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Electrocyclic Opening of 2 3-Benzo- and 2 3;7 8-Dibenzobicyclo[4.2

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hartree), than the formally anti-bonding degenerate x*MOs. The latter are stabilized by interaction with the three
peripheral silicon atoms. These are the "o-bridged-n" interactions described and depicted by Jackson. Allen et al.J31
the importance of which, however, has been discounted in
some other analyses of the bonding in l.l4]
As we have pointed out in the case of 3 vs. 4,l5]
the difference in behavior of corresponding polycyclic carbon
and silicon ring systems can be attributed to differences in
strain contributions. While three-membered silicon ring
systems are more strained than their carbon counterparts,
the opposite is true for four-membered rings.l5I Thus, the
ring-opening process, which, in effect, converts the threemembered rings in 3 into the four-membered ring in 4,
results in a higher relief of strain energy than would be
possible with the corresponding carbocyclic system.
Hence, bicyclobutane prefers a closed structure akin to 3.
The same situation pertains in 2 with its four-membered
rings and roughly 100" SilSi2Si3 bond angles. The geometry with a short SilSi3 distance, which we find not to be
minimum, would be higher in energy (see Table 2) due to
the presence of three 3-membered silicon rings.
Despite the singlet diradical character of the bridgehead silicons and the weak SilSi3 bonding, the strain
energy in 2 (62.2 kcal mol-' at pseudopotential MC-SCF21G*)1io-'21is relatively low, particularly in comparison
with the strain energy of [I.l.l]propellane 1 (102 kcal
mol -').12n.c1
This difference also is apparent in the much
higher strain energy of bicyclo[l.l.l]pentane 6 (67 kcal
mol - ')Izc1 than bicycle[ 1,l.l]pentasilane 5 (37 kcal rnol - '
at 3-21G*).13b.''1 While the bridgehead-bridgehead disH H
tance in 6 (1.88 A) is much longer than that in -1
(1.60 A),12.31
the two separations are closer in 2 (ca. 2.73 A)
and in 5 (2.92 A),"31especially when the inherently larger
silicon distances are taken into account.
In view of the relatively low strain energy predicted for
2 and the considerable recent experimental success in the
and 5,Ll31as well as many small-ring
preparation of 1, 3,@b1
silicon systems, the synthesis of appropriate derivatives of
4 affords an experimental challenge. Generally similar
conclusions have been reached by Alien et al.'3b1 and by
Nagase et al.l'ol who also have studied these systems.
Received: July 31, 1987 [Z 2385 IE]
German version: Angew. Chem. 99 (1987) 1312
[ 11 a) For a comprehensive analysis of the bonding in hydrocarbons, see K.
B. Wiberg, R. F. W. Bader, C. D. H. Lau, J . Am. Chem. SOC.109 (1987)
985: b) Other analyses of the bonding in I include: W. Stohrer, R. Hoffmann, ibrd. 94 (1972) 779; M. D. Newton, J. M. Schulman, ibrd. 94
(1972) 773; N. D. Epiotis, ibid. 106 (1984) 3170; P. Politzer, K. Jayasuriya, J. Mol. Sfruct. 135 (1986) 245; R. P. Messmer, P. A. Schultz, ibrd.
108 (1984) 7407, and Ref. [21-[41.
[2] a) K. B. Wiberg, Acc. Chem. Res. 17 (1985) 379, and references cited
therein; b) K. B. Wiberg, ibid. I05 (1983) 1227; c) K. B. Wiberg, Angew.
Chern. 98 (1986) 312; Angew. Chem. Int. Ed. Engl. 25 (1986) 312.
[3] a) J. E. Jackson, L. C. Allen, J . Am. Chem. Sor. 109 (1984) 591: b) D. B.
Kitchen, J. E. Jackson, L. C . Allen, to be published.
[4] a) A. B. Pierini, H. F. Reale, J. Medrano, J. Mol. Stmet. 148 (1986) 109;
b) D. Feller, E. R. Davidson, J. Am. Chem. SOC.109 (1987) 4133.
0 VCH VerlagsgesellsrhaJt mbH, 0-6940 Weinheim, 1987
P. von R. Schleyer, A. F. Sax, J. Kalcher, R. Janoschek, Angew. Chem. 99
(1987) 374; Angew. Chem. Int. Ed. Engl. 26 (1987) 364, and references
cited therein.
a) For a theoretical study of 3 (but not 4) see S. Collins, R. Dutler, A.
