Synthesis of ethoxy dibenzooxaphosphorin oxides through palladium-catalyzed C(sp2)-H activation/C-O formation

Article abstract of DOI:10.3762/bjoc.10.120

We report an efficient Pd-catalyzed C(sp²)-H activation/C-O bond formation for the synthesis of ethoxy dibenzooxaphosphorin oxides from 2-(aryl)arylphosphonic acid monoethyl esters under aerobic conditions.

Full text of DOI:10.3762/bjoc.10.120

Synthesis of ethoxy dibenzooxaphosphorin oxides
through palladium-catalyzed
C(sp2)–H activation/C–O formation
Seohyun Shin‡1,2, Dongjin Kang‡1,2, Woo Hyung Jeon1,2
and Phil Ho Lee*1,2
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1220–1227.
1Center for Catalytic Organic Reactions, National Creative Research
Laboratory (NCRL), Chuncheon 200-701, Republic of Korea and
2Department of Chemistry, Kangwon National University, Chuncheon
200-701, Republic of Korea
doi:10.3762/bjoc.10.120
Received: 08 February 2014
Accepted: 09 April 2014
Published: 23 May 2014
Email:
Phil Ho Lee* - phlee@kangwon.ac.kr
This article is part of the Thematic Series "Organophosphorus chemistry"
and is dedicated to Professor Bong Rae Cho (Korea University) on the
occasion of his honorable retirement.
* Corresponding author ‡ Equal contributors
Keywords:
Guest Editor: P. R. Hanson
C–H activation; catalysis; cyclization; palladium; phosphorus
heterocyclic compound
© 2014 Shin et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We report an efficient Pd-catalyzed C(sp2)–H activation/C–O bond formation for the synthesis of ethoxy dibenzooxaphosphorin
oxides from 2-(aryl)arylphosphonic acid monoethyl esters under aerobic conditions.
Introduction
Unreactive C(sp2)–H and C(sp3)–H bonds are ubiquitous in C–H activation/C–N formation has been widely used for the
organic compounds [1-7], so that the development of methods synthesis of azaheterocycles, the preparation of oxaheterocy-
for the transition metal-catalyzed C–H activation is one of the cles via C–H activation/C–O formation has been described a lot
challenging goals in organic synthesis. Especially, the develop- less, because the energy correlation between the HOMO of the
ment of synthetic methods of C–heteroatom bond formation via Pd–O bond and the LUMO of the Pd–C bond is unfavorable
C–H activation has received attention owing to the omnipres- and the Pd–O bond has a significantly ionic character [18-23].
ence of heterocyclic compounds in nature [8]. Recently, it has To expand this scope, we are interested in the development of
been demonstrated that the intramolecular bond formation C–H activation/C–O formation by means of new directing
between a heteroatom and a vicinal unreactive C–H is an effi- groups. Recently, a variety of C–H activations by using new
cient method for the synthesis of heterocycles [9-17]. Although phosphoryl-related directing groups have been reported by our
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[24-32] and other groups [33-41]. More recently, we developed the Pd(II)/Pd(IV) catalytic cycle, gave 2a in 30% yield in
a method allowing for synthetic access to benzoxaphosphole 1- t-butanol (80 °C for 16 h; Table 1, entry 1) [19,43-47]. In addi-
and 2-oxides starting from phosphonic and phosphinic acids via tion, various bases were examined. Although NaOAc, CsOAc,
Pd-catalyzed C(sp2 and sp3)–H activation/C–O formation [42]. CsF and CsOPiv afforded 2a in yields ranging from 42% to
In this context, we herein report the synthetic method of alkoxy 52%, KOAc gave the best result (57%) in the presence of
dibenzooxaphosphorin oxides from 2-(aryl)arylphosphonic acid PhI(OAc)2 in tert-butanol (see Supporting Information File 1).
monoesters via Pd-catalyzed C(sp2)–H activation/C–O forma- tert-Butanol gave the best result among the solvents DCE,
tion (Scheme 1).
dioxane, ACN, t-AmOH, DMF, HFIP, THF, toluene, TFA and
MeOH (see Supporting Information File 1). With this prelimi-
nary result in hand, we investigated a variety of organic acids as
ligands in an effort to improve the catalytic efficiency
(Scheme 3). However, these attempts provided no improve-
ment (Table 1, entries 2–4). Finally, we discovered that easily
accessible monoprotected amino acids, which have recently
been established as efficient ligands in C–H activations [48-50],
increased the yield (Table 1, entries 5–10). Among the investi-
gated ligands, N-acetyl-L-leucine (L9) gave the best results
(Table 1, entry 10). After examination of the reaction tempera-
ture (Table 1, entries 11–13) and time (Table 1, entries 14–16),
the oxidative cyclization using PhI(OAc)2 (2 equiv) and KOAc
(2 equiv) in the presence of Pd(OAc)2 (10 mol %) and L9
Scheme 1: Synthesis of alkoxy dibenzooxaphosphorin oxides by
C(sp2)–H activation/C–O formation.
