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The lowest excited singlet states of 1-arylbutadienes: photochemical implications
Volume 46, number 3
CIKMICAL
15 March 1977
PHYSICS LETTERS
THE LOWEST EXCITED SINGLET STATES OF I-ARYLBUTADIENES: PHOTOCHEMICAL IMPLICATIONS Peter J. BALDRY Chemistry
Department.
Freetow.
Sierra Leone
Fourah Ray ColIege, University
of Sierra Leone,
and John A. BARLTROP Damson Perrins Laboratory. Oxford, UK Received 6 September 1976 Revised mdnuxrrpt received 22 November
1976
PPP cnIcul&ion$ on I-arylbutadlcnes prcdlct il forblddcn transition lowest cxltcd Gngiet states have different cAxMed charge dcnsttics,
slightly below the first allowed transition. implying different types of photochemical
1. Introduction There has recently been considerable intcrcst in the nature of the lowest cxcitcd states of polyenes and sw-dlphenylpolyencs. The lowest excited smglet state had previously been assumed zo have B, svmmetry, corrcspondmg to the allowed UV transition, but several MO calculations predict the lowest excited ‘A, state to lit below the ‘B, [I-S]. There is some experimental evidence for the IA, strlte being lower [6-IO], though other evidence indicates the t B, state to be lower [3.4,1 I]. WChave carned out calculations on I-arylbutadiencs in connection with our studies of their photochemistry [ 12,I 31, which show that they might bc particularly favourablc compounds for the observation of a lowest IA,-like state.
2. Method of calculation Calculations wcrc carried out on I-arylbutadiemzs (fig. 1) where the aryl groups were (1) phenyl, (2) 3m:thylphenyl, (3) 4-methyl-phenyl, (4), 2,4,6-t+ methylphenyl, (5) 3-rnethoxyphenyl, (6) 4-methoxyphenyl, and (7) 4-dimethylaminophenyl. A standard 430
The two reaction.
“k&&t 1
3
Fig. 1.
n-electron PPP program was used [ 141, with contiguration interaction (CI) involving all singly-excited configurations. Parameters for carbon and nitrogen were taken from Beveridge and Hinze [IS] and for oxygen from Hinzc [16]. An inductive model was used for the rncthyl group: the valence-state ionisation potcntial, Q, was reduced from 11 .I6 cV to 10.16 eV for the carbon atom bearing the methyl group. For the hmdered molecu!es, cis-( 1) and (4), the benzene rmg was assumed twisted out of the plane of the diene unit by 30”, and the value of fl for the Ar-C-I bond was reduced from -2.35 to -2.04 eV. Bond lengths and angles were taken from the tables published by the Chemical Society [ 17 1.
3. Results In table 1, the calculated energies and oscillator
Volume 46. number 3
I’HYSICSLETTERS
CHEMICAL
Table 1 Calculated and observed singlet energies (in eV) Compound
Calculated energies (f)
Obsewud energy (cl
(1) cis-(l) a) (21
4.56 (0) 4.63 (0)
4.64 (1 .L66) 4.77(1.052)
4.52(0.300) 4.56(O.OI2) 4.66 (0.002) 4 SO(O.124) 4.53 (0.005) 4 43 (0.074)
4.67 (0.974) 4x53(1.258) 4.77(1.175) 4.65U.135) 4.60(1.279) 4.46(1.239)
4.44 (23 300) 4.68(12 8OWj 4.38(25 200) 4.38(27600) 4.59(16 300) 4.44 (24 400)
(3) (4) a) (5) (6) (7)
?) Twisted about &-C-I
_--bond by 30”.
4.30(26 800) 3.86 (30 000) .----
sirengths arc compared with the lowest energy absorp tion bads in the UV spectra (from ref. [ 133). Electron densities at the carbon atoms in the dicnc part of the molecule are given in table 2 for the two lowest singlet excited states (atoms numbered as in fig. 1).
