I d . Eng. Chern. Prod. Res. Dew. 1980, 19. 349-364 (177) Tsuchiya, T., Kwila. J.. Igeta, H., Snieckus. V.. J. Chem. Soc.,Chem. Commun., 640 (1974). (7781 Tsuchiya, T.. KuiM. J.. Osawa. K., J. Chem. Soc., Chem. C o m n . , 250 (1976). (179) Tsuchiya, T., KuriM. J.. Snieckus. Q., J. Org. Chem., 42, 1856(1977). (180) Tsuchya, T.. Sash&, H., Sawanishi, H.. Chem. pharm. Bull., 28,2880 (1978). (181) Wawronek. S.. Org. Prep. Pmced. Int., 6, 69 (1974).
340
(182) Wawzonek, S., Aelong, D.. McKillip. W. J., Org. Prep. Proced. Int., 8, 215 (1976). (183) Wawzonek. S.. Kellen. J. N.. J. Org. Chem., 38, 2058 (1973). (184) Yarnashila. Y., Masurnura. M.. Tetrahedron Len.. 1765 (1979).
Received for reuiew March 3, 1980 Accepted April 7 , 1980
The Cycloproparenes Brian Halton Department
Of
Chernlstiy, Vlctorla UnIvemW of Welllngton, Wellington, New Zealand
Cyclopropabenzene(bicyclo[4,1,O]hepta-l,3,5-triene).its derivatives and homologueslhe cycloproparenes-are reviewed from the synthetic, chemical, structural, and spectroscopic viewpoints. The fascinationof these highly strained molecules has led to a more detailed examination of the concepts of aromatic bond fixation, and the chemical properties recorded provae for the utilization of the compounds in organic synthesis. Introduction The existence of cyclopropabenzene (1) as the parent
5
la
5
lb
and most highly strained memher ofthe orth6-fused rycklkal)enzene series has been firmly esmtilished for some 15 years ( I ) . As a result of impnwed synthetic methods and Separation procedures, other aromatic systems ranying 1.2-methylene fusion and cyclopropahenzenes strained even more from fusion ui a second carbncyclic system have recently liecome available. The fascination ot’ these hydrocarbons and their derivatives lies in the desire to establish the limib of stress, strain, and distortion that can he imposed upon the benzenoid framework and to drlineate the consequential influences on bonding, structure, and chemical reactivity. The purpose of the prrsent cnntribution is to provide comprehensive, yet critical, coverage of the literature from the time that the early work was assessed ( 2 ) and t o indicate the likely direction ot’further research. The reader is referred to the earlier review VJ and also to a more recent acrount which highlights the major developments in cycloproparene chemktry (3). Chemical Abstracts has been reviewed from April I972 through 1)ecemtier 1979. Synthesis of t h e Cycloproparcnes A. Historical Approaches. The earliest claim of a rycloproparene was made by De and I h t t in 1930. These authors (4) proposed that the decomposition ot‘a series of aryliminosemicarbarones 2, derived frnm 9,10phenanthroquinone and 1-aryliminoguanidines, led to the iminocyclopropa[/]phenanthrenes 3. A reinvestigation ( 5 ) of these decompitions has failed to repnrdure the original finding, and the sole characterizalile products were the 4-arylsemicarbazunes 4. In contrast, the addition ot’ 4erondary diazoalkanes to the quinone imides 5 and 8 do give compounds whose properties match those reported by Mustafa and Kame1 in 1952 (6).However, the products are not the cycloproparenes 6, 9 and I O but the unrear0196-4321/80/1219-0349801 OO/O
Brian Halton was born in Lancashire, England, in 1941. He received his B.Sc.(Hons) in chemistry in 1963 at Southampton Uniuersity and Ph.D. in organic chemistry in 1966 from the same institution. After a year of postdoctoral research at the University of Florida, he was appointed to the faculty as Assistant Professor. He transferred to the faculty at Victoria Uniuersity of Wellington in 1968 and now holds the position of Reader in the Chemistry Department. In 1972 he spent six months as Visting Lecturer at the Uniuersity of New South Wales and was on study leave at the Uniuerstiy of Reading during 1975. Dr. Halton has authored and coauthored ouer 40 scientific papers, has been a contributor to the Chemical Society Specialist Periodical Report “AlicyclicChemistry”, and has coauthored a text, “Organic Photochemistry”, with Dr. J. M. Coxon of Canterbury Uniuersity. His research interests lie in the sphere of unnatural products with particular emphasis on the chemistry of highly strained organic molecules; he was awarded the N.Z. Association of Scientists’ Research Medal in 1974. He is a Fellow of the New Zealand Institute of Chemistry, Chairman of its Wellington Regional Committee, and a member of its National Council. He is married and has two children.
2
3
4
R = aryl ranged bicyclo[4,1,O]heptene derivatives 7, 11, and 12, respectively (7) (Scheme I); despite the application of a variety of conditions, aromatization of these products could not he induced. 0 1980 American Chemical Society
350
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Scheme I
Scheme I1 S ,02Ph
N
HNS02Ph
HkS02Ph
NS02Ph
6
5
R'
P?