Rauk, J. Am. G e m . SOC.109 (1987) 2564; b) for the X-ray structure of a
derivative of 3, see R. Jones, D. J. Williams, Y . Kabe, S . Masamune,
Angew. Chem. 98 (1986) 176; Angew. Chem. Int. Ed. Engl. 2.5 (1986) 79;
c) for a theoretical study of 4 (but not 3), see T. Dabisch, W. Schoeller,
J. Chem. SOC.Chem. Commun. 1986, 896.
The standard H F calculations were carried out with the Gaussian 82
program; see W. J. Hehre, L. Radom, P. von R. Schleyer, J. A. Pople: Ab
lnifio Molecular Orbital 7heory. Wiley, New York 1986. The MC-SCF
treatment employed was similar to that described in Ref. [5].
The MC-SCF wave-function obtained for 4 is quite similar,
y M ~ = 0 . 9vh5 0.30 v.,[S], yh/wa denotes that configuration in which the
bonding/antibonding 0-MO of the Si I-Si3 bridge is doubly occupied.
A. E. Reed, F. Weinhold, J. Chem. Phys. 83 (1985) 1736; A. E. Reed, R.
B. Weinstock, F. Weinhold, ibid. 83 (1985) 735 These bond orders may
vary somewhat at higher levels of theory (the 3-21G* bond orders for 2
are: Si 1 Si3 = 0.245, Si I Si2 = 0.834), but should be interpreted on a relative, rather than on an absolute basis.
S . Nagase, T. Kudo (Organometallics 6 (1987) 2456) give a strain energy
of 70 kcal mol-' for 2 (SCF/6-31G* calculations). These authors call
attention to the negatively charged bridgehead silicon, Sil, in 2. However, this is generally found when a silicon is bound to several SiHl or
SiH2 groups. For example, the 3-21G natural charges 191 on the central
Si atoms are: -0.106 in 2 (-0.163 at 3-21G*) but -0.378 in Si(SiH,)4,
+0.019 in 5 , +0.028 in 3, +0.072 in 4, and +0.19 in HSi(SiH,)3. In all
these molecules, the SiHz silicons have charges +0.34+0.1 and the SiHi
silicons +0.55+.02.
Ref. [3b] gives 71.3 and 37.4 kcal m o l - ' for 2 and 5 , respectively, using
a pseudopotential SCF-3 IG* (spd) basis. Our pseudopotential SCF21G* value for 2 (72.4 kcal mol-I) agrees well; also the SCF 69 kcal
mol- ' estimate of Nagase [lo]. However, an electron-correlation-corrected ring-strain energy was evaluated by "two-configuration (u2,
per-SiSi-bond" wave functions. The decrease of total energy for a normal SiSi bond is 4.7 kcal mol-' whereas that for the diradical central
SiSi bond in 2 is 14.9 kcal m o l '.~ Thus, S C F overestimates ring strain
energies in 2, and also in 4, by more than 10 kcal m o l - ' .
All these strain energies are evaluated by the use of the unusual homodesmotic equations [3b, 5 , 101.
The first preparation of a derivative of 5 has been reported by S.Masamune et a\. (Fflh Inf. Symp. Organosilicon Chem.. St. Louis, MO, USA,
June 1987).
Electrocyclic Opening of 2,343enzo- and
2,3 ;7,8-Dibenzobicyclo[4.2.O]octa-2,4,7-triene;
Benzoannelated Transition States**
By Wolfam Grimme.* Johann Lex, and Thomas Schmidt
Dedicated to Professor Emanuel Vogel on the occasion of
his 60th birthday
The electrons participating in pericyclic reactions are
aromatically stabilized in the transition state before they
form new bonds in the product. When an electron pair of a
benzene ring participates in a pericyclic reaction, the transition state is benzoannelated and "isoconjugate" with
naphthalene, whereas the product has lost all aromaticity.
Hence, energy-rich intermediates which rapidly undergo
further reaction can be generated relatively easily by pericyclic reactions with benzenoid double bonds. Frequently,
the reverse of their formation is the preferred reaction, and
it requires special analytical methods in order to detect the
occurrence of these processes.
