3946 exo,endo epimer 14c was suggested from high resolution nmr in the region of the sharp methyl doublets of 14a and 14b ( 7 8.8-9.1). Each of the two doublets appeared to contain a barely resolved further doublet of low intensity similar to those of 1Oc. However, this was not investigated further by glc. In a further experiment the reaction mixture was investigated by nmr for a possible epimerization of 14a and 14b under the experimental conditions. While this method was not very accurate, a change in the ratio of 14a :14b was not discernible after 3 days. Comments on Debromination in Acetone. The major by-product in these reactions was the parent ketone and diacetone alcohol. For example, 2,4-dibromo-2-methyl-3-pentanone (6b) and furan afforded the adduct 8 (35 %), 2-methyl-3-pentanone (51 %), diacetone alcohol (7 %), and an unidentified compound (7 Debromination under rigorously dry conditions using acetone (distilled from (i) KMnOa and (ii) P,OIO)and furan (refluxed over Na for 30 hr) in a 50:50 mixture (v/v) led to recovery of a,cu'-dibromo ketone.
z).
Comments on Debromination in Glyme. Initially, the zinccopper couple was added in one batch, when it was found that substantial amounts of CY,^ '-dibromo ketone could be recovered at the end of the reaction. In order to bring about complete debromination it is advantageous to introduce the zinc-copper couple in several portions. Under these conditions the cycloaddition is very efficient and provides a most simple route to seven-membered unsaturated ketones.
Acknowledgments. We thank the Petroleum Research Fund, administered by the American Chemical Society, for support of our work, Dr. A. G. Loudon for recording mass spectra, and Mrs. Maie-Anne Barrow and Mr. C. J. Cooksey for technical assistance. The combined glc-mass spectral analyses were kindly performed by Dr. B. Willhalm of Firmenich & Cie, Geneva.
Secondary Deuterium Isotope Effects in Allene Cycloadditions Sheng-Hong Dai'" and William R. Dolbier, Jr.* lb Contribution from the Department of Chemistry, Unirjersity of Florida, Gainesville, Florida 32601. Receioed July 29, 1971 Abstract: Intramolecular kinetic secondary deuterium isotope effects were obtained for the reactions of allenetetracyanoethylene oxide, hexachlorocyclopentadiene, l,l-dz with acrylonitrile, l,l-dichloro-2,2-difluoroethylene, and 5,5-dimethoxytetrachlorocyclopentadieneand for the dimerizations of allene-l,l-d2 and 1,2-cyclononadiene1-d. Zntermolecular effects were obtained for the acrylonitrile and hexachlorocyclopentadiene reactions as well as for the allene dimerization. The results lead to the conclusion that all [2 21 cycloaddition processes of allenes, including dimerizations, proceed uia a multistep pathway, while the [2 31 and [2 41 cycloadditions proceed uia
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concerted mechanisms.
W
ithin the past few years, a surge of general interest in the chemistry of allenic compounds has generated a great deal of mechanistic investigations in the cycloadditions of allenes. A number of kinetic2 and s t e r e o c h e m i ~ a l studies ~ ~ , ~ are now available, with the stereochemical results seeming to suggest that allene cycloadditions are quite stereoselective. Some observations of stereospecific [2 21 cycloadditions led to conclusions that such reactions were multicenter, concerted reactions,2b with allene acting in an antarafacial manner similar to the way that ketene has been demonstrated to b e h a ~ e ,while ~ most preferred to explain their results on the basis of multistep mechanisms.
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(1) (a) Taken in part from the Ph.D. dissertation of S.-H. Dai, University of Florida, June 1971; (b) Alfred P. Sloan Foundation Fellow, 1970-1972. (2) (a) A. Roedig, Angew. Chem., Int. Ed. Engl., 8, 150 (1969); (b) E. F. Kiefer and M. Y . Okamura, J . Amer. Chem. Soc., 90, 4187 (1968). (3) (a) J. E. Baldwin and U.V. Roy, J . Chem. SOC.D, 1225 (1969); (b) W. R. Moore, R. D. Bach, and T. M. Ozretich, J . Amer. Chem. SOC., 91,5918 (1969); ( c ) A. Roedig and N . Defzer, JustusLiebigs Ann. Chem., 710, 1 (1967); (d) J. J. Gajewski and W. A . Black, Terrahedron Lett., 899 (1970); (e) T. L. Jacobs, J. R. McClenon, and 0. J. Muscio, Jr., J . Amer. Chem. Soc., 91, 6038 (1969); (f) E. V. Dehmlow, Tetrahedron Lett., 4283 (1969). (4) (a) R. Huisgen and P. Otto, J . Amer. Chem. SOC.,90, 5342 (1968); (b) R . Huisgen, L. A. Feiler, and P. Otto, Tetrahedron Lett., 4485 (1968); (c) W. T. Brady and R. Roe, Jr., J . Amer. Chem. Soc., 92,4618 (1970); (d) H . M. Frey and N. S. Issacs, J . Chem. SOC.B, 830 (1970); (e) J. E. Baldwin and J. A. Kapecki, J . Amer. Chem. SOC., 92, 4868 (1970); (f) R. Montaigne and L. Ghosez, Angew. Chem., Int. Ed. Engl., 7, 221 (1968).