N 'S02Ph
7 HNSOZPh
NS02Ph
I
11
HNS02Ph
PhS02YH
fi
NS02Ph
9
/\ S ,OzPh N
15
10
/
16
S , 02Ph N
17
f
7'
R'
Ph N
I
Ph
R4
'S02Ph
11
R'
Ph
a
12
b
B. Photolysis of 3H-Indazoles a n d Spiro-3Hpyrazoles. In view of the results outlined above, it can be reasonably claimed that cycloproparene chemistry began in 1964 with the isolation and characterization of the cyclopropabenzene 14 by Anet and Anet (8). As a route
Me02C &Me
13 Me
Me02C
14
to cyclopropabenzenes the photolysis of 3H-indazoles,e.g., 13, was received favorably (2). However, the limitation of the method to the synthesis of geminally disubstituted derivatives (3-monosubstituted indazoles exist in the alternative aromatic 1H-tautomeric form) and difficulties associated with the preparation of the substrates have resulted in the method receiving scant attention recently (9). The spiro-3H-pyrazole route developed by Durr and Schrader (Scheme 11) (IO),and until quite recently the only mode of entry into the cyclopropa[u]naphthalene series, viz. 15d 18d, has now been extended to provide the first spirocyclopropabenzenes 19a-c (11, 12). The yields are satisfactory and no other synthetic procedure currently employed for cycloproparene synthesis is as adaptable to the preparation of these interesting derivatives. As a result of the detailed studies undertaken by the group a t Saarbrucken (11-14) the limitations and mechanistic implications of the method have been deduced (Scheme 11). With R5and R6of the spirocycle 15 as car-
-
R4
19
18
C
d
Ph Ph Ph Ph
RZ Ph H p-BrC,H, Ph
R3
R4
Ph Ph H Ph p-BrC,H, Ph benzo fused
bomethoxy substituents cyclopropabenzenes 18a-d are produced even when R3R4corresponds to benzo fusion. = (CH&],19a-c are obtained; the However, with 15 [R5R6 benzo-fused 15d yields 20d. Thus when benzo fusion is present, substituent control is evident since only the benzospirene 20d is obtained. I t is thought that the absence of electron-withdrawing carbomethoxy substituents on the hetero-ring of 15 allows the fused benzene ring to decrease the facility for the [1,5]carbon shift to such an extent that the alternative course of reaction to give 20 is observed (Scheme 11). Further support (12, 14) for the pathway from 15 to 18/19 (Scheme 11) stems from the thermal rearrangements of the diazaspirenes 15; 3Hindazoles 17 or, when R5= H, the lH-isomers 21 are obtained. However, despite rigorous attempts (13),17 has eluded detection in the photoprocess with either 366- or 313-nm irradiation. As a synthetic procedure the longer wavelength photolysis is to be recommended since the cycloproparene products are photolabile (A, -300 nm). C. Bicyclo[4,1,0]heptenes as Precursors. The dehydrohalogenation route to cyclopropabenzenes has received considerable attention since the last review (2) and halogenated bicyclo[4,1,0]heptenes can now be regarded as precursors pur excellence. By employing tetrahalocyclopropenes in Diels-Alder cycloadditions with appropriate buta-1,3-dienes (15), the bicycloheptenes 22 are readily available. Dehydrohalogenation is normally effected by employing potassium tert-butoxide in either tetrahydrofuran or dimethyl sulfoxide and with phenyl substituents a t the 2- and 5-positions of 22 the requisite gem-dihalocyclopropabenzenes 23a-c are obtained in 70-95% isolated yield (16, 17). In the absence of substituents which can stabilize the development of carbanionic character at the 2- and 5-positions of 22 the elimination is not always easy to effect (16),and although cy-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Scheme I11
R'
R'
351
I
26
IC
22
1
R'
zR*2 2
1 R'
Scheme IV
23a, R' = Ph; Rz = H; Y=Z=Cl b, R' = Ph; R Z = H; Y=Z=F c, R' = Ph; R 2 = H; Y = Cl;Z= F d, R' = R Z = H; Y=Z=F e, R' = R Z = H; Y = C1; Z = F Ph
Ph
mPh "Ph
Ph
Ph
Ph
Ph
24
25
88%
clopropabenzenes can often be isolated the yields are significantly reduced (440 nm) the isolated yield of 1,6-di-iodocycloheptatriene from 1 is increased to 64% (71) while in the complete absence of light only o-iodobenzyl iodide is formed (70). With thiocyanogen, 1 also gives a 1,6-disubstituted cycloheptatriene in high (61%) yield on photolysis (Scheme XIII) (72). The available evidence clearly implicates radical pathways with 1. and .SCN being involved. The 1,6-functionalized cycloheptatrienes obtained by these workers have been utilized (see for example Scheme XIV) for conversion into other valuable, and difficulty accessible, cycloheptatriene derivatives (70-72), thus demonstrating the synthetic potential of the cycloproparenes. The reaction between the methano[ lolannulene derivative 35 and bromine also leads to addition across the cyclopropene sp2 centers but in this instance 70, the Br
& 0
0
0
71
product of addition of two molar equivalents of bromine, is isolated (73). Although the yield of 70 is low (18%),the compound has been elegantly utilized to provide (in seven steps) the syn-bismethano[l4]annulene71 for the first time (73). It is tempting to suggest that the yield of 70 may be enhanced under photochemical conditions. Cyclopropabenzene has been recognized as a dienophile for some time (74) as is illustrated in Scheme XI for the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
