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Inductive charge dispersal in the solvolysis of 3-substituted bicyclo[1.1.1] pentyl bromides
Te~ahedron biters. Vol32,
0040 4039191 $3 oo+ 00 Pcrgamon Press plc
No 42. pp 5945-5948.1991
hmted III Great Bntam
INDUCTIVE
CHARGE
3-SUBSTITUTED
DISPERSAL
IN THE
SOLVOLYSIS
BICYCLO[l.l.l]PENTYL
OF
BROMIDES
* Cyril
Institute
A. Grob
of Organic
St.Johanns-Ring
Ernest
School
of Physlcal Bedford
KEY WORDS:
ABSTRACT:
Chemistry,
X1 Yang
University
19, 4056 Basel,
W. Della
and
Sciences, Park,
Inductlvlty;
and Cheng
Dennis
K. Taylor
The Flanders
South
Australia,
Carbenlum
of Base1
Switzerland
Ions;
University
of S.A.
5042
Graded
Bridging.
Solvolysis rates and products of 3-substituted bxyclo[l.l.l1pentyl
branldes indicate extremely strong inductive charge dispersal in the ionization to bxyclo[l
.l.l
lpentyl-l-cations,the precursors of 3-methylenecyclobutls.
Recent studies have shown that the solvolys~s rates (log k) of several bl- and trlcyclic halides and sulfonates correlate linearly with the inductive constants d: of neighql . The reaction constantPI, boring substltuents according to the equation log k = pI d, the so-called lnductlvlty, gauges the sensitlvlty of the rate to the inductive effect of substltuents at various posltlons and 1s a measure of charge dispersal in the resulting carbenium ion. Since 6: values were derived from the pKa of 4-substituted qulnuclldlnium 2 , in which sterlc, cotqugatlve and hypercoqugatlve effects are negli-
perchlorates -1
gible or absent, these correlatrons indicate that relative lonlzation rates are controlled only by the inductive effect of substituents.
5945
5946
Inductlvlty depends not only on the distance between the substituent and the cationlc center. It also varies with the structure of the molecule. Thx
is illustrated by the
pI values for the solvolysls in 80% ethanol of the 7%xbstituted 2-norbomyl, 2-bicyclo[2.2.2]octyl and 2-adamantyl p-toluenesulfonates2, 3 and 4, namely -2.00 3, -1.50 4 and -0.80 5, respectively. Thus, lnductrvlty varies widely although the substituents and the reaction center are connected by the same W-like partial structure. Furthermore, the solvolysls rate ratio of the parent compounds 2, 2 and 4 (R = H) 1s 2278 : 322 : 1. These striking differences were ascribed to differential 1,3-brldqmnq of the catlonic center by the carbon atcm to the rear of the C-0&
bond, 1-e. C6 m
the 2-norbomyl
ion pair 5. 1,3-Bridging 1s a qraded bonding interaction and therefore generates varylnq strain. Brldqinq strain is smallest in the cation -5 from 2, as evidenced by the large 9 I value of -2.00, and largest in the cation 5 from 4 (pI = -0.80).
bNs 5
6
-0Ns
s
1
Polar 1,2-, 1,3- and 1,4-interactionswere studied ln the ion pairs 8 derived from 6
2-, 3- and 4-monosubstltutedbxyclo[2.2.2]octyl p-nitrobanzenesulfonates1
. As expect-
ed, mnductlvlty was highest at C2 where the direct inductive effect coincides with the cf bonds. However, xnductlvlty was marginally hlqher at C4 than at C3 despite the sllqhtly larger Cl-C4 distance of 2.59 2 ccanparedto the Cl-C3 distance of 2.53 8 7. This result can be ratlonallzed by charge dispersal lnvolvinq a shift of electron density fran the back-lobe of C4 tmrds
the electrophlllc center Cl, as illustrated in 8. But, to Judge
by the size of QI at C4 of 8, the interaction 1s relatively weak.
6
Q
Br
%” 0 CH2
CH2
11
12 -
It was therefore of interest to study the lnductlvlty at C3 of 3-substituted bl8 derlvatlves, such as the brcmudes 2 . Not only 1s the Cl-C3 distance 10 shortened from ca. 2.5 to ca 1.9 8 '. Siqnlficantly, recent calculations suggest that
cyclo[l.l.llpentyl
the Cl-C3 Internuclear distance in the cation x
(R = H) 1s ca. 1.54 8. Also, calcula-
5947
tlons by Wlberg I1 have shown that bmdmg
between c1 and C3 causes only a modest in-
crease of angle stram, in lme with the reported high solvolytx reactlvlty of bicyclo[l.l.llpentyl halldes 12. In addition, NMR studies of bxyclo[l.l.llpsntyl systems pomt to strong spin-couplmg between brIdgehead subst=tuents('H, "F) When the brcmdes
9a-91 bable) were hydrolyzed m
13.
