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Optical activity and stereochemistry of 4a,4b-dihydropentahelicene
Volume 17, number 4
OPTICAL
ACTIVITY
15 December
CHEMICAL PHYSICS LETTERS
AND ST~RE~C~E~ISTRY
1972
OF 4a, 4~D~~~DR~PE~TA~EL~C~~E
Received 2 October
1972
In the photocyditation reaction of c&ii-fl-naphthyl ethylene with circuhrly polarized ii&t an opticaLly active dihydro aromatic’ (DHP) has been detected as an intermediate. The chirality of the DHP evidences the nans configuration of the tertiary hydrogens. It f&lows that the ring closure reaction takes place in the excited singlet state of the olcfin according to the Woodward-Hoffmann rules.
where the first excited sindet state S, of the cisisomer is involved {6,?], the question remains as to
f. Introduction
In the photochemical cyclization of cis-aryiethylenes to aromatics dihydropraducts (DHP) of the following structure
whether the cyclization &%Ibe attributed solely to c&S1 (adiabatic process, fig. 1, step A) or whether the.“hot” ground state molecules act as reactive species (diabatic process, fig. 1, step B) *. This problem can be readily solved by an investigation of the
stereochemistry of the tertiary hydrogens in 4a,4b position of the DHP involved, which presents a hitherto ~n~vest~gated approach of this field. R
are formed as intermediates [ I]. The degree of De
formation decreases at lower temperature while the fluorescence yield of the cðylenes increases f&3], ~~0~~ the phot~hem~c~ behaviour and the therm& stability of the IMP derivatives have been
* It is tacitly assumed that similar conditions in I7 f for stilbene will be found throughout benzologues of stilbene.
as described ti@ series of
studied in detail [4,.5] one principd mechanistic as-
S,
has not been adequately clarified until now. In a cyclization of type I -+ 11 pect
DuP
sia
I
Fig, 1. DLfferent processes‘in the photocyc&.?ation ~le~yien~: A ridi&tic, B.diabatic,
‘.
:
of cis-
Volume
17, number 4
15 December
CHEMICAL PHYSICS LETTERS
According to the Woodward-Hoffmann orbital symmetry the tram configuration
rules of of the 4a,Sb-
hydrogens can be expected if the DHP is formed by mechanism A, the cis configuration being formed on applying mechanism B. In the tram-DHP the molecule will lack any element of symmetry regardless of whether the substituents R and R’ are present or not and, therefore, must be chiral. With mechanism B, however, chiral products are obtained only if the double bond of the parent &isomer is substituted by two different aryl groups. Thus, the chirality of riHP compounds formed by asymmetric synthesis using circularly polarized light (CPL) should be examined by means of their optical activity. The precursors of the DHP must display a symmetry of C2.
2. Results and discussion Di-pnaphthyl ethylene (I) was c&en because of the thermal stability of the corresponding DHF II [8]. The following experiments were carried out with left (LCL) and right-handed (RCL) circularly polarized light: (a) Irradiation of cis-I in iso-octane with LCL or RCL yielded optically active 4a,4b_dihydropentahelicene (II). The values of the optical rotatory power of the resulting solutions (see section 3) are given in table 1. Values presented here are values of typical experiments. Repeating the irradiations under identical conditions the resulting optical rotatory power is reproducable within an error limit of less than 10%. The specific rotation cannot be calculated because the extent of optical induction is unknown. Differences between LCL and RCL experiments might be due Table 1 Optical rotatory power of DHP solutions aftx irradiation with CPL. Concentration of II: 1.3 X 10d2 M/k?, light path 10 cm, T= 2O”C, solvent: benzene
1972
to the inaccurate position of the quarter wave plate. Since II is strongly fluorescent [8] no CD or OF@ spectra could be recorded.