357
up to 130 "C, are produced. It will be interesting to see if the application of ultra-high pressures brings more success and whether other cycloproparenescan be induced to undergo cycloaddition. In the hope of detecting r-bond localization in 1, attempts have been made to isolate r complexes with metal carbonyls (80). 1 itself gave only intractable materials but 32 affords the novel insertion product 75 together with
Scheme XV R3
73
R3
0
a, R' = R Z = R 3 = C0,Et;b, R' = CN; R 2 R3 = CO,Et;c,R' = H; R' = R3 = +R2 C0,Et;d, R' = C0,Et; R Z = R 3 = H 32
R'
75
% yield
74a, b, c, d,
61 70 41 33
formation of 1,6-methano[101annulene from reaction with butadiene (60). The reactions recorded with the cyclic dienes a-pyrone and 1,2,4,5-tetrazine-3,6-dicarboxylate yield thermally labile cycloadducts which undergo decarboxylation and deazetation, respectively (2). However, with 4,5-dibromo-o-benzoquinone the crystalline adduct 72 is obtained (75). Compound 72 represents the first cycloaddition product of cyclopropabenzene in which the stereochemicaloutcome has been capable of determination and the dicarbonyl bridge has been assigned trans with respect to the three-membered ring (75). In the expectation of partial bond localization, this result is fully consistent with a concerted [,6, + ,4,] process involving l a
Br'
la
-
"0 L
V
72
as depicted and not the alternative [,2, + ,4,] reaction with the bond shift isomer l b (76). However, it must be noted that the reaction was performed in heated acetonitrile and an alternative nonconcerted pathway could be in operation. The potential of 1 as a dienophile has been further exploited recently as depicted by Scheme XV. The 1,2,4triazines 73a-d undergo cycloaddition with 1 to give methanoaza[lO]annulenes 74a-d (77, 78). Whereas the syntheses of 74a,b were effected at slightly elevated temperatures and normal pressure, the analogues 74c,d could only be obtained (77, 78) with the use of ultra-high pressure. The reaction of 1 with the parent triazine (73; R' = R2 = R3 = H) has not yet been effected under any conditions. The significance of these results lies in the ability of 1 to act as a synthon for compounds otherwise accessible only in low yields from long and complex synthetic procedures. Attempted cycloaddition reactions of 1 with p-toluenesulfonyl and p-nitrophenyl azides, diphenylnitrilimine, and ethyl diazoacetate (77), as well as 23a and phenyl azide (22),at normal pressures have thus far failed. However, a recent report shows that 1 does suffer dipolar cycloaddition across the bridge bond with aryl nitrile oxides at atmospheric pressure (79); bridged norcaradienes, stable
76
small amounts of the dione 76. No evidence for the desired complexes was observed. Birch reduction of 1 gives ring-cleaved products exclusively as is to be expected and l-methylcyclohexa-l,4-diene, toluene, and 1,2-diphenylethaneare obtained in 98% yield in a ratio of 62:28:10 (51). With the availability of the 3-halocyclopropabenzenes 30a,b (27-30) there exists the possibility of dehydrohalogenation to give the severely strained 2,3- and 3,4dehydrocyclopropabenzenes. However, reports of these studies have yet to appear (29). The Cyclopropabenzenyl Cation, Anion, and Radical Until recently the only evidence available to support the existence of cycloproparenyl cations came from the mass spectral fragmentation patterns of cycloproparene derivatives (2). However, SSC-EH-MO (Self Consistent
Charge Extended Huckel Molecular Orbital) calculations led to the prediction that the parent cation should be stabilized by charge delocalization, and the authors suggested that the species should be capable of isolation (81). Chemical evidence in support of cation formation was subsequently obtained from a series of reactions in which the integrity of the cyclopropabenzene framework is maintained. The gem-dichlorocyclopropabenzene23a and its naphthalene analogue 25 undergo exchange of the halo substituents with Grignard or organolithium reagents (21, 20). In this way 77 (R = Et) and 78 (R = Et) have been Ph
I
Ph
77
Ph
Ph I
78
R'
79a,
R' = Pr'; R' = M e
b, R' = Me;
RZ = H characterized. With the sterically more demanding isopropyl magnesium bromide, Grignard reduction occurs and, while the presence of 77 (R = Pr') is inferred, only the product of thermal rearrangement is isolated; hydrogen atom transfer occurs through a five-membered transition state to yield the styrene 79a. The thermal lability of 77
358
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Scheme XVI Ph
I
Scheme XVII Ph 1
23a
1,R=H 32,R R = benzo fused
I
Ph
Ph
80
M
R
I
Ph
6h
(R = Pr') is paralleled by the gem-diphenyl derivatives 77 (R = Ph) and 78 (R = Ph) (2,20,21). These reactions are readily rationalized by ionization of the gem-dihalocyclopropabenzene, e.g. 23a, and subsequent nucleophilic capture of the cyclopropabenzenyl cation, e.g, 80, thus formed (Scheme XVI). With methyl Grignard an unexpected outcome is recorded; styrene 79b is obtained only as the minor (18%) product of reaction from 23a. The major product (45%) is the fluorene 81a (21). In like manner R