An example of a hidden pericyclic reaction with participation of a benzene ring is the electrocyclic ring-opening
of benzonorcaradiene 1 to give the bicyclo[5.4.0]undeca[*] Prof. Dr. W. Grimme, Dr. J. Lex, Dr. T. Schmidt
lnstitut fur Organische Chemie der Universitat
Greinstrasse 4, D-5000 Koln 41 (FRG)
I**] This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
0570-0833/87/1212-1268 $ 02.50/0
Angew. Chem. In[. Ed. Engl. 26 (1987) No. 12
1,3,6,8,IO-pentaene 2."] The reaction already occurs at
room temperature, but the product with the o-quinodimethane partial structure undergoes back reaction so rapidly
that its detection is only possible via the racemization of
the benzonorcaradiene, which is very rapid on the N M R
time scale at 180°C.
We report here o n the corresponding ring opening of
benzo- and dibenzobicyclo[4.2.0]octa-2,4,7-triene.The synthesis of the 2,3-benzobicyc10[4.2.0]octa-2,4,7-triene-2'-carbonitrile 4 was achieved starting from the photoproduct
3''I of P-naphthonitrile and ethyl vinyl ether; 3 was obtained in 63% yield as a 5 : 1 mixture of the anti- and synisomer. Since several steps are involved in the formation of
3 and another skeletal structure has been proposed for the
product of the corresponding reaction with methyl vinyl
ether,"' we have confirmed the structure of 3 by an X-ray
diffraction analysis of the Diels-Alder adduct 5.l4]Compound 5 is formed upon heating a solution of 3 in hexachlorocyclopentadiene at 100°C for 14 h and can be isolated by chromatography (CHCI,, silica gel) and purified
by recrystallization from hexane. The ether 3 eliminates
ethanol on heating to 105°C (3 h) in a mixture of glacial
acetic acid, 85% phosphoric acid and water ( 5 :2.75 :1
parts by volume) and affords the benzobicyclo[4.2.0]octatriene derivative 4 (72%). Physical data of 4 and the other
new compounds are given in Table 1.
Above 100°C, 4 rearranges uniformly in a first-order
reaction into 2'-benzocyclooctatetraenecarbonitrile 7 ; the
NMR-spectroscopically measured rate constant in CD3CN
at 110.3"C is k,=(3.80+-0.06)~IO-'s-'. The energy-rich
primary product 6 of the electrocyclic opening of the sixmembered ring rapidly forms the arene by a double-bond
shift in the eight-membered ring. To check whether recyclization to the starting compound is a competing process, 4
was resolved into the enantiomers by chromatography on
microcrystalline t r i a c e t y l c e l l ~ l o s e The
. ~ ~ ~decrease in rota-360, c=0.44) in CH,CN at
tion of (-)-4 ([a]575=
110.3 "C, too, is a first-order process with the rate constant
s-'. Since it is found, within the
limits of error, that k,=ka, it must be concluded that back
reaction of 6 to racemic 4 does not take place. Likewise
the intermolecular trapping reaction with reactive dienophiles cannot compete with the double-bond shift in 6, so
the occurrence of the mechanistically plausible intermediate 6 had to be substantiated in another way.
& - Nca
In 2,3;7,8-dibenzobicyclo[4.2.0]octa-2,4,7-triene
816] the
electrocyclic opening should occur analogously to that in 4
with participation of an electron pair of the benzene ring
annelated to the six-membered ring. In the intermediate 9,
however, the double-bond shift in the eight-membered ring
is degenerate and leads to 9' ; here a stabilization can only
occur through recyclization. If the racemization of 8 or its
automerization to 8', both of which are possible only via
the o-quinoid intermediate, occurs under similar conditions as the rearrangement of 4, the formation of such an
intermediate can also be expected in the case of the latter.
