Studies on the Reaction of Cyclopentadienyltri-n- butyltin with Diethyl Azodicarboxylate

Wojciech J. Kinart"'*, Cezary M. Kinartb, Monika Kozaka, Zdzislaw Kinartb

aDepartment of Organic Chemistry, University of Lodz, 90-136 Lodz, Narutowicza 68, Poland hDepartment of Chemical Education, University of Lodz, 90-236 Lodz, Pomorska 163, Poland

ABSTRACT

The introduction of the tributylstannyl group at the cyclopentadiene ring affects its products with diethyl azodicarboxylate (DEAD). The reaction leads to the M-ene product in contrast to the analogous addition with cyclopentadiene. The obtained product undergoes dimerisation after purification on silica and then oligomerisation.

INTRODUCTION

The story begins in 1921, when Diels and Back /I/, pursuing a line of investigation that had been initiated earlier by Curtius, published a paper on the reactions of diethyl azodicarboxylate. Curtius had found that one could convert the functions to amide functions by reactions with primary amines.

Et02C-N=N-C02Et + 2RNH2 • RHNOC-N=N-CONHR + 2 EtOH (1) (2)

Scheme 1

Diels and Back found that although some aromatic amines conformed to this pattern of substitution, an exception emerged in the case of ß-naphtylamine, which gave an entirely different type of reaction, namely an addition leading to product 3 (Scheme 2).

257 Vol. 29, No. 5, 2006 Studies on the Reaction of Cyclopentadienyltri-n-butyltin With Diethyl Azodicarboxylate

Et02C. .NHC02Et Ν

,ΝΗ

(3)

Scheme 2

The nature of the reaction as an addition rather than a substitution was apparent just from the elemental analysis of the product, but Diels and Back expended a considerable effort to prove the structure by independent synthesis. The obvious next step was to try the reaction of cyclopentadiene with diethyl azodicarboxylate. However, as reported by Diels, Blom and Koll 121, the actual product 6 was again a new type, which resulted from cycladdition of Q and C4 of the diene to the N=N bond.

C02Et C02Et Ν I (4) Ν (i) || Ν II Ν I C02Et w w C02Et

NC02Et

Λ NC02Et

(6)

Scheme 3

This result seems in turn to raise the question whether any modification of the reaction conditions might lead to the adduct 5 .

258 Kinart et al. Main Group Metal Chemistry

EXPERIMENTAL

NMR spectra were recorded at 26.8°C on a Bruker Avance DRX-500 spectrometer operating at 2 500.13/125.77 MHz (B0 = 11.7 T) for solution in [ H] . Chemical shifts are in δ values and reported in ppm downfield from TMS used as internal standard. Coupling constants J are given in Hz (first- order analysis). TLC analysis was conducted on silica-gel foils 60 F254 from Merc (Darmstadt, Germany) and spots were visualized by UV (254 nm) irradiation. Silica-gel type 60 (0.063-0.200 nm) from Aldrich was applied for the gravity-column chromatography. Petroleum ether refers to the fraction of bp 40-60°C. Melting point (uncorrected) was determined on a Boetius heating stage/microscope. Cyclopentadienyltri-n-butyltin was purchased from Gelest, Inc. and used after distillation. All other commercial reagents were provided by Aldrich and used without further purification.

Diethyl l-(cyclopenta-2,4-dienyl)-hydrazo-l,2-dicarboxylate, its dimer 11 and its oligomer 12

The solution of cyclopentadienyltri-n-butyltin (710.3 mg, 2 mmol) and diethyl azodicarboxylate (348 mg, 2 mmol) in 10 mL of benzene was stored for 24 hours at room temperature (after this the orange color of the mixture resulting from DEAD had disappeared). The product was chromatographed using the mixture of petroleum ether-ethyl acetate (8:2, v/v). It was obtained in quantitative yield and was identified as 11 diethyl N-[8-(diethoxy-N,N-dicarbonylhydrazine)-3a,4,7,7a-tetrahydro-lH-4,7-methano-inden-l-yl)]- N,N -dicarboxylate. Its NMR spectrum exhibited the following values of chemical shifts: 'H NMR (CDCI3)-

