70-6667 Mccombs, Douglas Arthur, 1942- SYNTHESIS AND
SYNTHESIS AND REARRANGEMENTS OF PENTADIENYL AND HEPTATRIENYL CARBANIONS
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McCOMBS, Douglas Arthur, 1942- SYNTHESIS AND REARRANGEMENTS OF PENTADIENYL AND HEPTATRIENYL CARB ANIONS.
University of Arizona, Ph.D., 1969 Chemistry, organic
University Microfilms, Inc., Ann Arbor, Michigan SYNTHESIS AND REARRANGEMENTS OF PENTADIENYL
AND HEPTATRIENYL CARBANIONS
by
Douglas Arthur McCombs
A Dissertation Submitted to the Faculty of the
DEPARTMENT OF CHEMISTRY
In Partial Fulfillment of the Requirements For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
19 6 9 THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
I hereby recommend that this dissertation prepared under my direction by Douglas Arthur McCombs entitled SYNTHESIS AND REARRANGEMENTS OF PENTADIENYL
AND HEPTATRIENYL CARBANIONS be accepted as fulfilling the dissertation requirement of the degree of DOCTOR OF PHILOSOPHY
J-JLV dj Dissertation Director Date
After inspection of the final copy of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:"
~K- Cnicy
UjW
V*
fThis approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination. The inclusion of this sheet bound into the library copy of the dissertation is evidence of satisfactory performance at the final examination. STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an advanced degree at "The University of Arizona and is deposited in the University Library to be made available to bor rowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided that accurate acknowledgment of source is made. Requests for permission for extended quotation from or re production of this manuscript in whole or in part may be granted by the head of the major department or the Dean of the Graduate College when in his judgment the proposed use of the material is in the in terests of scholarship. In all other instances, however, permission must be obtained from the author.
SIGNED: ACKNOWLEDGMENTS
The author wishes to acknowledge Dr. R. B. Bates for his suggestions and guidance throughout the preparation of this work and Mrs. J, L. Cude for the final typing of the manuscript.
iii TABLE OP CONTENTS
Page
LIST OF ILLUSTRATIONS v
ABSTRACT vii
INTRODUCTION . 1
Pentadienyl Carbanions ..... 1 Heptatrienyl Carbanions ...... 4
EXPERIMENTAL 6
1,4-Cyclooctadiene 6 Cyclooctadienyllithium (1) 7 Protonation of Cyclooctadienyllithium (1).. . 8 cis-Bicyclof3.3.0]oct-2-ene (3) ...... 8 1.4-Cycloheptadiene 8 Cycloheptadienyllithium (9a) .... 9 Protonation of Cycloheptadienyllithium (9a) 10 Heptatrienyl lithium (8a) 10 n-Heptane From Heptatrienyllithium (8a) 11 1-Methylheptatrienyllithium (8b) 11 2,6-Dimethylheptatrienyllithium (8c) 12 6-Methylcycloheptadienyllithium (9b) ..... 12 1.5-Dimethylcycloheptadienyllithium (9c) 12 1,3,5-Trimethylcycloheptadienyllithium (9d) ...... 13 6-n-Butylcycloheptadienyllithium (9e) ...... 13
DISCUSSION 14
Formation of Cyclooctadienyllithium (1).... 14 NMR Studies of Cyclooctadienyllithium (1) . 14 Rearrangement of Cyclooctadienyllithium (1) 17 Cycloheptadienyllithium (9a) 22 NMR Studies of Cycloheptadienyllithium (9a) ...... 24 Heptatrienyl lithiums 27 NMR Studies of Heptatrienyllithiums ...... 28 Cyclization of Heptatrienyl Anions ... 29
APPENDIX: KINETIC DATA 44
LIST OF REFERENCES . . 45
iv LIST OF ILLUSTRATIONS
Figure Page
1. NMR spectrum of 1,4-cyclooctadiene 15
2. NMR spectrum of anion (1) (35°) ...... 15
3. NMR parameters for anion (l)...... 16
4. NMR parameters for anion (2) 16
5. NMR spectrum of anions (l) and (2) (35°) 18
6. NMR spectrum of (3) 18
7. Graph of the (l) to (2) conversion (35°). Points are experimental and curves are computer dravm 20
8. NMR spectrum of 1,4-cycloheptadiene 23
9. NMR spectrum of anion (9a) (35°) 23
10. NMR parameters for anion (9a) 25
11. Protonation of anions (l), (7), and (9a) 26
12. NMR spectrum of 1,3,6-heptatriene 31
13. NMR spectrum of anions (8a) and (9a) (-30°) ...... 31
14. NMR parameters for anion (8a) 32
15. NMR spectrum of 1,3,6-octatriene ...... 33
16. NMR spectrum of anion (8b) (-30°) ...... 33
17. NMR parameters for anion (8b) 34
18. NMR spectrum of 2,6-dimethyl-1,3,6-heptatriene 35
19. NMR spectrum of anions (8c) and (9c) (-40°) 35
20. NMR parameters for anion (8c) 36
21. NMR spectrum of anions (8a) and (9a) (-30°) ...... 37
v vi
LIST OF ILLUSTRATIONS—Continued
ige
NMR spectrum of(9b)(35°) 37
NMR spectrum of anion (9c) (35°) . 38
NMR parameters for anion (9b) 39
NMR parameters for anion (9c) 39
NMR spectrum of 2,4,6-trimethyl-l,3,6-heptatriene . . 40
NMR spectrum of anion (9d) (35°) 40
NMR parameters for anion (9d) 41
NMR spectrum of cycloheptatriene 42
NMR spectrum of anion (9e)(35°) 42
NMR parameters for anion (9e) 43 ABSTRACT
The first synthesis of lithium salts of three heptatrienyl carbanions in high concentration (ca 20%) is reported; when 1,3,6- heptatrienes in tetrahydrofuran are treated with n-butyllithium quan titative yields of heptatrienyl anions are observed. Nuclear magnetic resonance studies showed that the preferred shape of these anions at low temperatures (-30°) is the fully extended shape rather than the all-cis helical form predicted by Hoffmann and Olofson.