4, rn.p. 64-66OC; ' H - N M R : 6 = 7 . 2 8 (AB, Av=71 Hz, J=7.8 Hz; H-3',4'),
7.27 (d, J = 1.9 Hz; H-1'). 6.13 (AB, A v = 6 4 Hz, J = 9 . 9 Hz: H-4, S ) , 6.09 (AB,
Av=SO Hz, 5 ~ 2 . 7Hz; H-7, 8). 4.12 (d, J = 4 . 5 Hz; H-I), 3.68 (t, J = 4 . 5 Hz;
H-6); "C-NMR: 6 = 140.82 (C-3), 140.66 (C-7), 138.82 (C-8), 132.87 (C-2),
130.92 (C-3'), 130.47 (C-l', S), 128.46 (C-4'). 124.16 (C-4), 118.93 (CN), 110.22
(C-2'), 43.78 (C-I), 42.97 (C-6): UV (hexane): 1,,,=275 nrn ( ~ = 3 9 3 0 ) ,285
(3360) sh, 302 (1330) sh, 313 (890) sh
5, rn.p. 173°C; 'H-NMR: 6=1.65 (d, J = 1.4 Hz, 1 H), 7.39 (AB, Av= 114.5
Hz, J = 7 . 9 Hz, 2H), 4.38 (m, 1 H), 3.52 (AB, Av=47 Hz, J = 9 Hz, 2H-Et),
3.18 (m, 1 H), 3.06 (AB, Av= 18 Hz, J = 8 . 4 Hz, 2H), 2.66 (m. 1 H), 1.97 (AB,
Av=SI Hz, J = 13.4 Hz, 2H), 1.22 ( t . 3H-Et)
7, 'H-NMR: & = 7.30 ( S , I H), 7.28 (AB, A v e 3 9 . 3 H z , J = 9 Hz, 2 H), 6.32 (AB,
Av=37.5 Hz, J = 1 2 Hz, 4H), 5.90 (s, 2 H ) ; UV (hexane): A,,,=275
(&=2300) sh
8 - D , 'H-NMR: 6=7.4-6.8 (m, XH), 6.02 (d, J = 4 . 7 Hz, 1 H), 4.51 (AB,
A v = 3 4 Hz, J = 6 Hz, 2 H )
8'-D, 'H-NMR: 6=7.4-6.8 (m, XH), 6.13 (AB, Av=15.6 Hz, J = 9 Hz, 2H),
4.73 ( 5 , 1 H)
Ed. Engl. 26 (1987) No. 12
The position marked with a black dot is deuteriated.
Table I . Some physical data of the compounds 4, 5 , 7 , 8 - D , and 8'-D. The
NMR spectra were recorded at 90 MHz in CCI4 (those of 4 and 5 at 300
MHz in CDCl, and CCI4+CD2CI2, respectively).
Angew. Chem. Inr.
We first investigated the automerization by means of the
compound 8 - D deuteriated in position 4. This was obtained from 4-bromo-2,3;7,8-dibenzobicyclo[4.2.0]octa2,4,7-trieneC7I by metal-halogen exchange with n-butyllithium in T H F at -70°C and subsequent hydrolysis with
DZO. N M R spectroscopic studies revealed that 8 - D is in
equilibrium with its positional isomer 8'-D at 100°C in
CCI4. The equilibrium constant is K = 1.13 and the rate
constant for the formation of the equilibrium was found to
be (ka+ki,)=(7.45+0.06)x
s - ' at 100.6"C. Thus the
automerization of 8 takes place just as easily as the rearrangement of 4, indicating the formation of an o-quinoid
intermediate in both cases.
The question as to whether in the intermediate 9 the recyclization to 8 can compete with the energetically neutral
double-bond shift to give 9' was determined with help of
enantiomerically enriched 8. Partial hydroboration of racemic 8 with diisopinocampheylborane[81(0.5 molar equivalents, prepared from (-)-a-pinene) in 1.5 M ethereal solu-
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"C furnished optically active (+)-8 with
in benzene). Its racemization in benzene at 100.6"C is a reaction of first order with the rate
constant k a = ( 7 . 6 7 f 0 . 0 3 ) x
s-'. Thus, the racemization is equally rapid as the establishment of the equilibrium between the labeled isomers, and the o-quinoid intermediate 9 preferentially undergoes a double bond shift to
give 9'.
The free enthalpies of activation for the electrocyclic
ring opening of the benzoannelated cyclohexadienes in the
compounds 1, 4, and 8 can be compared with those for
the parent compounds. Table 2 shows that the participation
= 108.4 (c==
Table 2. Free enthalpies of activation for the ring-opening of cyclohexadienes and their benzoannelated derivatives.
T I"C1
[kcal mol-'1
[kcal mol-'1
7.7 191 20.2 [I]
20.4 [lo]
compounds. Perhaps the best known species of this type
are the borazines"] and related derivatives. These have
many of the characteristics of aromatic compounds exemplified by the close similarity in physical properties of borazine and benzene. In sharp contrast, boron-phosphorus
compounds are not nearly as thoroughly studied and no
stable boron-phosphorus analogue of borazine has been
described. We have recently published the syntheses and
structural studies of a monomeric phosphinoborane"] and
some phosphinideneborate~~~~
to gain some insight on the
nature of BP single and multiple bonds. We now report a
further addition to the known classes of BP compounds in
the form of the first structurally characterized BP analogue
of a borazine; this is the title compound (MesBPC6H,,),, 1
(Mes=2,4,6-Me,C6H2, C,Hl> =cyclohexyl), which has
been characterized by X-ray crystallography, and "P- and
"B-NMR spectroscopy.
