β elimination of a phosphonate group from an alkoxyl radical - Intramolecular acylation using acylphosphonate derivatives as carbonyl group acceptors

Article abstract of DOI:10.1139/v05-099

The possibility of β elimination of a phosphonate group in radical reactions was studied. The facile β elimination of the phosphonate group from an alkoxyl radical was observed for the first time, whereas the β elimination of the phosphonate group from an

Full text of DOI:10.1139/v05-099

917
␤ Elimination of a phosphonate group from an
alkoxyl radical — Intramolecular acylation using
acylphosphonate derivatives as carbonyl group
acceptors1
Chang Ho Cho and Sunggak Kim
Abstract: The possibility of β elimination of a phosphonate group in radical reactions was studied. The facile β elimi-
nation of the phosphonate group from an alkoxyl radical was observed for the first time, whereas the β elimination of
the phosphonate group from an aminyl and an alkyl radical did not occur. On the basis of our findings, the use of an
acylphosphonate as a carbonyl group radical acceptor was investigated. Radical cyclization of the acylphosphonate in
the presence of hexamethylditin in benzene at 300 nm for 2 h gave a cyclopentanone or a cyclohexanone derivative in
good yield without the formation of a direct reduction product. The reaction can be carried out in the presence of a
catalytic amount of hexamethylditin (0.2 equiv.) under similar conditions. In addition, an alkyl phosphonothiolformate
group can act as an alkylthiocarbonyl group equivalent radical acceptor, providing ready access to a thiolactone synthe-
sis.
Key words: radical, β elimination, acylation, cyclization, acylphosphonate.
Résumé : On a étudié la possibilité d’une élimination β d’un groupe phosphonate au cours de réactions radicalaires.
On a observé pour la première fois une élimination β facile d’un groupe phosphonate à partir d’un radical alkoxyle
alors qu’il ne se produit pas d’élimination β d’un groupe phosphonate à partir d’un radical aminyle ou d’un radical al-
kyle. Sur la base de ces observations, on a étudié l’utilisation d’un acylphosphonate comme groupe carbonyle accepteur
de radical. La cyclisation radicalaire de l’acylphosphonate, en présence d’hexaméthyldiétain, dans le benzène, à
300 nm, pendant deux heures, conduit à la formation d’une cyclopentanone ou d’une cyclohexanone, avec un bon ren-
dement, sans formation d’un produit de réduction directe. Il est possible d’effectuer la réaction dans des conditions
semblables, en présence d’une quantité catalytique d’hexaméthyldiétain (0,2 équiv.). De plus, un groupe phosphono-
thiolformiate d’alkyle peut agir comme groupe alkylthiocarbonyle équivalent à l’accepteur radicalaire et fournir ainsi un
accès facile à une synthèse de thiolactone.
Mots clés : radical, β élimination, acylation, cyclisation, acylphosphonate.
[Traduit par la Rédaction] Cho and Kim 921
Introduction
difficult owing to the high π-bond strengths of C=O bonds
(2, 3). Since β fragmentations of alkoxyl radicals are much
faster than the additions of alkyl radicals to carbonyl groups,
it is anticipated that carbonyl group derivatives cannot be
used effectively as radical acceptors to achieve radical-
mediated acylations. Thus, a limited number of carbonyl
group radical acceptors has been utilized in intramolecular
acylations and includes acyl sulfides, acyl selenides (4), and
acylgermanes (5, 6). It is noteworthy that a mixed car-
boxylic phosphonic anhydride is a good radical acceptor in
radical cyclizations but fails to function as the carbonyl
group acceptor because the β fragmentation of the alkoxyl
radical cleaves the carbon–carbon bond rather than the car-
bon–oxygen bond, as shown in Scheme 1 (7). In addition,
several indirect approaches using a nitrile (8) and an oxime
ether group have been developed (9, 10). In the case of
intermolecular acylation, the use of carbonyl group deriva-
tives as acceptors is uncommon and only several intermole-
cular radical carboxylations have been reported (11). We
developed highly efficient indirect acylation approaches us-
ing sulfonyl oxime ethers (10). This indirect approach is at-
The carbonyl group is one of the central functional groups
in organic chemistry and can be readily prepared by treat-
ment of carboxylic acid derivatives with organometallic
compounds (1). However, the radical version of this conven-
tional acylation reaction has not been successful to date be-
cause additions of alkyl radicals to C=O bonds are extremely
Received 20 December 2004. Published on the NRC Research
Press Web site at http://canjchem.nrc.ca on 27 July 2005.