Results and Discussion
First, a wide range of 2-(aryl)arylphosphonic acid monoethyl (30 mol %) gave the best result under aerobic conditions,
esters were efficiently prepared by a Suzuki reaction of affording 2a in 61% yield (isolated yield 55%, Table 1, entry
2-bromoiodoarenes with arylboronic acids, a lithium bromide 16). Both Pd(TFA)2 and Pd(OTf)2∙H2O gave inferior results
exchange reaction of 2-bromobiaryls followed by diethylphos- compared to Pd(OAc)2 (Table 1, entries 17 and 18).
phinylation with diethyl chlorophosphate, and the C–O cleavage
of diethyl 2-(aryl)arylphosphonates by using L-Selectride To ascertain the scope of the Pd-catalyzed C–H activation
(Scheme 2).
followed by the C–O formation, a wide range of
2-(aryl)phenylphosphonic acid monoethyl esters 1 were exam-
The C–H activation/C–O formation of 2-(phenyl)phenylphos- ined under the optimized reaction conditions (Scheme 4).
phonic acid monoethyl ester (1a) was examined with a variety Phenylphosphonic acid monoethyl ester 1b with a 2-methyl
of oxidants and bases in the presence of Pd(OAc)2. A multitude group on the phenyl ring was transformed to the desired diben-
of oxidants such as K2S2O8, BQ, benzoyl peroxide, PhI(TFA)2, zooxaphosphorin oxide 2b in 53% yield. Phenylphosphonic
Cu(OAc)2, CuCl2, CuBr, AgOAc, Ag2CO3 and Ag2O did not acid monoethyl esters (1c) with a 3-methyl group were selec-
produce the cyclized product 2a (see Supporting Information tively converted to the cyclized products (2c) in 66% yield due
File 1). However, PhI(OAc)2, which is an efficient oxidant for to steric effects. In the case of 4-tert-butyl, the desired product
Scheme 2: Preparation of 2-(aryl)arylphosphonic acid monoethyl esters.
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Table 1: Optimization studies for the cyclization of 2-(phenyl)phenylphosphonic acid monoethyl esters.
entry
cat. Pd
ligand
T [°C]
t [h]
yielda [%]
1
2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(OAc)2
10 mol % Pd(TFA)2
10 mol % Pd(OTf)2·H2O
—
80
80
16
16
16
16
16
16
16
16
16
16
16
16
16
4
30
23
30 mol % L1
30 mol % L2
30 mol % L3
30 mol % L4
30 mol % L5
30 mol % L6
30 mol % L7
30 mol % L8
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
30 mol % L9
3
80
34
4
80
28
5
80
48
6
80
48
7
80
54
8
80
53
9
80
51
10
11
12
13
14
15
16
17
18
80
57
60
20
100
120
100
100
100
100
100
61
50
45
8
51
12
12
12
61(55)
53
45
aYields were determined by 1H NMR with CH2Br2 as an internal standard. The number in parentheses is the isolated yield.
Scheme 3: A variety of organic acids and monoprotected amino acids as ligands.
2e was obtained in 65% yield. Substrate 1f, characterized by an 4-phenyl group on the phenyl ring turned out to be compatible
electron-donating 4-methoxy group, was cyclized to diben- with the reaction conditions. As anticipated, 2-naphthyl-substi-
zooxaphosphorin oxide 2f in 65% yield under aerobic condi- tuted phenylphosphonic acid monoethyl ester 1i underwent the
tions. The present method worked equally well with 3,4- Pd-catalyzed oxidative cyclization regioselectively at the steri-
dimethoxyphenyl-substituted phenylphosphonic acid monoethyl cally less hindered position to afford the desired diben-
ester 1g. Phenylphosphonic acid monoethyl ester 1h with a zooxaphosphorin oxide 2i in 70% yield. We were pleased to
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Scheme 4: Cyclization of 2-arylphenylphosphonic acid monoethyl esters.