Journal of the American Chemical Society
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We wish to present a rather broad series of studies which seem to support the latter conclusion, and which seem to relegate allene to the category of being just another relatively reactive alkene.5 In these studies secondary deuterium isotope effects have been utilized as the major mechanistic probe. Intramolecular competitive studies provided information about the pzoduct-determining steps while intermolecular competition experiments gave knowledge of the rate-determining steps. A broad spectrum of cycloaddition reactions was 41 cycloinvestigated : two Diels-Alder reactions ([2 additions), the reactions of allene with hexachlorocyclopentadiene (1)6 and with 5,5-dimethoxytetrachlorocyclopentadiene (2); one [2 31 cycloaddition the 1,3-dipolar cycloaddition of tetracyanoethyleneoxide(3) with allene;' two [2 21 cycloadditions, the reaction of allene with acrylonitrile (7)8 and with, 1,ldichloro-2,2-difluoroethylene (9)9; and two dimeriza-
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(5) This work has been partially reported in a series of preliminary communications: W. R. Dolbier, Jr., and S.-H. Dai, J . Amer. Chem. Soc., 90, 5028 (1968); 92, 1774 (1970); Tetrahedron Lett., 4645 (1970). (6) H. Pledger, J . Org. Chem., 25, 278 (1960). (7) W. J. Linn and R. E. Bensen, J . Amer. Chem. SOC.,87, 3657 (1965). (8) H . N. Cripps, J. I60% yield. Its spectroscopic properties were identical with those reported previously.* Kinetic Intramolecular Isotope Effects in the Allene-7 System. After purification of the adduct of allene-l,l-dz and 7 a careful nmr spectrum, with multiple intergrations, was obtained. At least ten integrations were averaged t o obtain the value of the allylic (12) A . T. Morse and L. C. Leitch, J . Org. Chem., 23,990 (1958).
Dai, Dolbier
1 Deuterium Isotope Effects in Allene
Cycloadditions
3948 Table 11. Equilibrium Isotope Effects in the Allene-Acrylonitrile System Reaction temp, "C
Reaction time, hr
Nmr ratio, allyl/vin yl
287 f 5 280 i 5 280 i 5
15 34 48
3.26 f 0.02 3.28 f 0.04 3.26 + 0.04
kH/kD
0.92 f 0.01 0.93 i 0.01 0.92 i 0.01
to vinylic proton ratio for each run as seen in Table I. The intramolecular isotope effect could be easily calculated from this ratio and the value could be corrected for the small amount of allene-dl component.
sealed in a thick-walled tube in the presence of a small amount of hydroquinone. The tube was heated in a manner similar to that described for the allene-7 reaction at temperatures varying from 162 to 200" for periods from 40 t o 48 hr. About 1.3 g of volatile starting materials was recovered after the reaction. The remaining product mixture was analyzed and the components were isolated by glpc (10% tricresyl phosphate, 15 ft X 0.25 in. column at 75"). There were two main components in a ratio of 2:3. the earlier eluting component being the dimer of 9 while the latter component was 2,2-difluoro-3,3-dichloromethylenecyclobutane(10): bp 123"; nmr 6 3.31 (t, 2), 5.46 (m, I), and 5.75 ppm (m, 1). The nmr indicated that this isomer comprised at least 95% of the [2 21 adduct mixture. Kinetic Intramolecular Isotope Effects in the Allene-9 System. The method employed was identical with that used in the allene-7 system. See Table IV for the results.