4. Discussion Our calculations on I-arylbutadienes show two low-lying singlet excited states of similar energy. ‘l-he upper state in each compound corresponds to the ‘E%,,state in symmetrical potyenes; the transition is strongly allowed, and arises essentially from $, where $, is the highest filled orbital, and + &*I, rLnl + 1 is the lowest unfilled orbital. There is reasonable agreement between the energy calculated for the transition to this state and the longest wavelength
I.5 Lfarch 1977
UV absorption energy. Slighily lower in energy, thcrc is calculated to be a weakly-allowed transition, essen+ I$,, +z), and this tiaI[y (9, _ 1 -+@,+,)-(&~ lower state corresponds to the excited *A, state in symmetrical polyenes. Most recent calculations on unsubstitutcd poiyenes have predicted the 1 IF& state to be below the 2 I$ state when using singlc-excitation C!, but the 2 I$ state to bc lower than I *ES, after extended CI [t-4]. However, Koutccki [ 181 has shown that, in scmiempirical calculations, &herelative energies of A and B states after double-excitation CI arc very sensitive to the approximation used for Yij, the repukion integrals *. This makes the vahdity of results based on cxtended CI uncertain; for example, KouteckJj found for butadiene and hexatriene that extended CI could predict either 1 IB, or 2 *A as the lowest excited sin&% depending on the formu fa for calculating yii [Is!, while Gavin and Rice found that single-excitation CI gave closer agreement with the observed UV than extended CI [3 1, and Shinoda ct al. predict that the 1 I B, state lies below the 2 I Ap even after extended CI, using a different formula for -yjj from other workers [IY I. in view of this uncertainty, it issignificant that our caIculations on i -arylbutadienes show the 2 I$-like stztc to lie below the 1 * B,-like state after only sit-@-excitation CL We were unable to Terry out a complctz CI calculation on the symmetrically-substituted 1,4-diphenylbuta* Some recent calculations are ab initio [i 1, for which no comparable analysis has been given. NevertiteIess,different approximations in evaluating rep&ion integrals for these calculations could lead to simdx changes in A and B state energies.
Table 2 Electron denqxticsin St (‘Aglike.) and Sz (‘Bu-like) --
Compound
c-3
c-2
C-l
C4
s2
SI
s2
Sl
s2 -
Sl
s2
0.783
0.953
0.922
1.015
(2) (3) (4) a)
0.762 0.821 0.771 0.796
0.940 0.947 0.984
0.907 0.950 0.879
1.OOP 1.024 1.028
0.858 0.850 0.879 0.859
0.95 1 0.942 0.939 0963
0.743 0.732 0.783 0.732
0.916 a.902 0.906 0.939
0.939 0.942
1.039 1.023
0.963
0.763
0.940
0.806
1.020 0.957
0.864
(5)
(6)
0.777
0.979
0.902
1.Q25
0.87 1 0.865
0.948 0.961
0.769 0.741
0.9 16 0.936
(7)
0.810
1.006
0.886
1.035
0.898
0.973
0.783
0.962
Sl (1) c~s-(I)
a)
a) Twisted about Ar-C-l bond by 30”. 431
CHEMICALPHYSICSLJXTERS ause of limits in computer storage available, the 30 towcst-energy singly-excited configurns we found the I ‘8, state to lie below 2 IA by V. Our rest&s indicate that the energy oft a e like state relative to the 1 lB,-like state is much irt I -ary~butadienes than in unsubstituted or tricat diphenyl-substituted polyenes, and it th&cfore; be possible to observe the 2 lAg I&rm,especially since their low symmetry the 1 f Ag (ground state) + 2 IA, transition less JZ experimental evidence for a lowest IA, excited in po)yerres or o,tidiphenylpolyenes is as contraictory as the theoretical results. The principal evie comes from two-photon absorption spectra [a], -resn!ution spectroscopy [ 7, I 01, and from O-O band positions in UV and fluorescence, and ffuorescence lifetimes [6,7]. Unfortunately the fluorescence of 0.arylbutadienes shows little or no vibrational structure and, except for (5) and (7). is very weak, and we were unabr’e to obtain spectroscopic evidence for the 2 th, state lying below the 1 lH,. Hudson and Kohler hay: drawn attention to differences in bond order between 2 IA, and 1 I%, slates, and the possible photochemical consequences f ?,I0J, pr!ncipally differences in relative proportions of C&Gtrims isomerisation. However, conformational effect: in the ground state, and the existence of other photo. chemical reactions, could make such differences dir% cult to identify unambiguously +. Our calculations also show differences in bond orders between I$-like and ‘B,-like states, but of more rignificance are the differences in electron density shown in table 2. ‘This shuws substantial positive charges on the diene carbon atoms. especially C-l and C-4, in the ‘$-like state S,, and much smaller charges in S, (l%,-like): generally positive at C-l and C-3, more positive at C-4, and negative at C-2. The implication is that St should react preferentialfy with nucleophiles at C-l and C-4, while S, should be less reactive towards nucleophiles, and react with Rlectrophiies at C-2. There is a further difference in substituent effects: the effect of increasing the eiectrondonating character of the substituent is to incrcose the electron density on C-l to Cd, so that St will become less reactive and S, more reactive to” For eumptrs of polycne photochemistry
where groundstate conformations play a major role, see ref. f2Oj.