obtain 2j by a Pd-catalyzed oxidative cyclization of 1-naphthyl- 4-Methylphenylphosphonic acid monoethyl esters 3a and 3b
substituted phenylphosphonic acid monoethyl ester 1j. with a 3-methyl- and 3,4-dimethoxyphenyl group at 2-position
2-(Aryl)phenylphosphonic acid monoethyl esters 1k, 1l and 1m turned out to be compatible with the Pd-catalyzed oxidative
with an electron-withdrawing fluoro or chloro group on the cyclization. There are no regioisomers formed due to steric
phenyl ring were subjected to the oxidative cyclization to effects. Substrate 3c bearing a chloro group was selectively
deliver the desired products 2k, 2l and 2m in yields ranging cyclized to afford 4c in 64% yield. To our delight, the present
from 54% and 64%. In particular, the tolerance of the chloro method worked equally well even if a fluoro group on the
groups may be of importance for a subsequent catalytic cross- phenyl ring is present. 3-Fluorophenylphosphonic acid
coupling reaction. Substrate 1n, which contains a 2-thiophenyl monoethyl esters 3d, 3e and 3f with 3-methyl-, 3,4-dimethoxy
moiety, was subjected to the cyclization affording 2n in 52% and 3-chlorophenyl groups at the 2-position selectively under-
yield. The preparation of 2-arylphenylphosphonic acid went the oxidative cyclization to give the corresponding
monoethyl esters with a nitro, difluoro, or ethoxycarbonyl group cyclized products 4d, 4e and 4f in yields ranging from 50% and
failed.
63%.
Next, the Pd-catalyzed oxidative cyclization of 2-(aryl)aryl- We carried out kinetic isotope effect (KIE) studies to prove the
phosphonic acid monoethyl esters 3 were examined to demon- reaction mechanism (see Scheme 8). The required deuterium-
strate the efficiency of the present method (Scheme 5). labeled 2-(phenyl)phenylphosphonic acid monoethyl ester
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Scheme 5: Cyclization of 2-(aryl)arylphosphonic acid monoethyl esters.
1a-[D5] was efficiently prepared by a Suzuki reaction of deuter- exchange reaction of 2‘-bromo-2-iodo-1,1‘-biphenyl (10) and
ated bromobenzene (6) with 2-bromophenylboronic acid (5), a the treatment of D2O, diethylphosphinylation with diethyl
lithium bromide exchange reaction of 2-bromo deuterated chlorophosphate, and C–O cleavage of diethyl 2-(phenyl)-
biphenyl 7 followed by diethylphosphinylation with diethyl phenylphosphonate by using L-Selectride (Scheme 7).
chlorophosphate, and C–O cleavage of diethyl 2-(phenyl)-
phenylphosphonate by using L-Selectride (Scheme 6). In addi- In the case of an intermolecular competition reaction using 1a
tion, the deuterium-labeled 2-(phenyl)phenylphosphonic acid and 1a-[D5], a KIE was detected (kH/kD = 1.0; Scheme 8, reac-
monoethyl ester 1a-[D1] was obtained by the lithium bromide tion 1) [51,52]. Also, an intramolecular competition reaction
Scheme 6: Preparation of 1a-[D5].
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Scheme 7: Preparation of 1a-[D1].
Scheme 8: Studies with isotopically labelled compounds.
using 1a-[D1] was carried out to give KIE (kH/kD = 0.6; Scheme 8), we hypothesize that the C–O reductive elimination
Scheme 8, reaction 2). These results indicate that the C–H step is the rate-determining step. A feasible mechanism
cleavage at the ortho-position of 2-(phenyl)phenylphosphonic involving the Pd(II)/Pd(IV) catalytic cycle is described in
acid monoethyl ester is not involved in the rate-limiting step Scheme 9. The C–H activation might be efficiently accelerated
and the C–H bond metallation is reversible.
by the N–H activation propelled by N-Ac-L-Leu-OH (L9) as a
ligand [53-55], resulting in the formation of palladacycle III.
To elucidate the mechanism of the present reaction, the reac- Thereafter, ethoxy dibenzooxaphosphorin oxide 2a is obtained
tion was conducted with a stoichiometric amount of Pd(OAc)2 from the oxidation of the Pd(II) to Pd(IV) species IV and the
and without the oxidant PhI(OAc)2. However, no cyclized prod- subsequent C–O reductive elimination.
uct was observed. This result indicates that the C–O reductive
elimination from Pd(II) is not favorable. Because both the inter- Conclusion
molecular and intramolecular competition experiments exhib- In this paper, we have developed an efficient synthetic method
ited no significant kinetic isotope effect (kH/kD = 1.0 and 0.6; for a wide range of ethoxy dibenzooxaphosphorin oxides
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Scheme 9: A plausible mechanism.
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