A New Entry of Practical and Chemoselective Iodocarbenoids for Carbonyl Iodomethylenation

Article abstract of DOI:10.1002/jccs.201600875

Direct oxidative addition of CHI₃ to the Mg-TiCl₄ bimetallic species resulted in the generation of a highly chemoselective and practically convenient iodomethylenetitanium complex, which efficiently effected condensation even with enolizable or inert carbonyl compounds, such as sterically congested ketones, to provide vinyl iodide compounds.

Full text of DOI:10.1002/jccs.201600875

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
A New Entry of Practical and Chemoselective Iodocarbenoids for Carbonyl
Iodomethylenation
Tu-Hsin Yan and Mei-Yuan Chen
Chemistry Department, National Chung-Hsing University, Taichung, Taiwan
(Received: December 20, 2016; Accepted: February 6, 2017; DOI: 10.1002/jccs.201600875)
Direct oxidative addition of CHI3 to the Mg-TiCl4 bimetallic species resulted in the generation of
a highly chemoselective and practically convenient iodomethylenetitanium complex, which effi-
ciently effected condensation even with enolizable or inert carbonyl compounds, such as sterically
congested ketones, to provide vinyl iodide compounds
Keywords: Vinyl iodide; Oxidative addition; Carbenoids; Methylenation; Bimetallic species.
INTRODUCTION
Formation of C = C bonds by means of metallo-
carbonyl compounds, such as sterically congested
ketones, to provide vinyl iodide compounds.
carbenes constitutes a particularly valuable strategy.
The importance of vinyl iodides as building blocks for
the construction of useful molecules such as
N-vinylheterocycles and aryl vinyl ethers, alkenyl sul-
fides, enamides, alkylidene complexes,1 acetylenes,2 and
dienes3 makes their availability by such processes
important. In spite of the vast number of titanium-car-
bene-mediated carbonyl-olefinations,4,5 remarkably few
reactions of iodocarbenoids are described except for the
CHI3/CrCl2-6a and Ph3P = CHI-6bpromoted iodo-
methylenation of aldehydes or ketones with iodoforms.
These processes do not generally extend to enolizable
and sterically hindered ketones, and the use of very
expensive CrCl2 is unavoidable. In addition to
carbonyl-iodomethylenation, several syntheses of vinyl
iodides employ iodination of alkenylboronic acid or
NaHMDS-promoted coupling of CH2I2 with
ArCH2Br.7 In searching for a new and practical strat-
egy based on the concept of the C–I bond activation
promoted by the highly active Ti-Mg bimetallic species,
we turned our attention to the elaboration of iodoform
initiated by simple metal–iodine exchanges.
RESULTS AND DISCUSSION
Believing a Mg-Ti inorganic-Grignard reagent5,8
might be more reactive than either titanium
(II) complexes9a or Mg-Hg-species,9b which had been
used to activate gem-dihalides, we explored the oxida-
tive addition of CHI3 to the Mg/TiCl4 system. Expect-
edly, as the data in Table 1 show, exposure of the
simple cyclic ketone 1a to TiCl4/Mg/CHI3 in CH2Cl2
and the electron-pair donor (EPD) additive dimethox-
yethane (DME) at 0ꢀC did indeed produce the desired
vinyl iodide 1b (Table 1).
As the data in Table 1 show, adding a solution of
1b (1.0 mmol) in DME (1.2 mL) to a mixture of CHI3
(1 mmol), TiCl4 (3 mmol), and Mg (8 mmol) in 3 mL
of CH2Cl2 at 0ꢀC followed by stirring at 0ꢀC for 2 h
gave a 75% yield of 1b (entry 1). Most revealing was
the effect of the amount of Mg relative to titanium
chloride on this process. Use of a 2:6 TiCl4/Mg ratio
improved the vinyl iodide formation. The iodomethyle-
nation product 1b was obtained in 90% yield (entry 2).
Replacing DME with tetrahydrofuran (THF) gave a
slightly diminished yield of 70% (entries 3 and 4).
Replacing DME with other additives including acetoni-
trile, NEt3, and pyridine gave no reaction (entry 5).
Extension of these observations to other traps confirms
their generality.