\
W
P
recorded (Table I) are fully consistent with the substituted cyclopropabenzenyl cation 80. Ion 80 exhibits a singlet resonance for H3 and H4 (9.22 ppm) which is deshielded by ca. 1.5 ppm when compared with 23a and C1 (131.2 ppm) appears 71 ppm downfield of its sp3 counterpart in 23a. By employing the same techniques the ions 82 and 83 have also been generated under conditions conducive Ph
R
I
I
I
,R
I
83
Ph
R
82 84 to long life (86,87). The spectral assignments for 83 and h
Me
81a, R = H b, RR = benzo fused
25 gives 81b. While the route from the cyclopropabenzene to the fluorene is unknown, the reaction corresponds to monohalogen exchange and Grignard reduction (20,21). An even more unusual outcome is observed when 25 is treated with only a slight excess of bromo-Grignard reagent-the chlorine atoms of 25 are replaced by bromine atoms to give the isolable bromo analogue (20). Even more convincing evidence for the existence of cycloproparenyl cations come from the behavior of 23 with silver fluoride and aluminum hydride (or deuteride). With the former reagent 23a is converted to 23b (82) and the reaction can be terminated at the half exchange stage to give 23c (17),while with the latter reagent 23a,e yield the corresponding hydrocarbon (83). Reduction with lithium aluminum hydride cannot be stopped at the bis-exchange stage and reductive ring opening to cycloheptatrienyl and benzyl derivatives is observed (21). The exchange reactions are not restricted to functionalized cycloproparenes. Both 1 and 32 are themselves subject to abstraction of one methylene proton when subjected to trityl tetrafluoroborate and the corresponding aryl aldehyde is formed from nucleophilic capture of the cation by water (Scheme XVII) (84). The reaction with 1 exhibits first-order kinetics and a deuterium isotope effect of 6.5 has been recorded which clearly implicates the parent ion in this process. In view of the evidence supporting the existence of cyclopropabenzenyl cations as reactive intermediates, it is not suprising that representative examples have been characterized spectroscopically. When 23a is allowed to react with antimony pentachloride a moisture sensitive orange solid is produced which, unlike the precursor, is decomposed to 2,5-diphenylbenzoicacid. A solution of this solid in chlorosulfonic acid provides 'H and 13C NMR spectra identical with those obtained by dissolution of 23a in pre-cooled fluorosulfonic acid (85, 86), and the data
its precursor (Table I) have been shown to be correct from the data provided by the corresponding 2,5- and 3,4-dideuteriocyclopropabenzenes and the ion 85 derived from the former (18). Cyclopropanaphthalenylcations have yet to be characterized. The appearance of the aromatic protons of the cyclopropabenzenylcations at lower field than in the precursors is consistent with related examples (85,86), and the higher charge density at C3(4) compared with C2(5) is in agreement with calculations (81). By utilizing the changes in the 13Cchemical shifts upon ionization the charge distribution has been estimated (86). In ions 80 and 82 ca. 95% of the charge resides in the central ring system with little delocalization into the pendant phenyl substituents. Some 45% of the charge resides on the three-membered ring of each of the ions and this compares very favorably with the 45-50% predicted for the parent ion (81). In each case there is a significant contribution from back bonding by the halo substituent as illustrated by 84. The parent cyclopropabenzenyl cation has yet to be generated under conditions suitable for long life, but the isolation of 23e (17) is encouraging since the molecule is suitably substituted for preferential replacement of the chlorine atom by hydrogen and subsequent fluoride ion loss. A detailed analysis of the spectroscopic data of the parent ion would be welcomed. It is worthy of mention that decomposition of 85 with D
D I
I
85
?
I
D
o-'"" 9
I
D
water yields benzoic acid labeled at C2 and C5 (18). This
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
359
Table I. 'H and I3C NMR Parameters for Some Cyclopropabenzenes and Cyclopropabenzenyl Cations" comnound
H 2 (5)
H3 (4)
C l a (5a)
c3 (4)
c1
129.1
60.1
16,86
ref
7.70(s)
131.6
C2 (5) 130.4
9.22 (s)
150.4
146.4
147.5
131.2
85,86
7.58(s)
126.1
128.6
132.0
101.6
83,86
9.38 (s)
134.7
130.8
148.7
147.2
86
7.45 ( m )
129.5
116.0
134.7
100.3
18, 86, 87
9.20 ( m )
141.1
119.8
153.8
148.1
18, 86, 87
Ph
23a Ph I
Ph
80
Ph
23b
Ph
82
7.45( m )
eF 23d
8.40 ( m )
L-,
a3 Chemical shifts are i: 0.1 ppm downfield from internal Me$ external Me,& for cations. a
shows that the ion suffers nucleophilic attack only at C1. The SCC-EH-MO calculations (81) predict that the cyclopropabenzenyl anion 87 should be the least stable of the derived species but not antiaromatic. Excellent evidence for the existence of 87 has been obtained by Eaborn and his collaborators (88). These authors have shown that 1 is more acidic then cyclobutabenzenesince lithiation can be effected at low temperatures and the intermediate converted into the silyl derivative 86 in a yield of 36%.