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Table 111. Intermolecular Isotope Effects in the Allene and Acrylonitrile System Allene composition before reaction
Allene dolda
Nmr ratio, allyl/vinyl
Adduct doldr
dr 50.8 i 0.03 do 49.2 i 0.03 dq 50.2 i 0.03 do49.8 i 0.03
0.97 i 0.02
4.01 f 0.03
1.00 i 0.3
1.03 f 0.05
0.99 i 0.02
3.94 i 0.03
1.04 i 0.02
1.05 f 0.04
knlkn
Av 1.04 i. 0.05
Table IV.
Intramolecular Isotope Effects in the Allene-9 System
Reaction Reaction temp, time, "C hr
190 170 170 162 200
Reaction of Allene with Hexachlorocyclopentadiene.6 Allene .O g) was transferred cia vacuum line to a pyrolysis tube (-10-ml capacity) containing an 1.5-2.0molar excess of 1. The tube was sealed under vacuum, wrapped with glass wool, and heated in a tube furnace for the desired time. Then after cooling to -78", the tube was opened and the product purified by molecular distillation (115" (0.5 mm)). Further purification could be done by a rapid distillation. Yields of purified 1,2,3,4,7,7-hexachloro-5methylene-2-norbornene were generally -80%: nmr 6 3.12(quart., 2 H), 5.41(sext, 1 H), and 4.25ppm (sext, 1 H). Kinetic Intramolecular Isotope Effects in Allene-1 System. The method employed was identical with that used in the allene-7 system. Table V contains the results. Intermolecular Isotope Effects in Allene-1 System. Using a large excess (21-fold) of allene-do and -drwith 1 the reactions and purifications were identical with those for other such reactions (-1
Nmr ratio, allyl/vinyl
48 48 48 40 48
kH/krmra
b 1.15 f 0.03 1.15 i 0.04 1.24 f 0.03 1.15 f 0.04 Av 1.17 i 0.04
1.00 i 0.01 1.12 i 0.02 1.12 i 0.03 1.19 i 0.03 1.12 f 0.OY
d]and 2.8% dospecies present in a k H / k n value corrected for 13.5 allene-l,l-d2. * Control run with 100% allene-do. When the adduct is heated to 287" for 4.5 hr, the nmr ratio diminishes to 1.01 i 0.02. Table V. Intramolecular Isotope Effects in the Allene-1 System ~~
a
~~
~
Reaction temp, "C
Reaction time, hr
Allene deuterium content
Nmr ratio, allyl/vinyla
150 i 3 150 i 3
12 15
1.006 f 0.006 0.894 i 0.004
0.89 + 0.01
145 i 2
12
0.87 i 0.01
0.87 f 0.01
145 i 2
15
0.93 i 0.01
0.93 i 0.01
135 i 1
12
100% do 91.2% dz 7.03% di 91.2% dz 7.03% di 91.2zdz 7.03% dl 91.2% dz 7.03% di
0.923 f 0.004
0.92 f 0.01
kH/kDcor
Heating the product mixture at temperatures as high as 200" caused virtually no change in the nmr ratio.
Equilibrium Isotope Effects in the Allene-7 System. The adduct of allene-dr and 7 was heated to 280-287' for 15-48 hr in a sealed tube. The results are shown in Table 11.
Intermolecular Isotope Effects with Allene-7 System. A mixture of allene-do and allene-d4 was allowed to react with a fourfold excess of acrylonitrile in a manner similar to that described above. However, the temperature of the reactions was maintained a t 190-210" so as to be able to control the rate of the reaction. The reactions were terminated when the conversion t o adduct 8 was only -10%. The allyl/vinyl proton ratio was determined by nmr as described earlier and, with the three protons of 7 being used as an internal proton standard, the values of k H / k D can easily be obtained algebraically. They are summarized in Table 111. Reaction of Allene and l,l-Dichloro-2,2-difl~oroethylene.~ A mixture of 0.5 g (0.013mol) of allene and 2.5 g (0.019mol) of 9 was
Journal of the Ameriacn Chemical Society
except that the excess allene was recovered. An internal proton standard CHCI, was used, weighing out appropriate amounts of adduct and CHCh t o give near identical peak integrations. The relative amount of heavy and light adduct could then be deduced knowing the moles of total adduct (from weighing) and the moles of light adduct (from adduct/CHCl3 proton ratio). Then k d k D = (fH/fD)adduct X (d4/do)starting a ~ ~ e n e .Table VI shows the results. 1,2,3,4,7,7-Hexachloro-5-(chloromethyl-~~~-2-nor~rnene. AllylI,I-d213 chloride (-0.75 g) and an approximate twofold excess of 1 (4.5 g) were sealed in a 15-ml,thick-walled tube. After heating at 130" for 15 hr in a tube furnace, the tube was cooled and opened.
