- Email: [email protected]
Photon can tremendously accelerate the alkyl iodides’ elimination in water
Accepted Manuscript Photon can tremendously accelerate the alkyl iodides’ elimination in water Wenbo Liu, Chao-Jun Li PII: DOI: Reference:
S0040-4039(15)00319-6 http://dx.doi.org/10.1016/j.tetlet.2015.02.048 TETL 45917
To appear in:
Tetrahedron Letters
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.
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Photon can tremendously accelerate the alkyl iodides' elimination in water Wenbo Liu and Chao-Jun Li*
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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 Available online
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. 2009 Elsevier Ltd. All rights reserved.
Keywords: Elimination reaction Alkyl iodides Water UV light Kinetics
Since Breslow's landmark research1 on the extraordinary DielsAlder 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 investigating unique features of organic reactions in water.6, 7 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.
To begin our research, we selected the secondary cyclohexyl iodide as the substrate. We conducted the reactions with 1 mmol alkyl iodide and 1 mmol NaOtBu in 1 mL water at 50 0C. For comparison, two parallel reactions, with UV irradiation and without UV irradiation, were run for 2 hrs. The amount of the elimination product, i.e cyclohexene, was determined through 1H 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.
——— Corresponding author. Tel.: +1-514-398-5457; fax: +1-514-398-3797; e-mail: [email protected]
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-10iododecane 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 iodideelimination 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 the reaction temperature at 80 0C. 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
NaOH Na2CO3 NEt3 DBU K3PO4
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.
Relative amount of cyclohexene without UV 0.02 0.01 0.02 0.01 0.01
Relative amount of cyclohexene with UV 1.01 1.42 3.33 0.62 0.82
Acceleration rate 51 142 167 62 82
Table 2: Impact of different solvents on the elimination of cyclohexyl iodide with and without UV irradiation
Base
CH3CN PhCl PhCF3 p-dioxane DMSO
Relative amount of cyclohexene without UV 0.06 0.16 0.19 0.05 0.06
Relative amount of cyclohexene with UV 0.28 0.58 0.29 0.48 0.26
Acceleration rate 4.67 3.63 1.53 9.60 4.34
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
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 389390, 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. 6. Herrerías, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007, 107, 2546. 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. 8. Unpublished results
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. 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. 14. Kropp, P. J. Acc. Chem. Res. 1984, 17, 131.
Supplementary information Experimental details, representative procedures and characterization spectra are enclosed in another separate file.
S0040-4039(15)00319-6 http://dx.doi.org/10.1016/j.tetlet.2015.02.048 TETL 45917
To appear in:
Tetrahedron Letters
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 Wenbo Liu and Chao-Jun Li*
Fonts or abstract dimensions should not be changed or altered.
Leave this area blank for abstract info.
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 Available online
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. 2009 Elsevier Ltd. All rights reserved.
Keywords: Elimination reaction Alkyl iodides Water UV light Kinetics
Since Breslow's landmark research1 on the extraordinary DielsAlder 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 investigating unique features of organic reactions in water.6, 7 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.
To begin our research, we selected the secondary cyclohexyl iodide as the substrate. We conducted the reactions with 1 mmol alkyl iodide and 1 mmol NaOtBu in 1 mL water at 50 0C. For comparison, two parallel reactions, with UV irradiation and without UV irradiation, were run for 2 hrs. The amount of the elimination product, i.e cyclohexene, was determined through 1H 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.
——— Corresponding author. Tel.: +1-514-398-5457; fax: +1-514-398-3797; e-mail: [email protected]
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-10iododecane 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 iodideelimination 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 the reaction temperature at 80 0C. 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
NaOH Na2CO3 NEt3 DBU K3PO4
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.
Relative amount of cyclohexene without UV 0.02 0.01 0.02 0.01 0.01
Relative amount of cyclohexene with UV 1.01 1.42 3.33 0.62 0.82
Acceleration rate 51 142 167 62 82
Table 2: Impact of different solvents on the elimination of cyclohexyl iodide with and without UV irradiation
Base
CH3CN PhCl PhCF3 p-dioxane DMSO
Relative amount of cyclohexene without UV 0.06 0.16 0.19 0.05 0.06
Relative amount of cyclohexene with UV 0.28 0.58 0.29 0.48 0.26
Acceleration rate 4.67 3.63 1.53 9.60 4.34
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
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 389390, 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. 6. Herrerías, C. I.; Yao, X.; Li, Z.; Li, C.-J. Chem. Rev. 2007, 107, 2546. 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. 8. Unpublished results
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. 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. 14. Kropp, P. J. Acc. Chem. Res. 1984, 17, 131.
Supplementary information Experimental details, representative procedures and characterization spectra are enclosed in another separate file.