Photon can tremendously accelerate the alkyl iodides' elimination in water

Article abstract of DOI:10.1016/j.tetlet.2015.02.048

Elimination of the alkyl halides in water is very difficult due to the heterogeneous nature and the limitation of base strength. We discovered that ultra-violet (UV) light can enhance the elimination rate of alkyl iodides, including primary, secondary, and tertiary iodides in water dramatically for the first time. We propose a tandem radical-carbocation reaction mechanism to rationalize this special property of alkyl iodides.

Full text of DOI:10.1016/j.tetlet.2015.02.048

Accepted Manuscript
Photon can tremendously accelerate the alkyl iodides’ elimination in water
Wenbo Liu, Chao-Jun Li
PII:
S0040-4039(15)00319-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2015.02.048
TETL 45917
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
17 December 2014
4 February 2015
13 February 2015
Please cite this article as: Liu, W., Li, C-J., Photon can tremendously accelerate the alkyl iodides’ elimination in
water, Tetrahedron Letters (2015), doi: http://dx.doi.org/10.1016/j.tetlet.2015.02.048
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Graphical Abstract
To create your abstract, type over the instructions in the template box below.
Photon can tremendously accelerate the
alkyl iodides' elimination in water
Leave this area blank for abstract info.
Wenbo Liu and Chao-Jun Li*
Fonts or abstract dimensions should not be changed or altered.

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Photon can tremendously accelerate the alkyl iodides' elimination in water
Wenbo Liua and Chao-Jun Lia 
a
Department of Chemistry and FQRNT Center for Green Chemistry and Catalysis, McGill University, 801 Sherbrooke Street West, Montreal, QC H3A 0B8,
Canada
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Elimination of the alkyl halides in water is very difficult due to the heterogeneous nature and the
limitation of base strength. We discovered that Ultra-Violet (UV) light can enhance the
elimination rate of alkyl iodides, including primary, secondary, and tertiary iodides in water
dramatically for the first time. We propose a tandem radical-carbocation reaction mechanism to
rationalize this special property of alkyl iodides.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Elimination reaction
Alkyl iodides
Water
UV light
Kinetics
Since Breslow's landmark research1 on the extraordinary Diels-
Alder reaction rate difference between organic solvents and
water, tremendous effort has been focused on the behaviors of
various reactions in aqueous media.2, 3 The elimination of alkyl
halides to generate alkenes is one type of fundamental reactions
taught in introductory organic chemistry.4 Most elimination
reactions,5 including E1, E2 and E1cb (Figure 1), occur in
organic solvents facilely in the presence of a stoichiometric
amount of base; whereas the elimination in water is sluggish and
difficult due to its heterogeneous nature and the limitation of base
strength (vide infra). Our laboratory has a long interest in
To begin our research, we selected the secondary cyclohexyl
iodide as the substrate. We conducted the reactions with 1 mmol
0
alkyl iodide and 1 mmol NaOtBu in 1 mL water at 50 C. For
comparison, two parallel reactions, with UV irradiation and
without UV irradiation, were run for 2 hrs. The amount of the
1
elimination product, i.e cyclohexene, was determined through H
NMR analysis of the reaction mixture. As shown in Figure 2, the
observed overall reaction rate difference9 between the two
reactions is 130 folds. It has been known that a radical
intermediate can disproportionate10 to form the eliminate product.
In our case, UV light is obviously able to cleave the C-I bond
homolytically, producing the radical intermediate. Therefore, we
also investigated how UV light affected the elimination of
cyclohexyl bromide and cyclohexyl chloride. To our surprise,
under the irradiation of UV, the elimination rate of cyclohexyl
bromide is only 5.6 times faster (Figure 3a) than the one without
UV and the cyclohexyl chloride can not be affected by UV
(Figure 3b) at all.
7
investigating unique features of organic reactions in water.6,
Recently, during our study of coupling reactions in water assisted
by ultraviolet light (UV),8 we found that UV can tremendously
facilitate the elimination of alkyl iodides in water. On the other
hand, without the assistance of UV, the elimination rate of alkyl
iodide, especially primary and secondary alkyl iodides, is
extremely slow. This interesting observation led us to conduct
further systematic research on the photon-assisted alkyl halide
elimination and herein we wish to report our results.
———
 Corresponding author. Tel.: +1-514-398-5457; fax: +1-514-398-3797; e-mail: cj.li@mcgill.ca