Two new electrocyclic carbanion reactions were observed.
Several heptatrienyl carbanions were found to rearrange to cyclohepta- dienyl carbanions at -30°, and cyclooctadienyl anion was observed to rearrange to £iis-bicyclo[3.3.0]octenyl anion at 35°.
vii INTRODUCTION
PentadienyI and heptatrienyl carbanions have been proposed as intermediates in base-catalyzed diene and triene isomerizations and rearrangements (Birch, Shoukry, and Stansfield 1961; Slaugh 1967;
Zuech, Crain, and Kleinschmidt 1968). Until recently (Bates, Gosse- link, and Kaczynski 1967a), no convenient method of obtaining such carbanions in high concentrations was available. It was the purpose of this work to prepare in high concentration and to characterize several proposed carbanion intermediates and to study their reactions.
Pentadienyl Carbanions
Slaugh (1967) observed the high-yield metal hydride-catalyzed rearrangements of 1,3- and 1,5-cyclooctadienes to cjLs-bicyclo[3.3.0]- oct-2-ene (3). He suggested two possible mechanisms for these con versions. One involves the formation of the eyelooctadieny1 anion (1) from 1,3-cyclooctadiene and its rearrangement to cis-bicyclof3.3.01- octenyl anion (2).
(1)
1 © (1) • CO (2)
(2) + a) + CO o (3)
Anion (2) would then abstract a proton from 1,3-cyclooctadiene to form
the final product (3) and more cyclooctadienyl anion (l). The other mechanism proposed requires a hydride addition-elimination process.
First, 1,3-cyclooctadiene was converted to 1,5-cyclooctadiene (the less
thermodynamically stable diene) which added potassium hydride. The re sulting anion (4) then underwent a transannular ring closure to the bicyclo anion (5) which eliminated potassium hydride to form (3).
© K +
(5) 3
The suggestion was made that since no rearrangement to a bicyclo derivative was observed in the potassium t-butoxide-catalyzed rearrangement of 1,5-cyclooctadiene to 1,3-cyclooctadiene, the re arrangement was facilitated by formation of the anion on the metal hydride surface.
Of the three possible planar shapes (U, Sickle, and W) of the pentadienyl carbanions (Bates, Carnighan, and Staples 1963), a bent
U-shape (6) is the only possible form for the cyclooctadienyl anion 2 (1). For maximum overlap of the p-orbitals on the five adjacent sp hybridized carbons, the five carbons must be coplanar and their
© © //
(6)
p-orbitals must be perpendicular to this plane. Models indicate, how ever, that the five adjacent p-orbitals cannot possibly be perpendicu lar to that plane. Thus, the cyclooctadienyl anion (1), if prepared, could only approximate the U-shape. The anion would be expected to be quite unstable with respect to 6,6-dimethylcyclohexadienyl anion (7).
CH3 CH3
(7) (9a) 4
It was decided to attempt synthesis of cyclooctadienyl anion
(1) in order to study its structure by NMR and to determine which, if
either, of Slaugh's (1967) mechanisms was more likely. If (l) were the
intermediate in the first mechanism, then it was of interest to deter
mine if the metal hydride surface were necessary for rearrangement. If
(l) were not the intermediate, then its synthesis should give the same
result as the potassium t-butoxide equilibrations of 1,3- and 1,5-cyclo-
octadienes.
In view of the fact that the cycloheptadienyl anion (9a) would
have similar geometry to that of the cyclooctadienyl anion (1), it was
thought that the cycloheptadienyl anion (9a) might, if the anion re
arrangement proposed by Slaugh (1967) were correct, rearrange to the
cis_-bicyclo[3.2.0]heptenyl anion. This rearrangement would then pro
vide a convenient route to cis-bicyclo^.2.0lhept-2-ene.
It was decided to synthesize the cycloheptadienyl anion (9a),
to determine its structure by NMR and to study some of its reactions.