86
87
for neutral compounds and f 0.1 ppm downfield from Scheme XVIII
89
1
90
/
-cs/
HZ01
1
Cleavage of the C-Si bond of 86 occurs with aqueous sodium hydroxide and the reaction follows first-order kinetics; anion 87 is clearly involved. In fact 86 reacts some 64 times more rapidly then benzyltrimethylsilane and this indicates that the pK, of 1 is ca. 36 (cf. pK, of toluene = 41). The cyclopropabenzenylradical is expected (81)to have a stability which lies between those of the (isolable) cation and the (observed) anion. However, no report of a radical has yet been recorded despite the availability of cyclopropabenzene derivatives which appear amenable to radical decomposition. 1-Oxocycloproparenes Although reports to substantiate the transient existence of 1-oxocyclopropabenzenes,e.q., 90, in solution were recorded some ten years ago (89), the sensitivity of the compounds toward nucleophiles and electrophiles has thus
88
far precluded formal characterization either of the parent compound or one of its derivatives. Nonetheless, Chapman and his group (90)have obtained small amounts of a material at 8 K from the photodecompositions of 88 and 89 (Scheme XVIII). This compound exhibits a carbonyl stretching frequency at 1838 cm-' in the infrared and is tentatively assigned as the ketone 90. In support of this
90
II
0
assignment is the observation that the compound suffers decarbonylation to benzyne on further photolysis. Ketone 90 has also been proposed as a possible intermediate in
360
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
Table 11. Structural Parameters of Some Cycloproparenesa
+ Ph e
1.333
1.385 (1.389)
1.417 (1.421)
1.392
1.519 (1.520)
126.6 (126.3)
109.3 (109.6)
1.35
1.39 (1.42)
1.40 (1.38)
1.39
1.47 (1.45)
128 (124)
(110)
1.339
1.355
1.423
1.368
1.337
1.437
64.0 (64.0)
98
62 (63)
99
C02Me
18b
":;.:
108
Ph
23a
m:
6
1.411
1.439
1.52
126.1
111.5
1.504
124.9
114.7
101
62.9
100
32 a
Bond lengths are in Angstrom units and bond angles in degrees.
the thermal decomposition of cyclopentadienylideneketene since fluorenone is isolated in 14% yield (91). Physical and Theoretical Aspects of the Cycloproparenes The remarkable successes that have been achieved in the synthesis and utilization of a wide variety of cycloproparene derivatives have served to widen the horizons previously envisaged and have provided even greater stimulus for further investigations. Not surprisingly, a number of the cycloproparene derivatives have provided valuable physicochemical parameters and others have been the subjects of accurate structure determinations. The concept of bond fixation, first advanced by Mills and Nixon (92)50 years ago, has been subjected to detailed scrutiny in recent times. Although the experimental data on which the original hypothesis was based was subsequently shown to be ambiguous (93), an early theoretical study of the bond lengths and bond angles in indan and tetralin led Coulson and Longuet-Higgins (94) to conclude that bond length alteration should occur in indan in the direction indicated by 91 and be more pronounced in the
Ql3 91
more strained members of the cycloalkabenzene series. As was discussed in the earlier article (2),more recent theoretical investigations (81, 95) have cast doubt on the direction of bond localization, i.e. la vs. lb. While there have been no more detailed theoretical studies, e.g., by ab initio methods, the hybridization of 1 has been examined (96) by the maximum overlap method but the results obtained are limited in that the spin-spin coupling constant J'sC-~ is not reproduced accurately [ J I ~ Cobsd - ~ (113), 170 Hz; calcd (96), 152.2 Hz]. Benzohomovalenes, including a variety of cycloproparenes, have also been the subject of chemical graph theory (97). Despite the conflict in the theoretical predictions, accurate crystallographic data are now available for the cycloproparenes 18b (98), 23a (99), and 32 ( l o o ) ,and a
microwave structure determination has been performed on 23d (101). The results are summarized in Table I1 and it is apparent that the cyclopropabenzene framework suffers from considerable bond length and bond angle deformation; the bicyclic framework is essentially planar in all cases examined. The bond lengths recorded indicate some bond localization, but the most striking feature is the absence of bond fixation in accord with either of the Kekul6 structures l a or lb; with the exception of 23a, each molecule examined has three adjacent short bonds (bonds b, a, b'; Table 11). The bridge bonds (bond a, Table 11) are shorter than that of benzene (1.395 A) and that predicted (1.420 A) (951,and the effect is most pronounced in the gem-disubstituted derivatives including the fluorinated compound 23d. As is to be expected, bond a is longer than than the double bond of cyclopropene (1.296 A) (102) and shorter than the single bond of cyclopropane (1.510 A) (103), but its relationship to these values is remarkably similar to those between the bond length of benzene and the accepted carbon-carbon double and single bond values. Thus is appears that the internal strain of the cycloproparenes is accommodated by severe distortion without any tendency toward a formalized bond fixed structure. The apparent [6 + 41 cycloaddition of 1 with 3,4-dibromo-o-benzoquinone to give 72 could, therefore, simply reflect minimum steric interference in the transition state while providing relief to the distortion (and strain) present in 1. The heat formation (-1.4 kcal mol-') and structure of 23d has been calculated by the MNDO (Modified Neglect of Diatomic Overlap) method (104). While the gross structural features are reproduced well the calculations significantly overestimate the length of the bridge bond (calcd, 1.420 A; obsd, 1.339 A). The strain energy of cyclopropabenzene estimated (105) as 45.5 kcal mol-' greater than that of cyclopropane has now been determined (100) as 68 kcal mol-' from the silver ion catalyzed methanolysis reaction which yields 64 (ZR = OMe). This value is significantly higher than the total strain energy of cyclopropene (52.6 kcal mol-') (106) but comparable to that of 1,3-dimethylbicyclo[l,l,O] butane