!5 March 1977
:;iards elcctrophiles as the substituent becomes more electron-donating. Furthermore, substituents in the 3-position of the ring lead to anomalously high electron densities in S). I-phenylbutadiene has been shown to undergo photoaddition to hydroxylic solvcnts 1I 2 ] ; a future pubhcation [ 133 will discuss the mechantnism of this reaction in the‘light of resutis reported here.
111R.J. Buenkcr and J.L. Whitten, J. Chem. Fhys. 49 (1968) 5381; P.A. Clark and LG. Csiemadia, J. Chem. Phys. 56 (1972) 275s; T.H. Dunning Jr., R.P. Hosteny and I. Shavitt, J. Am.
Chem. Sot. 95 (1973) 5087.
r21P.A. Clark, J. Chem. Fhys. 54 (1971) 45;
K. Schulten and M. Karplus, Chem. Phys. Letters 14 (1972) 30s. 131 R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 60 (1974) 3231.
141 F.W.E. Knoop and L.J. Oosterhoff, Chem. Phys. Letters 22 (1973) 247.
ISI A.M. Schaffer, W.H. Waddeli and R.S. Becker, J. Am. Chem. Sot. 96 (1974) 2063. 161 BS. Hudson and B-E. Kohier. Chem. Phvs. Letters 14 (1972) 299.
B.S. Hudson and B.E. Kohler, J. Chem. flays. 59 (1973) 4984. R.L. Swofford and W.M. McClain, J. Chem. Phys. 59 (1973) 5740. K. Mandal and T.N. Misra, Chem. phys. Letters 27 (1974) 57; R. McDiarmid, Chem. Phys. Letters 34 (1975) 130. R.L. Christensen and B.E. Kohler, J. Chem. Phys. 63 (1975) 1837. T.A. Moore and P.-S. Song, Chem. Phys. Letters 19 (1973) 128; B.S. Hudson and B.E. Kohler, Chem. Phys. Letters 23 (1973) 139; R.M. GavavinJr., S. Risemberg and S.A. Rice, J. Chem. PhYS. 18 (1973) 3160; R. McDiarmid, 3. C&m. Ph% 64 (1976) S 14. P.J. BaYdry, J. Chem. Sot. Perkin I (197.5) 1913. P.‘J. Baldry, to be published. LE. Bloor and B.R. Gilson, QCPE No. 71, Quantum Chemistry Program Exchange, Indiana University, Bloomington, Indiana. [ 1S j D.L. Beverilrgeand J. Hinze, J. Am. Chem. Sot. 93 (1971) 3107. [ 16f J, Hinze and H.H. Jaffh, 1. Am, Chem. Sot. 84 (1962) 540;
Volume 46, number 3
CHUMCAL PHYSICS LlXTERS
J. Hinzc, MA. Whitchcad and H.H. Mf& 3. Am. Chem. Sot. 85 (1963) 148. [ 171 L.E. Sutton, D.G. Jenkin, A.D. hfitchell and L.C. Cross, eds., Tables of Interatomic Distances and Configurations in RfolecuIes and Ions, Special Publication No. 11 (The Chemical Society, London, 1958). [IS] J. Koutcck9, J. Chem. Phys. 47 (1967) 1501.
15 March 1977
[ 191 H. Shinoda, H. Tatematsu and T. h¶iyazaki, Buif. Chem. Sot. Japan 46 (1973) 2950. 1201 A. Fadwa, L. Brodsty and S. CIough, 5. Am. Chem. Sot. 94 (I 972) 6767; P..f. Vroegop, J. Lugtenburg and E. Havinga, Tetrahedron 29 (1973) 1393; P. Courtot and R. Rumin, I. Chem. Sot. Chem. Commua. (1974) 168.
