introduction of these carbenes in [i + 2] cycloaddition reactions. For this purpose, we studied the possibility of generating (2-furyl)chlorocarbene (FCC) and ...
(2-FURYL)- AND (2-THEINYL)CHLOROCARBENES -- NEW CARBENES WITH HETEROCYCLIC SUBSTITUENTS* I. E. Dolgii, K. N. Shavrin, I. V. Krylova, and O. M. Nefedov
UDC 542.91:547.512:547.73:547.72
According to Schecter et al. [3], 2-furylcarbenes generated by the pyrolysis of sodium salts of 2-oxomethylfuran tosylhydrazones even in the presence of active olefin acceptors predominantly undergo intramolecular isomerization to the corresponding 4-oxo-3-penten-lynes. 2-Thienylcarbenes obtained by a similar pathway are somewhat more stable [3]. We should expect that the introduction of a chlorine atom to the carbene carbon atom of furylcarbenes or thienylcarbenes would increase their s~ability and thereby permit the introduction of these carbenes in [i + 2] cycloaddition reactions. For this purpose, we studied the possibility of generating (2-furyl)chlorocarbene (FCC) and (2-thienyl)chlorocarbene (TCC) by the action of t-BuOK in the presence of dibenz-18-crown-6 at --20~ or KOH in the presence of triethylbenzylammonium chloride (TEBAC) at 20~ on 2-dichloromethylfuran (la) or 2-dichloromethylthiophene (Ib), respectively. FCC and TCC generated under these conditions in the presence of a 7-10-fold molar excess of olefin (II) add at the C=C bond to form the corresponding 2-furyl- or 2-thienylchlorocyclopropanes (III) in yields from 26 to 67%. In this case, the formation of 12% 2-(chloromethylene)-5-tert-butoxy-2,5-dihydrofuran (IV) is noted in the case of FCC upon its generation by the action of t-BuOK on (Ia) in addition to chlorocyclopropanes (III, Z = 0), even in the case of rather active (II), apparently due to the consecutive addition of t-BuO- and H+ to the bipolar form of FCC [2] R
V
RI
t-BuOX /2--~~ _ ~ _ R,+ x=o
~ --~j--C[-ICls qX
N C = C / -R/' \R3
(I)
(If)
\ ^ u / CI I RI3 R
l~on
/~,, =(;/
/ t-BuO/\O (IV)
H~
\CI,
R1
/J-xX A R, ' \ x / cl, ~, (III)
X = O ~ S; R=RI=H, R2=Ra=Me (a);R=R2=Me, R I = R = H (b);R=R~=R~=Me, :RI=H (c);R=R~=R~=R~=Me~). Under these conditions, FCC and TCC react with cis-2-butene (lib) with complete steric specificity, which, according to Skell's hypothesis [4], indicates their singlet ground state. In this case, a mixture of cis and trans cyclopropane adducts (lllb) is formed as a consequence of the presence of two inequivalent substituents in these carbenes,+ in which the cis/trans ratio is 1:1.8 in the case of l-chloro-l-(2-furyl)-2,3-dimethylcyclopropane and 1:1.2 in the case of l-chloro-(2-thienyl)2,3-dimethylcyclopropane. The determination of the configuration of the minor isomer as cis and of the major isomer as trans was made on the basis of an examination of the deshielding effect of electron-withdrawing groups in analogous gem-chlorophenylcyclopropanes [5]. The method of Moss [6] was used to establish the philicity of FCC entailing the competitive reactions of this carbene generated from (la) by the action of KOH in the presence *For preliminary communications, see [i, 2]. #The isomers are indicated relative to the heterocyclic substituent. N. D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR, Moscow. Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. i0, pp. 2380-2385, October, 1986. Original article submitted April 17, 1986.
