diazonium salts," J. Org. Chem., 34, No. 4, 854-857 (1969). ... Mechanisms of reactions of brominated diolefins of the bicyclo[3.3.1]nonane series were studied.
5.
R. M. Elofson and F. F. Cadallah, "Substituent effects in the polarography of aromatic diazonium salts," J. Org. Chem., 34, No. 4, 854-857 (1969). 6. M. Siskin, "One-electron reductions of aromatic cations," Methods Free Radicals Chem., !, 83-131 (1972). 7. V. D. Pokhodenko, A. A. Beloded, and V. G. Koshechko, Redox Reactions of Free Radicals [in Russian], Nauka. Dumka, Kiev (1977). 8. R. A. Marcus, "Chemical and electrochemical electron transfer theory," Annu. Rev. Phys. Chem., 15, 155-196 (1964). 9. F. De Yong and D. N. Reinhout, "Stability and reactivity of crown ether complexes," Adv. Phys. Org. Chem., 17, 279-434 (1980). i0. O. B. Nagy, N. W. Muanda, and J. B. Nagy, "Quantitative evaluation of solute-solvent interactions," J. Phys. Chem., 83, No. 15, 1961-1970 (1979). Ii. V. E. Kampar, V. R. Kokars, and O. Ya. Neiland, "Efficiency of neutral w-donors with acceptors -- onium cations and their electron affinity, 2. Phenyldiazonium salts," Zh. Obshch. Khim., 47, No. 4, 858-862 (1977). 12. L. Eberson, "Electron transfer reactions in organic chemistry," Adv. Phys. Org. Chem., 18, 79-185 (1982). 13. A. A. Polyakova, K. A. Bilevich, N. N. Buhnov, et al., "One-electron transfer reaction with the participation of the pyrilium cation," Dokl. Akad. Nauk SSSR, 212, No. 2, 370373 (1973). 14. B. K. Bandlish and N. J. Shine, "Ion radicals. 37. Preparation and isolation of cation radical tetrafluoroborates by the use of nitrosonium tetrafluoroborates," J. Org. Chem., 42, No. 3, 561-563 (1977). 15. L. Michaelis and S. Granick, "The polymerization of the free radicals of the Wurster dye type: the dlmeric resonance bond," J. Am. Chem. Soc., 65, No. 9, 1747-1755 (1943). 16. Ya. M. Kolotyrkin (editor), Electrochemistry of Metals in Nonaqueous Solvents [in Russian], Mir, Moscow (1974),
MECHANISM OF TRANSANNULAR BROMINATION REACTIONS OF DIOLEFINS OF THE BICYCLO[3.3.1]NONANE SERIES P. A. Krasutskii, A. B. Khotkevich, Yu. A. Serguchev, and A. G. Yurchenko
UDC 547.678+547.442
Mechanisms of reactions of brominated diolefins of the bicyclo[3.3.1]nonane series were studied. The reaction of transannular cyclization into adamantane derivatives proceeds by a molecular-ionic mechanism, and the rate of reaction obeys a thirdorder kinetic equation. An analysis of the calculated thermodynamic parameters of the transition state shows that intermediate complexes %rith charge transfer of the diolefin...Bra and diolefin...Br, type are formed.
One of the specific trends of electrophilic reactions with nonconjugated olefins is homoallylic addition leading to cyclic and polycyclic products. In several cases these reactions find application as methods of synthesis of natural compounds and other products [i, 2]. Therefore, the establishment of mechanisms of transannular transformations is very important, since eventually it will be possible to carry out the reactions of polyolefinic systems in the required direction. It should be noted that at present no correct data are available in the literature on the mechanisms of transannular reactions of polyolefins. It was suggested in review [3] that the formation of products of transannular cyclization indicates an ionic or radical mechanism. The investigations in [4, 5] also indicate a possible ionic mechanism for this type of reactions, at least in the case of iodination. In the present work, we continued the study on the mechanism of the transannular additions of halogens to diolefins of the bicyclo[3.3.1]nonane series. In contrast to the Kiev Polytechnic Institute. Institute of Organic Chemistry, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 21, No. i, pp. 52-56, January-February, 1985. Original article submitted January 26, 1984. 48
0040-5760/85/2101-0048509.50
~ 1985 Plenum Publishing Corporation
iodination reaction, in which the product of the transannular addition is often the only stable component of the reaction mixture [6, 7], bromination products are determined mainly by kinetic factors. The investigation was carried with 3,7-dimethylenebicyclo[3.3.1]nonane (1), 3-methyl-7methylenebicyclo[3.3.1]-2-nonane (2), and 7-methylenebicyclo[3.3.1]-2-nonane (3). The bromination of compounds i and 2 in nonpolar aprotic solvents proceeds with the formation of l-bromomethyl-3-bromoadamantane (4) [8] and trans-l,4-dibromo-3-methyladamantane (5), respectively, as the only products:
L~..~ 3r2 GH28r !