Heptatrienyl Carbanions
Zuech et al. (1968) reported the base-catalyzed isomerizations
and cyclizations of various octatrienes with alkali metal salts of.
alkyl amides. Isomerizations of 1,3,6- and 1,3,7-octatrienes with
piperidinosodium yielded a substantial amount of methyl-substituted
cycloheptatrienes. The authors suggested that a plausible pathway for
the cyclization was through a heptatrienyl anion (8b) which cyclized
to give the corresponding cycloheptadienyl anion (9b). The authors © (8b)
CH
©
(9b)
also reported the base-catalyzed isomerization of trans,trans,cis-lt5,9- cyclododecatriene to bicyclo[5.5.O]dodecadienes, which is consistent with the octatrienyl studies.
Since heptatrienyl anions had only been postulated at this
point, it was decided to prepare several heptatrienyl carbanions in high concentration and to observe their NMR spectra and reactions.
Also, if these anions were the intermediates Zuech et al.(1968) proposed
in the octatriene rearrangements, then one might be able to observe the
transformation of heptatrienyl anions to cycloheptadienyl anions by NMR. EXPERIMENTAL
The procedures used for the preparations of pentadienyl and heptatrienyl anions in this study are essentially those of previous
authors (Gosselink 1966; Bates, Gosselink, and Kaczynski 1967a) with
the exception that the heptatrienyl anions were prepared by mixing the corresponding 1,3,6-heptatriene and tetrahydrofuran (THF) at -78° with subsequent addition of n-butyllithium in hexane at this temperature.
The anions formed were then warmed to -30° for nuclear magnetic reson ance (NMR)studies.
All NMR spectra were run on a Varian HA-100 spectrometer at
100 MHz with the ^-protons in THF (Y6.4) used as an internal standard for anion spectra. All vapor phase chromatography (VPC) was done on a
Varian Aerograph Model 90-P with a Varian Aerograph Model 20 recorder.
Mass spectral data were obtained on a Hitachi-Perkin-Elmer RMU-6E
Double Focusing Mass Spectrometer. Kinetic data were analyzed on a
Pace TR-10 Analog Computer.
1,4-Cyclooctadiene
In a 500-ml Erlenmeyer flask fitted with a magnetic stirrer bar, gas bubbler, and vent was placed commercial (Aldrich Chemical
Company) 1,5-cyclooctadiene (108 g, 1.0 mole). Commercial (Matheson
Company) dry hydrogen bromide gas from a lecture bottle was then passed
through the bubbler until 60 g (0.74 mole) of gas was taken up by the
6 diene. The mixture was allowed to stir overnight and was used in the following procedure without purification.
The crude product from the above procedure was placed in a
500-ml, one-neck, round-bottom flask equipped with a reflux condenser.
Potassium hydroxide (75 g, 1.6 moles) and 95% ethanol were added to the flask and the mixture was brought to reflux. After two hours, the flask was cooled and water (200 ml) was added to dissolve the solid material. The resulting two-phase mixture was then extracted with
pentane. The pentane layer was dried over anhydrous magnesium sulfate
and the solvent partially evaporated. Four components were collected by VPC (207o of a 20%, solution of silver nitrate in polypropylene gly col on 60/80 Chromosorb W) at 110°. The desired product, 1,4-cyclo- octadiene, was the third component collected (identified by its NMR spectrum) and the other three were, respectively, cis-bicyclof3.3.0l- oct-2-ene (3), 1,3-cyclooctadiene, and 1,5-cyclooctadiene (identified by comparison of VPC and NMR data with authentic samples).
Cyclooctadienyllithium (1)
Commercial (Foote Mineral Company) n-butyllithium in hexane
(1.1 ml, 1.6 mmol) was added to 1,4-cyclooctadiene (0.15 ml, 1.5 mmol) in an NMR tube. The resulting mixture was chilled in a Dry Ice-acetone bath and commercial (Stohler Isotope Chemicals) perdeuterotetrahydro- furan (THF-d ) (0.3 ml) was added. The mixture was allowed to come to Oc room temperature. After the mixture had separated into two layers
(10-15 min), the top layer (mostly hexane) was discarded. The only
product visible by NMR was cyclooctadienyl anion (1). 8
Protonation of Cyclooctadienyllithium (l)
The anion was prepared as described above. Upon separation of the mixture into two layers, the top layer was discarded and the bottom layer was poured slowly into water (5 ml). The two-phase mixture was extracted twice with pentane (5 ml). The pentane layers were combined, washed six times with water, and dried over anhydrous magnesium sulfate.
The pentane solution of the protonated products was analyzed by VPC (silver nitrate column) at 110°. The three components collected were identified by their NMR spectra and were, respectively, cis- bicyclo[3.3.0]oct-2-ene (3) (9%), 1,3-cyclooctadiene (4670), and 1,4- cyclooctadiene (45%). cis-Bicyclo[3.3.0]oct-2-ene (3)
Cyclooctadienyllithium (l) was prepared in an NMR tube in the previously described manner. After separation of the two layers, the top layer was drawn off and the bottom layer was washed twice with
0.3-ml portions of dry cyclohexane. The bottom layer was then allowed to stand at room temperature for 24 hours. The only product visible in the lower layer after 24 hours was £is-bicyclo[3.3.0]oct-2-ene (3), as determined by NMR.