70% dloxane the correspondmg
3-methylenecyclobutanols11 were the sole products. Rate constants at 70°, measured m 80% ethanol are listed m
the Table. In the Figure lcq k values are plotted agamst
correspondmg mductlve substltuent constants @
2.
TABLE. Firstarder Rate Constants for the Brcmdes -9a-91 in 80% (V/V) Ethanol, Extrapolated to 70~ c.
R=
k [s-'1
R=
k [s-l]
a)
H
2.08 x 10-l
e)
cH2cl
2.37 x 1O-5
b)
1.08 x 1O-2 3.79 x 10-3
9)
cH2ms
C)
m3 CH20H
d)
C6H5
6.17 x 1O-4
e)
CH20Ac
6.55 x 1O-5
log
h)
cXm3
1.31 x 10-5 x -7 10 9.60
1)
OAc
5.49 x 10-7
k
-6-
FIGURE. Plot of log k for the Bromides 9a-91 against 6: --
the
5948
The correlation of log k with 9
1s faxly gccd (r = 0.96) conslderlng that extrapo-
lations fran 25O and 130° are involved. This result lndlcates that ior&zation rates are 1, 3-5 . previous studies
controlled by the lnductlve effect of the substltuents, as m
In the present case the parent and the substituted bxyclo[l.l.l]pentyl cations x
are
the first lntermedlates, rather than the methylenecyclobutonlumions l2, which are the precursors on the alcohols -11. This follows frcxnthe fact that the points for -9d and 9-s lie close to the correlation line, lndxatlng that C6H5 and OAc exert their inductive rather than their con]ugatlve effects. The latter should cause rate enhancements if ion12 atlon led directly to the coqugated cations 12, R = C6H5 of OAc.
It also follows that
ring-opening of the cations -10 to the cations 2 occurs in a subsequent fast step, in ac14 , since no alcohols derived from -10 were obtamned. cordance with ab initlo predictions The PI value of -2.63, calculated frcm the plot, 1s the largest value'observed in the solvolysis of 7-substituted halldes and sulfonates to date. It surpasses the 9I for the formation of the norbomyl-2-catlon 5. This high lnductlvlty reflects graded brldglng in the bxyclo[l.l.l]pentyl cation, symbolized by the dotted line in tO,Brldglng decreases as the electron-wlthdrawlngpower of the substltuent increases. REFERENCES
1.
Grob, C.A.; Act. Chem. Res. 1983, T_6,426.
2.
Grob, C.A.; Schaub, B.; Schlageter, M-G.; Helv.Chlm.Acta 1980, $i, 57.
3.
Bielmann, R.; Fuso, F.; Grob, C.A.; Helv.Chlm.Acta 1988, ?j, 312.
4.
Grob, C.A.; Sawlewicz, P.; Helv.Chlm.Acta 1984, 57, 1906.
5.
Grob, C.A.; Wlttwer, G.; RamaRao, K.; Helv.Chun.Acta 1985, @,
6.
Bielmann, R.; Grob, LA.;
7.
Yokoseki, A.; Kuchltsu, K.; Merino Y.; Bull.Ch~.Soc.Japan 1970, $3, 2017.
651.
Kury, D.; Yao, G.W.; Helv.Chim.Acta 1985, 58, 2158.
8.
Ph.D. Dissertation of Yang, Cheng Xi, Base1 1987.
9.
Chiang, J-F.; Bauer, S.H.; J.Am.Chem.soC. 1970, 9_2,1614. Padwa, A.; Shefter, E.; Alexander, E.; lbld. 1968, 92, 3717.
10.
Dslla, E.W.; Gill, P.M.W.; Schlesser, C.H.; J.Org.Chem. 1988, 53, 4384.
11.
Wlberg, K.B.;
12.
Wkerg,
1967, 2, K.B.; Van mndt wllllams; J.AIII.C~~ILSOC.
Taylor,
D.K.; Aust.J.Chem. 1990, 22, 945.
13.
Tetrahedron L.&t. 1985, 2_6,599. 3373.
Della,
BarfIeld, M.; Della, E-W.; Pigou, P.E.; J.Am.C!hein.So~. 1984, 106, 5051. &rfleld, M.; Della, E.W.; Pigou, P-E.; Walter, S.R.; ibid. 1982, I_@, 3549.
14.
E.W.;
Della, E.W.; Schlesser, C.H.; J.Chem.Res. 1989, 172.
