G. Yamamoro, M. Oki, J. Chem. SOC. Chem. Commun. 1974, 713. a) F . Suzuki, Thesis, The University of Tokyo, 1975; b) F. Suzuki, M. Oki, H . Nakanishi, Bull. Chem. SOC. Jpn. 47, 3114 (1974). E . B. Barnett, M. A. Mathews, J. Chem. SOC. 123, 2556 (1923). All the new compounds gave correct elemental analyses. The 3.5-dimethylbenzyl group in these compounds showed evidence for restricted rotation about the C,,-CH2 bond: both the methyl groups and the o-H atoms became nonequivalent at low temperatures. Coalescence temperatures were about 50°C and the barriers ca. 16 kcal/mol. 'H-NMR (CDCla, -20°C): 6= 1.87(3 H, s,m-CHa), 2.38 (3 H, s, m-CHa),

s, IC-H), and 6.1-7.5 (11 H, m, aromatic). 'H-NMR (CDC13, -20°C): 6= 1.84 (3H, s, m-CHI), 2.18 (3H, s, l-CH,), 2.39 (3H, s, m-CHJ), 2.58 (3H, s, 4-CH3), 5.21 and 5.65 (2H, AB-q, J=18Hz, CHI), 5.59 ( lH , s, lC-H), and 6.0-7.5 (11H, m, aromatic). G. Yamamoto, M. 6 k i , Bull. Chem. SOC. Jpn. 48, 3686 (1975). IH-NMR (CDCla, -20°C): 6= 1.88(3 H,s,m-CH3),2.39 (3 H, s, m-CHs), 2.88 (3H, s, I-OCHs), 3.86 (3H, s, 4-OCH,), 5.31 and 5.54 (2H, ABq, J=18Hz, CH2), 5.89 ( lH , s, 10-H), and 6.2-7.5 ( l l H , m, aromatic). No sign of the existence of an a p form was found, probably due to the steric effect which makes the up form unstable relative to sc forms. Thus the rate process observed by the NMR spectra is assumed to be a (+)-sc+( - )-sc interconversion.

2.42 (3H, s, 4-CHs), 2.85 (3H, S, I-CHs), 5.43 (2H, S, CHI), 5.62 ( lH ,

Tetra-teut-butylcyclopentadienone[ 'I

By Giinther Maier and Stephan Pfriem"] While searching for precursors for the photochemical syn-

thesis of tetra-tert-butyl-cyclobutadiene and/or -tetrahed- raner'l our choice fell upon tetra-tert-butylcyclopentadienone (3). By systematic exploitation of the "antiaromatic"-type reactivity characteristic of the cyclopentadienone ring system, it proves possible to obtain this first example of a compound having four adjacent tert-butyl groups in an eclipsed arrange- ment.

Action of bromine on the tri-tert-butyl derivative (l)[3J led, in spite of steric hindrance, to a dibromide which readily eliminates hydrogen bromide on treatment with potassium hydroxide to form the bromo dienone (2). Replacement of the halogen in (2) by the alkyl group of tert-butyllithium is possible only under "forbidden" conditions, i.e. in 1,2- dimethoxyethane at room temperature (- 50°C should not

really be exceeded if reaction of the alkyllithium with solvent is to be avoided[41).

The spectroscopic data of (2) and (3) are listed in Table 1. Bromination of (I) is observed to give first the 2,Sdibromo derivative ( 4 ) ['H-NMR (CDC13): 6=4.88 (ring H), 1.58, 1.50, 1.38; I3C-NMR ([D6]-benzene): ring c atoms at 6=200.40, 159.22, 144.04,84.34, 52.19; IR (CC14): 1750cm-'] which rearranges in solution to the a$-unsaturated isomer ( 5 ) ['H-NMR (CDCI3): 6=4.68 (ring H), 1.60, 1.47, 1.18;

82.70, 61.90; IR (CCI4): 1710cm-']. Whether the bromine atoms in ( 4 ) and ( 5 ) are cis or trans has not been established.

I3C-NMR (C6D6): ring c atoms at 6= 197.80, 171.21, 149.10,

The unusual reaction conditions for the step ( 2 ) + (3) have their reason in the inability of the primary addition product (7) to eliminate a bromide ion. In a preceding step, it must undergo either a 1,5 shift of a tert-butyl group [ (7)+ (6)]-this is favored by enolate formation in (6)[51-

Table 1. Properties of (2) and ( 3 ) . The isolated compounds gave correct elemental analyses.