1.4-Cyclohep tad iene
The method used was a refinement of that used by ter Borg and
Bickel (1961) to prepare 1,3-cycloheptadiene from cycloheptatriene. In a 500-ml, three-neck, round-bottom flask equipped with a paddle stirrer,
Dry Ice condenser, and dropping funnel was distilled 300 ml of ammonia. 9
To the ammonia was added sodium (14 g, 0.61 mole) in small chunks with stirring. After solution was complete, commercial (Aldrich Chemical
Company) cycloheptatriene (21.8 g, 0,24 mole) was added dropwise with stirring over a five-minute period. The resulting red solution was allowed to stir an additional 20 minutes before being added dropwise to one liter of cold (-78°) absolute ethanol. Immediately after this ad dition, water (200 ml) was added slowly to the mixture. The resulting
two-phase mixture was quickly extracted with two 100-ml portions of pentane. The pentane layers were combined, washed five times with
100-ml portions of water, and dried over anhydrous magnesium sulfate.
The solvent was evaporated, leaving 20 ntl of a yellow liquid. Three components were collected by VPC (Carbowax 20M on 50/60 firebrick col umn) at 100°. The expected diene was the second component collected
(identified by its NMR spectrum) and the other components were, respec tively, cycloheptene and 1,3-cycloheptadiene (identified by their NMR spectra). The ratio of 1,3-cycloheptadiene to 1,4-cycloheptadiene was
3:1.
Cycloheptadienyllithium (9a)
The anion was prepared by two methods. The first method was from 1,4-cycloheptadiene as previously described in the preparation of eyelooctadienyllithium (l). Two layers separated 10-15 minutes after
the mixture was brought to room temperature.
The second method used to prepare cycloheptadienyllithium (9a)
involved rearrangement of the acyclic anion from 1,3,6-heptatriene. In
an NMR tube were placed commercial (Chemical Samples Company) 10
1,3,6-heptatriene (0.15 ml, 1.2 mmol) and dry THF (0.35 ml). The mix ture was chilled in a Dry Ice-acetone bath and commercial (Foote Min eral Company) 1.6 M n-butyllithium in hexane (1.1 ml, 1.6 mmol) was added. A yellow precipitate formed immediately. Upon warming to -50°, two layers separated and at room temperature the only product visible by NMR was cycloheptadienyllithium (9a).
Protonation. of Cycloheptadienyllithium (9a)
Cycloheptadienyllithium (9a) was prepared from 1,3,6-heptatriene as described above. The top layer was discarded and the bottom layer was poured into 40 ml of 107= ammonium chloride solution. The resulting mixture was extracted twice with pentane. The pentane layers were com bined, washed eight times with water, and dried over anhydrous magnesium sulfate. Analysis by VPC (Carbowax 20M column) at 100° showed a 72:28 ratio of 1,3-cycloheptadiene to 1,4-cycloheptadiene.
Heptatrienyllithium (8a)
Heptatrienyllithium (8a) was prepared as previously described in the preparation of cycloheptadienyllithium (9a) from 1,3,6-heptatriene.
The initial reaction mixture was kept at -78° in an NMR tube until be ing placed in the probe at -55°. Signals from a 70:30 mixture of hepta trienyllithium (8a) and cycloheptadieny]lithium (9a) were observed at
-30°. 11
n-Heptane From Heptatrienyllithium (8a)
Heptatrienyllithium (8a) was prepared as described above. The
reaction mixture was kept at -78°, however, for 14 hours before the
top layer was discarded.
The solid bottom layer (still at -78°) was treated with a 50:50
mixture (10 ml) of ethanol and ether (also chilled to -78°) and stirred
until no solid material remained. The resulting solution was extracted
with water, dried over anhydrous magnesium sulfate, and stripped of
solvent.
The protonation product was diluted with absolute ethanol and
hydrogenated at atmospheric pressure and room temperature with platinum
black catalyst until no more hydrogen was absorbed. Pentane was added
to the ethanol solution and the resulting mixture was extracted eight
times with 10-ml portions of water and dried over anhydrous magnesium
sulfate.
The pentane solution from the hydrogenation reaction was ana
lyzed by comparison of VPC data with authentic samples of n-heptane and
cycloheptane on two different columns (Silicone Rubber GE SE-30, and
Carbowax 20M) at 100° (helium flow rate, 60 ml/min). Retention times for n-he.pt.aue and cycloheptane, respectively, were 2.9 minutes and 5.4
minutes on GE SE-30 and 0.3 minute and 0.8 minute on Carbowax 20M. The n-heptane to cycloheptane ratio was 89:11.
1-Methylheptatrienyllithium (8b)
The anion was prepared from commercial (Aldrich Chemical Com
pany) 1,3,6-octatriene as described for heptatrienyllithium (8a). The 12
NMR tube was kept in a Dry Ice-acetone bath until being placed in the probe at -60°. The probe was then warmed to -30° and signals from
1-methylheptatrienyllithium (8b) were observed.
