equiv) at ambient temperature,
a range of unactivated terminal and internal alkenes underwent
hydrotrifluoromethylation with good regioselectivity (Scheme1).
Terminal alkenes bearing amide (3b-3g) and ester (3h-3q)
functionalities were tolerated in this NHBC-initiated radical
process. Sulfonamide (3r), sulfonate (3s), and oxygenated alkyl
(3u-3y) substituted alkenes also furnished the corresponding
CF3 adducts. Notably, the unsaturated carboxylic acid (3t) which
was susceptible to decomposition in other RAE conditions could
be well-adopted in this catalytic system. In addition, alanine and
boldenone-derived terminal alkenes were compatible to afford
the trifluoromethylated products in high yields (3z and 3aa).
Internal alkenes also transformed into the desired products 3bb
Scheme 2. Substrate Scope of the Aryl Migration Reaction with Condition B
(diastereomeric
ratio
of
3:1)
and
3cc.
Moreover,
Furthermore, this fluorine-bearing RAE protocol was applied to
the fluoroalkylation of diallylic and alkynyl substrates (Scheme 3).
NHBC ester-derived •RF precursors were employed in
intramolecular cyclization processes with diallyl sulphonamides,
and diesters to afford the corresponding fluorinated products
(7a−7f) in good diastereomeric ratios (1.5:1 to 13:1).
perfluoroalkylated NHBC esters 1j and 1k were prepared and
reacted with unactivated olefins in the same manner to afford
the corresponding products 3dd and 3ee in high yields.
With slight variation of the optimized conditions, we expanded
the scope of this NHBC strategy to difunctionalization of
unactivated olefins.18 Using heteroaryl-substituted unsaturated
tertiary alcohol, the distal migration induced by radical
Phenylpropiolate
8 was also submitted to the standard
conditions to furnish chromenones 9a and 9b.19
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10.1002/anie.201812192
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COMMUNICATION
In summary, we have developed a leaving group assisted
strategy for generating a portfolio of fluoroalkyl radicals from
redox-active N-hydroxybenzimidoyl chloride esters, which has
been
successfully
employed
in
the
photoinduced
hydrofluorination and arylation of unactivated alkenes.
Compared with the commonly used fluorinating reagents, the
fluorine-bearing NHBC esters are air-stable non-volatile
crystalline solid, readily achievable in large quantities from their
fluorocarboxylic acid, and easy to initiate under mild
photocatalytic conditions. Further study of NHBC-derived RAEs
is underway in our laboratory.
Acknowledgements
Professor Fengling Qing and Jinbo Hu (SIOC) are gratefully
acknowledged for helpful discussion. This work was supported
by the National Natural Science Foundation of China (No.
21472082, No. 21402088and No. 21772085), the Fundamental
Research Funds for the Central Universities (No.
020514380148), the National Thousand Young Talents Program,
the Jiangsu Specially-Appointed Professor Plan, and the NSF of
Jiangsu Province (BK20170631) in China.
Scheme 3. Further Elaborations.
To gain insight into this fluoroalkyl radical formation, we
conducted computational studies using DFT calculations (M06-
2X with solvation).20-22 We compared the thermodynamics for the
generation of nitrogen radical or oxygen radical. As shown in
Figure 2A, when Sanford’s CF3-NHPI reagent gets an electron,
the formation of PhthN radical and TFA anion is favored by 14.0
kcal mol-1 as compared to the formation of TFA radical and
PhthN anion. This is consistent with their experiments.12 In the
case of our CF3-bearing NHBC ester, the radical formation
selectivity is reversed (Figure 2B). Calculations indicate that the
chloro-imine anion is not a minimum in the potential energy
surface, which decomposes into benzonitrile and chloride
spontaneously. Since chloride is a good leaving group, this
process is highly exergonic, making the formation of TFA radical
1.2 kcal mol-1 lower in Gibbs free energy than that for TFA anion.
Therefore, CF3 radical would be generated predominantly from
the decarboxylation of TFA radical, while a small amount of
chlorine radical would be formed from the decomposition of
chloro-imine radical. This accounts for our experimental results
and confirms our hypothesis on the leaving group assisting
effect. Similar trends are observed for perflluoroalkyl radicals
C2F5• and C3F7•, which are favored by 0.3 and 1.0 kcal mol-1 in
Gibbs free energy respectively. In the case of diflluoromethyl
radical, a 7.3 kcal mol-1 Gibbs free energy difference was
expected, indicating that the generation of •CF2H is extremely
favorable for its NHBC ester.
Keywords: Fluoroalkylation • photoredox • olefins • imidoyl
chloride
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Entry for the Table of Contents
COMMUNICATION
Weigang Zhang, Zhenlei Zou, Yuanheng
Wang
,
Yi Wang,* Yong Liang,*
Zhengguang Wu,* Youxuan Zheng and
Yi Pan
Page No. – Page No.
A General Leaving Group-Assisted
The newly developed N-hydroxybenzimidoyl chloride (NHBC) ester provides a
general leaving group-assisted strategy to generate a portfolio of fluoroalkyl
radicals, which can be successfully applied in photoinduced decarboxylative
hydrofluoroalkylation and heteroarylation of unactivated olefins.
Strategy
Fluoroalkylations
Hydroxybenzimidoyl Chloride Esters
for
Photoinduced
using N-
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