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9 1987 Plenum Publishing Corporation
TABLE i Parameter
lg
(k/ko+i.O)
Isobutylene IcisrZ-Butenel2-Methyl-2- 2,3-Dimethyl-2(IIa) (lib) butene (llc) butene (lid)
i
0,362
t.16
1,64
of TEBAC with pairs of methyl-substituted ethylenes (II) taken in 10-fold molar excess with isobutylene (lla) as the standard. The k/ko ratio was obtained from the gas-liquid chromatographic analysis data and, in the case of (lla) and (llb), from the PMR spectral data. The values obtained for k/ko and log(k/ko + 1.0) are given in Table i. The experimental selectivity index (SI) of FCC was found according to Moss [6] to be 0.95 and the calculated SI was found equal to 1.37. The significant difference between the calculated and experimental SI values may be explained upon examination of the reaction of the p-orbitals of the carbene site with the heterocycle z-orbitals. FCC may clearly exist in either conformation A, in which the plane of the furan ring (FR) is perpendicular to the vacant p-orbital of the carbene or conformation B, in which the heterocycle and the p-orbital of the carbene site lie in the same plane. In this case, the stabilizing interaction of the carbene and FR orbitals is characteristic for conformation A. However, Dreiding models show that maximum overlap of the orbitals of the reacting species is hindered by considerable steric hindrance in the case of FCC in conformation A. This hindrance leads to the predominant reaction of FCC with double bonds in conformations providing for minimal steric hindrance despite lower stabilization energy relative to conformation A. However, since the OR+ substituent constant used in calculating the SI is related to reactions of carbenes in the most stable state [6], i.e., in conformation A in the case of FCC, a significant discrepancy should be observed between the calculated and observed SI values for carbenes reacting in less stable conformations (forms close to B in the case of FCC), as found in the case of the reaction of FCC with olefins. Judging from the calculated SI value (1.37), FCC is close to an ambiphilic carbene, while according to the experimental data (SI = 0.95) it has pronounced electrophilic properties. This accounts for the lask of success in attempts to achieve the addition of FCC at the double bonds of olefins with reduced nucelophilicity such as dimethyl furmarate. The structures of the previously undescribed adducts of FCC and TCC to olefins, namely, the corresponding 2-furyl- and 2-thienylchlorocyclopropanes (III) were supported by IR, PMR and mass spectrometry. We should note the low stability of i-(2-furyl)-l-chlorocyclopropanes (III, X = 0), which form tars insoluble in organic solvents over several days at 0-5~ while l-(2-thineyl)-l-chlorocyclopropanes (III, X = S ) r e m a i n unaltered under ~hese conditions. EXPERIMENTAL Analysis of the starting compounds and reaction products was carried out by gas-liquld chromatography on an LKhM-8MD chromatograph with flame ionization or katharometer detection and an 1-02 integrator using a 200 • 0.2 cm column packed with 5% OV-17 or Se-30 on Chromaton N-AW-DMCS and a 30 ml/min helium gas carrier flow rate. The PMR spectra were taken on a Tesla BS-467 spectrometer at 60 MHz and Bruker WP-250 spectrometer at 250 MHz for 10-20% solutions in CCI~ or CDCIs with TMS as the internal standard. The IR spectra were taken neat or for CCI~ solutions on a UR-20 spectrometer. The mass spectra were taken on a Varlan MAT-CH-6 mass spectrometer. !
Pure-grade technical samples of isobutylene and cis-2-butene were used. S~nples of 2methyl-2-butene and 2,3-dlmethyl-2-butene were obtained according to our previous procedures
[7]. 2-Dichloromethylfuran (la). A sample of 0.i g (0.001 mole) pyrldlne was added to a solution of i0 g (0.i mole) furfural in 150 ml CH2CI~ at 0~ and then 22 g (0.I mole) PCI5 was added in portions. After 30 mln, the reaction mass was neutralized with 30 g NaHCOs, stirred for an additional 2 h at o~ and filtered. Distillation of the filtrate gave 11.2 g (75%) (la) [2]. 2-Dichloromethylthiopene (Ib), mp 61-62~ thiopenaldehyde by analogy to (Ia) [8].
(2 mm) was obtained in 60-70% yield from 2-
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