We here report the protocols whereby direct oxi-
dative addition of CHI3 to the Mg-TiCl4 bimetallic spe-
cies5 resulted in the generation of
a
highly
chemoselective and practically convenient iodomethyle-
netitanium complex (Scheme 1), which efficiently
effected condensation even with enolizable or inert
The use of ethylcyclohexanone 2a also gave satis-
*Corresponding authors. Email: thyan@mail.nchu.edu.tw
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Article
Yan and Chen
O
With an eye to extending these observations to
other less reactive ketones, we explored the iodomethy-
lenation of unsaturated ketones with CHI3. Fortu-
nately, exposing 1-acetylcyclohexene 9a to the
TiCl4-Mg-CHI3-DME system led to smooth iodo-
methylenation to give 82% of the desired 95:5 mixture
of (E)- and (Z)-vinyl iodides 9b (entry 8). On the other
hand, using the acetophenone 10a also gave satisfactory
results wherein the vinyliodide 10b was obtained in 75%
yield (entry 9). Changing the carbonyl compounds to
aldehydes, which are particularly prone to reduction
and/or dimerization, led to equally gratifying results.
Thus, reacting cyclohexanecarboxaldehyde 11a with
iodoform using a 2:6 TiCl4/Mg ratio led to smooth
iodomethylenation to give vinyl iodides 11b in 82%
yield as a 70:30 E and Z mixture (entry 10). The aro-
matic aldehyde 12a also gave satisfactory results. Iodo-
methylenation of 12a using a 3:8 TiCl4/Mg ratio
afforded a 90:10 mixture of (E)- and (Z)-vinyl iodides
12b in 75% yield (entry 11).
CHI
R'
R
R'
R
R
L
TiCl4/Mg
EPD additive
L = EPD or Cl
CHI3
IHC Ti
L
CHI
O
H
R
H
Scheme 1. TiCl4-Mg bimetallic complex-promoted
CHI transfer reaction of CHI3.
factory results with the iodoform-TiCl4-Mg system, giv-
ing 85% yield of 2b as a 90:10 E/Z mixture (Table 2,
entry 1). More gratifyingly, the reaction directly scales
up; thus, vinyliodide 2b was obtained in 80% yield on a
10-mmol scale using a 14 mmol of TiCl4 and 42 mmol
of Mg. Iodomethylenation onto the acyclic ketones 3a
and 4a was equally effective (entries 2 and 3). As
expected, the phenylthio group was completely unaf-
fected. The sterically congested ketone 5a presented no
difficulties. For the iodomethylenation of 5a, use of
5 equiv of TiCl4 and 4 equiv of Mg proved most satis-
factory, giving 78% yield of the desired vinyl iodide 5b
as a 90:10 E/Z mixture. Most pleasingly, due to the
non-basic character of the titanium–iodomethylene
complex, enolizable ketones10,11 also proved to be a sat-
isfactory trap (entries 6 and 7). Thus, 2-indanone 7a
and β-tetralone 8a gave 78% and 76% yields, respec-
tively, of the desired iodomethylenation products 7b
and 8b.
CONCLUSIONS
In conclusion, the successful application of the
Mg-TiCl4-promoted CHI transfer to various ketone
and aldehyde carbonyl groups illustrated the extraordi-
nary reactivity of this new titanium–iodomethylene
complex. This oxidative addition of CHI3 promoted by
the Mg-Ti bimetallic complex not only represents an
extremely simple, practical, and efficient synthesis of
vinyl iodides but also offers the opportunity to discover
other carbenoids derived from Mg-TiCl4-polyhaloalk-
anes.12 Not only is this iodocarbenoid non-basic but it
also seems highly nucleophilic. The novel reactivity
involved suggests several intriguing directions, which
are currently under active investigation.