433
CIKMICAL
15 March 1977
PHYSICS LETTERS
THE LOWEST EXCITED SINGLET STATES OF I-ARYLBUTADIENES: PHOTOCHEMICAL IMPLICATIONS Peter J. BALDRY Chemistry
Department.
Freetow.
Sierra Leone
Fourah Ray ColIege, University
of Sierra Leone,
and John A. BARLTROP Damson Perrins Laboratory. Oxford, UK Received 6 September 1976 Revised mdnuxrrpt received 22 November
1976
PPP cnIcul&ion$ on I-arylbutadlcnes prcdlct il forblddcn transition lowest cxltcd Gngiet states have different cAxMed charge dcnsttics,
slightly below the first allowed transition. implying different types of photochemical
1. Introduction There has recently been considerable intcrcst in the nature of the lowest cxcitcd states of polyenes and sw-dlphenylpolyencs. The lowest excited smglet state had previously been assumed zo have B, svmmetry, corrcspondmg to the allowed UV transition, but several MO calculations predict the lowest excited ‘A, state to lit below the ‘B, [I-S]. There is some experimental evidence for the IA, strlte being lower [6-IO], though other evidence indicates the t B, state to be lower [3.4,1 I]. WChave carned out calculations on I-arylbutadiencs in connection with our studies of their photochemistry [ 12,I 31, which show that they might bc particularly favourablc compounds for the observation of a lowest IA,-like state.
2. Method of calculation Calculations wcrc carried out on I-arylbutadiemzs (fig. 1) where the aryl groups were (1) phenyl, (2) 3m:thylphenyl, (3) 4-methyl-phenyl, (4), 2,4,6-t+ methylphenyl, (5) 3-rnethoxyphenyl, (6) 4-methoxyphenyl, and (7) 4-dimethylaminophenyl. A standard 430
The two reaction.
“k&&t 1
3
Fig. 1.
n-electron PPP program was used [ 141, with contiguration interaction (CI) involving all singly-excited configurations. Parameters for carbon and nitrogen were taken from Beveridge and Hinze [IS] and for oxygen from Hinzc [16]. An inductive model was used for the rncthyl group: the valence-state ionisation potcntial, Q, was reduced from 11 .I6 cV to 10.16 eV for the carbon atom bearing the methyl group. For the hmdered molecu!es, cis-( 1) and (4), the benzene rmg was assumed twisted out of the plane of the diene unit by 30”, and the value of fl for the Ar-C-I bond was reduced from -2.35 to -2.04 eV. Bond lengths and angles were taken from the tables published by the Chemical Society [ 17 1.
3. Results In table 1, the calculated energies and oscillator
Volume 46. number 3
I’HYSICSLETTERS
CHEMICAL
Table 1 Calculated and observed singlet energies (in eV) Compound
Calculated energies (f)
Obsewud energy (cl
(1) cis-(l) a) (21
4.56 (0) 4.63 (0)
4.64 (1 .L66) 4.77(1.052)
4.52(0.300) 4.56(O.OI2) 4.66 (0.002) 4 SO(O.124) 4.53 (0.005) 4 43 (0.074)
4.67 (0.974) 4x53(1.258) 4.77(1.175) 4.65U.135) 4.60(1.279) 4.46(1.239)
4.44 (23 300) 4.68(12 8OWj 4.38(25 200) 4.38(27600) 4.59(16 300) 4.44 (24 400)
(3) (4) a) (5) (6) (7)
?) Twisted about &-C-I
_--bond by 30”.
4.30(26 800) 3.86 (30 000) .----
sirengths arc compared with the lowest energy absorp tion bads in the UV spectra (from ref. [ 133). Electron densities at the carbon atoms in the dicnc part of the molecule are given in table 2 for the two lowest singlet excited states (atoms numbered as in fig. 1).