(Received IIIGermany 19 July1991)
0040 4039191 $3 oo+ 00 Pcrgamon Press plc
No 42. pp 5945-5948.1991
hmted III Great Bntam
INDUCTIVE
CHARGE
3-SUBSTITUTED
DISPERSAL
IN THE
SOLVOLYSIS
BICYCLO[l.l.l]PENTYL
OF
BROMIDES
* Cyril
Institute
A. Grob
of Organic
St.Johanns-Ring
Ernest
School
of Physlcal Bedford
KEY WORDS:
ABSTRACT:
Chemistry,
X1 Yang
University
19, 4056 Basel,
W. Della
and
Sciences, Park,
Inductlvlty;
and Cheng
Dennis
K. Taylor
The Flanders
South
Australia,
Carbenlum
of Base1
Switzerland
Ions;
University
of S.A.
5042
Graded
Bridging.
Solvolysis rates and products of 3-substituted bxyclo[l.l.l1pentyl
branldes indicate extremely strong inductive charge dispersal in the ionization to bxyclo[l
.l.l
lpentyl-l-cations,the precursors of 3-methylenecyclobutls.
Recent studies have shown that the solvolys~s rates (log k) of several bl- and trlcyclic halides and sulfonates correlate linearly with the inductive constants d: of neighql . The reaction constantPI, boring substltuents according to the equation log k = pI d, the so-called lnductlvlty, gauges the sensitlvlty of the rate to the inductive effect of substltuents at various posltlons and 1s a measure of charge dispersal in the resulting carbenium ion. Since 6: values were derived from the pKa of 4-substituted qulnuclldlnium 2 , in which sterlc, cotqugatlve and hypercoqugatlve effects are negli-
perchlorates -1
gible or absent, these correlatrons indicate that relative lonlzation rates are controlled only by the inductive effect of substituents.
5945
5946
Inductlvlty depends not only on the distance between the substituent and the cationlc center. It also varies with the structure of the molecule. Thx
is illustrated by the
pI values for the solvolysls in 80% ethanol of the 7%xbstituted 2-norbomyl, 2-bicyclo[2.2.2]octyl and 2-adamantyl p-toluenesulfonates2, 3 and 4, namely -2.00 3, -1.50 4 and -0.80 5, respectively. Thus, lnductrvlty varies widely although the substituents and the reaction center are connected by the same W-like partial structure. Furthermore, the solvolysls rate ratio of the parent compounds 2, 2 and 4 (R = H) 1s 2278 : 322 : 1. These striking differences were ascribed to differential 1,3-brldqmnq of the catlonic center by the carbon atcm to the rear of the C-0&
bond, 1-e. C6 m
the 2-norbomyl
ion pair 5. 1,3-Bridging 1s a qraded bonding interaction and therefore generates varylnq strain. Brldqinq strain is smallest in the cation -5 from 2, as evidenced by the large 9 I value of -2.00, and largest in the cation 5 from 4 (pI = -0.80).
bNs 5
6
-0Ns
s
1
Polar 1,2-, 1,3- and 1,4-interactionswere studied ln the ion pairs 8 derived from 6
2-, 3- and 4-monosubstltutedbxyclo[2.2.2]octyl p-nitrobanzenesulfonates1
. As expect-
ed, mnductlvlty was highest at C2 where the direct inductive effect coincides with the cf bonds. However, xnductlvlty was marginally hlqher at C4 than at C3 despite the sllqhtly larger Cl-C4 distance of 2.59 2 ccanparedto the Cl-C3 distance of 2.53 8 7. This result can be ratlonallzed by charge dispersal lnvolvinq a shift of electron density fran the back-lobe of C4 tmrds
the electrophlllc center Cl, as illustrated in 8. But, to Judge
by the size of QI at C4 of 8, the interaction 1s relatively weak.
6
Q
Br
%” 0 CH2
CH2
11
12 -
It was therefore of interest to study the lnductlvlty at C3 of 3-substituted bl8 derlvatlves, such as the brcmudes 2 . Not only 1s the Cl-C3 distance 10 shortened from ca. 2.5 to ca 1.9 8 '. Siqnlficantly, recent calculations suggest that
cyclo[l.l.llpentyl
the Cl-C3 Internuclear distance in the cation x
(R = H) 1s ca. 1.54 8. Also, calcula-
5947
tlons by Wlberg I1 have shown that bmdmg
between c1 and C3 causes only a modest in-
crease of angle stram, in lme with the reported high solvolytx reactlvlty of bicyclo[l.l.llpentyl halldes 12. In addition, NMR studies of bxyclo[l.l.llpsntyl systems pomt to strong spin-couplmg between brIdgehead subst=tuents('H, "F) When the brcmdes
9a-91 bable) were hydrolyzed m
13.