Ind. Eng. Chern. Prod. Res. Dev., Vol. 19, No. 3, 1980 361
NMR Data of the Cycloalkabenzenes, Table 111. o-Xylene and Cyclopropenea com- Cla (5a) C2 ( 5 ) C3 ( 4 ) Ccu (Jl3c-n) (J~Jc-H) ref pound 125.4 114.7 128.8 18.4 113, 115 : o a ( 168.5)b (170)
ma 5
0
03
145.6
122.1 (162) 144.0 124.4 (155.5)
a; DE
136.3
129.8
126.6
29.4 113, 115 (138) 126.2 33.8 113,115 (127)
126.0
19.4
115
2.3 117 (167) a The numbering system used here is for convenience only. Chemical shifts are in ppm relative to Me,Si. Values in parenthesis are the one bond I3C-H coupling constants in Hz. 108.7
(68.2 kcal mol-') (107) in which two CT bonds are broken. The strain energy of cyclopropa[b]naphthalene (32) has been measured likewise as 65-67 kcal mol-' from a methanolysis study and determined as 67.8 kcal mol-' from combustion data (100). Combustion data for the biscyclopropanaphthalene 38 place the strain energy at a minimum value of 166 kcal mol-' since clean combustion of the molecule was not easy to achieve (108). This value, well in excess of twice that of 1, must reflect further distortion due to the presence of two annelated three-membered rings in the molecule. It should be noted, however, that since both strain and distortion are associated with the cycloproparenes, the measured effects cannot be assigned exclusively to either feature. Thus the strain energies of 1 and 32 are ca. 68 kcal mol-' if the resonance energies are the same as those in benzene and naphthalene, respectively, and that of 38 L 166 kcal mol-' if the resonance energy of 38 is the same as that of naphthalene. The photoelectron spectrum of 1 has been examined (109) and the influence of the methylene group on the r-orbital energies assessed by the simple inductive hyperconjugative Huckel molecular orbital model. As a consequence, the methylene function had to be assigned a negative inductive effect which in turn leads to the conclusion that the highest occupied r-orbital [bl(r)] is destabilized by hyperconjugation and that the highest occupied u orbitals are localized in the carbon-carbon bonds of the three-membered ring. The 'H NMR spectra of the cycloproparenes indicate that the diamagnetic ring current is not significantly affected by the structural distortions present since the protons of the six-membered ring resonate in the usual aromatic region. The aromatic protons of 1 appear (110) as an AA'BB' system at 7.149 and 7.189 ppm. On the basis that the strongest long-range proton-proton spin-spin coupling will be observed between the nuclear protons at C2(5) and the distal methylene group protons (110, I l l ) , the signal at 7.149 ppm is assigned to H2(5) and that at 7.189 ppm to H3(4). The methylene protons of 1 appear as a singlet at 3.11 ppm and other cycloproparenes exhibit this signal in the range 3.0-3.5 ppm. The anomalously small value of J2,4 (Jmeta) for the cycloproparenes (Jmeta < J,,,) (2) has not been adequately rationalized although the distorted geometry of the ring system must be involved (112). The availability (113-115) of 13C NMR data has now allowed for comparison with other cycloalkabenzenes
Table IV. I3C NMR Assignments for the Cycloproparenes" compound no. C1 Cla (5a) C2 ( 5 ) 2 1 18.4 125.4 114.7 :
50
C3 (4) ref 128.8 113
1
m a m
0
55
32 18.6
123.4
112.3
136.7
115
38 19.9
122.8
113.5
140.1
38
39 19.3
122.1
112.7
136.2
34
40 19.2
122.8
110.0
145.5
39
43 19.9
119.6 135.9 148.0 (126.0)b (112.4) (121.0)
40
a Chemical shifts are in ppm downfield from internal Values in parentheses for this compound refer Me,Si. to the magnetically nonequivalent C5a, C5, and C4 atoms, respectively.
(Table 111) and it should be noted that the original (116) assignments for 1, cyclobutabenzene,and indan have been subject to correction. The data of Table I11 clearly show tht strain factors influence the carbon-13 chemical shifts at C2(5); enhanced shielding is associated with an increase of ring strain, cf. C2(5): indan, 124.4: cyclopropabenzene, 114.7 ppm. The effect is also reflected (113) in an almost linear increase in the one-bond carbon-13 proton coupling constants for the C2-H moiety in passing from indan (155.5 Hz) to 1 (168.5 Hz). A similar situation is observed with the carbon chemical shifts and one-bond 13C-H coupling of the benzylic groups. However, the present theories of carbon chemical shifts do not offer a simple explanation for these phenomena. It is also clear from the data of Table I11 that cyclopropabenzene is anomalous in exhibiting shielding at Cla(5a) (125.4 ppm) when the same carbon atoms of the remaining cycloalkabenzenes are deshielded. However, it must be noted that substantial cyclopropyl carbon shielding is associated with the unique bonding of the three-membered ring as is illustrated (117) by the values for the vinylic carbon atoms of cyclopropene (108.7 ppm) when compared with those of cyclobutene (137.2 pprn). The carbon atoms remote from the sites of ring fusion [C3(4)] are essentially unaffected by strain factors. One bond 13C-13Ccoupling constants for the Cla-C2 and Cla-C1 bonds of the cycloalkabenzenes have also been measured (118). Once again exceptional values are recorded for 1 due to the special bonding situation of the three-membered ring. However, these data have been rationalized on the basis of the Walsh model. The influence of strain is also apparent in the 13CNMR data of other cycloproparenes (Table IV). Thus the anomalously low value for the chemical shift of Cla(5a) is reproduced over a range of ca. 4 ppm and the anticipated low field signal due to C2(5) appears in the 110-115 ppm region of the spectra. The appearance of this latter signal can be regarded as characteristic of the cycloproparene framework. However, its precise position is not unduly influenced by additional strain as demonstrated by the values for 40 (110.0 ppm) and 43 (112.4 ppm) and, as Billups (3)has suggested, there appears to be a strict limitation on the use of the C2(5) chemical shifts as a criterion of strain. Carbon-13 NMR data are also available for 18a and its derived diacid, 18b, and 19a,b (119).