(6) (7) (8)

or an oxygen migration [ ( 7 ) + (8)]. However, such reactions require higher temperatures than are normally used in reac- tions with tert-butyllithium. Both (6) and (8) readily eliminate Bre to give the product (3).

Procedure

(2): To a solution of the dienone ( 1 ) (3.35g, 13.5mmol) in CC14 (125 ml)at room temperature is slowly added dropwise a mol. equiv. of a 1.55 M solution of Brz in CCI4. The almost quantitative addition is accompanied by decoloration of the solution. 6 N aqueous potassium hydroxide (150ml) is added, stirring is continued for 2d at room temperature, and the red cc14 phase is separated off, washed with water, and evapo- rated down after drying with MgS04. Recrystallization from light petroleum at -35°C affords (2) (3.55g, 80?b) as dark red crystals.

(3): To a solution of (2) (661 mg, 2.02mmol) in anhydrous 1,2-dimethoxyethane (125ml) under Nz at -10°C is added

~ ~~~~ ~~~~ ~

M. p. 'H-NMR I3C-NMR IR C"c1 (6) (4 [cm-'1

(2) 1 08-1 09 (CCI4): 1.46, Ring C atoms ([D6]-acetone): (KBr): 1.41, 1.30 193.93 (C=O), 179.03, 1701 (C-0),

173.75, 148.17, 1561 (C-C) 113.06 (C-Br)

( 3 ) 113-115 (CC14): 1.34 Ring C atoms ([DI2]-cyclo- (KBr): 1.24 hexane): 197.18 (C-0), 1688 (C=O)

175.77, 143.11

uv l n m l

MS [mkl

(cyclohexane): 434 (275), 200 (end ab- sorption, 19 200) (cyclohexane): 425 (1 86). 219 (20800)

328/326 (M+). 2711269 (M' -C4H9)

['I Prof. Dr. G. Maier, Dipl.-Chem. S. Pfriem Fachbereich Chemie der Universitat Lahnberge, D-3550 Marburg 1 (Germany) Present address: Institut fur Organische Chemie der Universitat, Hein- rich-Buff-Ring 58, D-6300 Lahn-Giessen (Germany)

within 5 min a solution of tert-butyllithium (3.07 mmol) in hexane. The mixture is maintained at this temperature for 15min and stirring then continued at room temperature for 2 d. Hydrolysis, extraction into light petroleum, evaporation

519 Angew. Chem. I n t . Ed. Engl. 17 (1978) No. 7

ofsolvent, and chromatography of the residue with light petro- leum on SOz gives (3) (135mg, 22 %) as yellow crystals.

Received: March 2, 1978 [Z 984a IE] German version: Angew. Chem. 90, 551 (1978)

CAS Registry numbers: ( I ) , 36319-94-5; (2), 66808-99-9; ( 3 ) , 66809-00-5; ( 4 1 , 66809-01-6; ( 5 ) , 66809-02-7

[ I ] Small Rings, Part 24. This work was supported by the Deutsche For- schungsgemeinschaft and the Fonds der Chemischen Industrie. We are indebted to Ms. U . Stanior for experimental assistance.-Part 23: G. Maier, U . Schafer, W Sauer, H. Harran, R . Matusch, J. F . M. Oth, Tetrahedron Lett. 1978, 1837.

[2] G. Maier, S . Pfriem, U . Schafer, R. Marusch, Angew. Chem. 90, 552 (1978); Angew. Chem. Int. Ed. Engl. 17, 520 (1978).

[3] G . Maier, F. Bosslet, Tetrahedron Lett. 1972, 1025. [4] U . Schollkopfin Houben/Weyl: Methoden der organischen Chemie, Vol.

XIII / l . Thieme, Stuttgart 1970, pp. 3-25, 87-253; M. Schlosser: Struktur und Reaktivitat polarer Organometalle. Springer, Berlin 1973.

[5] An analogous phenyl shift is considered as a possibility in the reaction of tetracyclone with phenyllithium: A. K . Youssef, M. A. Ogliaruso, J . Org. Chem. 37, 2601 (1972).

Tetra-tert-butyltetrahedrane[']

By Giinther Maier, Stephan Pfriem, Ulrich Schafer, and Rudolf Matusch[*]

The synthesis of a tetrahedrane is one of the most attractive challenges yet to be accomplished in organic chemistry. Many setbacks[*] and discouraging theoretical predictionsI3] have not deterred us from pursuing this goal.

