Tetrahedron Letters,Vo1.29,No.40,pp 5157-5160,1988 0040-4039/88 $3.00 + .OO Printed in Great Britain Pergamon Press olc

A 13C NMR STUDY ON THE TRIS(PHENYLETHYNYL)METHYL AND

ci,ci-BIS(PHENYLETHYNYL)BENZYL CATIONS: THE TRIS- AND BIS-ETHYNOLOGUES OF

THE TRIPHENYLMETHYL CATION

Koichi Komatsu,* Toshihiro Takai, Shuji Aonuma, and Ken'ichi Takeuchi

Department of Hydrocarbon Chemistry, Faculty of Engineering,

Kyoto University, Sakyo-ku, Kyoto 606, Japan

Summary: Comparison of the 13

C NMR data of the title cations with those of the triphenylmethyl cation and its monoethynologue indicates the increasing contribution of the allenyl-type resonance structure upon consecutive introduction of the ethynyl component.

The alkynylmethyl cation (A) has attracted considerable interest,'IL

since it can also be expressed as its mesomeric allenyl cation form (B) which

is structurally related to the vinyl cation. 3

The l3 C NMR spectroscopy

appears to be particularly suited for study of this type of carbocations, and

has been applied to estimate the relative contribution of the two mesomeric

structures by using the carefully chosen reference compounds. le

In contrast to

the monoalkynylmethyl cations, however, iittle has been studied on the di- and

trialkynylmethyl cations so far. lb In a series of systematic studies on the

ethynologues of triphenylmethane dyes, 2

Akiyama and coworkers have attempted

to isolate the salts of tris- and bis(phenylethynyl)methyl cations 1 and 2, but

instead they found an intriguing transformation of these cations into the

2,6_diphenylpyrylium ions. 2b

Here we report the preparation and the 13

C NMR study of the cations 1 and

2, which constitute, together with the previously reported cation 3, le

a series

of mono- to trisethynologues of the long-known triphenylmethyl cation 4.

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Although the alcohols 5 and 6 were once prepared and reported to exhibit

marked halochromism, 4 no spectral study has ever been carried out on the colored

species probably due to their extreme instability. However, when ca. 5 equiv

of FS03H-SbF5 (1:l) was added very carefully to a cooled mixture (-7B°C) of the

corresponding al.cohol in S02ClF under argon with sufficient agitation, there

resulted a deep blue-violet (from 5) or red-purple solution (from 6), which

exhibited the 13C NMR spectrum clearly assignable to the cations 1 or 2,

respectively. The assignments were confirmed by examining the cations having

the central carbon 90% enriched with 13C. ' The cations were unstable and

remained unchanged only at temperature below -78'C for about 8 hours. The 13C

chemical shift data for 1 and 2 are shown in Table 1 together with those of

related cations and neutral precursors for comparison.

R2 R3 R' RL

Rl 5 Ph-s.- Ph-I- Ph_=_- R'

R2-A-OH 6 Ph-z- Ph-z- Ph- Ph-s- Ph-G-

k3 7 Ph-=_- Ph- Ph- Ph-a- Ph-

8 Ph- Ph- Ph-

Table 1. 13C NMR Chemical Shiftsa of Cations and the Related Compounds

/---a-Phenyl Comp Ce

c8 cY C. C

ortho C meta

C' /-- 8-Phenyl

C. C C 7 lpS0 para ipso ortho meta para

lb= 139.8 111.4 138.4 - - - - 120.1 138.7 131.2 140.2

2bd 162.1 106.5 145.5 140.7 139.9 132.2 138.0 120.4 138.6 131.3 137.6

je* 186.8 105.9 159.1 138.7 139.4 131.4 144.0 119.9 138.7 130.7 137.6

4gh 211.8 - - 140.8 144.2 131.2 144.0 - - - -

si 55.7 86.5 83.1 - - - - 121.5 132.0 128.2 129.0

6i 65.7 89.1 84.9 141.9 125.7 128.6 128.4 121.8 131.6 128.0 128.3

7 fi 74.5 91.5 86.6 144.6 125.7 128.0 127.4 121.9 131.4 128.0 128.0

8hj 80.3 - - 150.1 128.6 128.6 128.6 - - - - ik

,', il 61.9 72.7 84.5 86.7 83.9 86.7 140.6 - 126.7 - 128.8 - 128.7 - 121.1 122.0 131.6 131.8 128.0 128.3 128.8 128.4

a A/ppm from TMS. b Measured in SO2ClF from 13C signal of (CD3)2CO (30.0 ppm) in a capillary held inside the IO-mm sample tube. c At -78O-6. d At -90°C. e In SO2 at -6O“C. f Ref. le. S In ClS03H. h Ref. 7: Original data are con- verted using 6cs2 193.7. i In CDC13. j In THF. k 6CCH3 52.4 ppm. 2 GoCH353.1 ppm

The marked deshielding of C, and CY in the cation 3 relative to the

precursor alcohol 7 has been interpreted as due to substantial contribution

from the mesomeric form B.le As clearly seen from Table 1, the C, signal is

dramatically shifted upfield upon consecutive introduction of the ethynyl group.

The C y signal also undergoes the upfield shift, but is still considerably

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deshielded in 1 as compared with that in the alcohol 5. Although the amount of

downfield shift relative to the precursors can not be directly correlated to

the charge density for the carbons of different hybridization, comparison of

these data indicates that the degree of positive charge delocalization to Cy

apparently increases as the number of the ethynyl group increases. 6

The charge densities on each of the phenyl carbons were estimated using the

equation, qi = (&CiR+ - &CiRoH)/160,7'8 and were summed up for each benzene ring to

give the results shown in Table 2. These data demonstrate that a phenyl group

at the cc-position constantly bears a larger amount of positive charge than that

at the y-position. Thus, the positive charge residing in the cross-conjugated

triynylmethyl system in the cation 1 amounts to about 0.45 of the total charge.