382 Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980 Table V. Electronic Spectral Data of Some Cycloproparenes compound
no.
solvent
hmam
nm
1 cyclohexane 252 258 264 270 27 7 43 hexane 264 270 276.5 40 cyclohexane 284 287.5 294 32 cyclohexane 220
44 hexane
228.5 282 330 38 cyclohexane 226 21 3 326 39 226.5 256 267 288 297 304 310 324 35 dioxane 268 302 377 387 396 406
loge
2.7 3.0 3.2 3.4 3.3 -3.1 -3.2 -3.2 -3.0 -3.0 2.8 4.7 4.7 3.5 ( 5 ) 3.3 4.9 3.8 3.8 4.9 3.5 3.7 3.1 (5) 3.3 3.3 3.5 (5) 3. I 4.9 4.1 3.0 3.1 3.1 2.9
Scheme XIX ref 60
"
R2'
H
97
100
*
99a. R' = H; R 2 = C1 b, R' = C1; R2= H
98a
95
98b
40
39
1
32, 33 23
38 34
36
The electronic spectra of the simple cycloproparenes (Table V) confirm that mono-annelation of a small ring to a benzenoid nucleus has very little influence on the aromatic chromophore. Thus 1, cyclobutabenzene, and o-xylene have similar ultraviolet spectra as do the cyclopropanaphthalenes and their ring opened o-dimethyl analogues. However, with the linear annelation of a second small ring to the benzenoid nucleus bathochromic shifts are recorded. The shift of the absorption maximum to longer wavelength was noted (120) some time ago for biscyclobuta[a,d]benzene [A, (EtOH) 276 (3.661, 280 (3.71) and 286 nm (log t 3.59)1 when compared with durene [A, (EtOH) 268.5 (2.86), 272.5 (2.83) and 278 nm (log t 2,89)], but the effect is much more pronounced for the linear cyclopropacyclobutabenzene 40 (Table V). The same situation does not apply to the angularly fused bisannelated systems. Thus 43, biscyclobuta[a,c]benzene and prehnitene (1,2,3,4-tetramethylbenzene)have very similar absorption maxima. This dichotomy of behavior has been rationalized (121) for the biscyclobutabenzenes and results from the hyperconjugative abilities of the fused rings and changes in the configurational composition of the lowest excited singlet state. The infrared spectra of the cycloproparenes are simple and reflect the symmetry of the molecules. Nevertheless, there is a characteristic band at ca. 1670 cm-' (1, 1666; 32, 1673, 44, 1687 cm-') due to the combination of a threemembered ring skeletal vibration with the aromatic double bond stretch. The electron impact mass spectra of the cycloproparenes invariably show a molecular ion and the primary fragmentation gives the corresponding cycloproparenyl cation/s. However, the loss of a hydrogen atom from the molecular ion is not straightforward. Buchs and his co-
workers (122, 123) have shown that complete hydrogen scrambling occurs prior to H.loss with the aid of specifically deuterated analogues. Moreover, by using 1 labeled at C1 with carbon-13, it has been found (123) that the carbon atoms of the molecular and (M-H)+ ions lose their positional identity before the integrity of the ring system is destroyed by the loss of ethylene. The involvement of cycloproparenes and their derived cations as fragmentation products from other systems has also been noted (124). Heteroatom Derivatives At the time of writing no examples of methylene fusion across the ortho positions of a heteroaromatic ring system, e.g. 92-94, have been recorded. Nevertheless, develop-
92
93
94, X = NH,0,S
ments in this area are to be encouraged since the concepts of bond localization could be more readily discernible in these systems. In contrast to the above, heteroatom fusion across the ortho sites of the benzene ring has been studied. 1-Azaand 1-thiacyclopropabenzenes,95 and 96, respectively,
95
96
have been proposed as reactive intermediates but the analogous oxygen derivative appears unknown. The gas phase pyrolysis of isatin (97) affords the iminocyclohexadienylidene 98a which interconverts quantitatively with 98b via 95 prior to ring contraction to cyanocyclopentadiene (Scheme XIX) as evidenced by labeling studies (125). Analogous results are obtained with benzo-annelated derivatives. However, photolysis of the benzotriazoles 99a,b yields products derived only from the corresponding diradicals 100a,b (126);crossover products are not observed and consequently ring closure of diradicals 100 is not involved. In like manner, the available evidence (127,128) suggests that 1,Zketocarbenes do not interconvert and the product mixtures obtained can be adequately accounted for without the intervention of a 1-oxacyclopropabenzeneintermediate. However, the sulfur atom, with its enhanced polarizability and greater size, undergoes bridging to give 96 during 1,2-thiocarbene interconversions (127, 129). Indeed the photolysis of 101 yields the heteroallene 102 and traces of a material ten-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 19, No. 3, 1980