Oil δΗ 1.08 (12H,m); 3.36 (4H, br s); 4.23 (8H, q, J=6.6 Hz); 6.00 (2H, d, J=0.8 Hz); 6.08 (2H,s); 6.39 (2H, br s); 7.50 (2H, br s). °C NMR (CDClj) δ 13.31,14.11, 26.59, 27.57, 43.49, 44.24, 61.86,61.99,62.29,62.46, 62.75, 124.98, 131.12, 156.80, 155.99. Within the next few weeks, we observed formation of oligomer 12 from dimer 11 in the form of yellow crystals, mp 101-103 °C. Its NMR spectrum exhibited the following

values of chemical shifts: 'H NMR (CDC13) (pentane) δΗ 1.25 (24H, m, 8xCH3), 1.64 (4H, m), 2.03 (4H, m),

2.85 (2H, m), 4.17 (16H, m, 8xOCH2), 5.34 (2H, br s), 5.95 (2H, m), 6.14 (2H, m), 6.23 (2H, m), 6.42 (2H, br s), 6.60 (2H, br s). t3 C NMR (CDCI3) δ 13.23 (CH3), 13.92 (CH3), 14.04 (CH3), 14.08 (CH3), 26.94, 43.69, 50.31, 53.09, 61.41, 61.52, 61.69, 61.94, 62.15, 62.49, 111.40, 128.85, 130.36, 132.41, 136.56, 153.58, 154.68, 155.31, 155.79,156.42,156.68. Low-resolution mass spectrum (MS) of 12 was taken using chemical ionization (CI) technique with a Finningan MAT 95 instrument (source temperature of ca. 200°C, reagent gas-isobutan, accelerating voltage of 4.8 kV). MS (CI) m/z (%) = 721.5 (M+l, very small) corresponding to trimer, 481.4 (44) corresponding to dimer 11 and 241.2 (100) corresponding to monomer.

RESULTS AND DISCUSSION

We have focused our attention on cyclopentadienyltri-n-butyltin as a possible starting material for such

259 Vol. 29, No. 5, 2006 Studies on the Reaction of Cyclopentadienyltri-n-butyltin With Diethyl Azodicarboxylate

synthesis. The cyclopentadienyltin (IV) compounds undergo Diels-Alder cycladditions with reactive dienophiles such as maleic anhydride, diethyl maleate, and diethyl acetylenedicarboxylate /3/, and an endoperoxide has been identified from the reaction with singlet oxygen /4/. The Diels-Alder reactions have frequently been employed for formation of a six-membered skeleton in synthesis of fine chemicals and pharmaceuticals. The cyclopentadienyltin (IV) compounds are usually yellow oils or crystals, which are sensitive to air, light, and moisture. On standing, they darken in color, which is probably due, at least in part, to Diels-Alder polymerisation. In the case of cyclopentadienyltri-n-buyltin, this process of dimerisation

occurs within few months. Its diluted solutions in CDC13 are considerably less susceptible to such process. No considerable growth of the concentration of the dimer in 0.2 Μ solution of cyclopentadienyltri-n-butyltin has been observed within two months. Consideration of the dissociation energies of the sp2C-H, sp3C-H, sp2C-Sn and sp3C-Sn bonds suggests that the vinylic isomers 8 and 9 should be more stable than 7, but all structural studies indicate that in gaseous, liquid, or solid state, the sp3C-bonded isomer 7 is the most stable. Only when the metal is at the 5(sp3)-position can the molecule benefit from C-Sn hyperconjugation with symmetrical butadiene LUMO.

Scheme 4

The five cyclopentadienyl protons show only one sharp signal at 6.14 ppm which has satellites at both sides originating from H-"7Sn and H-119Sn coupling /5/. Also 13C NMR spectrum of cyclopentadienyltri-n- butyltin revels one signal at 112.00 ppm and four other at 28.15, 16.30, 13.57 and 11.21. The NMR spectra of l,8-ditributylstannyl-3a,4,7,7a-tetrahydro-lH-4,7-methano-inden 10, which was obtained by storage of the monomer for a few months in a sealed flask at room temperature, are more complex.

The 'H NMR spectrum recorded by us in the present study of this dimer reveals the following signals: δΗ

(CDC13) 6.56 (2H, dq, J=5.3 and 1.3 Hz), 6.48 (2H, dq, J=5.3 and 1.3 Hz), 2.99 (2H, m), 1.62 (6H, m), 1.37 (6H, m), 1.15 (4H, m), 0.92 (15H, m). The I3C NMR spectrum reveals signals at 134.58, 125.14 and 112.00,