Table 1. Effect of various EPD additive on the TiCl4-Mg-
mediated iodomethylenation of cyclohexanone 1b
I
O
CHI3/Mg/TiCl4
CH2Cl2/EPD/0°C
1a
EXPERIMENTAL
1b
Typical procedure
Method A. A solution of cyclohexanone 1a
(0.098 g, 1 mmol) in DME (1.2 mL) was added drop-
wise to a suspension consisting of Mg (146 mg,
6 mmol), TiCl4 (0.21 mL, 2 mmol), CHI3 (0.394 g,
1 mmol), and CH2Cl2 (3 mL) at 0ꢀC. After stirring for
1–2 h at 0ꢀC, a saturated potassium carbonate solution
(10 mL) was added and the mixture was diluted with
CH2Cl2 (20 mL). The organic layer was separated,
dried, evaporated, and purified by chromatography on
TiCl4/Mg
Entry (equiv)
Yielda
(%); 1b
EPD additive
1
2
3
4
5
3:8
2:6
2:6
3:8
DME (1.2 mL)
DME (1.2 mL)
THF (1.2 mL)
THF (2.0 mL)
NEt3, CH3CN or
pyridine
75
90
55
70
0
2:6 or 3:8
a Isolated yield
2
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Carbonyl Iodomethylenation
Table 2. Iodomethylenation of ketones and aldehydes with CHI3-TiCl4-Mg/DME (1.2 mL) at 0ꢀC
Entry
1
Carbonyl compd (1 mmol)
TiCl4/Mg (equiv)
Methylene product
Yielda % (E/Z)
2:6
85–90:10b
80
O
CHI
14:42c
2b
2a
O
2
3
3:8
2:6
85–60:40b
83–75:25b
CHI
3a
3b
SPh
SPh
O
CHI
4b
4a
4
5
5:4
3:8
78–90:10b
O
CHI
5b
5a
73
O
CHI
6a
6b
CHI
O
6
7
3:8
3:8
78
CHI
7b
CHI
O
7a
76–65:35b
O
8a
8b
CHI
8
3:8
3:8
2:6
3:8
82–95:5b
O
9a
O
9b
9
75–85:15b
82–70:30d
75–90:10d
CHI
10a
O
10b
CHI
10
11
H
H
11b
11a
O
CHI
H
H
12b
12a
NOESY may provide a further confirmation.
a Isolated yield.
b E/Z ratio based upon 1H NMR and steric strain considerations.
c 10-mmol scale of 2a and 12 mL of DME.
d E/Z ratio based on the J-value between the two vinyl protons.
silica gel (elution with hexane) to give (iodomethylene)
cyclohexane 1b (0.200 g, 90% yield) as a colorless oil: 1H
NMR (400 MHz, CDCl3) δ 1.54–1.49 (m, 6 H), 2.28–2.24
(m, 4 H), 5.75–5.74 (m, 1 H); 13C NMR (100 MHz,
CDCl3) 26.0, 26.9, 27.9, 35.9, 37.2, 71.1, 151.2; HRMS-EI
Calcd for C7H11I: 221.9905. Found: 221.9911.
Method B. A solution of cyclohexanone 1a
(0.098 g, 1 mmol) in DME (1.2 mL) was added
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Article
Yan and Chen
dropwise to a suspension consisting of Mg (194 mg,
8 mmol), TiCl4 (0.32 mL, 3 mmol), CHI3 (0.394 g,
1 mmol), and CH2Cl2 (3 mL) at 0ꢀC. After stirring for
1–2 h at 0ꢀC, a saturated potassium carbonate solution
(10 mL) was added and the mixture was diluted with
CH2Cl2 (20 mL). The organic layer was separated,
dried, evaporated, and purified by chromatography on
silica gel (elution with hexane) to give (iodomethylene)
cyclohexane 1b (0.170 g, 75% yield) as a colorless oil:
Method C. A solution of cyclohexanone 1a
(0.098 g, 1 mmol) in THF (2.0 mL) was added drop-
wise to a suspension consisting of Mg (146 mg,
6 mmol), TiCl4 (0.21 mL, 2 mmol), CHI3 (0.394 g,
1 mmol), and CH2Cl2 (3 mL) at 0ꢀC. After stirring for
1–2 h at 0ꢀC, a saturated potassium carbonate solution
(10 mL) was added and the mixture was diluted with
CH2Cl2 (20 mL). The organic layer was separated,
dried, evaporated, and purified by chromatography on
silica gel (elution with hexane) to give (iodomethylene)
cyclohexane 1b (0.150 g, 70% yield) as a colorless oil.
1-(Iodomethylene)-2-methylcyclohexane 2b. Fol-
lowing the procedure of method A, iodomethylenation
of 2a (0.110 g, 1 mmol) afforded 2b (0.201 g, 85%) as a
colorless oil (Z- and E-isomers): 1H NMR (400 MHz,
CDCl3) 1.03 (d, J = 7.2 Hz, 3 H), 1.35–1.71 (m, 6 H),
1.94–2.01 (m, 1 H), 2.25–2.30 (m, 1 H), 2.62–2.65 (m,
1 H), 5.71 and 5.76 (2 bs, total 1 H, ratio ~10:90); Z-
and E-isomers: 13C NMR (100 MHz, CDCl3) 18.6,
25.0, 27.3, 36.1, 36.3, 40.2, 70.9, 155.1; HRMS-EI
Calcd for C8H13I: 236.0062. Found: 236.0060.
