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Bridged dibenzimidazolinylidenes as new derivatives of tetraaminoethylene
TetrahedronLetters,Vol.36, No. 16, pp. 2741-2744, 1995 ElsevierScienceLtd Printedin GreatBritain 0040-4039/95 $9.50+0.00
Pergamon 0040-4039(95)00386-X
Bridged Dibenzimidazolinylidenes as New Derivatives of Tetraaminoethylene Zhiqiang Shi and Randolph P. Thummel*
Department of Chemistry, University of Houston, Houston, Texas 77204-5641
Abstract: The deprotonation of N,N'-polymethylene bridged bis-benzimidazolium salts in the absence of air provides the corresponding bridged dibenzimidazolinylidenes which undergo a spontaneous chemiluminescent reaction with dioxygen to afford ureaphanes. In an earlier paper I we reported the preparation of the N,N'-polymethylene bridged bibenzimidazolium salts 1 and their conversion to ureaphanes 4 which we suspected to proceed through the neutral dibenzimidazolinylidene 2. The only previously known examples of such species were the tetra-N-phenyl derivative reported by Bourson 2 and the tetra-N-methyl derivative reported by Hiinig and coworkers. 3 Both species were prepared by deprotonation of the corresponding N,N'-disubstituted benzimidazolium salt. Somewhat surprisingly the analogous imidazolinylidene dimers could not be isolated.4 We had suspected that the dibenzimidazolinylidene 2 might exist in equilibrium with an analogous b/s-carbene 5 and the position of equilibrium would be sensitive to steric and conformational effects imposed by different bridge lengths. The recent report of a similar tr/s-carbene, which was sterically restrained from dimerization, lent encouragement to this proposal. 5 Scheme 1
/(CH2)~
/ (CH2)~ - 2e"
(CH2)n (pFs.)2 111(n =3)
\ y
/ (CH2)n
/ (CH2)~ CH3CN (CH2)n
2a,b
311(n =3) b (n = 4)
b (n = 4)
/ (CH2)n,~
/ (CH2)~
(CH2)n
(CH2)n
4a,b
Sa,b
2741
(Br')2
2742
In an effort to possibly generate the bL~-carbene by deprotonation, we prepared the N,N'polymethylene bridged bis-benzimidazolium salts 3. Thus treatment of benzimidazole (6) with 1,3dibromopropane or 1,4-dibromobutane provided the neutral species 7. Methylation with methyl iodide led to the tethered diquaternary salt 8 while treatment with a second equivalent of the dibromoalkane provided the doubly bridged salts 3. It is noteworthy that in the synthesis of 3a, a minor product 10a was formed in 4% yield.
H i~.. H
I (CH2)~, Br-(CH2)n-Br I~----~N~.H H_.4:~F~ (n = 3,4) N N~"~/
/ (CH2)~ Br'(CH2)n'Br. (n = 3,4)
7a (n = 3, 81%) b (n = 4, 83%)
N\
/N (CH2)n
+
lOa
(Br')2
3a (24%) b (11%)
CH311 (n=4) H H CH3N(~)N/~]~N(~) N"CH3
Nail
8 (80%)
Air
0 C H ~N ,II~N~.~N
o ~I,,N,CH 3
9185"/.)
When 3a or 3b is treated with sodium hydride in acetonitrile in the presence of air, the ureaphanes 4Lb were obtained in yields of 73% and 54% ~spectively. These compounds are identical with those reported earlier from the reduction of la,b. To isolate the dibenzimidazolinylideneintermediate, it was necessary to carry out the reduction of 2 or deprotonation of 3 with the scrupulous exclusion of oxygen. Thus the electrolysis of l a at -1.10 V initially gave rise to an intense red color which was presumably due to the formation of the cation radical. Upon further reduction, the color faded and when the current had fallen to zero, a yellow solution was obtained from which the air sensitive 2a was isolated in 62% yield. In the IH NMR the benzo-protons exhibited an AA'BB' pattern centered at 56.74 and 56.26. The bridge methylenes showed an eight proton muhiplet at 52.98 and a four proton multiplet at 51.36. In the 13C NMR the benzo-carbons showed lines at 128.4, 119.5 and 105.6 while the central C--C appeared at 140.5 and the bridge carbons showed lines at 48.7 and 28.7 ppm. The identical material could be obtained in 68% yield when 3a was treated with sodium hydride in acetonitrile under anaerobic conditions. Electrolysis of l b at -1.40 V gave an intense purple color which faded as the current decreased to zero affording an orange precipitate in 43% yield. The NMR spectra were very similar to 2a with IH signals at i~6.76, 6.23, 3.23, 1.36 and 13C signals at 142.3, 120.0, 105.3, 50.2, 29.5 ppm (one benzo carbon obscured by C6D6). A minor side product (ca. 15%) was also detected by NMR. When either 2a or 2b was exposed to air, intense chemiluminescence was observed, the compound's color slowly faded to white, and ureaphanes 4a, h were obtained. The analogous deprotonation of 8 in the presence of air led to the corresponding bis-urea 9 but the intermediate dibenzimidazolinylidenewas not isolated.