.
O
Brz
~
2
Br S Br
In the bromination of compound 3, besides the transannular cyclization product, trans-l,4-dibromoad~m~ntane (6), products of "anomalous" addition, 7-bromomethylbicyclo[3.3.1]nona-2,6diene (7a), and 7-bromomethylbicyclo[3.3.1]nona-2,7-diene (7a) are also formed: 6r
~2Br
7a
d
~G~zBr
7b
The structure of dibromides 5 and 6 is confirmed by the agreement between the observed PMR spectra and those calculated by the method described in [9, I0]. Products 7a, 7b are formed in a ratio of ~3:2, as determined by GLC and PMR spectra. A similar isomerization of the u-bond has already been observed in the halogenation of olefins [ii, 121. It is logical to assume that products 5 and 6 are formed in the electrophilic attack by bromine on the endocyclic C2--C3 double bond regioselectively at the exo position, while products 7a and 7b are formed in an attack by bromine on the exomethylene group. It is possible that theparallel reactions of the homoallylic and "anomalous" addition occur at comparable rates, as indicated by the almost equimolar ratios of products 6, 7a, and 7b in the reaction mixture. The kinetic measurements were carried out in CCI, at a concentration of (2-20).10 -3 moles/liter olefin and (1-6)'10 -3 mole/liter of bromine, by a method of standing flow, with spectrophotometric control of the reaction. The time of half-transformation for compound 1 was 20-50 msec, for 2, 0.5-1 sec, and for 3, 3-10 sec. The measurements were carried out at the wavelength of X = 417 nm, corresponding to the absorption maximum of the free bromine (e = 213) using the FEU-39A photomultiplier as detector. As the recorder for reactions i and 2, the memory storage oscillographs 8-9Awasused, and for compound 3, a self-recording apparatus. For compounds 2 and 3, a glass mixer with a time constant of ~50 sec was used, and for compound i, a mixer with stainless steel pistons, pne-m~tic feeding, and time constant of less than i msec. The low concentration of bromine in the solution prevented corrosion of the mixer material. The measurements were carried out in the temperature range of 280-325~ The rate of the brominatlon reaction of compounds 1-3 obeys the kinetic equation with the form W = k [o~n
]
[Br2p.
The observed rate constants of the third-order reaction, and the calculated activation parameters, together with the literature data on the bromination kinetics of cyclohexene in CCI, [13], are listed in Table i. The data in this table show that for the reactions studied, a decrease in the rate with increase in the temperature is characteristic. As in iodination [4, 5], this pattern indicates the formation of intermediate equilibrium complexes with a i:i or possibly 1:2, composition (CTC-I and CTC-2, respectively). The rate of reaction changes in the series of cyclohexene < 3 < 2 < i. The relative correspondence of the thermodynamic parameters of the bromination and iodination reactions [4, 5] indicates an analogy of their mechanisms, including a trimolecular transition state (8) and a synchronized limiting stage:
49
TABLE 1, Kinetic Cons=ants and Calculated Activation Parame t : e r s of Bromination of C o m p o u n d s 1-3
k, llte~
-AH",
-AS",
Eel. ram of
mole I .sec
k]'/mole
],J/mole
reaction~
Compound
?, K
8,86.106 6,74.10" 4,36.10 o 2,28.108
32•
186--10
l ,8. IOs
!
286,8 295 314 284 296 306,5 317 323
2,52. I ,50. 9,45. 6,02. 4,66.
