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On the conformation of β-bromo radicals
T~tr&&~n
LettorrNo. 34, pp
2953
-
2954,1975. Porgona Prom.
Printed in tieat
Britain.
ONlIIECONFcRMATICNOR fH!R~RADICALS DeannaNelsonand M.C.R.Syrmns Department of Chemistry, The University,Leicester,LEl 7F3I (Receivedin UK 16 June 1975; acceptedfor publication10 July 1975) During the past few yearswe have assignede.s.r.spectra to a- and B-brano-radicals(I
3S4 In both our assignments have been dismissed in favourof others. and II),lS2but recently the previousletter5a casehas beenpresentedin favourof our a-bromoand B-bromoassigrrments: herewe wish to defendour &bromo assignment and to proposean alternative structure for the speciesdetectedby Lloyd-et al. and assignedstructureIII.4
t 0 )C-Br 0 I
II
III
The speciesof interest was preparedfrom isobutylbromideby X-irradiation of an adamn4 tane "solution" . The resulting narrow-line isotropic spectrumcomprised a set of ten quartets, the quartetstructure beingassignedto hyperfine couplingto bromine(Aiso=6.7G)because featuresfor "Br and "Br were detected.The set of ten lines (Aiso= 21.4G)was analysedin termsof structure III,withAWe) = 21.4G,A(H1) = 21.4G+ A@$) = 42.8G. This structure is improbable on chemicalgrounds:bromineand iodineare expectedto resemblechlorine, whichcertainly adoptsconformation II, and mechanistic6 and N.M.R.'Istudies stronglysupporta synnretric or asynnnetric 'bridged" structure.Also, B-bromoradicalshave neverbeen detectedin fluidsolution, so why shouldthey give a well-defined e.s.r.spectrum in adamantane at -7l'C? Rapidelectron-spin relaxation cannotbe the cause,so it is necessary tQ postulate a high reactivity to explainthe negativeliquid-phase results. However,it is
generally accqted that f3-bromo radicalsare uninzolecularly very unstable:89g R2&CR2(Br) + and thisprocesswouldnot be preventedin amane.
Br + R2C = CR2
(1)
Ne. 34
w54
Finally, accurately
it is a curious coincidence results
Me2CH-&12radicals
isaaerise
electron
to the more stable Me3C* species chloride,
famed fraa alkyl branides and iodides
capture often exhibit
of which is a fuuction
a small residual
of the alkyl radical
their data, with A’H = 21.4G and A(“Br)
even at 77 K,l” and our own bmnide and iodide on irradiaAlso we and others have shown11’12
in the solid
state by dissociative
hyperfine coupling to halogen,
and the solvent.
= 6.7G.
We therefore
suggest that the
The sma.llreductian
inthe
from that for normal Me3C* (- 22.4G) is normal for these anim complexes. are in fair
agreement with expectation
bmnide in adamantane should give the same lO-coapcmentspecies,
prising
four lines separated by -ca. 6G.
al -*
This was indeed detected,
Ch aunealing, the brcmine splitting
radicals.
was lost,
proton coupling
The relative
line
If this is correct,
for Me3C. radicals.
t-butyl
(CH3)$.
the magnitude
This hypothesis accords with all
studied by Lloyd -et a1.4 is the radicalMe&...Br-.
intensities
is me that
It is knmn that
contains nine equivalent protms.
This would explain the presence of ten lines.
that alkyl radicals
species
that this species
cmfirm the formation of Me3C* fran isobutyl
tion at 77 K.
staggered cmformation
in hyperfine coupling to H(1) + H(2) that produces just ten narrow lines:
It seems far mre likely
studies
that the preferred
each component com-
in addition
to scnne “free”
as was observed by Lloyd -et
4
1.
S.P. Mishra, G.W. Neilson and M.C.R. Synvns, J.C.S.
2.
A.R. Lyme and M.C.R. .Symns, J. Amer. Chem. Sot., G.W. Neilson,
Faradav II,
1974, 70, 1165.
1971, 93, 7330;
S.P. Mishra andM.C.R. Synums, J.C.S.
Faradav II,
1975, a,
3.
R.F. Picme and M.T. Rogers, J. Chem. Phys., 1974, &
4.
R.V. Lloyd, D.E. Wood and M.T. Rogers, J. Amer. Chem. Sot.,
5.
R.J. Booth, S.P. Mishra, G.W. Neilscm andM.C.R. Symns, Tet. Letts.,
6.
P.S. Skell,
7.
J.H. Harris and P.B. Shevlin, J.C.S.
8.
R.K.
Friedlina,
9.
W.O.
Haag and E.I.
R.R. Pavlis,
1974, 96, 7130.
D.C. Lewis and K.J. Shea, J. Amer. Chem. Sot.,
1973, s,
6735.
1962, 31, 1.
1965, 3683.
P.B. Ayscough and H.E. Evans, J. Phys. Chem., 1964, 68, 3066.
11.
S.P. Mishra and M.C.R. Syrmms, J.C.S.
Perkin II,
1973, 391;
and M.C.R. Symms, Nature, 1974, 249, 341. E.P. Sprague and F. Williams,
1975......
Chem. Corma., 1973, 179.
H.N. Kosl and M.Y. Khelina, Russ. Chem. Rev., Heiba, Tet. Letts.,
363.
4814.
10.
12.
A.R. Lyons,
J. Chem. Phvs.,
1971, 54, 5425.