2.6-Dimethylheptatrienyllithium (8c)
The anion was prepared from commercial (Aldrich Chemical Com pany) 2,5-dimethyl-l,3,6-heptatriene as described for heptatrienyllithium
(8a). The NMR tube was kept at -78° until it was placed in the probe at -60°. The probe was then warmed slowly to -30° and signals resulting from a 1:9 mixture of 2,6-dimethylheptatrienyllithium (8c) and 1,5- dimethylcycloheptadienyllithium (9c) were observed.
6-Methylcycloheptadienyllithium (9b)
The preparation of 6-methylcycloheptadienyllithium (9b) was done as described for 1,3,6-heptatriene except that 1,3,6-octatriene was used instead. An orange precipitate formed immediately and two layers sepa rated well before the sample reached room temperature. At room tempera ture the only product visible by NMR was 6-methylcycloheptadienyllithi.um
(9b).
1,5-Dimethylcycloheptadienyllithium (9c)
The 1,5-dimethyl derivative of cycloheptadienyllithium (9c) was prepared from 2,6-dimethyl-l,3,6-heptatriene by the method described for 1,3,6-heptatriene. Two layers separated by the time the sample reached room temperature. At room temperature the only product visible by NMR was 1,5-dimethylcycloheptadienyllithium (9c). 13
1.3.5-Trimethylcycloheptadienyllithium (9d)
The anion was prepared from 2,4,6-trimethyl-l,3,6-heptatriene as previously described for heptatrienyllithium (8a). The NMR tube was brought to room temperature rapidly and then heated to 60° for one hour. Upon cooling, two layers separated. The anion (9d), as well as unreacted starting material, was visible by NMR,
»«• 6-n-Butylcycloheptadienyllithium (9e)
Commercial (Aldrich Chemical Company) cycloheptatriene (Figure
29; see Discussion section for all figures) (0.2 ml, 1.5 mmol) and n-butyllithium in hexane (1.1 ml, 1.5 mmol) were placed in an. NMR tube and chilled in a Dry Ice-acetone bath. Dry THF (0.4 ml) was added to the mixture and the mixture was warmed to room temperature. A dark green color appeared immediately and separation of two layers occurred after about 30 minutes. The NMR spectrum of the resulting mixture showed only 6-n-butylcycloheptadienyllithium (9e) (Figures 30 and 31). DISCUSSION
Formation of Cyclooctadienyllithium (1)
The preparation of cyclooctadienyllithium (1) from 1,4-cyclo- octadiene (Figure 1) went in quantitative yield as determined by NMR.
Its ease of formation was surprising considering the unfavorable p- orbital overlap in the anion. Attempts to prepare the anion from
1,3-cyclooctadiene were unsuccessful as expected, since 1,3-dienes either polymerize or do not react with n-butyllithium as rapidly as
THF does under these conditions (Bates et al. 1967a). In this case the latter situation occurred.
NMR Studies of Cyclooctadienyllithium (l)
The NMR spectrum of cyclooctadienyllithium (l) consists of three groups of signals: a downfield quartet CI'4.4) integrating to two protons, an upfield triplet (^7.7) integrating to one proton, and a broad multiplet C?7.l) integrating to two protons (Figure 2). The chemical shift and coupling constant assignments for cyclooctadienyl lithium (1) are given in Figure 3.
Anion (l) was determined to be roughly U-shaped by the coupling constants in its NMR spectrum, which are similar to those of 6,6- dimethylcyclohexadienyllithium (7) (Bates, Gosselink, and Kaczynski
1967b). A rather large difference in the chemical shifts for the end and central protons in the pentadienyl system is noted, however. In anion (7) the end protons absorb atf6.6 and the central proton absorbs
14 15
Figure 1. NMR spectrum of 1,4-cyclooctadiene.
I
Figure 2. NMR spectrum of anion (1) (35°). Chemical Shifts (T) Coupling Constants (Hz)
Ha 7.7 Jab 6.8
Hb 4.4 Jbc 9.0
He 7.1 Jac 0
Hd 7.3
Figure 3. NMR parameters for anion (1).
Ha
Chemical Shifts (f) Coupling Constants (Hz)
Ha 7.7 Jab 3.5
Hb 4.0
Figure 4. NMR parameters for anion (2). 17
atT6.1, while in anion (1) the end protons absorb att7.1 and the cen
tral proton absorbs at T7.7. This reversal of the chemical shifts sug
gests an increase in the double bond character at the ends of the
pentadienyl system for anion (l), and, coupled with the weakness of the
long-range coupling between the central and end protons of the penta
dienyl system, indicates a considerable deviation from perfect U-sTiape. '
The perfect U-shape would force the central and end protons to be in the
same plane as the five adjacent sp 2 hybridized carbons of the pentadi
enyl system, allowing a maximum coupling of 1.7 Hz between these two
types of protons (Barfield 1964). This result is observed for 6,6-
dimethylcyclohexadienyllithium (7) (Bates et al. 1967b). Protonation
of anion (l) gives a 1:1 ratio of 1,3- to 1,4-cyclooctadiene, while
protonation of anion (7) gave a 2:3 mixture of 1,3- to 1,4-diene (Bates
et al. 1967a), suggesting a greater electron density at the central
carbon of the pentadienyl system in (7). However, this difference may
be largely due to a steric effect rather than an electronic effect in
the case of anion (7).