Dedicated to Professor Howard Alper in recognition of his
outstanding contribution to organic chemistry.
C.H. Cho and S. Kim.2 Center for Molecular Design and
Synthesis and Department of Chemistry, School of Molecular
Science (BK21), Korea Advanced Institute of Science and
Technology, Daejeon 305-701, Korea.
1This article is part of a Special Issue dedicated to Professor
Howard Alper.
2Corresponding author (e-mail: skim@kaist.ac.kr).
Can. J. Chem. 83: 917–921 (2005)
doi: 10.1139/V05-099
© 2005 NRC Canada

918
Can. J. Chem. Vol. 83, 2005
Scheme 2. β Elimination of a diethyl phosphonate group.
Scheme 1. Intramolecular acylation approach.
1
R
SnBu
3
HN
N
N
Bu SnH/AIBN
3
3
O
X
X = OP(O)Ph
2
X
X
O
R
PO(OEt)
2
R
PO(OEt)
2
R
PO(OEt)
2
O
1
3a: R =Bu Sn
3
2
1
X = SPh, SePh, GePh
3
1
R = TBDPSO(CH )
3b: R =H (72%)
2 3
O
O
O
O
PPh
O
PPh
2
SMe
2
PPh
PPh
+
O
2
O
S
2
Bu SnH/AIBN
3
O
O
O
O
R
S
R
N
SMe
R
N
R
tractive since the carbonyl group can be easily generated by
oxidative cleavage or by hydrolysis.
Me
Me
4: R = PhCH
6 (35%)
7 (39%)
5
2
In connection with our continued interest on radical
acylation, we have searched for other efficient carbonyl
group radical acceptors and investigated the possibility of
using a phosphonate as a leaving group in radical reactions
(12).
Me
N
SMe
_
O
O
PO(OEt)
O
C
S
2
)
V-40 (0.3 equiv.
H
R
R
R
PO(OEt)
2
PO(OEt)
2
Results and discussion
8a: R= PhO(CH )
10a (89%)
10b (90%)
9
2 4
8b: R= TBDPSO(CH )
The β elimination of organophosphorous groups has not
been well-studied and several reports have appeared only in
recent years. The β elimination of a phosphinate group was
observed during a rearomatization process (13). The β elimi-
nation of a diphenylphosphinoyl group has been known in
radical allylation (14) and in tandem radical reactions in-
volving cyclization and the subsequent β elimination of the
diphenylphosphinoyl group (15).
2 3
Scheme 3. Radical cyclization of an acylphosphonate.
EtO C CO Et
O
2
2
EtO C CO Et
EtO C CO Et
2
2
2
2
_
(Me Sn)
P(OEt)
2
3
2
hν
C PO(OEt)
O
O
2
I
O
P(OEt)
2
11
12(91%)
The ease of β elimination generally depends on the nature
of the π bond formed and the strength of the σ bond broken.
Thus, it is expected that the β elimination would be easier
when the π bond formed is stronger (16). Based on this ra-
tionale, we initially studied the β elimination of a diethyl
phosphonate group from an aminyl radical (Scheme 2).
When azidophosphonate 1 was treated with Bu3SnH–AIBN
in refluxing benzene for 2 h (17), 3b was isolated in 72%
yield, clearly indicating that the β elimination of the phos-
phonate group from the aminyl radical in radical intermedi-
ate 2 did not occur, probably because of the relatively weak
π-bond strength of C=N bonds. Thus, our attention was
given next to the formation of a carbonyl bond as a result of
the β elimination of the phosphonate group, and we studied
the possibility of β elimination of the diethyl phosphonate
group from the alkyl radical using thiohydroxamate ester 4
(18). Radical reaction of 4 with Bu3SnH–AIBN in refluxing
benzene for 4 h afforded a mixture of 6 and 7 roughly in an
equal ratio without the β elimination of the phosphonate
group. Apparently, the phosphorous–oxygen bond in radical
intermediate 5 is strong and did not undergo cleavage. Thus,
5 reacted with Bu3SnH and the starting thiohydroxamate es-
ter 4 to afford a mixture of 6 and 7, respectively. Finally, the
β elimination of a phosphonate group from the alkoxyl radi-
cal was examined. As shown in Scheme 2, when 8a was
treated with V-40 (1,1′-azobis(cyclohexane-1-carbonitrile))
as initiator in chlorobenzene for 3 h (19), aldehyde 10a was
isolated in 89% yield. A similar result was obtained with 8b.