(b) The identification of the origin of the optical activity in experiment (a) was carried out by asymmetric photodestruction of racemic DHP II with LCL and RCL In this reaction using LCL for example (+>pentahelicene is formed by photo-oxidation while (-) -DHP is left in excess, the net optical rotatory power remaining nearly zero. For this reason the destruction was carried out to completion; conditions were chosen to achieve asymmetric phot*oxidation (see section 3). Solutions containing optically active pentahelicene [IO] were obtained by this method
with a rather low optical induction (see table 1). The rotational values obtained by asymmetric synthesis (a) may be attributed to some extend to optically active pentahelicene, the formation of which cannot be completely excluded. As shown in experiment (b), however, the resulting rotatory power of active pentahelicene formed by asymme.tric photcoxidation under the same concentration conditions is rather small. Therefore, small amounts of pentahelicene, if ever present in experiment (a), do not falsify the results given in table 1. The results show that the 4a,4b-hydrogens of the DJYIP11 must be in tram configuration because of its chirality. According to the Woodward-Hoffmann rules mechanism A is operative here. Thus, the phots chemical formation of the DHP II proceeds via the Sl-state of the DHP starting from the Si-state of the parent c&isomer (see fig. 1). Mallory’s suggestions of an exothermic reaction I + IL based on thermochemical data [7] are confirmed by the present results. Assumptions as a thermal Furthermore,
of a valence isomerization taking place reaction [9] should be carefully revised. it is shown that the cis-arylethylene I
is a prochiral compound maintaining its absolute configuration during the !ifetime of S, and it obviously has a considerable larger CD than the DHP II.
Optical activity Reaction
asymmetric synthesis of DHP II’
with RCL
with LCL
3. Experimental
+ 0.056O + 0.071” + o.oo?”
(a) Asymmetric synthesis of II. Solutions of fans-1 in iso-occtane (3.7 X 10-j M/Q) were irradiated in por-
+ 0.007O
asymmetric destruction
%7f3
-0.068” -0.093O -o.oo7°
of DHP II
6546
-0.008°
Q578
Q546
493
Vdlume
17, number 4
CHEMICALPHYSICSLETTERS
tions of 20 rn_Qin a quartz IJesse for 1.5 h*. Irradiation equipment: 50 W high-pressure mercury lamp (Hanau), line filter 365 run (Schott), linear polarizer IL 40 (Polacoate), u4 plate (B. Zalle). After reinoval of the soivent in vacuum at -20°C the residue was collected in, 1 mQ of benzene and kept at liquid nitrogen temperature. (b) Asymmetric photodcstruction of II. 2 mP of a solution of II in iso-octane was irradiated at -10°C for 6.5 h with LCL or RCL nt 436 nm in the presence of oxygen, equipment as described above, The loss of solvent for reasons of oxygen flushing was restituted to the final volume of I m
15 December 1972
Acknowledgement We wish to thank Professor Ernst Fischer for stimulating discussions in this field. Professor W. Sucrow kindly enabled us to carry out the measurements on the Perkin Elmer 141.
References [ 11 F.R. Stermitz, in: Organic photochemistry, ed. O.L. Chapman (New York, 1967). [2] S. Sharafy and K.A. Muszkat, J. Am. Chem. Sot. 93 (1971) 4119.
[3] J. Klueger, G. Fischer, E. Fischer, Ch. Gocdicke and H. Stegemeyer, Chem. Phys. Letters b (1971) 279. 141 K.A. Musakat and E. Fischer, J. Chem. Sot. B (1967) 662. t51 E.V. BLckburn, C.E. Loader rind C.J. Timmons, J. Chem. Sot. (1970) 163. 17 b (1962) 153. ISI H. Sregemeyer, Z. Naturforsch. 171 F.B. hfdlory, C.S. Wood and J.T. Gordon, J. Am. Chem. Sot. 86 (1964) 3094. 181 T. Knittel, G. Fischer and E. Fischer, Chem. Commun.
(1972) 84.
* Starting from the rru~zs isomer the DHP is built up via the cis form because of the well known wants *cis photoequilibrium of arylethylenes.
191 ti. G&ten and L. Klasinc, ‘Tetrahedron 24 (1968) 5499. ilO1 Ch. Goedicke and H. Stegemeyer, Tetrahedron Letters (1970) 937.
