- 5 "C. After 5 minutes' stirring at - 5 "C, the solution is ex- tracted with ether containing NaHC03, the extract washed with saturated NaHC03, and dried over Na2S04 (containing NaHC03) at 0 "C. After removal of solvent from the extracts (aspirator with a cold trap (C02/ether)) the residue is dis- tilled in uacuo into a cold trap (C02/ether). (2a)-(2c) are yellow liquids, whose absorption spectra are almost identical; yield 35% (based on conc. HCl), 93% and 88%, respectively; b.p. 30 "C/l tomf4], 40 "C/2 torr (46 "C/4 torrI5]), and 30 "C/ 2 torr (42 'C/SO torr[']), respectively.

Received: June 5, 1979 [Z 300 IE] German version: Angew. Chem. 91, 852 (1979)

CAS-Registry numbers: (la), 35358-33-9; ( lb ) , 1792-81-0; ( lc) , 5341-95-7; (2a). 71607-19-7; (Zb), 71607- 20-0 (Zc), 71607-21-1

lated and characterized. We have now synthesized and char- acterized 4-tert-butyl-3-tert-butylimino-l,2-dioxetane (3a) as the first compound of type (3). Besides assessing its thermal stability and chemi-energization efficiency, we thought that (3a) might isomerize to (5); this reaction would proceed via the elusive 1,4-dioxyl radical (4)[31.

N-tBu 1Bu

[ I ] a) Reviews: W. Adam, Adv. Heterocycl. Chem. 21,437 (1977); K. A . Horn, 1.- y . Koo. S. P. Schmidt, C. B. Schuster, Mol. Photochem. 9, 1 (1978/1979); b) W. H. Richardson. F. C. Montgomery, P. Slusser. M. B. Yelvington, J. Am. Chem. SOC. 97, 2819 (1975); c) for 1,Zdioxetane containing N atom(s), the CIEEL (Chemically Initiated Electron Exchange Luminescence) mechanism was proposed: F. McCapra, J. Chem. Soc. Chem. Commun. 1977, 946 J:y. Koo, C. B. Schuster, J. Am. Chem. SOC. 99, 6107 (1977).

[2] Compared with the chemiluminescence of luminol in aqueous solution: J. Lee, H. H . Seliger. Photochem. Photobiol. 4, 1015 (1965); 15, 227 (1972); J. Lee, A. S. Wesley, J. F. Ferguson, H. H. Seliger in E H. Johnson, Y Haneda: Bioluminescence in Progress. Princeton University Press, Princeton 1966, p. 35.

I31 Cf. W. Adam, 1. Erden, Angew. Chem. 90,223 (1978); Angew. Chem. Int. Ed. Engl. 17, 211 (1978).

[4] In spite of its extreme sensitivity to hydrolysis (Za) has been characterized: A,,, (CCI4)=390nm(sh)(Iog~=1.95), 375 (2.20), 36012.24). 340(2.18), 335 (2.08); i.,,, (neat)=2950 w, 1670 s, 1380 vs, 830 cm-' ; 'H-NMR (CCI,): S=2.44 (m, 4H, CHd, 6.03 (m, 2H, CHO).

[ 5 ] L. P. Kuhn, L. DeAngelis, J . Am. Chem. Soc. 76, 328 (1954); L. P. Kuhn, R. Wright, L. DeAngelis, i bd . 78, 271 9 (1 956).

A 3-Imino-1,2-dioxetane via Singlet Oxygenation of a Ketenimine'"] By Waldemar Adam, Ottorino De Lucchi, Helmut Quast, Rautgundis Recktenwald, and Faris Yany['I

The synthesis of stable 1,2-dioxetanes ( I ) has been abun- dantly documented and extensively utilized for the mechan- istic exploration of chemi-energized electronic excitation"]. The considerably less stable a-peroxylactones (2), which are

R V R 0-0 R p 0-0 R k N - I L 0-0

the postulated reaction intermediates in luciferin biolumines- cence, produce electronic excitation through the novel elec- tron exchange mechanism (CIEEL, i. e. chemically initiated electron exchange luminescence)f2I. They have also been iso-

[*I Prof. Dr. W. Adam (NIH Career Awardee. 1975-1980), Dr. 0. De Lucchi, Dr. F. Yany Department of Chemistry, University of Puerto Rico Rio Piedras, Puerto Rico 00931 (USA) Prof. Dr. H. Quast, Dipl.-Chem. R. Recktenwald Institut fur Organische Chemie der Universitat Am Hubland, D-8700 Wurzburg (Germany)

I**] Acknowledgments are made to the Donors of the Petroleum Research Fund (1 1022-ACl). administered by the American Chemical Society, the National Science Foundation (CHE-78-12621). the National Institutes of Health (GM- 00141-05 and RR-8102-07), and the Deutsche Forschungsgemeinschaft (Qu 25/ 6) for financial support.