4. Discussion Our calculations on I-arylbutadienes show two low-lying singlet excited states of similar energy. ‘l-he upper state in each compound corresponds to the ‘E%,,state in symmetrical potyenes; the transition is strongly allowed, and arises essentially from $, where $, is the highest filled orbital, and + &*I, rLnl + 1 is the lowest unfilled orbital. There is reasonable agreement between the energy calculated for the transition to this state and the longest wavelength
I.5 Lfarch 1977
UV absorption energy. Slighily lower in energy, thcrc is calculated to be a weakly-allowed transition, essen+ I$,, +z), and this tiaI[y (9, _ 1 -+@,+,)-(&~ lower state corresponds to the excited *A, state in symmetrical polyenes. Most recent calculations on unsubstitutcd poiyenes have predicted the 1 IF& state to be below the 2 I$ state when using singlc-excitation C!, but the 2 I$ state to bc lower than I *ES, after extended CI [t-4]. However, Koutccki [ 181 has shown that, in scmiempirical calculations, &herelative energies of A and B states after double-excitation CI arc very sensitive to the approximation used for Yij, the repukion integrals *. This makes the vahdity of results based on cxtended CI uncertain; for example, KouteckJj found for butadiene and hexatriene that extended CI could predict either 1 IB, or 2 *A as the lowest excited sin&% depending on the formu fa for calculating yii [Is!, while Gavin and Rice found that single-excitation CI gave closer agreement with the observed UV than extended CI [3 1, and Shinoda ct al. predict that the 1 I B, state lies below the 2 I Ap even after extended CI, using a different formula for -yjj from other workers [IY I. in view of this uncertainty, it issignificant that our caIculations on i -arylbutadienes show the 2 I$-like stztc to lie below the 1 * B,-like state after only sit-@-excitation CL We were unable to Terry out a complctz CI calculation on the symmetrically-substituted 1,4-diphenylbuta* Some recent calculations are ab initio [i 1, for which no comparable analysis has been given. NevertiteIess,different approximations in evaluating rep&ion integrals for these calculations could lead to simdx changes in A and B state energies.
Table 2 Electron denqxticsin St (‘Aglike.) and Sz (‘Bu-like) --
Compound
c-3
c-2
C-l
C4
s2
SI
s2
Sl
s2 -
Sl
s2
0.783
0.953
0.922
1.015
(2) (3) (4) a)
0.762 0.821 0.771 0.796
0.940 0.947 0.984
0.907 0.950 0.879
1.OOP 1.024 1.028
0.858 0.850 0.879 0.859
0.95 1 0.942 0.939 0963
0.743 0.732 0.783 0.732
0.916 a.902 0.906 0.939
0.939 0.942
1.039 1.023
0.963
0.763
0.940
0.806
1.020 0.957
0.864
(5)
(6)
0.777
0.979
0.902
1.Q25
0.87 1 0.865
0.948 0.961
0.769 0.741
0.9 16 0.936
(7)
0.810
1.006
0.886
1.035
0.898
0.973
0.783
0.962
Sl (1) c~s-(I)
a)
a) Twisted about Ar-C-l bond by 30”. 431
CHEMICALPHYSICSLJXTERS ause of limits in computer storage available, the 30 towcst-energy singly-excited configurns we found the I ‘8, state to lie below 2 IA by V. Our rest&s indicate that the energy oft a e like state relative to the 1 lB,-like state is much irt I -ary~butadienes than in unsubstituted or tricat diphenyl-substituted polyenes, and it th&cfore; be possible to observe the 2 lAg I&rm,especially since their low symmetry the 1 f Ag (ground state) + 2 IA, transition less JZ experimental evidence for a lowest IA, excited in po)yerres or o,tidiphenylpolyenes is as contraictory as the theoretical results. The principal evie comes from two-photon absorption spectra [a], -resn!ution spectroscopy [ 7, I 01, and from O-O band positions in UV and fluorescence, and ffuorescence lifetimes [6,7]. Unfortunately the fluorescence of 0.arylbutadienes shows little or no vibrational structure and, except for (5) and (7). is very weak, and we were unabr’e to obtain spectroscopic evidence for the 2 th, state lying below the 1 lH,. Hudson and Kohler hay: drawn attention to differences in bond order between 2 IA, and 1 I%, slates, and the possible photochemical consequences f ?,I0J, pr!ncipally differences in relative proportions of C&Gtrims isomerisation. However, conformational effect: in the ground state, and the existence of other photo. chemical reactions, could make such differences dir% cult to identify unambiguously +. Our calculations also show differences in bond orders between I$-like and ‘B,-like states, but of more rignificance are the differences in electron density shown in table 2. ‘This shuws substantial positive charges on the diene carbon atoms. especially C-l and C-4, in the ‘$-like state S,, and much smaller charges in S, (l%,-like): generally positive at C-l and C-3, more positive at C-4, and negative at C-2. The implication is that St should react preferentialfy with nucleophiles at C-l and C-4, while S, should be less reactive towards nucleophiles, and react with Rlectrophiies at C-2. There is a further difference in substituent effects: the effect of increasing the eiectrondonating character of the substituent is to incrcose the electron density on C-l to Cd, so that St will become less reactive and S, more reactive to” For eumptrs of polycne photochemistry
where groundstate conformations play a major role, see ref. f2Oj.