OPTICAL
ACTIVITY
15 December
CHEMICAL PHYSICS LETTERS
AND ST~RE~C~E~ISTRY
1972
OF 4a, 4~D~~~DR~PE~TA~EL~C~~E
Received 2 October
1972
In the photocyditation reaction of c&ii-fl-naphthyl ethylene with circuhrly polarized ii&t an opticaLly active dihydro aromatic’ (DHP) has been detected as an intermediate. The chirality of the DHP evidences the nans configuration of the tertiary hydrogens. It f&lows that the ring closure reaction takes place in the excited singlet state of the olcfin according to the Woodward-Hoffmann rules.
where the first excited sindet state S, of the cisisomer is involved {6,?], the question remains as to
f. Introduction
In the photochemical cyclization of cis-aryiethylenes to aromatics dihydropraducts (DHP) of the following structure
whether the cyclization &%Ibe attributed solely to c&S1 (adiabatic process, fig. 1, step A) or whether the.“hot” ground state molecules act as reactive species (diabatic process, fig. 1, step B) *. This problem can be readily solved by an investigation of the
stereochemistry of the tertiary hydrogens in 4a,4b position of the DHP involved, which presents a hitherto ~n~vest~gated approach of this field. R
are formed as intermediates [ I]. The degree of De
formation decreases at lower temperature while the fluorescence yield of the cðylenes increases f&3], ~~0~~ the phot~hem~c~ behaviour and the therm& stability of the IMP derivatives have been
* It is tacitly assumed that similar conditions in I7 f for stilbene will be found throughout benzologues of stilbene.
as described ti@ series of
studied in detail [4,.5] one principd mechanistic as-
S,
has not been adequately clarified until now. In a cyclization of type I -+ 11 pect
DuP
sia
I
Fig, 1. DLfferent processes‘in the photocyc&.?ation ~le~yien~: A ridi&tic, B.diabatic,
‘.
:
of cis-
Volume
17, number 4
15 December
CHEMICAL PHYSICS LETTERS
According to the Woodward-Hoffmann orbital symmetry the tram configuration
rules of of the 4a,Sb-
hydrogens can be expected if the DHP is formed by mechanism A, the cis configuration being formed on applying mechanism B. In the tram-DHP the molecule will lack any element of symmetry regardless of whether the substituents R and R’ are present or not and, therefore, must be chiral. With mechanism B, however, chiral products are obtained only if the double bond of the parent &isomer is substituted by two different aryl groups. Thus, the chirality of riHP compounds formed by asymmetric synthesis using circularly polarized light (CPL) should be examined by means of their optical activity. The precursors of the DHP must display a symmetry of C2.
2. Results and discussion Di-pnaphthyl ethylene (I) was c&en because of the thermal stability of the corresponding DHF II [8]. The following experiments were carried out with left (LCL) and right-handed (RCL) circularly polarized light: (a) Irradiation of cis-I in iso-octane with LCL or RCL yielded optically active 4a,4b_dihydropentahelicene (II). The values of the optical rotatory power of the resulting solutions (see section 3) are given in table 1. Values presented here are values of typical experiments. Repeating the irradiations under identical conditions the resulting optical rotatory power is reproducable within an error limit of less than 10%. The specific rotation cannot be calculated because the extent of optical induction is unknown. Differences between LCL and RCL experiments might be due Table 1 Optical rotatory power of DHP solutions aftx irradiation with CPL. Concentration of II: 1.3 X 10d2 M/k?, light path 10 cm, T= 2O”C, solvent: benzene
1972
to the inaccurate position of the quarter wave plate. Since II is strongly fluorescent [8] no CD or OF@ spectra could be recorded.