Table 2. The Empirical Charge Densities per Benzene Ring

Cation u-Phenyl y-Phenyl 1.000 -cqph

1 0.183 0.451

2 0.275 0.178 0.369

3 0.281 0.173 0.265

4 0.266 0.202

The deep blue coloration was observed also upon treatment of the alcohol 5

with other Br#nsted or Lewis acids such as CF3C02H, SbC15, SnC14, and Ph3C+BF4-

in CH2C12. However, the major species was shown by 13C NMR to be either the

precursor alcohol 5 or decomposed materials. Only when treated with a large

excess (ca. 17 equiv) of BF3-0Et2 in CH2C12 at -78"C, the cation 1 was shown to

be present in equilibrium with 5 in a ratio of ca. 1:2. Similarly the cation 2

was generated, together with a considerable amount of decomposition products.

The generation of 1 and 2 was chemically confirmed by formation of the methyl

ether 9 and 10 in yields of 28% and 68%, respectively, upon quenching these

solutions with K2C03/MeOH.

Finally, the visible spectra of the cations 1 and 2 generated by BF3'0Et2

were recorded by the use of stopped-flow/rapid-scan technique. The maximum

absorptions of 1 and 2 observed at 0.05-0.8 second after generation are shown in

Table 3 together with those reported for 3 and 4.1d The results demonstrate

that the longer-wavelength absorptions are gradually red-shifted in accord with

the widening of r-conjugation system upon consecutive introduction of the

phenylethynyl group.

Acknowledgement: Encouragement by Professor Emeritus Kunio Okamoto of Kyoto

University and by Professor Shuzo Akiyama of Nagasaki University is gratefully

acknowledged. This work was supported by Grant-in-Aid for Special Project

Research from Ministry of Education, Science and Culture (No. 62124035).

5 160

Table 3. Visible Absorptions of the Cations

Cation

la 2a

jb qb

h /nm (log s) Solvent

547 (-4.6) 632 (-4.9) 508 (~4.7) 571 (-4.8) CH2C12

449 (4.42) 504 (4.57) 403 ( 4.59) 429 (4.59) FS03H-SbF5

a Obtained by mixing a 6~10~~ M solution of alcohol with

a 3x10-' M solution of BF3.OEt2 in CH2C12 at 19'C: Values

of extinction coefficient were estimated assuming the

quantitative generation of cations. b Ref. Id.

References and Notes:

1.

2.

3.

4.

5.

6.

7.

8.

a) H. G. Richey, Jr., J. C. Philips, and L. E. Rennick, J. Am. Chem. Sot.,

87, 1381 (1965); b) H. G. Richey, Jr., L. E. Rennick, A. S. Kushner, J. M.

Richey, and J. C. Philips, ibid AI 87, 4017 (1965); c) C. U. Pittman, Jr.,

and G. A. Olah, ibid., 87, 5632 (1965); d) G. A. Olah, C. U. Pittman, Jr.,

R. Waack, and M. Doran, ibid., 88, 1488 (1966); e) G. A. Olah, R. J. Spear,

P. W. Westerman, and J-M. Denis, ibid., 96, 5855 (1974); f) H.Mayr and R.

Schneider, Chem. Ber., 115, 3470 (1982); g) G. K. S. Prakash, V. V.

Krishnamurthy, G. A. Olah, and D. G. Farnum, J. Am. Chem. Sot., 107, 3928

(1985); h) V. V. Krishnamurthy, G. K. S. Prakash, P. S. Iyer, and G. A. Olah,

ibid LI 108, 1575 (1986).

a) S. Akiyama, K. Yoshida, M. Hayashida, K. Nakashima, S. Nakatsuji, and M.

, 1981, 311; b) S. Nakatsuji, K. Nakashima, K. Yamamura,

and S. Akiyama, Tetrahedron Lett., 25, 5143 (1984); c) S. Nakatsuji, N.

Okamoto, K. Nakashima, and S. Akiyama, Chem. Lett., 1986, 329; d) S.Akiyama,

S. Nakatsuji, K. Nakashima, and M. Watanabe, J. Chem. Sot., Chem. Commun.,

1987, 710.

H. G. Richey, Jr., and J. M. Richey, "Carbonium Ions," Vol. 2, G. A. Olah

and P. v. R. Schleyer, Ed., Wiley-Interscience, New York (1970), p. 899.

K. Hess and W. Weltzien, w, 54B, 2511 (1921).

The corresponding alcohols were prepared from PhCX-13C02H and Ph13C02H via

esterification and reaction with PhCzCLi.

The values of one bond 13C-13C coupling constant (JC,_Cg/H~) were measured

as follows: 1, 104.3; 2, 96.8; 5, 86.5; 6, 82.1. The difference in Jca_cB

between the cation and the precursor alcohol also appears to increase as

the more ethynyl group is introduced.

G. J. Ray, R. J. Kurland, and A. K. Colter, Tetrahedron, 27, 735 (1971).

D. G. Farnum, "Adv. Phys. Org. Chem.," Vol.11, V. Gold and D. Bothell, Ed.,

Academic Press, New York (1975), p. 123.

(Received in Japan 31 May 1988)