70% dloxane the correspondmg
3-methylenecyclobutanols11 were the sole products. Rate constants at 70°, measured m 80% ethanol are listed m
the Table. In the Figure lcq k values are plotted agamst
correspondmg mductlve substltuent constants @
2.
TABLE. Firstarder Rate Constants for the Brcmdes -9a-91 in 80% (V/V) Ethanol, Extrapolated to 70~ c.
R=
k [s-'1
R=
k [s-l]
a)
H
2.08 x 10-l
e)
cH2cl
2.37 x 1O-5
b)
1.08 x 1O-2 3.79 x 10-3
9)
cH2ms
C)
m3 CH20H
d)
C6H5
6.17 x 1O-4
e)
CH20Ac
6.55 x 1O-5
log
h)
cXm3
1.31 x 10-5 x -7 10 9.60
1)
OAc
5.49 x 10-7
k
-6-
FIGURE. Plot of log k for the Bromides 9a-91 against 6: --
the
5948
The correlation of log k with 9
1s faxly gccd (r = 0.96) conslderlng that extrapo-
lations fran 25O and 130° are involved. This result lndlcates that ior&zation rates are 1, 3-5 . previous studies
controlled by the lnductlve effect of the substltuents, as m
In the present case the parent and the substituted bxyclo[l.l.l]pentyl cations x
are
the first lntermedlates, rather than the methylenecyclobutonlumions l2, which are the precursors on the alcohols -11. This follows frcxnthe fact that the points for -9d and 9-s lie close to the correlation line, lndxatlng that C6H5 and OAc exert their inductive rather than their con]ugatlve effects. The latter should cause rate enhancements if ion12 atlon led directly to the coqugated cations 12, R = C6H5 of OAc.
It also follows that
ring-opening of the cations -10 to the cations 2 occurs in a subsequent fast step, in ac14 , since no alcohols derived from -10 were obtamned. cordance with ab initlo predictions The PI value of -2.63, calculated frcm the plot, 1s the largest value'observed in the solvolysis of 7-substituted halldes and sulfonates to date. It surpasses the 9I for the formation of the norbomyl-2-catlon 5. This high lnductlvlty reflects graded brldglng in the bxyclo[l.l.l]pentyl cation, symbolized by the dotted line in tO,Brldglng decreases as the electron-wlthdrawlngpower of the substltuent increases. REFERENCES
1.
Grob, C.A.; Act. Chem. Res. 1983, T_6,426.
2.
Grob, C.A.; Schaub, B.; Schlageter, M-G.; Helv.Chlm.Acta 1980, $i, 57.
3.
Bielmann, R.; Fuso, F.; Grob, C.A.; Helv.Chlm.Acta 1988, ?j, 312.
4.
Grob, C.A.; Sawlewicz, P.; Helv.Chlm.Acta 1984, 57, 1906.
5.
Grob, C.A.; Wlttwer, G.; RamaRao, K.; Helv.Chun.Acta 1985, @,
6.
Bielmann, R.; Grob, LA.;
7.
Yokoseki, A.; Kuchltsu, K.; Merino Y.; Bull.Ch~.Soc.Japan 1970, $3, 2017.
651.
Kury, D.; Yao, G.W.; Helv.Chim.Acta 1985, 58, 2158.
8.
Ph.D. Dissertation of Yang, Cheng Xi, Base1 1987.
9.
Chiang, J-F.; Bauer, S.H.; J.Am.Chem.soC. 1970, 9_2,1614. Padwa, A.; Shefter, E.; Alexander, E.; lbld. 1968, 92, 3717.
10.
Dslla, E.W.; Gill, P.M.W.; Schlesser, C.H.; J.Org.Chem. 1988, 53, 4384.
11.
Wlberg, K.B.;
12.
Wkerg,
1967, 2, K.B.; Van mndt wllllams; J.AIII.C~~ILSOC.
Taylor,
D.K.; Aust.J.Chem. 1990, 22, 945.
13.
Tetrahedron L.&t. 1985, 2_6,599. 3373.
Della,
BarfIeld, M.; Della, E-W.; Pigou, P.E.; J.Am.C!hein.So~. 1984, 106, 5051. &rfleld, M.; Della, E.W.; Pigou, P-E.; Walter, S.R.; ibid. 1982, I_@, 3549.
14.
E.W.;
Della, E.W.; Schlesser, C.H.; J.Chem.Res. 1989, 172.
(Received IIIGermany 19 July1991)