2
Tetrahedron
To confirm whether that UV light can dramatically enhance
the elimination of alkyl iodides in water is a general feature of
alkyl iodides, we also tested the primary iodide and tertiary
iodide. Thus, 1-iodododecane was selected as the substrate, and
was subjected to the same conditions as those of cyclohexyl
iodide. As control experiments, we also tested 1-bromododecane
and 1-chloroheptane. To our surprise, no elimination product was
observed in all three cases in water under regular thermal
conditions of the reaction temperature with more than 99% of the
starting materials were recovered. When these three primary
halides were subjected to UV irradiation, after 2 hrs, no
elimination product could be detected for the 1-bromododecane
(Figure 5a) and 1-chloroheptane (Figure 5b) whereas 16.8% of
the elimination product was observed with a 40% conversion for
1-iodododecane (Figure 4). We also synthesized the 1-chloro-10-
iododecane to further test the chemo-selectivity between primary
chloride and primary iodide. As expected, under the standard
conditions, 1-chloro-10-iododecane can only produce the iodide-
elimination product, without forming any chloride-elimination
product at all (Figure 5c).
Compared to primary halides and secondary halides, tertiary
halides are much easier to eliminate, therefore we would not
expect too much change caused by UV light. However, we were
still able to detect the acceleration effect of UV on the
elimination rate of alkyl halides. UV could increase the tertiary
iodide elimination rate 10.4 times (Figure 6) while the values for
the tertiary bromide and tertiary chloride were 6.5 and 1.1,
respectively (Figure 7).
Figure 8: Elimation reaction yield of 1-iodohexadecane versus time
with and without UV irradiation
To get further insight of the effect of UV on elimination rate
enhancement, we selected 1-iodohexadodecane to investigate the
reaction progress against time with and without the UV
irradiation. We obtained the yields of the elimination product
during the first 5 hrs and composed the graph in Figure 8. The
experiments showed that in the absence of UV, there was no
elimination product. In the presence of UV, the reaction rate

3
could be accelerated tremendously and reached a plateau in 4
hours (49%).
In order to further exclude the possibility that the dramatic rate
difference may be caused by the disproportion of the radical
intermediate, we conducted the independent control experiments
by using a stoichiometric amount of a radical initiator AIBN.
Since AIBN can only be activated at a higher temperature, we set
0
the reaction temperature at 80 C. As shown in Figure 9 and
Figure 10, under the assistance of AIBN, only 1.5 fold and 2.8
fold accelerations were observed for cyclohexyl iodide and
cyclohexyl bromide, respectively (compared to 130 fold and 5.6
fold with and without UV). Thus it is quite obvious that the rate
enhancement under UV irradiation for alkyl iodides is not only
caused by the radical disproportion.
Figure 11: Proposed mechanism to rationalize the observation
In order to further understand how other parameters such as
base and solvent could affect this photo-accelerated alkyl iodides'
elimination, by using cyclohexyl iodide as a model substrate, we
also investigated the impact of another five bases and another
five aprotic solvents, the results of which were compiled in Table
1 and Table 2, respectively. These results demonstrate that: (1)
UV promoted rate enhancement of cyclohexyl iodide's
elimination in water is less dependent on the base although
different bases result in various rate accelerations; (2) the
distinguished rate enhancement could only occur in water instead
of the aprotic solvents.
Table 1: Impact of different bases on the elimination of
cyclohexyl iodide with and without UV irradiation
Base
Relative amount Relative amount
Acceleration
rate
of cyclohexene
without UV
0.02
of cyclohexene
with UV
1.01
NaOH
Na2CO3
NEt3
51
142
167
62
0.01
1.42
0.02
3.33
DBU
0.01
0.62
K3PO4
0.01
0.82
82
Therefore, extremely large enhancements of elimination rates
under UV irradiation in water (including both primary and
secondary iodides and significant enhancement for tertiary
iodide) appear to be a general and unique feature of alkyl iodides.
Because the UV lamp that we employed in our study bears a full
wavelength spectrum, it has the potential to homolytically break
C-I, C-Br and C-Cl bonds to form the corresponding carbon
radicals. By analyzing the control reactions between iodides,
chlorides and bromides under UV as well as the control
experiments with the radical initiator, it can be concluded the
elimination product of the alkyl iodides is not only produced
from the disproportion pathway of carbon radical since C-Br
bond also has the potential to form the carbon radical both under
our UV irradiation and radical initiator conditions. As concluded
by Kropp's seminal work11-14 on the photochemistry of alkyl
halides, the appropriate radius size and electronegativity of iodine
render the further electron transfer between the carbon and iodine
radicals facile in highly polar solvents such as water and
alcohols. Therefore, we proposed the following mechanism by
taking cyclohexyl iodide as an example to rationalize this
interesting feature of alkyl iodides in Figure 11. First, the carbon
iodide bond can be homolytically cleaved with the input of UV
light to give a transient carbon and iodine radical pair. Due to the
presence of highly polar solvent, i.e water, and the suitable radius
size and electronegativity of iodine, further single electron
transfer occurred faster than the collapse of the radical pair cage.
After the formation of the carbon cation, proton abstraction was
followed to result in the elimination product.
Table 2: Impact of different solvents on the elimination of
cyclohexyl iodide with and without UV irradiation
Base
Relative amount Relative amount
Acceleration
rate
of cyclohexene
without UV
0.06
of cyclohexene
with UV
0.28
CH3CN
PhCl
PhCF3
p-dioxane
DMSO
4.67
3.63
1.53
9.60
4.34
0.16
0.58
0.19
0.29
0.05
0.48
0.06
0.26
In summary, we have discovered that UV light can enhance the
elimination rate of primary, secondary alkyl iodides dramatically
and tertiary alkyl iodides significantly in water for the first time.
A tandem radical-carbocation reaction pathway to rationalize this
observation has been proposed for this interesting effect. Further
research and applications of this chemistry are proceeding in our
laboratory.
Acknowledgments
We are grateful to the Canada Research Chair (Tier 1)
foundation, FRQNT, and NSERC for their support of our
research. We thank Dr. Lu Li for the technical assistance of gas
calibration.