Heating of the N-tert-butyl imidate (6) together with an excess of methyllithium in ether or tert-butyllithium in hex- ane effected demethanolation, affording high yields of the ketenimine (7) I4l. Tetraphenylporphyrin-sensitized photo-oxy- genation of (7) (0.02 M solution in CFCI,, CH2CI2, or xylene at - 78 "C, 150-W sodium street lamp, continuous purging with a slow stream of dry oxygen) led to complete consump- tion of (7) within 50 min.

Subambient 'H-NMR monitoring confirmed the presence of a thermally labile product, which exhibited proton reson- ances at 6=1.22 (9H, s), 1.60 (9H, s), and 5.59 (lH, s), con- sistent with structure (3a). On warm-up to ca. 30 "C, the sole products were 2,2-dimethylpropanal (9) and tert-butyl iso- cyanate (10). So far we have no evidence for formation of the oxazetidinone (5)I'I. On flash distillation of a CH2C12 solu- tion, the iminodioxetane (3a) codistilled with the solvent, showing that a monomeric material was on hand; however, extensive decomposition into aldehyde and isocyanate oc- curred. Isolation of (3a) in pure form was not attempted in view of its inherent explosive nature. Iodometric titration confirms that (3a) is formed in ca. 95% yield. Reaction with NaI in acetone/acetic acid gave the expected N-tert-butyl 2- hydroxy-3,3-dimethylbutyramide @)I6].

N/fBu I1

0

J tBuyH + c

/C H-C< 0 0 ii

tBu, NH-tBu

HO

These experimental facts clearly establish the proposed 3- imino-1,2-dioxetane structure (3a) for the singlet oxygena- tion product of the ketenimine (7). Furthermore, although no direct chemiluminescence could be observed in the de- composition of (3a), in the presence of fluorescers such as rubrene, pyrene, 9,10-diphenyl- or 9,1O-dibromoanthracene, appreciably enhanced chemiluminescence was detected. In view of the thermal instability of (3a) (half-lives are of the order of a few seconds at 0 "C), it was difficult to quantita- tively estimate the singlet and triplet excitation yieldscx1. However, our qualitative observations reveal that the excita- tion yields are much lower (ca. 1000-fold) than those ob-

788 0 Verlag Chemie, CmbH, 6940 Weinheim, 1979 0570-0833/79/1010-0788 $ 02.50/0 Angew Chem. In/. Ed. EngI I N (fY7Y) No. 1 0

served for the u-peroxylactones (2) and that the electron ex- change mechanism does not operate[2].

On the basis of our preliminary results, we conclude that the 3-imino-I ,2-dioxetanes (3) are thermally much less stable than the u-peroxylactones and are inefficient sources for chemi-energizing electronically excited products. In this con- text, it is of interest that the 3-imino-1,2-dioxetanes (3) de- rived from N-phenyl- and N-p-tolyl-diphenylketenimines and N-p-tolyl-tert-butylketenimine via singlet oxygenation were too unstable for 'H-NMR observation even at - 60 "C; only the carbonyl compound and isocyanate produced by fragmentation could be detected.

Received: August 17, 1979 [ Z 301 IE] German version: Angew. Chem. 91, 855 (1979)

CAS Registry numbers: (3a). 71607-22-2; (5). 71607-23-3; 161, 71607-24-4; (7). 71607-25-5; (8), 5510-45-2; (9). 630-19-3; (10). 1609-86-5

[ I ] a) W. Adam, Adv. Heterocycl. Chem. 21, 437 (1977); b) K. A . Horn, J . -Y. Xoo. S. P. Schmidt, G. 8. Schuster, Mol. Photochem. 9, 1 (1978/1979).

121 a) S. P. Schmidt, G. B. Schuster, J. Am. Chem. SOC. 100, 1966 (1978); b) W. Adam. 0. Cueto, F. Yany, J. Am. Chem. Soc. 100, 2587 (1978).

131 W. W Richardson, J. H. Burns. M . E. Price, R. Crawford, M. Foster, P. Sluss- er, J H. Anderegg, J . Am. Chem. SOC. 100.1596 (1978).

141 (71, b.p. 40-45"C/20 torr; 'H-NMR (CCI4), S=1.07 (1-Bu). 1.20 (N-t-Bu), 3.52 (C H), 2010 (C- G N ) . 1395 and 1370 cm ~ ' (1-Bu). Also direct dehydration of N-tert-butyl 3,3-di- methylbutyramide by successive treatment with n-butyllithium, benzenesul- fonyl chloride, and again n-butyllithium affords the ketenimine in moderate yields. (7) also results from the reaction between 1,3-di-terr-butylaziridinone and lriphenylphosphane C R Xrow, Angew. Chem. 83, 455 (1971); Angew. Chem Int. Ed. Engl. 10, 435 (1971).