2743
Tetraaminoethylenes react with Rh(I) to give bis-carbenoid adducts. 6 Thus
when
dibenzimidazolinylidene 2a is treated with Rh(I) 1,5-cyclooctadienyl chloride, a 1:1 adduct is obtained which gives a base peak in the FAB mass spectrum at 527 [M-CI]÷ and shows IH and 13C NMR spectra consistent with structure 11. The carbenoid carbon appears as a weak signal at 195 ppm..The structure of adduct 11 was confirmed by x-ray analysis and is illustrated in Figure 1. We still do not have evidence for the involvement of a b/s-carbene intermediate since a mono-carbene derived from a single deprotonation of 3 could attack the remaining benzimidazolium ion to lead to 2 without the involvement of 5.
Rh(COD)CI •
11
L..J 2-'
Figure 1. ORTEP diagram of Rh(I) adduct 11. We were interested in how the tetracation 10a might behave upon deprotonation. When lt)a is treated with sodium hydride in the presence of air a 69% yield of the trimethylene-bridged benzimidazolone tetramer 12 is obtained. In solution this material is conformationally mobile at room temperature on the NMR time scale and thus its geminal methylene protons are equivalent. Structure 12 is somewhat deceptive since the benzimidazolone carbonyls do not point towards one another as was ascertained by an X-ray crystal analysis. Figure 2 illustrates the molecular structure of 12 and it is evident that the benzimidazolone rings alternately point up and down. We are testing the ability of this material to organize itself around an appropriate guest such as a metal cation. 7
(Br ")4 1011
12 (69%)
2744
>2.0034 20 G
: =-
I
Figure 2. X-ray structure of tetramer 12
Figure 3. ESR spectrum of cation radical 13
It is noteworthy that while 1 is a good electron acceptor, 2 is an excellent electron donor. When equimolar quantities of l a and 2a are combined in acetonitrile, single electron transfer occurs and the coproportionation product is obtained. This species is a deep red, air sensitive cation radical 13 which shows a strong ESR signal (figure 3). Future studies will be directed towards the study of salts of this and other related cation radicals.
la
+
2a
CH3CN
M (PF6")
13
Acknowledgement. We would like to thank the Robert A. Welch Foundation and the National Science Foundation (CHE-9224686) for financial support of this work, Dr. James Korp for assistance with the Xray structure determination, and Prof. David Hoffman for use of the dry box.
References 1. Shi, Z.; Thummel, R. P. Tetrahedron Len. 1994, 35, 33. 2. Bourson, J. Ball. Soc. Chim. France 1971, 3541. 3. (a) Htinig, S.; Scheutzow, D.; Schlaf, H.; Quast, H. LiebigsAnn. Chent 1972, 765, 110. (b) Htinig, S.; Scheutzow, D.; Schlaf, H.; Quast, H. LiebigsAnn. Chent 1972, 765, 126. 4. (a) SchOnherr, H.-J.; Wanzlick, H.-W. Chem. Ber. 1970, 103, 1037. (b) Sch~3nherr, H.-J.; Wanzlick, H.-
W. Liebigs Ann. Chem. 1970, 731, 176. 5. Dias, H. V. R.; Jin, W. Tetrahedron Lett. 1994, 35, 1365. 6. Hitchcock, P. B.; Lappert, M. F.; Terreros, P.; Wainwright, K. P. J. Chem. Soc., Chem. Commun. 1980, 1180. 7. (a) Htay, M. M.; Meth-Cohn, O. Tetrahedron Lett. 1976, 79. (b) Htay, M. M.; Meth-Cohn, O. Tetrahedron Lett. 1976, 469.