!5 March 1977
:;iards elcctrophiles as the substituent becomes more electron-donating. Furthermore, substituents in the 3-position of the ring lead to anomalously high electron densities in S). I-phenylbutadiene has been shown to undergo photoaddition to hydroxylic solvcnts 1I 2 ] ; a future pubhcation [ 133 will discuss the mechantnism of this reaction in the‘light of resutis reported here.
111R.J. Buenkcr and J.L. Whitten, J. Chem. Fhys. 49 (1968) 5381; P.A. Clark and LG. Csiemadia, J. Chem. Phys. 56 (1972) 275s; T.H. Dunning Jr., R.P. Hosteny and I. Shavitt, J. Am.
Chem. Sot. 95 (1973) 5087.
r21P.A. Clark, J. Chem. Fhys. 54 (1971) 45;
K. Schulten and M. Karplus, Chem. Phys. Letters 14 (1972) 30s. 131 R.M. Gavin Jr. and S.A. Rice, J. Chem. Phys. 60 (1974) 3231.
141 F.W.E. Knoop and L.J. Oosterhoff, Chem. Phys. Letters 22 (1973) 247.
ISI A.M. Schaffer, W.H. Waddeli and R.S. Becker, J. Am. Chem. Sot. 96 (1974) 2063. 161 BS. Hudson and B-E. Kohier. Chem. Phvs. Letters 14 (1972) 299.
B.S. Hudson and B.E. Kohler, J. Chem. flays. 59 (1973) 4984. R.L. Swofford and W.M. McClain, J. Chem. Phys. 59 (1973) 5740. K. Mandal and T.N. Misra, Chem. phys. Letters 27 (1974) 57; R. McDiarmid, Chem. Phys. Letters 34 (1975) 130. R.L. Christensen and B.E. Kohler, J. Chem. Phys. 63 (1975) 1837. T.A. Moore and P.-S. Song, Chem. Phys. Letters 19 (1973) 128; B.S. Hudson and B.E. Kohler, Chem. Phys. Letters 23 (1973) 139; R.M. GavavinJr., S. Risemberg and S.A. Rice, J. Chem. PhYS. 18 (1973) 3160; R. McDiarmid, 3. C&m. Ph% 64 (1976) S 14. P.J. BaYdry, J. Chem. Sot. Perkin I (197.5) 1913. P.‘J. Baldry, to be published. LE. Bloor and B.R. Gilson, QCPE No. 71, Quantum Chemistry Program Exchange, Indiana University, Bloomington, Indiana. [ 1S j D.L. Beverilrgeand J. Hinze, J. Am. Chem. Sot. 93 (1971) 3107. [ 16f J, Hinze and H.H. Jaffh, 1. Am, Chem. Sot. 84 (1962) 540;
Volume 46, number 3
CHUMCAL PHYSICS LlXTERS
J. Hinzc, MA. Whitchcad and H.H. Mf& 3. Am. Chem. Sot. 85 (1963) 148. [ 171 L.E. Sutton, D.G. Jenkin, A.D. hfitchell and L.C. Cross, eds., Tables of Interatomic Distances and Configurations in RfolecuIes and Ions, Special Publication No. 11 (The Chemical Society, London, 1958). [IS] J. Koutcck9, J. Chem. Phys. 47 (1967) 1501.
15 March 1977
[ 191 H. Shinoda, H. Tatematsu and T. h¶iyazaki, Buif. Chem. Sot. Japan 46 (1973) 2950. 1201 A. Fadwa, L. Brodsty and S. CIough, 5. Am. Chem. Sot. 94 (I 972) 6767; P..f. Vroegop, J. Lugtenburg and E. Havinga, Tetrahedron 29 (1973) 1393; P. Courtot and R. Rumin, I. Chem. Sot. Chem. Commua. (1974) 168.
433