Evidently, the β elimination is energetically favorable be-
cause of the formation of a strong carbonyl bond along with
cleavage of a relatively weak carbon–phosphorous bond.
O
Based on our findings, we studied the feasibility of using
an acylphosphonate as a carbonyl group radical acceptor in
intramolecular radical acylations (Scheme 3) (12). Acyl and
bis(acyl)phosphine oxides have been employed as photo-
chemical sources of acyl radicals for use as initiators in
polymerization reactions (20). Also, acylphosphonates were
used as acylating agents of alcohols, amines, and enolates
(21). Acyldiethylphosphonates were conveniently prepared
by treatment of acid chlorides with triethyl phosphite in di-
chloromethane at room temperature and were quite stable to
silica gel column chromatographic purification (22). Radical
cyclization of acylphosphonate 11 in the presence of hex-
amethylditin at 300 nm (1.1 equiv.) for 2 h in benzene gave
cyclopentanone 12 in 91% yield without the formation of the
direct reduction product. The reaction can be carried out in
the presence of a catalytic amount of hexamethylditin
(0.2 equiv.) to afford 12 in 88% yield under similar condi-
tions because the initially generated phosphonate radical re-
acts with an alkyl iodide to generate an alkyl radical. Thus,
the remaining reactions were carried out in benzene using
0.2 equiv. of hexamethylditin at 300 nm for 3 to 4 h.
Table 1 summarizes the experimental results and illus-
trates the efficiency of the acylphosphonates as the carbonyl
group radical acceptor. The reaction was normally clean,
yielding the corresponding cyclopentanones and cyclohex-
anones in high yield. For most of the cases observed in this
study, there was no indication of the presence of the direct
reduction product and other side products. Tandem radical
© 2005 NRC Canada

Cho and Kim
919
Table 1. Radical cyclization of acylphosphonates.a
Scheme 4. Comparative studies of an acyl sulfide, an
acylgermane, and an acylphosphonate.
Yield (%)b
Entry
Product
Substrate
E
E
E
(Me3Sn)2
COX
X = PO(OEt)2
COX
EtO C
2
CO Et
2
EtO C
2
CO Et
2
hν
I
O
c
1
18a: X = PO(OEt)2
18b: X = SPh
19
X = GePh3
71 (82)
20 (75%)
COPO(OEt)
Ph
I
2
O
18c: X = GePh3
X = SPh
E = CO2Et
MeO C
2
E
Ph
MeO C
2
61
COPO(OEt)
2
2
20 (65%) + 21 (15%)
O
O
21 (92%)
I
Ph
Ph
COPO(OEt)
Scheme 5. Radical addition and cyclization of an acylphosphonate.
3
4
5
72
69
2
E
E
E
E
E
E
I
O
X
-PO(OEt)
2
CO Et
2
CO Et
2
X
C PO(OEt)
O
COPO(OEt)
2
2
X
COPO(OEt)
O
2
I
O
24
22
23
H
EtO C
CO Et
2
2
EtO C
2
CO Et
2
parative experiment was also carried out with acylgermane
18c. Irradiation of a benzene solution of 18c in the presence
of hexamethylditin (0.2 equiv.) at 300 nm for 3 h afforded a
65:15 mixture of 20 and 21, indicating higher reactivity of
the acylphosphonate relative to the acylgermane as the
carbonyl group radical acceptor. Since an approximate rate
constant for 5-exo cyclization of a primary alkyl radical
to an acylgermane is known to be 7 × 106 s–1 (5c), the
approximate rate constant for the same cyclization to the
acylphosphonate would be 3 × 107 s–1. Apparently, the
acylphosphonate is regarded as the most reactive among pre-
viously reported carbonyl group acceptors.
I
COPO(OEt)
2
d
79
O
EtO C
2
CO Et
2
EtO C
2
CO Et
2
EtO C
2
CO Et
2
EtO C
2
CO Et
2
77
6
I
COPO(OEt)
2
O
aThe reaction was carried out with 0.2 equiv. of (Me3Sn)2 in benzene at
300 nm for 2 h.
R
E
E
R
E
R
bThe yield was not optimized.
O
X
(Me Sn)
3
c(Me3Sn)2 (1.1 equiv.).
COX
2
COX
[1]
dStereochemistry of starting material is E only. Diastereomeric mixture
(1.2:1) of the product.
hν
I
15
14
13a: X = PO(OEt)
2
13b: X = SPh
cyclizations worked equally well. Not only alkyl radicals but
also alkenyl radicals reacted with acylphosphonates to give
cyclopentanone derivatives (Table 1, entries 4–6).