Rearrangement of Cyclooctadienyllithium (1)
Upon standing in the probe (35°), the signals due to (1) were
observed to decrease, and new signals at T4.0 and XI.1 appeared. These
new signals were determined to be due to ci£-bicyclo[3.3.0]octenyl anion
(2) (Figure 5), since protonation of the reaction mixture after one day
yielded only £is-bicyclo[3.3.0]oct-2-ene (3) (Figure 6). NMR parameters
for anion (2) are given in Figure 4. The signals observed for anion (2)
are similar to those observed by Katz and Rosenberger (1962) for the 18
o§>
Figure 5. NMR spectrum of anions (1) and (2) (35°).
00
Figure 6. NMR spectrum of (3). 19 pentalene dianion (11). The end protons in the allyl system of anion
(2) form a doublet attl.l and the corresponding protons in the 3> w (2) (11)
pentalene dianion (11) give a doublet at "£5.0; the central proton in the allyl system of anion (2) gives a triplet at 1*4.0 and the corres ponding protons on anion (11) give a triplet at T4.3. Katz and Rosen- berger (1962) observed a coupling constant of 3.0 Hz between the two types of protons in anion (11). In anion (2) the coupling constant between the central and end protons in the allyl system is 3.5 Hz. No coupling was observed between the end protons on the allyl system in
(2) and the bridgehead protons. This result is undoubtedly due to the magnitude of the dihedral angle between the respective hydrogens.
The (1) to (2) conversion was followed by integration of the
T4.4 quartet of (1) and the T4.0 triplet of (2). By means of relations
(12) and (13), the rearrangement was determined to have a first-order - 1 m 1 rate constant of 5.2 x 10 sec and a half life of 80 minutes at 35°
(Figure 7). Anion (2) was observed to abstract protons from an unknown source nearly as fast as it formed (pseudo first-order rate constant,
3.7 x 10 sec ), The maximum concentration of anion (2) was roughly
507. at 135 minutes. The protonation was followed by integration of the
Tli.l singlet of £is^bicyclo[3.3.0]oct-2-ene (3). The protons did not 20
MINUTES
Figure 7. Graph of the (l) to (2) conversion (35°), Points are experimental and curves are computer drawn. 21 come from THF, since when the reaction was run in THF-d_, the mass O spectrum of the resulting hydrocarbon showed no incorporation of deu terium.
2.3 , C1 . 0.69 • • — •• I 1 Aft MOM f* SS k = fc2 1 fcl l0S °2 ^ " k
(12) (13)
It is quite likely that the (1) to (2) conversion is the cycli- zation step Slaugh (1967) described in his first mechanism; the 190° temperature Slaugh (1967) used is clearly not necessary for this step, but was probably required for proton abstraction from the initial dienes.
The suggestion (Slaugh 1967) that a metal hydride surface facilitates the rearrangement is probably incorrect; in t-butoxide-catalyzed isomcr- izations of cyclooctadienes, the anion (1) simply does not exist long enough to cyclize to (2), since the reaction medium is much more acidic.
The disrotatory (1) to (2) conversion is thermally favored
(Woodward and Hoffmann 1965) and is related to the thermal (100°) con version of cisi,cls.,cis.-l,3l5-cyclononatriene to cisj-b icyclo[4.3.0 ]nona-
2,4-diene (Glass, Whatthey, and Winstein 1965) and the cyclopentadienyl- cyclopentenyl carbonium ion rearrangement (Sorensen 1967). Anion (1) can be kept at -78° in daylight for at least a week, further indicating the thermal rather than photochemical nature of the rearrangement.
Anion (1) cannot be seen by NMR after standing 12 hours at 35°.
Therefore, the equilibrium lies toward anion (2) to at least 95%. As suming that the equilibrium concentrations for (1) and (2) are 57. and 22
95%, respectively, anion (2) is at least 1.5 kcal per mole more stable
than anion (1). Thus, the equilibrium between the pentadienyl and
cyclopentenyl systems lies to the right in this case at least. Penta
dienyl lithium does not close, even when heated to 100°, so the equilib
rium may lie to the right only in the cyclooctadienyl system.
If the equilibrium between pentadienyl anion (14) and cyclo
pentenyl anion (15) favors the pentadienyl anion (14), then one would
expect the cyclopentenyl anion (15) to undergo ring opening to form pen
tadienyl anion (14). Attempts to prepare the allyl anion (15) by the
method of Winstein et al. (1967) were unsuccessful. When 3-methoxy-
cyclopentene was treated with sodium-potassium alloy in dry 1,2-di-
methoxyethane (DME), the ensuing reaction yielded only a brown precipi
tate instead of the desired anion.
(14) (15)
Cycloheptadienyllithium (9a)
The reaction of 1,4-cycloheptadiene (Figure 8) with n-butyl-
lithium went as expected to cycloheptadienyllithium (9a). Anion (9a) was also prepared in quantitative yield from 1,3,6-h.eptatriene (10a)
(see below). As with the reaction of n-butyllithium and 1,3-cyclo-
octadiene, the reaction of 1,3-cycloheptadiene with n-butyllithium gave
no pentadienyl anion, since cleavage of THF was faster. However, upon 23
>-»>
1 1 •• I •. • I " h j. I.I.I—
Figure 8. NMR spectrum of 1,4-cycloheptadiene.