Attempting the isolation of a tetrahedrane ( 1 ) raises a dilemma: valence-bond isomerization to cyclobutadiene (3) and fragmentation into two molecules of acetylene are orbital symmetry forbidden, thus imparting some degree of kinetic stability to ( 1 ) in spite of its high strain energy [540-574 kJ/mol (1 29-1 37 kcal/mol), i. e. 88-96 kJ/mol per skeletal bondI3g1]. Nevertheless, ( 1 ) can also undergo ring opening to the diradical (2). The energy barrier of this reaction path decides whether a tetrahedrane can be obtained. In the tetra-

19 .A.

methyl series there are indications that the bicyclobutanediyl diradical does not undergo ring closure to give the tetrahedrane but instead suffers rupture of the backbone bond to give cy~lobutadiene[~"~. Moreover, if it is also borne in mind that the tert-butyl groups will considerably lower the thermostabi- lity of a C-C bond[51, the prospect of preparing a (CH3)3C- substituted derivative of ( 1 ) appears very remote.

Our experimental results are therefore all the more astonish- ing: tetra-tert-butylcyclopentadienone (4)"l exhibits entirely different photochemical behavior from the corresponding com- pound containing just one tert-butyl group less [ ( 4 ) , H in place of tBu at C-21. While the latter cyclizes to housenone

[*I Prof. Dr. G. Maier, Dipl.-Chem. S. Pfriem, DipLChem. U. Schafer Fachbereich Chemie der Universitat Lahnberge, D-3550 Marburg 1 (Germany) Present address: Institut fur Organische Chemie der Universitat, Heinrich Buff-Ring 58, D-6300 Lahn-Giessen (Germany) Priv.-Doz. Dr. R. Matusch Institut fur Pharmazeutische Chemie Marbacher Weg 6, D-3550 Marburg 1 (Germany)

on irradiation with 405-nm light in argon at 10K[4b1, ( 4 ) does not undergo an analogous reaction (bridge formation between C-2 and C-5). However, if dienone ( 4 ) isolated in argon is excited by 254-nm light then the IR spectrum shows exclusive criss-cross addition to give the tricyclopentanone (5)-the carbonyl absorption of ( 4 ) diminishes and the dou- ble band corresponding to ( 5 ) appears. On prolonged irradia- tion, carbon monoxide is evolved (2135 cm- I), and the typical band of ketene (6) appears, which is also photolabile and liberates further CO, albeit very slowly (a small amount of (6) is still present after 3 weeks).

IR spectroscopy is insufficiently specific for identifying the hydrocarbons formed in this way. On GC/MS analysis of the photolytic product evaporated from the cold finger, only di-tert-butylacetylene (9) can be positively identified. The reaction sequence detected in argon can also be observed by IR spectroscopy in a Rigisolve matrix at -196°C or in solution between -130°C and room temperature; the final product contains only little acetylene (9) at low temperature but consists almost entirely of that compound under normal conditions.

Truly spectacular results were obtained from a long series of low-temperature 'H-, and especially ' 3C-NMR spectra. These measurements, which were indispensable for continuous checks of the irradiation experiments, served not only to opti- mize the reaction conditions, but also as a guideline in the isolation of the products. Thus the 'H-NMR signals of ( 5 ) , ( 6 ) , and (9) appear on irradiation of ( 4 ) in solution at room temperature. However, if the reaction is carried out at - 100°C in [Dlo]-diethyl ether, an additional signal appears at S = 1.21. This singlet belongs to a hydrocarbon which can be isolated by chromatography and gives three signals a t 6=32.26, 28.33, and 10.20 in the I3C-NMR spectrum. Such a spectrum is in complete agreement with that expected for tetra-tert-butyltetrahedrane (8)[@. In view of the downfield shift due to a tert-butyl group (AS =: 25) which we have noted in numerous model substances, a value of S = - 15 is calculated for the C atoms in the unsubstituted parent compound (l)['I .

The tetrahedrane (8) forms colorless crystals which are air- stable. The mass spectrum shows the correct molecular mass (field ionization, M + : m/e=276.1), and high resolution con- firms the composition C2oH36 (calc. 276.281 5; exp. 276.2814). The compound melts at 135 "C, a property which would make a tetrahedrane structure appear most unlikely if there were not the combined evidence of all the spectroscopic data listed in Table Final confirmation is expected from an X-ray structure analysis[8b1. Almost equally informative is the ther- mal behavior of (8)['1: heating of the tetrahedrane in cyclo- siland'o] effects quantitative isomerization at 130°C to give tetra-tert-butylcyclobutadiene (7). The colorless solution of (8) becomes yellowish orange owing to an absorption maxi- mum at 425 nm (E = 38) in the spectrum of (7). At the same

520 Angew. Chem. Ins. Ed. Engl. 17 (1978) N o . 7