260 Kinart et al. Main Group Metal Chemistry

corresponding to two pairs of vinylic carbons C2, C3 and C5 and C6 instead of the single signal at 112.00 corresponding to the monomer. Discussed results are analogous to these observed by Nakagawa and later by Onaka for cyclopentadiene and its dimer /6/. We have been particularly interested in studies of reactions of allyltin compounds and organotin phenoxides with different enophiles. We have studied reactions of different allyltin compounds with 4- phenyl-l,2,4-triazoline-3,5-dione, diethyl azodicarboxylate (DEAD) and singlet oxygen, and organotin phenoxides with (DEAD), bis(trichloroethyl) azodicarboxylate and diethyl acetlenedicarboxylate. All studied reactions with (DEAD) led only to the formation of M-ene product. Additionally, we have observed that allyltin compounds and organotin phenoxides are considerably more active than the parent olefins and phenols Π/. Therefore, we were curious whether substitution of cyclopentadiene ring in position 1 by tributylstannyl group would favor the formation of the M-ene product at the expense of the cycladdition product. In order to explain this effect, we have prepared an equimolar mixture of cyclopentadienyltri-n- butyltin and diethyl azodicarboxylate in benzene. After disappearance of the color of the , the solvent was removed and the product was isolated by gradient chromatography (light petroleum-ethyl acetate (8/2 v/v) as eluent) and identified by NMR spectroscopy. Obtained compound has been identified as a dimer of the M-ene product. No trace of the simple M-ene product could be detected in the reaction mixture. Presumably, the dimerisation reaction was very fast and the newly formed M-ene product was immediately converted into dimer.

(11)

Scheme 6

It has been identified as diethyl N-[8-(diethoxy-N,N-dicarbonylhydrazine)-3a,4,7,7a-tetrahydro-lH-4,7- methano-inden-1 -yl)]-hydrazine-N,N -dicarboxylate 11. Within a few days, we have observed formation of

261 Vol. 29, No. 5, 2006 Studies on the Reaction of Cyclopentadienyltri-n-butyltin With Diethyl Azodicarboxylate the oligomer from dimer. Assuming that the last reaction is of second order we have carried out kinetic measurements. The progress of the reaction at 298 Κ was monitored by NMR by measuring the decrease of the intensities of signals at δ 6.00 and 3.36 ppm corresponding to C2-H and C3-H vinyl protons and C4-H,

C7-H, C3a-H and C7a-H protons respectively. We have prepared three samples of dimer 11 of masses 30, 110 and 150 mg and we have dissolved them in NMR tubes in CDC13 to obtain solutions of volume equal to 0.7 cm3. We have measured times corresponding to the decrease of the initial intensities by 50 %. The rate constant found from these measurements was equal to approximately 3.910"5 dm3 mol'1 s"1. The oligomer 12 obtained from 11 was characterised by mass spectroscopy and NMR spectroscopy. It is known that upon being heated, cyclopentadiene polymerises by successive Diels-Alder addition reactions. These cyclopentadiene oligomers have been identified as endo- and exo-cyclopentadiene dimer, endo- cyclopentadiene trimer and endo-cyclopentadiene tetramer /8/. We assume that oligomerisation of 11 leads to the formation of tetramer or a greater oligomer. However, the prediction of its structure will require further studies. The analysis of obtained data seems to indicate that the oligomer 12 contains the unit of endo- dicyclopentadiene, substituted by two diethyl hydrazinedicarboxylate groups in positions 1 and 8. The proton magnetic resonance spectra of cyclopentadiene oligomers were previously reported by Foster /8/ and Kochi 191.

REFERENCES

1. O. Diels and J. Back, Chem. Ber. 54, 213-226 (1921) 2. O. Diels, J.H. Blom and C. Koll, Ann. 443, 242-262 (1925). 3. H. Gilman and L.A. Gist, J. Org. Chem. 22, 250-255 (1957). 4. G.O. Schenck, E.A. Körner von Gustorf and H. Köller, Angew. Chem. 73, 707-708 (1961). 5. H.P. Fritz and C.S. Kraiter, J. Organomet. Chem. 1, 323-327 (1964). 6. I. Imachi and M. Onaka, Tetrahedron Lett. 45, 4943-4946 (2004).; K. Nakagawa, S. Iwase, Y. Ishii, S. Hamanaka and M. Ogawa, Bull. Chem. Soc.Jpn. 50, 2391-2395 (1977). 7. W.J. Kinart and C.M. Kinart, J. Organomet. Chem. 691, 1441-1451 (2006).(and references therein). 8. R.G. Foster and M.C. Mclvor, J. Chem. Soc. (B), 1969, 188-192. 9. R.G. Salomon and J.K. Kochi, J. Am. Chem. Soc. 96, 1137-1144 (1974).

262