Figure 9. NMR spectrum of anion (9a) (35°). 24 treatment of cycloheptatriene with n-butyllithium (Hafner and Rellens- mann 1962) in THF, a quantitative yield of 6-n-butylcycloheptadienyl- lithium (9e) was formed.
NMR Studies of Cycloheptadienyllithium (9a)
The NMR spectrum of cycloheptadienyllithium (9a) (Figures 9 and
10) is quite similar to that of cyclooctadienyllithium (l). As in anion (1), the order of the chemical shifts for the central and end pro tons in the pentadienyl system in anion (9a) are reversed as compared with the corresponding protons in 6,6-dimethylcyclohexadienyllithium
(7). Again, this chemical shift reversal may be due to greater double bond character at the ends of the pentadienyl system. The coupling constants between protons in cycloheptadienyllithium (9a) (Figure 10) are very similar to those of anion (1), The weakness of the long-range coupling between the central and end protons of the pentadienyl system indicates, as in anion (1), a large deviation from planarity.
Upon standing in the probe at 35°, ho rearrangement of anion
(9a) was observed. In fact, even when heated to 100° anion (9a) showed no tendency to cyclize, but appeared to polymerize instead. The reason for no rearrangement in this case is probably that the bicyclo[3.2.0]- heptenyl system is too strained and the equilibrium is driven toward the pentadienyl anion.
Protonation of anion (9a) (Figure 11) with excess ammonium chloride solution gave a 72:28 mixture of 1,3- and 1,4-cycloheptadienes.
This result deviates considerably from the protonation of anion (1), which gave a 1,3- to 1,4-diene ratio of 1:1. The 72:28 1,3- to 1,4-diene Chemical Shifts CO Coupling. Constants (Hz)
Ha 6.9 Jab 7
Hb 4.4 Jbc 10
He 6.2 Jed 5
Hd 7.6 Jac 0
Figure 10. NMR parameters for anion (9a). CH, CH, CH CH CH, ,CH
+
—
(7) 607.
*0*0
(9 a) 727. 287.
*0*0 (1) 507. 507.
Figure 11. Protonation of anions (1), (7), and (9a). 27
ratio for anion (9a) further indicates that the 2:3 ratio of 1,3- to
1,4-dienes in the protonation of 6,6-dimethylcyclohexadienyllithium
(7) may be largely due to a steric effect.
Heptatrienyllithiums
Lithium salts of heptatrienyl anions were prepared in THF solu
tions by a method similar to the preparation of pentadienyl carbanions
(Bates et al. 1967a). Since 1,3-dienes give addition products with
n-butyllithium instead of pentadienyllithiums, it was thought that
1,3,6-trienes might undergo addition, but, in fact, proton abstraction
was found to go faster for trienes (lOa-d) (Figures 12, 15, 18, and 26).
At -50°, trienes (10a) and (10b) gave good yields of heptatrienyl
anions (8a) and (8b) (Figures 13 and 16); the only impurities detected
by NMR were the rearrangement products (9a) and (9b) (Figures 21, 22,
and 24), which form slowly at thi^ temperature. It was necessary to
warm (10c) to -30° before it reacted rapidly, and NMR showed the result
ing solution to.contain more (9c) than (8c) (Figures 19, 23, and 25).
Triene (lOd) reacted rapidly only at room temperature or above, giving
a good yield of (9d) (Figures 27 and 28); (8d) was not observed.
(Figures 12 through 31 may be found at the end of the Discussion sec
tion.) (10) (11)
h h fa fa a H H H H
b H H H CH
c H H CH3 CH3 d H CH3 CH3 CH3
NMR Studies of Heptatrienyllithiums
The NMR parameters for (8a-c) (Figures 14, 17, and 20) are characteristic of ionic structures, as expected by analogy with allyl- lithiums (West, Purmort, and McKinley 1968) and pentadienyllithiums
(Bates et al. 1967b). The decrease in electron density on the carbon atoms bearing the negative charge is reflected in chemical shift de creases in the series allyl (end protons,^8.5) (West et al. 1968), pentadienyl (end protons, ?7»1; central proton, ^5.9) (Bates et al.
1967b), and heptatrienyl (end protons, "C6.7; internal protons,15.5).
From the relatively large values (12 Hz) observed for the internal vicinal coupling constants in (8a), the extended conformation (16) ap pears to be favored rather than the helical all-cis form (17) antici pated by Hoffmann and Olofson (1966); however, the low activation 29
energy for cyclization described below indicates that the all-cis con
formation is also of low energy,
(16) (17)
Protonation of anion (8a) at -70° by ethanol-ether followed by
hydrogenation gave a heptane-cycloheptane ratio of 9:1, further indi
cating the extent of cyclization at this stage and showing the feasi
bility of carrying out reactions on this anion before it cyclizes.