(1-Iodoprop-1-en-2-yl)cyclohexane 3b. Following
the procedure of method B, iodomethylenation of 3a
(0.126 g, 1 mmol) afforded 3b (0.225 g, 85%) as a col-
orless oil l (Z- and E-isomers): 1H NMR (400 MHz,
CDCl3) 1.08–1.75 (m, 10 H), 1.73 and 1.76 (2 s, total
3 H, ratio ~1:1), 2.06–2.14 and 2.44–2.55 (2 m, total
1 H, ratio ~1:1), 5.72 and 5.90 (2 bs, total 1 H, ratio
~40:60); Z- and E-isomers: 13C NMR (100 MHz,
CDCl3) 20.1 and 22.6; 26.0 and 26.3; 30.0 and 31.8;
46.6 and 48.2; 72.6 and 74.9; 151.2 and 152.9; HRMS-
EI Calcd for C9H15I: 250.0218. Found: 250.0212.
2-(Iodomethylene)-4-(phenylthio)butane 4b. Follow-
ing the procedure of method A, iodomethylenation of
4a (0.180 g, 1 mmol) afforded 4b (0.252 g, 83%) as a
colorless oil (Z- and E-isomers): 1H NMR (400 MHz,
CDCl3) 1.82 and 1.90 (2 s, total 3 H, ratio ~2.4:1),
2.47–2.53 (m, 2 H), 2.96–3.00 (m, 2 H), 5.93 and 5.96
(2 bs, total 1 H, ratio ~25:75), 7.39–7.18 (m, 5 H); Z-
and E-isomers: 13C NMR (100 MHz, CDCl3) 23.5
and 23.6; 30.5 and 31.9; 38.5 and 38.8; 76.8; 126.1 and
126.2; 128.8 and 128.9; 129.5, and 129.6; 145.3 and
145.6 ; HRMS-EI Calcd for C11H13IS: 303.9783.
Found: 303.9788.
1-Iodo-2,3,3-trimethylbut-1-ene 5b. Adding
a
~0.9 M solution of 5a in DME to a mixture of CHI3
(1 mmol), TiCl4 (5 mmol) and Mg (4 mmol) in 3 mL of
CH2Cl2 at 0ꢀC followed by stirring at 0ꢀC for 2 h
afforded 5b (0.175 g, 78%) as a colorless oil (Z- and E-
isomers): 1H NMR (400 MHz, CDCl3) 1.08 (s, 9 H),
1.86 (s, 3 H), 5.92 and 6.02 (2 bs, total 1 H, ratio
~10:90); Z- and E-isomers 13C NMR (100 MHz,
CDCl3) 21.9, 28.9, 38.9, 75.4, 155.1; HRMS-EI Calcd
for C7H13I: 224.0062. Found: 224.0071.
1,4-bis(Iodomethylene)cyclohexane 6b. Following
the procedure of method B, iodomethylenation of 6a
(0.112 g, 1 mmol) afforded 6b (0.263 g, 73%) as a col-
orless oil: 1H NMR (400 MHz, CDCl3) 2.31 (s, 4 H),
5.94 (s, 2 H); 13C NMR (100 MHz, CDCl3) 35.7, 36.9,
73.2, 148.7; HRMS-EI Calcd for C8H10I2: 359.8872.
Found: 359.8870.
2-(Iodomethylene)indane 7b. Following the proce-
dure of method B, iodomethylenation of 7a (0.132 g,
1 mmol) afforded 7b (0.200 g, 78%) as a colorless oil:
1H NMR (400 MHz, CDCl3) 3.64 (s, 2 H), 3.74 (s,
2 H), 6.16 (s, 1 H), 7.14–7.24 (m, 4 H); 13C NMR
(100 MHz, CDCl3) 40.2, 44.1, 71.5, 124.6, 124.8, 125.9,
126.7, 126.8, 140.5, 151.8; HRMS-EI Calcd for
C10H9I: 255.9749. Found: 255.9743.