151 We would expect a carbonyl band at f a . 17Xb1800 cm ' for (5); see C. Kresze, A . Trede, Tetrahedron I Y . 133 (1963).

161 CX). 52% yield m.p. 91-92°C (92°C [7]). The 'H-NMR and IR spectra were identical with those of an authentic sample.

(71 D. Giraud-Clinet. 1. Anafol, C. R. Acad. Sci. Ser. C 262, 224 (1966). [8] N . J. Turro, P. Lechtken, G. B. Schuster. J. Orell, H:C. Steinmetzer, W.

H); IR (CCL) i.=2960-2860 (aliphatic C

Adam. J. Am. Chem. SOC. 96. 1627 (1974).

1,4-Di-tert-butylsilabenzene: Generation and Trapping Reactions By Gotrfried Markl and Peter Hofmeister['I

Now that the heteroatom homologues of pyridine have proved to be stable aromatic compounds, increasing atten- tion is being turned to the question whether the higher homologues of benzene, i. e. silabenzene and germabenzene, are capable of existence.

Theoretical calculations on silabenzene have been per- formed by Dewar et al. (MIND0/3)['1, by Schlegel et al. (ab initio, GAUSSIAN 70 and FORCE program, STO 3G basis set)[21, and by Blustin (ab initio, Frost's Floating Spherical Gaussian Orbital (FSGO) Model)I3l. These studies give val- ues of bond lengths and angles showing some degree of di- vergence; calculations by Dewar et al. and Schlegel et al. show silabenzene to have a singlet ground state and an aro- matic stabilization energy of an order comparable with that of benzene.

According to the ab initio calculations, silabenzene exhi- bits strong polarization with a low electron density on silicon and an enhanced electron density in positions 2, 4, and 6-as expected from the electronegativities xc = 2.50 (2.55) and

The problems encountered in the silabenzene system are therefore due mainly to its pronounced electrophilic reactivi- ty, arising from the 6 + character of silicon. The silabenzene ring must be synthesized under conditions avoiding the pres-

x s i d . 7 4 (1.90).

['I Prof. Dr. G. Markl, P. Hofmeister Institut fur Chemie, Fakultat fur Chemie und Pharmazie der Universitat Universitatsstrasse 31, D-8400 Regensburg (Germany)

ence and/or formation of nucleophiles capable of addition to the Si-C bond.

Barton et aZ.l4l have presented evidence supporting the for- mation of I-methyl-I-silabenzene (2), on elimination of HC1 from ( I ) with the sterically hindered base LiN(Si(CH3),)*, by trapping of (2) with hexafluoro-2-butyne to give the l-sila- barrelene derivative (3).

H

Simultaneous formation of I-[N,N-bis(trimethylsily1)ami- no]-l-methyl-l-sila-2,4-cyclohexadiene suggests that the elimination is accompanied by substitution at the silicon atom.

Starting from 1,l -di-n-butyl-4-tert-butyl-4-methoxy- 1,4- dihydrostannin, we have prepared 1 ,4-di-tert-butyl-l -chloro- 1 -sila-2,4-cyclohexadiene (4) by a method resembling one which we recently described['].

The sole product of reaction, apart from LiC1, of (4) with lithium diisopropylamide in n-pentane at 0 "C is a colorless crystalline compound, m.p. 204 "C, shown by analytical and spectroscopic data (Tables 1 and 2) to be the dimeric silaben- zene (6), having a 1,3-disilacyclobutane structure, which was predicted by Dewar et al.['I.

:1 H H ,

Is 1,4-di-tert-butylsiIabenzene (5) an intermediate of dimer formation?

No diisopropylamine substitution product is formed on hydrogen chloride elimination; in contrast to the case with ( I ) , the steric effect of the tert-butyl group attached to silicon prevents both substitution of (4) by LiN(iPr)2 and the con- ceivable addition of LiN(iPr)* or HN(iPr)2 to the Si-C dou- ble bond of (5).

This result rules out formation of (6) by a pathway other than dimerization of intermediate silabenzene (5). The Si-C double bond of (5) thus behaves in an entirely analogous manner to silaethenes, which likewise dimerize to 1,3-disila- cyclobutanes['I.

Support for elimination of HCl from (4) to give (5) is also provided by the reaction of 4-tert-butyl-I-chloro-I-methylsi- la-2,4-cyclohexadiene (7) with lithium diethylamide which, in contrast to (4), gives almost exclusively the substitution product, which then isomerizes to the 2,5-diene (8).

Formation of (5) from (4) is proved just as unequivocally by the reaction of (4) with LiN(iPr)2 in n-pentane in the pres- ence of 1,3-dienes (1,3-butadiene, isoprene, 2,3-dimethyl-I ,3-

Angem: Chem. In/. Ed. Engl. IX ( IY?Y) No. 10 0 Verlag Chemie, CmbH, 6940 Weinheim, 1979 05?0-0833/79/1010-07X9 $ 02.50/0 789