A.R. Lyons, S.P. Mishra
LettorrNo. 34, pp
2953
-
2954,1975. Porgona Prom.
Printed in tieat
Britain.
ONlIIECONFcRMATICNOR fH!R~RADICALS DeannaNelsonand M.C.R.Syrmns Department of Chemistry, The University,Leicester,LEl 7F3I (Receivedin UK 16 June 1975; acceptedfor publication10 July 1975) During the past few yearswe have assignede.s.r.spectra to a- and B-brano-radicals(I
3S4 In both our assignments have been dismissed in favourof others. and II),lS2but recently the previousletter5a casehas beenpresentedin favourof our a-bromoand B-bromoassigrrments: herewe wish to defendour &bromo assignment and to proposean alternative structure for the speciesdetectedby Lloyd-et al. and assignedstructureIII.4
t 0 )C-Br 0 I
II
III
The speciesof interest was preparedfrom isobutylbromideby X-irradiation of an adamn4 tane "solution" . The resulting narrow-line isotropic spectrumcomprised a set of ten quartets, the quartetstructure beingassignedto hyperfine couplingto bromine(Aiso=6.7G)because featuresfor "Br and "Br were detected.The set of ten lines (Aiso= 21.4G)was analysedin termsof structure III,withAWe) = 21.4G,A(H1) = 21.4G+ A@$) = 42.8G. This structure is improbable on chemicalgrounds:bromineand iodineare expectedto resemblechlorine, whichcertainly adoptsconformation II, and mechanistic6 and N.M.R.'Istudies stronglysupporta synnretric or asynnnetric 'bridged" structure.Also, B-bromoradicalshave neverbeen detectedin fluidsolution, so why shouldthey give a well-defined e.s.r.spectrum in adamantane at -7l'C? Rapidelectron-spin relaxation cannotbe the cause,so it is necessary tQ postulate a high reactivity to explainthe negativeliquid-phase results. However,it is
generally accqted that f3-bromo radicalsare uninzolecularly very unstable:89g R2&CR2(Br) + and thisprocesswouldnot be preventedin amane.
Br + R2C = CR2
(1)
Ne. 34
w54
Finally, accurately
it is a curious coincidence results
Me2CH-&12radicals
isaaerise
electron
to the more stable Me3C* species chloride,
famed fraa alkyl branides and iodides
capture often exhibit
of which is a fuuction
a small residual
of the alkyl radical
their data, with A’H = 21.4G and A(“Br)
even at 77 K,l” and our own bmnide and iodide on irradiaAlso we and others have shown11’12
in the solid
state by dissociative
hyperfine coupling to halogen,
and the solvent.
= 6.7G.
We therefore
suggest that the
The sma.llreductian
inthe
from that for normal Me3C* (- 22.4G) is normal for these anim complexes. are in fair
agreement with expectation
bmnide in adamantane should give the same lO-coapcmentspecies,
prising
four lines separated by -ca. 6G.
al -*
This was indeed detected,
Ch aunealing, the brcmine splitting
radicals.
was lost,
proton coupling
The relative
line
If this is correct,
for Me3C. radicals.
t-butyl
(CH3)$.
the magnitude
This hypothesis accords with all
studied by Lloyd -et a1.4 is the radicalMe&...Br-.
intensities
is me that
It is knmn that
contains nine equivalent protms.
This would explain the presence of ten lines.
that alkyl radicals
species
that this species
cmfirm the formation of Me3C* fran isobutyl
tion at 77 K.
staggered cmformation
in hyperfine coupling to H(1) + H(2) that produces just ten narrow lines:
It seems far mre likely
studies
that the preferred
each component com-
in addition
to scnne “free”
as was observed by Lloyd -et
4
1.
S.P. Mishra, G.W. Neilson and M.C.R. Synvns, J.C.S.
2.
A.R. Lyme and M.C.R. .Symns, J. Amer. Chem. Sot., G.W. Neilson,
Faradav II,
1974, 70, 1165.
1971, 93, 7330;
S.P. Mishra andM.C.R. Synums, J.C.S.
Faradav II,
1975, a,
3.
R.F. Picme and M.T. Rogers, J. Chem. Phys., 1974, &
4.
R.V. Lloyd, D.E. Wood and M.T. Rogers, J. Amer. Chem. Sot.,
5.
R.J. Booth, S.P. Mishra, G.W. Neilscm andM.C.R. Symns, Tet. Letts.,
6.
P.S. Skell,
7.
J.H. Harris and P.B. Shevlin, J.C.S.
8.
R.K.
Friedlina,
9.
W.O.
Haag and E.I.
R.R. Pavlis,
1974, 96, 7130.
D.C. Lewis and K.J. Shea, J. Amer. Chem. Sot.,
1973, s,
6735.
1962, 31, 1.
1965, 3683.
P.B. Ayscough and H.E. Evans, J. Phys. Chem., 1964, 68, 3066.
11.
S.P. Mishra and M.C.R. Syrmms, J.C.S.
Perkin II,
1973, 391;
and M.C.R. Symms, Nature, 1974, 249, 341. E.P. Sprague and F. Williams,
1975......
Chem. Corma., 1973, 179.
H.N. Kosl and M.Y. Khelina, Russ. Chem. Rev., Heiba, Tet. Letts.,
363.
4814.
10.
12.
A.R. Lyons,
J. Chem. Phvs.,
1971, 54, 5425.
A.R. Lyons, S.P. Mishra