37•
230-+ lO
6. IOs
0,9. I0 ~
280
,., z
I0 ~ I06 I0 = I04 I04
288 308,5 318,5
4,22.103 2,21. IOs 1,135. IOs
49+4
310~I0
298
24,9
49
340
Br~Br
8r~Br~
.. 9
•8Hz. Brz F
8r
.
:""'CHz 1 B, r CTC -/
]
.-~
.
9
.
"-~.'.'CH,i
CTC -2
8
The ionic pair (9), whose formation is inevitable because of stereochemical features of t h e transannular reactions, is one of the intermediate reaction products. The specific property of the diolefins studied, in particular of compound I, is the combination of energetic and stereoelectronic factors favorable for a synchronous process, which may lower the degree of charge separation in the CTC to values which can be reached in a molecular interaction. These factors include the steric proximity of double bonds and the o-overlap of the p-orbitals [14, 15], the inappreciable strain in the formation of o-bonds of the adamantane structure [16], and also the readily attained antiperiplanarity of the Br-Cll and C3-CI0 bonds in the transition state for retaining the orbital symmetry. The decrease in the rate of halogenation on transition from compound i to 3 is explained by both entropic and enthalpic factors. The entropic unfavorableness of the homoallylic addition to compounds 2 and 3 consists in the need for an electrophilic attack at the endocyclic %-bond only. An attack at the exocyclic double bond does not lead to a transannular cyclizatlon product, since in this case a strained structure of noradamantane should be formed. For compound 3 this attack occurs in products 7a and 7b, which is explained by the decrease in the rate of the homoallylic addition to values of the rates of an "anomalous" addition [ii, 12]. However, the more pronounced asymmetry of the ~-bonds in compound 1 leads to a higher charge separation in CTc-i and CTC?2 (6~s > 6+.~o or 6t7 > 6~,I), and hence to a mere ready + is ~ require ~ d for the transannular cyclization,on theC3 or attainment of "barrier " charge, (~c) C7 atoms. For compounds i and 2, the following inequalities occur for the charge on the C3 + > + + > + atom in CTC-I and C T C - 2 : 6 C 3 ~CIO and 6C3 6C2 , while for compound 3, the relationship ~t3 = ~C2 holds. In an analysis of the with the well-lnvestigated respect to the identity of which the direction of the 50
mechanisms of the transformations studied, an analogy is observed fragmentation reactions of adamantane derivatives [17-19], with structures of the corresponding transition states A and B, in charge transfer is opposite:
~r~-.~-B r.. ~,
A
BrZn-~GH
B
The transannular cyclization reaction of (I) proceeds in nonpolar solvents by the action of mild electrophilic reagents, while the fragmentation of l-bromomethyl-3-bromoadamantane (4) proceeds in boiling dlmethylformamide [17]. This is explained by the fact that in the case of A, the readily polarizable w-bonds transform into stable o-bonds, while in the case of B the opposite situation occurs. We can therefore assume the feasibility of the polyhomoallylic addition reaction of electrophilic reagents to a hypothetical compound (I0), while the reverse fragmentation process of dibromide (ii) scarcely occurs:
/o
H
The values of enthalpy of activation of the reaction which were found are of special interest. As known [20], anegative enthalpy of activation is characteristic of a process with a successive exothermal formation of equilibrium compounds. In our case, these intermediates may be charge transfer complexes CTC-I and CTC-2. From an analysis of the literature data of the complexation of olefins with halogens [4, 5, 21-23], we can estimate the enthalpy of formation of CTC-I between bromine and the diolefins studied as a value with an order of magnitude of --20 kJ/mole. The absolute value of the negative enthalpy of activation of the reaction (--32 t o - 4 9 kJ/mole) considerably exceeds this value, even if we take into account the correction for the possible temperature dependence of the entropy of the reaction [20]. The data obtained provide a very convincing proof for the existence in the systems studied of diolefin--brumine complexes with a 1:2 composition. In the scheme of the mechanism of the bromination process of compound i under consideration, the problem of the mechanism of cation (9) stabilization and the postactivatlonal stage of the process still remain unclear. An ion pair mechanism with an intramolecular attack of the anionoid particle (Brs-) on the cationold center may be possible, in analogy with homoallylic interaction of norbornadiene and methylenebornene with sulfenyl chloride derivatives [24, 25]. LITERATURE CITED i. 2.
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