R
E
R
E
O
E = CO Me
2
COX
+
R = CH CH Ph
2
2
It is noteworthy that reaction of 13a under similar condi-
tions gave cyclohexanone 16 as a sole product, whereas 13b,
using a thioester as acceptor, gave a mixture of 16 and 17b
in 46% and 33% yield, respectively (4) (eq. [1]). Apparently,
the latter is formed as a result of the formation of stable rad-
ical intermediate 15b owing to the facile β fragmentation of
the alkoxyl radical 14b. The result obtained here clearly in-
dicates that the β elimination of the phosphonate group in 9
is very fast and highly efficient. Even more exciting results
were obtained with 18 (Scheme 4). When 18a was treated
with hexamethylditin (0.2 equiv.) at 300 nm for 2 h, gratify-
ingly, 20 was obtained exclusively, whereas 21 was reported
to be a sole product from the radical reaction of acyl sulfide
18b (4). Apparently, 6-exo ring closure to the acylphos-
phonate group is much faster than 5-exo ring closure to the
C=C bond, whereas 5-exo ring closure to the C=C bond is
much faster than 6-exo ring closure to the thioester group.
To strengthen the efficiency of the acylphosphonate, a com-
16 (69%) from 13a
16 (46%) from 13b
-
17b (33%)
Based on the highly efficient and fast addition of the alkyl
radical onto the acylphosphonate, we next studied sequential
radical reactions involving intermolecular radical addition to
the alkenyl and the alkynyl bond and subsequent cyclization
(Scheme 5). This approach provides a ready access to β-
functionalized cyclopentanones and cyclohexanones (23).
Addition of a phenylsulfanyl radical and a phenylsulfonyl
radical to an alkenyl group in 22 was followed by
cyclization to afford cyclopentanone 24 along with the gen-
eration of the diethyl phosphonate radical for the chain prop-
agation. Similarly, various electrophilic alkyl radicals from
activated olefins bearing α-electron-withdrawing groups
reacted smoothly with 22 to yield 24 in high yields. Appar-
© 2005 NRC Canada

920
Can. J. Chem. Vol. 83, 2005
Table 2. Radical addition and cyclization of acylphosphonates.
ently, the fast addition of the alkyl radical to the acyl-
phosphonate obviates the problem of the quenching of
radical intermediate 23 by the alkyl iodide prior to the
cyclization. The experimental results are summarized in Ta-
ble 2. The present approach can be applied to the formation
of cyclohexanone derivatives (eq. [2]). Treatment of 25 with
ethyl iodoacetate and hexamethylditin (0.2 equiv.) in ben-
zene at 300 nm for 5 h afforded 26a in 86% yield. A similar
result was obtained with iodomethyl phenyl sulfone under
the same conditions. Furthermore, the present approach can
be applied to alkynyl-acylphosphonate 27. When 27 was re-
acted with thiophenol and phenylsulfonyl bromide in the
presence of AIBN in refluxing benzene for 3 h, the desired
products 28a and 28b were obtained in 86% and 90% yield,
respectively (eq. [3]).
EtO C
CO Et
2
2
EtO C
CO Et
2
2
X-Y
COPO(OEt)
2
X
O
22
24
X-Y
PhS-H
(TMS)3Si-H
PhSO2-Br
C6F13-I
Conditiona
Time (h)
Yield (%)
A
A
10
2
89
75
A
B
B
B
B
B
2
5
5
5
5
5
90
85
78
75
72
73
EtOOCCH2-I
EtOOCCH2-SC(=S)OEt
PhSO2CH2-I
E
E
E
E
E
E
[2]
-PO(OEt)
2
X-Y
NCCH2-I
COPO(OEt)
2
C PO(OEt)
O
aMethod A: AIBN, C6H6, reflux; Method B: 0.2 equiv. of (Me3Sn)2,
C6H6, hv = 300 nm.
2
O
X
X
26a: X = CH COOEt (86%)
X-Y: EtOOCCH I,
25
2
2
26b: X = CH SO Ph (85%)
PhSO CH I
2
2
2
2
Scheme 6. Radical addition and cyclization of phosphonoformate
and phosphonothiolformate.