(Received in USA 3 January 1995; revised 27 February 1995; accepted 28 February 1995)
Pergamon 0040-4039(95)00386-X
Bridged Dibenzimidazolinylidenes as New Derivatives of Tetraaminoethylene Zhiqiang Shi and Randolph P. Thummel*
Department of Chemistry, University of Houston, Houston, Texas 77204-5641
Abstract: The deprotonation of N,N'-polymethylene bridged bis-benzimidazolium salts in the absence of air provides the corresponding bridged dibenzimidazolinylidenes which undergo a spontaneous chemiluminescent reaction with dioxygen to afford ureaphanes. In an earlier paper I we reported the preparation of the N,N'-polymethylene bridged bibenzimidazolium salts 1 and their conversion to ureaphanes 4 which we suspected to proceed through the neutral dibenzimidazolinylidene 2. The only previously known examples of such species were the tetra-N-phenyl derivative reported by Bourson 2 and the tetra-N-methyl derivative reported by Hiinig and coworkers. 3 Both species were prepared by deprotonation of the corresponding N,N'-disubstituted benzimidazolium salt. Somewhat surprisingly the analogous imidazolinylidene dimers could not be isolated.4 We had suspected that the dibenzimidazolinylidene 2 might exist in equilibrium with an analogous b/s-carbene 5 and the position of equilibrium would be sensitive to steric and conformational effects imposed by different bridge lengths. The recent report of a similar tr/s-carbene, which was sterically restrained from dimerization, lent encouragement to this proposal. 5 Scheme 1
/(CH2)~
/ (CH2)~ - 2e"
(CH2)n (pFs.)2 111(n =3)
\ y
/ (CH2)n
/ (CH2)~ CH3CN (CH2)n
2a,b
311(n =3) b (n = 4)
b (n = 4)
/ (CH2)n,~
/ (CH2)~
(CH2)n
(CH2)n
4a,b
Sa,b
2741
(Br')2
2742
In an effort to possibly generate the bL~-carbene by deprotonation, we prepared the N,N'polymethylene bridged bis-benzimidazolium salts 3. Thus treatment of benzimidazole (6) with 1,3dibromopropane or 1,4-dibromobutane provided the neutral species 7. Methylation with methyl iodide led to the tethered diquaternary salt 8 while treatment with a second equivalent of the dibromoalkane provided the doubly bridged salts 3. It is noteworthy that in the synthesis of 3a, a minor product 10a was formed in 4% yield.
H i~.. H
I (CH2)~, Br-(CH2)n-Br I~----~N~.H H_.4:~F~ (n = 3,4) N N~"~/
/ (CH2)~ Br'(CH2)n'Br. (n = 3,4)
7a (n = 3, 81%) b (n = 4, 83%)
N\
/N (CH2)n
+
lOa
(Br')2
3a (24%) b (11%)
CH311 (n=4) H H CH3N(~)N/~]~N(~) N"CH3
Nail
8 (80%)
Air
0 C H ~N ,II~N~.~N
o ~I,,N,CH 3
9185"/.)
When 3a or 3b is treated with sodium hydride in acetonitrile in the presence of air, the ureaphanes 4Lb were obtained in yields of 73% and 54% ~spectively. These compounds are identical with those reported earlier from the reduction of la,b. To isolate the dibenzimidazolinylideneintermediate, it was necessary to carry out the reduction of 2 or deprotonation of 3 with the scrupulous exclusion of oxygen. Thus the electrolysis of l a at -1.10 V initially gave rise to an intense red color which was presumably due to the formation of the cation radical. Upon further reduction, the color faded and when the current had fallen to zero, a yellow solution was obtained from which the air sensitive 2a was isolated in 62% yield. In the IH NMR the benzo-protons exhibited an AA'BB' pattern centered at 56.74 and 56.26. The bridge methylenes showed an eight proton muhiplet at 52.98 and a four proton multiplet at 51.36. In the 13C NMR the benzo-carbons showed lines at 128.4, 119.5 and 105.6 while the central C--C appeared at 140.5 and the bridge carbons showed lines at 48.7 and 28.7 ppm. The identical material could be obtained in 68% yield when 3a was treated with sodium hydride in acetonitrile under anaerobic conditions. Electrolysis of l b at -1.40 V gave an intense purple color which faded as the current decreased to zero affording an orange precipitate in 43% yield. The NMR spectra were very similar to 2a with IH signals at i~6.76, 6.23, 3.23, 1.36 and 13C signals at 142.3, 120.0, 105.3, 50.2, 29.5 ppm (one benzo carbon obscured by C6D6). A minor side product (ca. 15%) was also detected by NMR. When either 2a or 2b was exposed to air, intense chemiluminescence was observed, the compound's color slowly faded to white, and ureaphanes 4a, h were obtained. The analogous deprotonation of 8 in the presence of air led to the corresponding bis-urea 9 but the intermediate dibenzimidazolinylidenewas not isolated.