1 94:11 1 M a y 31, 1972
(13) R. D. Schuetz and F. W. Millard, J . O r g . Chem., 24, 297 (1959).
3949 Table VI.
Intermolecular Isotope Effects in the Allene-l System
Mole ratio allene/diene
Reaction temp, "C
Reaction time, hr
kA/kDcor
21.1 21.1
135 f 3 135 f 3
19 10
0.93 f 0.04 0.88 ?r 0.04
Dimerization of Allene. The procedure described optimizes the yield of 1,2-dimethylenecyclobutane (11) while minimizing the over conversion of allene.lO To a 15-ml, thick-walled tube were added 2.4 g of benzene and 0.8 g of allene cia vacuum line. After sealing in uacuo, the tube was wrapped in glass wool and heated at 130" for 14 hr, whence the unreacted allene (95%) was recovered by vacuum line transfer from -78 t o - 195". Of the residue, 91 % was determined by glpc (lo%, 10 ft X 0.25 in. Carbowax 1500 column at 80") to be 11. The only other components in the mixture were the three trimers. The dimer was collected by glpc and shown to display the previously reported spectral characteristics:15 nmr (CCla) 6 2.63 (sext, 4 H), 4.69 (sext, 2 H), and 5.13 ppm (sext, 2 H); uv (EtOH), , ,A 255 mp (e 10,100), 246 (12,300), and 240 (11,000). Intramolecular Isotope Effects for Allene Dimerization. The procedure followed was essentially the same as that employed for the allene-7 system. Table IX shows the results.16 Intermolecular Isotope Effects for the Allene Dimerization System. The allene-do and -d4composition before reaction was determined as described earlier and was used to calculate the statistical isotopic composition of dimer if k H / k D = 1. The actual isotopic composi-
Table VII. Intramolecular Isotope Effects in the Allene-2 System Reaction time, hr 10 10 10 10
Table VIII.
Reaction temp, "C 125 125 125 125
f 3 f3 f3 3~
3
Nmr ratio, vinyl/allyl 1.09 1.10 1.09 1.10
f f f f
0.01 0.01 0.02 0.02
kH/kD
0.92 0.91 0.92 0.91 Av 0 . 9 2
f 0.01 f 0.01 &
0.02
f 0.02 f 0.02
Intramolecular Isotope Effects in the Allene-3 System
Mole ratio, allene/3
Reaction temp, "C
Reaction time, hr
Nmr ratio, vinyl/allyl
3.20 3.42 2.95
130-135 130-135 130-135
10 10 10
0.99 f 0.01 1.07 i 0.01 1.06 ==! 0.01
kH/kDeor
0.93 0.92 0.94 Av 0.93
f 0.01 f 0.01
f 0.01 f 0.01
Table IX. Intramolecular Isotope Effects for the The liquid product was distilled and a pale yellow oil obtained: Dimerization of Allenea bp 96-99' (0.01 mm); nmr 6 2.36 (quart., JAB = 3.8 cps, JAX = 12.5 cps, 1 H), 3.17 (q, JBX= 8.6 cps, JAX = 12.5 cps, 1 H), and Allene Allene = 3.8 cps, JAX = 8.6 cps, 1 H). 3.62 ppm (9, JAB Nmr ratio, deuterium Nmr ratio, deuterium 1,2,3,4,7,7-Hexachloro-5-(methylene-d~)-2-norbornene (18).14 A allyl/vinyl content: % allyl/vinyl mixture of 1,2,3,4,7,7-hexachloro-5-(chloromethy1-d~)-2-norbornene content? % (3.0 g, 8.6 mmol), KOH (0.56 g, 10 mmol), and 20 ml of absolute 100 do 1.00 f 0 . 0 3 95.7d2 1.13 f 0.01 EtOH was refluxed for 3 hr. The solution was then filtered and 95.5 dz 1.16 f 0.02 8.30 dl solvent distilled a t reduced pressure. The product was collected 3.86dI 95.5 dz 1.14 f 0.01 (2.2 g, 8 2 z ) as a colorless, viscous oil: bp 76-80" (0.010 mm); nmr 3.72 d1 ( C C 1 4 ) 6 2 . 9 8 ( q , J ~=~ 15.lcps,2H),5.32and5.68(m,