E
E
E
E
E
E
-PO(OEt)
Y
2
X
C
O
X
X
[3]
Y
COPO(OEt)
PO(OEt)
COPO(OEt)
2
2
2
C PO(OEt)
O
2
Y
O
O
X
X
28a: X = SPh:
PhSH/AIBN
31
30
29a: X = O
29b: X = S
27
E/Z = 1:12, 86%
PhSO Br/AIBN
28b: X = SO Ph:
2
2
E/Z = 1:32, 90%
was obtained, yielding 36b in 88% yield. It is evident that
intermediate radical 34 underwent bromine and iodine atom
transfer rather than cyclization. To enforce the cyclization of
36a, the solution of 36a was irradiated at 300 nm in the
presence of hexamethylditin, but lactone 35 was not formed.
The failure of the cyclization may be due to the unfavorable
E-conformation of 34 and (or) the low reactivity of the
alkoxycarbonyl group as a radical acceptor. Since the differ-
ences in energy are generally smaller for the E and Z confor-
mations in thiol esters than for the corresponding carboxylic
esters (25), radical cyclization of 29b was studied. Irradia-
tion of a benzene solution of 29b, ethyl iodoacetate, and
hexamethylditin (0.5 equiv.) at 300 nm for 3 h afforded a
mixture of addition product 38 and thiolactone 39 in 15%
and 42% yield, respectively (eq. [6]). Furthermore, reaction
of 38 in the presence of hexamethylditin at 300 nm for 3 h
gave thiolactone 39 in 88% yield. From the results obtained
here, two features are noteworthy. The addition of an alkyl
radical onto the phosphonothiolformate group occurred
smoothly, and the diethyl phosphonate group was eliminated
preferentially relative to the alkylthio group.
We next studied several interesting variations using a
phosphonoformate and a phosphonothiolformate as radical
acceptors to explore whether the present acylation could be
further extended to the synthesis of lactone 31a and
thiolactone 31b, respectively, via intermediate 30
(Scheme 6). Diethylphosphonoformate 29a was prepared by
the treatment of chloroformate 33a with triethyl phosphite in
dichloromethane at room temperature for 2 h (eq. [4]). 29a
was purified by passing it through a short column of silica
gel. Similarly, diethylphosphonothiolformate 29b was pre-
pared from 32b by a two-step procedure (24). Treatment of
32b with diphosgene afforded 33b, which was further
treated with triethyl phosphite to give alkyl phosphono-
thiolformate 29b in 75% yield.
X
C
O
P(OEt)3
CH Cl
diphosgene
CH2Cl2
XH
X
[4]
PO(OEt)2
COCl
2
2
32a: X = O
32b: X = S
33
29a (83%)
29b (75%)
In conclusion, we have shown the facile β elimination of a
phosphonate group from an alkoxyl radical and its applica-
tion to intramolecular acylation approach, in which we dem-
onstrated that the acylphosphonate was a highly efficient
carbonyl group radical acceptor and more reactive than the
previously known acyl sulfide and acylgermane. In addition,
an alkyl phosphonothiolformate group can act as an
Reaction of 29a with phenylsulfonyl bromide in the pres-
ence of AIBN (0.2 equiv.) in benzene at 80 °C for 6 h did
not afford lactone 35 but gave addition product 36a in 84%
yield (eq. [5]). When the reaction was carried out with ethyl
iodoacetate and hexamethylditin at 300 nm, a similar result
© 2005 NRC Canada

Cho and Kim
921
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-PO(OEt)
O
2
O
O
[5]
X
X
C PO(OEt)
COPO(OEt)
2
2
X
O
O
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35
29a
34
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PhSO Br/AIBN
2
EtO CCH I/(Me Sn)
2
2
2
3
O
COPO(OEt)
2
Y
36a: X = SO Ph, Y = Br (84%)
2
36b: X = EtO CCH , Y = I (88%)
2
2
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X
-PO(OEt)
S
2
S
S
[6]
X
C PO(OEt)
O
COPO(OEt)
2
2
X
O
39 (42%)
37
29b
12. For a preliminary report, see: S. Kim, C.H. Cho, and C.J. Lim.
EtO CCH I/(Me Sn)
2
2
2
3
88%
hν/(Me Sn)
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3
2
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X
COPO(OEt)
2
Y
38: X = EtO CCH , Y = I (15%)
2
2
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ing a ready access to a thiolactone synthesis.3
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Acknowledgments
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We thank the Center for Molecular Design and Synthesis
(CMDS) and Brain Korea 21 Project for financial support.
19. S. Kim, C.J. Lim, S.-E. Song, and H.-Y. Kang. Synlett, 688
(2001).
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rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml.
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