2743
Tetraaminoethylenes react with Rh(I) to give bis-carbenoid adducts. 6 Thus
when
dibenzimidazolinylidene 2a is treated with Rh(I) 1,5-cyclooctadienyl chloride, a 1:1 adduct is obtained which gives a base peak in the FAB mass spectrum at 527 [M-CI]÷ and shows IH and 13C NMR spectra consistent with structure 11. The carbenoid carbon appears as a weak signal at 195 ppm..The structure of adduct 11 was confirmed by x-ray analysis and is illustrated in Figure 1. We still do not have evidence for the involvement of a b/s-carbene intermediate since a mono-carbene derived from a single deprotonation of 3 could attack the remaining benzimidazolium ion to lead to 2 without the involvement of 5.
Rh(COD)CI •
11
L..J 2-'
Figure 1. ORTEP diagram of Rh(I) adduct 11. We were interested in how the tetracation 10a might behave upon deprotonation. When lt)a is treated with sodium hydride in the presence of air a 69% yield of the trimethylene-bridged benzimidazolone tetramer 12 is obtained. In solution this material is conformationally mobile at room temperature on the NMR time scale and thus its geminal methylene protons are equivalent. Structure 12 is somewhat deceptive since the benzimidazolone carbonyls do not point towards one another as was ascertained by an X-ray crystal analysis. Figure 2 illustrates the molecular structure of 12 and it is evident that the benzimidazolone rings alternately point up and down. We are testing the ability of this material to organize itself around an appropriate guest such as a metal cation. 7
(Br ")4 1011
12 (69%)
2744
>2.0034 20 G
: =-
I
Figure 2. X-ray structure of tetramer 12
Figure 3. ESR spectrum of cation radical 13
It is noteworthy that while 1 is a good electron acceptor, 2 is an excellent electron donor. When equimolar quantities of l a and 2a are combined in acetonitrile, single electron transfer occurs and the coproportionation product is obtained. This species is a deep red, air sensitive cation radical 13 which shows a strong ESR signal (figure 3). Future studies will be directed towards the study of salts of this and other related cation radicals.
la
+
2a
CH3CN
M (PF6")
13
Acknowledgement. We would like to thank the Robert A. Welch Foundation and the National Science Foundation (CHE-9224686) for financial support of this work, Dr. James Korp for assistance with the Xray structure determination, and Prof. David Hoffman for use of the dry box.
References 1. Shi, Z.; Thummel, R. P. Tetrahedron Len. 1994, 35, 33. 2. Bourson, J. Ball. Soc. Chim. France 1971, 3541. 3. (a) Htinig, S.; Scheutzow, D.; Schlaf, H.; Quast, H. LiebigsAnn. Chent 1972, 765, 110. (b) Htinig, S.; Scheutzow, D.; Schlaf, H.; Quast, H. LiebigsAnn. Chent 1972, 765, 126. 4. (a) SchOnherr, H.-J.; Wanzlick, H.-W. Chem. Ber. 1970, 103, 1037. (b) Sch~3nherr, H.-J.; Wanzlick, H.-
W. Liebigs Ann. Chem. 1970, 731, 176. 5. Dias, H. V. R.; Jin, W. Tetrahedron Lett. 1994, 35, 1365. 6. Hitchcock, P. B.; Lappert, M. F.; Terreros, P.; Wainwright, K. P. J. Chem. Soc., Chem. Commun. 1980, 1180. 7. (a) Htay, M. M.; Meth-Cohn, O. Tetrahedron Lett. 1976, 79. (b) Htay, M. M.; Meth-Cohn, O. Tetrahedron Lett. 1976, 469.
(Received in USA 3 January 1995; revised 27 February 1995; accepted 28 February 1995)