2-(Iodomethylene)-1,2,3,4-tetrahydronaphthalene
8b. Following the procedure of method B, iodomethy-
lenation of 8a (0.146 g, 1 mmol) afforded 8b (0.205 g,
76%) as a colorless oil (Z- and E-isomers): 1H NMR
(400 MHz, CDCl3) 2.50–2.53 and 2.57-–2.60 (2 m,
total 2 H, ratio ~35:65), 2.78–2.81 and 283–2.86 (2 m,
total 2 H, ratio ~1:1.6), 3.55 (s, 2 H), 6.05 and 6.07
(2 s, total 1 H, ratio ~35:65), 7.08-7.17 (m, 4 H); Z-
and E-isomers: 13C NMR (100 MHz, CDCl3) 29.4
and 30.6; 33.7 and 34.3; 38.8 and 39.6; 73.2 and 73.5;
125.8 and 126.2; 126.3 and 127.3; 127.8 and 128.2;
128.6 and 135.0; 135.8 and 136.8; 137.6 and 147.3;
HRMS-EI Calcd for C11H11I: 269.9905. Found:
269.9901.
(E)-1-(1-Iodoprop-1-en-2-yl)cyclohexene 9b. Fol-
lowing the procedure of method B, iodomethylenation
4
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Carbonyl Iodomethylenation
of 9a (0.124 g, 1 mmol) afforded 9b (0.203 g, 82%, E/Z
>95:5) as a colorless oil (Z- and E-isomers): 1H NMR
(400 MHz, CDCl3) 1.54-2.16 (m, 11 H), 5.93–5.94 (m,
1 H), 6.22–6.23 (m, 1 H); Z- and E-isomers 13C NMR
(100 MHz, CDCl3) 21.9, 22.2, 22.6, 25.8, 26.2, 76.3,
126.1, 135.5, 147.2; HRMS-EI Calcd for C9H13I:
248.0062. Found: 248.0065.
(1-Iodoprop-1-en-2-yl)benzene 10b. Following the
procedure of method B, iodomethylenation of 10a
(0.120 g, 1 mmol) afforded 10b (0.183 g, 75%) as a col-
orless oil (Z- and E-isomers): 1H NMR (400 MHz,
CDCl3) 2.20 and 2.26 (d, J = 2.8), total 3 H, ratio
~1:4), 6.25 and 6.50 (q, J = 2.8), total 1 H, ratio
~15:85), 7.13-7.38 (m, 5 H); Z- and E-isomers: 13C
NMR (100 MHz, CDCl3) 24.4 and 26.5; 79.1; 126.2
and 127.0; 127.4 and 127.6; 127.8 and 128.2; 128.4 and
128.5; 141.4 and 147.1; HRMS-EI Calcd for C9H9I:
243.9748. Found: 243.9752.
2-(Iodopvinyl)cyclohexane 11b. Following the pro-
cedure of method A, iodomethylenation of 11a
(0.112 g, 1 mmol) afforded 11b (0.194 g, 82%) as a col-
orless oil (Z- and E-isomers): 1H NMR (400 MHz,
CDCl3) 1.04-1.38 (m, 5 H), 1.63–1.71 (m, 5 H),
1.94–2.05 and 2.24–2.35 (2 m, total 1 H, ratio ~30:70),
5.95 (dd, J = 14.0, 1.2 Hz, 0.69 H, E), 5.96 (dd,
J = 8.4, 7.2 Hz, 0.31 H, Z), 6.04 (dd, J = 7.2, 0.6 Hz,
0.31 H, Z), 6.46 (dd, J = 14.0, 7.0 Hz, 0.69 H, E): Z-
and E-isomers: 13C NMR (100 MHz, CDCl3) 25.5 and
25.7; 25.86 and 25.9; 31.3 and 32.0; 43.6 and 44.5; 73.2
and 74.5; 146.2 and 152.1; HRMS-EI Calcd for
C8H13I: 236.0062. Found: 236.0066.
2-(Iodopvinyl)benzene 12b. Following the proce-
dure of method B, iodomethylenation of 12a (0.106 g,
1 mmol) afforded 12b (0.173 g, 75%) as a colorless oil
(Z- and E-isomers): 1H NMR (400 MHz, CDCl3) 6.55
(d, J = 8.4) and 6.83 (d, J = 15.2), total 1 H, ratio
~10:90), 7.26–7.62 (m, 6 H); Z- and E-isomers: 13C
NMR (100 MHz, CDCl3) 79.2 (Z and E); 126.0 (Z and
E); 128.1 and 128.3; 128.5 and 128.7; 137.7 and 138.7;
145.0 (Z and E); HRMS-EI Calcd for C8H13I:
229.9592. Found: 229.9586.
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ACKNOWLEDGMENT
We thank the Ministry of Science and Technol-
ogy, Republic of China, for generous support.
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