4
Tetrahedron
References and notes
9. We determined the average reaction rate ratio to compare the
effect of UV irradiation by employing the follwing method: after
the reaction was run for a given time, it was terminated and 1 mL
CDCl3 was added to extract the organic compounds, which was
followed by adding 10 L benzyl alcohol as the internal
standard. The reaction rate difference is calculated based on the
integration raito of the characteristic peak of the elimination
product.
1. Rideout, D. C.; Breslow, R. J. Am. Chem. Soc. 1980, 102, 7816;
Breslow, R. Acc. Chem. Res. 1991, 24, 159.
2. Li, C.-J. Chem. Rev. 2005, 105, 3095; Organic Reactions in
Water, Ed. Lindstrom, U. M., Blackwell, Oxford, UK, 2007. Li,
C.-J., Chan, T.-H. Comprehensive Organic Reactions in Aqueous
Media, John Wiley & Son, New York, 2007.
3. Li, C.-J.; Chen, L. Chem. Soc. Rev. 2006, 35, 68.
4. Please see Bruice, P. Y. Organic Chemistry, 5th Ed., page 389-
390, Pearson, 2007; McMurry, J. Organic Chemistry, 8th Ed.,
Cengage Learning, 2011; Solomons, T. W. G.; Fryhle, C. B.
Organic Chemistry, 11th Ed., Wiley, New York, 2013.
5. Please see Smith, M. B. March's Advance Organic Chemistry, 7th
Ed., Chapter 17, WIley, New Yor, 2013.
10. Please see Smith, M. B. March's Advanced Organic Chemistry
7th Ed, p 246, Wiley, New York, 2013.
11. Kropp, P. J.; Poindexter, G. S.; Pienta, N. J.; Hamilton, D. C. J.
Am. Chem. Soc. 1976, 98, 8135.
12. Kropp, P. J.; Worsham, P. R.; Davidson, R. I.; Jones, T. H. J.
Am. Chem. Soc. 1982, 104, 3972.
13. Kropp, P. J.; McNeely, S. A.; Davis, R. D. J. Am. Chem. Soc.
1983, 105, 6907.
6. Herrerías, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007, 107,
2546.
14. Kropp, P. J. Acc. Chem. Res. 1984, 17, 131.
7. For examples, see: Li, C.-J. Acc. Chem. Res. 2010, 43, 581. Li,
C.-J.; Meng, Y. J. Am. Chem. Soc. 2000, 122, 9538; Keh, C. C.
K.; Wei, C.; Li, C.-J. J. Am. Chem. Soc. 2003, 125, 4062; Wei,
C.; Li, C.-J. Green Chemistry, 2002, 4, 39; Zhou, F.; Li, C.-J.
Nature Commun., 2014, 5, 4254.
Supplementary information
Experimental details, representative procedures and
characterization spectra are enclosed in another separate file.
8. Unpublished results