(b) The identification of the origin of the optical activity in experiment (a) was carried out by asymmetric photodestruction of racemic DHP II with LCL and RCL In this reaction using LCL for example (+>pentahelicene is formed by photo-oxidation while (-) -DHP is left in excess, the net optical rotatory power remaining nearly zero. For this reason the destruction was carried out to completion; conditions were chosen to achieve asymmetric phot*oxidation (see section 3). Solutions containing optically active pentahelicene [IO] were obtained by this method
with a rather low optical induction (see table 1). The rotational values obtained by asymmetric synthesis (a) may be attributed to some extend to optically active pentahelicene, the formation of which cannot be completely excluded. As shown in experiment (b), however, the resulting rotatory power of active pentahelicene formed by asymme.tric photcoxidation under the same concentration conditions is rather small. Therefore, small amounts of pentahelicene, if ever present in experiment (a), do not falsify the results given in table 1. The results show that the 4a,4b-hydrogens of the DJYIP11 must be in tram configuration because of its chirality. According to the Woodward-Hoffmann rules mechanism A is operative here. Thus, the phots chemical formation of the DHP II proceeds via the Sl-state of the DHP starting from the Si-state of the parent c&isomer (see fig. 1). Mallory’s suggestions of an exothermic reaction I + IL based on thermochemical data [7] are confirmed by the present results. Assumptions as a thermal Furthermore,
of a valence isomerization taking place reaction [9] should be carefully revised. it is shown that the cis-arylethylene I
is a prochiral compound maintaining its absolute configuration during the !ifetime of S, and it obviously has a considerable larger CD than the DHP II.
Optical activity Reaction
asymmetric synthesis of DHP II’
with RCL
with LCL
3. Experimental
+ 0.056O + 0.071” + o.oo?”
(a) Asymmetric synthesis of II. Solutions of fans-1 in iso-occtane (3.7 X 10-j M/Q) were irradiated in por-
+ 0.007O
asymmetric destruction
%7f3
-0.068” -0.093O -o.oo7°
of DHP II
6546
-0.008°
Q578
Q546
493
Vdlume
17, number 4
CHEMICALPHYSICSLETTERS
tions of 20 rn_Qin a quartz IJesse for 1.5 h*. Irradiation equipment: 50 W high-pressure mercury lamp (Hanau), line filter 365 run (Schott), linear polarizer IL 40 (Polacoate), u4 plate (B. Zalle). After reinoval of the soivent in vacuum at -20°C the residue was collected in, 1 mQ of benzene and kept at liquid nitrogen temperature. (b) Asymmetric photodcstruction of II. 2 mP of a solution of II in iso-octane was irradiated at -10°C for 6.5 h with LCL or RCL nt 436 nm in the presence of oxygen, equipment as described above, The loss of solvent for reasons of oxygen flushing was restituted to the final volume of I m
15 December 1972
Acknowledgement We wish to thank Professor Ernst Fischer for stimulating discussions in this field. Professor W. Sucrow kindly enabled us to carry out the measurements on the Perkin Elmer 141.
References [ 11 F.R. Stermitz, in: Organic photochemistry, ed. O.L. Chapman (New York, 1967). [2] S. Sharafy and K.A. Muszkat, J. Am. Chem. Sot. 93 (1971) 4119.
[3] J. Klueger, G. Fischer, E. Fischer, Ch. Gocdicke and H. Stegemeyer, Chem. Phys. Letters b (1971) 279. 141 K.A. Musakat and E. Fischer, J. Chem. Sot. B (1967) 662. t51 E.V. BLckburn, C.E. Loader rind C.J. Timmons, J. Chem. Sot. (1970) 163. 17 b (1962) 153. ISI H. Sregemeyer, Z. Naturforsch. 171 F.B. hfdlory, C.S. Wood and J.T. Gordon, J. Am. Chem. Sot. 86 (1964) 3094. 181 T. Knittel, G. Fischer and E. Fischer, Chem. Commun.
(1972) 84.
* Starting from the rru~zs isomer the DHP is built up via the cis form because of the well known wants *cis photoequilibrium of arylethylenes.
191 ti. G&ten and L. Klasinc, ‘Tetrahedron 24 (1968) 5499. ilO1 Ch. Goedicke and H. Stegemeyer, Tetrahedron Letters (1970) 937.