19.2 [I I]
tion of a benzenoid electron pair increases the free enthalpy of actiuation by 11f 1 kcal mol-'. Incremental calculations,I''] on the other hand, indicate that the loss of
aromaticity increases the free enthalpy of reaction of the
benzoannelated compounds by 25 k 1 kcal mol -'. The introductory concept that benzoannelated transition states of
pericyclic reactions are still aromatically stabilized is thus
Received: August 18, 1987 [Z 2403 IE]
German version: Angew. Chem. 99 (1987) 1277
The B3P3compound 1 was formed as pale yellow crystals in moderate yield during the attempted synthesis of
MesB(PH-C6Hl ')'. The X-ray crystal str~cture'~'
is illustrated in Figure 1. It consists of a
in which all the BP bonds are essentially equal, averaging
1.84 A in length. This distance is considerably shorter than
[I] E. Vogel, D. Wendisch, W. R. Roth, Angew. Chem. 76 (1964) 432; Angew. Chem. Int. Ed. Engl. 3 (1964) 443.
[2] K. Mizuno, C. Pac, H. Sakurai, J. Chem. SOC.Perkin Trans. I 1975, 2221.
[3] T. R. Chamberlain, J. J. McCullough, Can. J. Chem. 51 (1973) 2578.
[4] Triclinic crystals, space group Pi, Z=2, a=7.833(1), b = 10.818(2),
c = 14.195(3) A, a=71.84(2)", p=79.77(2)", ~ = 6 9 . 5 9 ( 2 ) ~pcalcd=
1.549 g
cm-', 3652 reflections observed; Re0.033. Further details of the crystal
structure investigation are available on request from the Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-75 14 EggensteinLeopoldshafen 2 (FRG), on quoting the depository number CSD-52735,
the names of the authors, and the journal citation.
[S] We thank Prof. Dr. K G .Klurner, Bochum, for carrying out the separation.
[6] M. P. Cava, D. R. Napier, J. Am. Chem. SOC.78 (1956) 500.
[7] M. P. Cava, D. R. Napier, J. Am. Chem. SOC.79(1957) 1701.
[8] G. Zweifel, H. C. Brown, J. A m . Chem. SOC.86 (1964) 393.
191 M. B. Rubin, J. A m . Chem. SOC.103 (1981) 7781.
[lo] E. Vogel, H. Kiefer, W. R. Roth, Angew. Chem. 76 (1964) 432; Angew.
Chem. int. Ed. Engl. 3 (1964) 442.
[ 111 M.-E. Giinther, R. Aydin, W. Buchmeier, B. Engelen, H. Giinther, Chem.
Eer. 117 (1984) 1069.
[I21 S. W. Benson: Thermochemical Kinetics, Wiley, New York 1968.
By H . V . Rasika Dias and Phirip P. Power*
The isoelectronic nature of the BN and CC pairs has
generated considerable interest in various boron-nitrogen
[*] Prof. P. P. Power, H. V. R. Dias
Department of Chemistry, University of California
Davis, California 95616 (USA)
This work was supported by the National Science Foundation and the
A. P. Sloan Foundation.
0 VCH Verlagsgesellschaji mbH, D-6940 Weinheim. 1987
d c4
Synthesis and X-Ray Structure of
A Boron-Phosphorus Analogue of Borazine**
c40 C41
Fig. I . Two views of the crystal structure of 1 (H atoms omitted for clarity).
Selected bond distances [A] and angles ["I: B1-P1 1.838(6), P1-B3 1.833(6),
B3-P3 1.835(6), P3-B2 l.S51(6), B2-P2 1.837(7), P2-Bl 1.844(7), B1-C7
1.577(8), B2-C22 1.570(8), B3-C37 1.577(8), PI-Cl 1.837(6), P2-Cl6 1.842(7),
P3-C31 1.836(6); PI-Bl-P2 114.8(3), P2-B2-P3 113.8(3), Pl-B3-P3 116.0(3),
BI-Pl-B3 124.2(3), Bl-P2-B2 126.0(3), B2-P3-B3 125.0(3).
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Angew. Chem. Int. Ed. Engl. 26 (1987) No. 12
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benz, electrocyclic, opening, dibenzobicyclo
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