Cyclization of Heptatrienyl Anions
When solutions containing (8a-c) were warmed to -30° or above,
quantitative first-order cyclization to cycloheptadienyl anions (9a-c)
was observed. First-order rate constants and half lives for these re
arrangements are given in Table 1. Proton abstraction was faster for
the less substituted heptatrienes since separation of layers occurred
at lower temperatures. This result is expected if one considers the
destabilization effect that alkyl substitution has on carbanions (Birch
et aim 1961). These rearrangements were followed by integration of the f5.5 multiplets for the heptatrienyl anions (8a-c) and the f6.9 multi-
plets for the cycloheptadienyl anions (9a-c). This type of rearrange
ment presumably occurs in a conrotatory manner (Woodward and Hoffmann 30
1965). The carbanion equilibrium favors the cyclized forms (9a-d) to the extent that no uncyclized anion was detected by NMR, even in the extreme case d in which a 1°, 2°, 2% 1° heptatrienyl anion closes to a 3°, 3°, 3° cycloheptadienyl anion.
Table 1. Rate constants and half lives for the rearrangements of anions (8a-c) to anions (9a-c) (-30°).
Anion k t^ (min) Product
(8a) 3.2 sec"1 13 (9a)
(8b) 1.6 x 10"1 sec"1 300 (9b)
(8c) 2.0 x 10"1 sec"1 200 (9c) 31
Figure 12. NMR spectrum of 1,3,6-heptatriene,
Figure 13. NMR spectrum of anions (8a) and (9a) (-30°), 32
Chemical Shifts (K) Coupling Constants (Hz)
Ha 5.5 Jac 12
Hb 6.6 Jad 12
He 6.8 Jbc 2
Hd 3.9 Jbd 16
He 4.0 Jed 8
Figure 14. NMR parameters for anion (8a). 33
Figure 15. NMR spectrum of 1,3,6-octatriene.
Figure 16. NMR spectrum of anion (8b) (-30°), 34
Hd1 He Hd
f CH, He 3
Hb' Ha' Ha Hb
Chemical Shifts CO Coupling Constants (Hz)
Ha 5.6 Jad 12
Ha1 5.5 Jae 12
Hb 6.7 Ja'd' 12
He 7.1 Ja1 e 12
Hd 4.3 Jbc 2
He 4.2 Jbd 16
Hf 4.4 Jb'f 7
Hg 8.6 Jed 3
Figure 17. NMR parameters for anion (8b). 35
h ! 1 ' 1 ! '1 1 "1 " 1 • ' • O <00 XO MO 160 « >-M»Ki « p p 0
1, f I L 1 1—1 1 1 : 1 : 1 : 1 r 1 r 1
Figure 18. NMR spectrum of 2,6-dimethyl-l,3,6-heptatriene.
Figure 19. NMR spectrum of anions (8c) and (9c) (-40°). H H Ha Hb
Chemical Shifts (V) Coupling Constants (Hz)
Ha 5.5 Jad 12
Hb 7.4 Jbc 3
He 7.7
Hd 4.0
Figure 20. NMR parameters for anion (8c). 37
V
Figure 21. NMR spectrum of anions (8a) and (9a) (-30°)
Figure 22. NMR spectrum of (9b) (35°). 38
1 \-\-\ :• 1 = 1 — :' 1 '""I1 '' io «o xo no too 0 >-N»H» «
s/ i J A# 1 •—; 1 H— h—r-—1—-—1 r-H——; 1.:.:, —1—
Figure 23. NMR spectrum of anion (9c) (35°). Chemical Shifts (?) Coupling Constants (Hz)
Ha 6.9 Jab 7.5
Hb 4.5 Jab' 7.5
Hb1 4.4 Jbc 10
He 6.4 Jbc* 10
He' 6.1 Jed 4
Figure 24. NMR parameters for anion (9b).
H H
CH,
Hb Ha
Chemical Shifts (T*) Coupling Constants (Hz)
Ha 7.3 Jab 7.5
Hb 4.6
He 8.5
Hd 7.8
Figure 25. NMR parameters for anion (9c). 40 A
>-H>
Figure 26. NMR spectrum of 2,4,6-trimethyl-l,3,6-heptatriene.
Figure 27. NMR spectrum of anion (9d) (35°) 'H H Hd CH
Hb CH„a
Chemical Shifts (T)
Hb 4.6
Hd 7.8
Figure 28. NMR parameters for anion (9d). 42
Figure 29. NMR spectrum of cycloheptatriene.
_A_
Figure 30. NMR spectrum of anion (9e) (35°) 43
H ii-Bu
He1 He
Hb' Hb
Chemical Shifts CO Coupling Constants (Hz)
Ha 6.9 Jab 7.5
Hb 4.5 Jbc 10
Hb' 4.4 Jed 4
He 6.2
He1 6.1
Figure 31. NMR parameters for anion (9e). APPENDIX
KINETIC DATA
Table 1-A. Kinetic data for the (l) to (2) rearrangement.
Relative Areas Time (min) t4.4 (1) f4.0 (2) T4.7 (3)
10 645 24 0
15 500 25 20
30 470 56 25
65 312 103 33
135 147 140 100
225 90 70 158
360 66 33 330
555 10 24 380
720 0 10 401
1320 0 0 405
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