u
101
96
102
tatively identified as 96 from infrared spectroscopic data (130). Nomenclature Cycloproparene has been used throughout this review for convenience purposes only. However, before “fusion nomenclature” (Le,,the prefix cyclopropa-) can be used “at least two rings of five or more members” must be present (I.U.P.A.C. Rule A 21.3). Thus while 1H-cyclopropa[a]naphthalene is correct for 44, 1H-cyclopropabenzene is incorrect for 1. The Chemical Abstracts Service and I.U.P.A.C. are unanimous in the nomenclature for 1 as bicyclo[4,1,0]hepta-l,3,5-trieneeAs will be appreciated, any strict adherence to these rules provides nomenclature for the parent member of the series which not only differs from that of the higher homologues but also could be taken to imply a bond localized structure. In addition, compounds 95 and 96 are also systematically named as 7-aza- and 7-thiabicyclo[4,1,0]hepta1,3,5-triene. Acknowledgments I am indebted to :Professors E. Vogel (University of Cologne), P. Muller (University of Geneva), and W. E. Billups (Rice University), and Drs. J. M. Muchowski (Syntex Research) and R. Okazaki (University of Tokyo) for providing information prior to publication. The contribution of my co-workers, whose names appear in the literature citations, cannot be underestimated. Financial support from the New Zealand Universities Grant Committee, the British Council, and Victoria University is gratefully acknowledged. Literature Cited Vogel, E., Grimme, W., Korte, S., Tetrahedron Lett., 3625 (1965). Halton. B.. Chem. Rev.. 73. 113 11973). Billups, W. E., Acc. Chem.’Res.,‘ll, 245 (1978). De, S. C., Dutt, D. N., J . Indian Chem. SOC.,7, 537 (1930), but see also Perkin, W. H., J. Chem. SOC.,1 (1888). Haiton, B., Harrison, S. A. R., Spangler. C. W., Aust. J. Chem., 28, 681 (1975). Mustafa, A,, Kamel, M., J . Am. Chem. SOC., 75, 2939 (1953). Jones, G. W., Kerur, D. R., Yamazaki, T., Shechter, H., Woolhouse, A. D., Haiton, B., J . Org. Chem., 39, 492 (1974); Pinkus, A. G., Tsuji, J., ibid., 39, 497 (1974). Anet, R., Anet, F. A. L., J . Am. Chem. SOC.,88, 525 (1964). Schrader, L., Tetrahedron, 29, 1833 (1973). Durr, H., Schrader, L., Angew. Chem. Int. Ed. Engl., 8, 446 (1969); Chem. Ber., 103, 1334 (1970). Dum, H., Schmitz, H., Angew. Chem. Int. Ed. Engl.. 14, 647 (1975). DW, H., Ranade, A. C., Halberstadt, I., Tetrahedron Left., 3041 (1974). Luddecke, E., Rau, H., Durr, H., Schmitz, H., Tetrahedron, 33, 2677 (1977). Durr, H., Sergio, R., Chem. Ber., 107, 2027 (1974). Law, D. F. C., Tobey, S. W., J . Am. Chem. SOC.,90, 2376 (1968). Halton, B., Milsom, P. J., Woolhouse, A. D., J . Chem. Sac., Perkin Trans. I , 731 (1977); Halton, E., and Milsom, P. J.. J . Chem. SOC., Chem. Commun., 814 (1971). Muller, P., Etienne, R., m e r , J., Plneda, N., Schlpoff, M., Tetrahedron Lett., 3151 (1978), Helv. Chim. Acta, 81, 2482 (1978). Muller, P., Pfyffer, J., Wentrup-Byne, E., Burger, U., Helv. Chim. Acta., 61, 2081 (1978). Browne, A. R., Halton, E., J . Chem. SOC.,Chem. Commun., 1341 (1972). Erowne, A. R., Halton, E., J . Chem. SOC.,Perkin Trans, 7 , 1177 (1977). Halton, B., Woolhouse, A. D., Milsom, P. J., J . Chem. SOC., Perkin Trans. 7 , 735 (1977). Halton, B., unpublished observations. Vogel, E., personal communication. Billups, W. E., Bbkeney, A. J., Chow, W. Y., J . Chem. SOC.,Chem. Commun., 1461 (1971); Org. Synth., 55, 12 (1976). Restien, J., anther, Ii., Angew. Chem. Int. Ed. Engl., 13, 276 (1974). Eanwell. M. G., Blattner, R., Browne, A. R., Cralg, J. T., Halton, E., J . Chem. Soc., Perkin Trans. 7 , 2165 (1977). Kumar, A., Tayal, S. R., Devaprabhakara, D., Tetrahedron Lett., 863 (1976). Garratt, P. J.. Koller. W., Tetrahedron Lett., 4177 (1976). Blllups, W. E., Chamberlain, W. T., Asim, M. Y., Tetrahedron Lett.. 571 (1977). ~
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:pyas.C. S.. Copper. M. A,. I"'
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,",*,"",.
,n
