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Model Calculation on the Stereoselectivity of the Triplet Photoreaction of 1 2-Dimethyltrimethylene.

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Table 2. Desrlylation of 4 (SiR,
=
~~
SiMe,).
R'
Reaction
conditions
TIC]
6A:6B
Yield I%]
6A 6 8
( - )-phenylmenthyl
TBAF. T H F
TBAF. THF. HMPT [a]
TBAF, T H F
CsF, DMFjTHF
TBAF, THF
- 78
- 78
31:69
34:66
28:72
31:69
71:29
26
25
23
13
55
(-)-phenylmenthy)
(-)-phenylmenthyl
(-)-phenylmenthyl
(+)-phenylmenthyl
-105
- 70
- 78
58
49
60
29
22
[a] Hexamethylphosphoric triamide
diastereomers (S,S,S)-6A and (R,R,R)-6B (R' = (-)-phenylmenthyl) can be separated easily by flash chromatography. The
transformation with the (+ )-phenylmenthyl ester 4e yielded the
(S,S,S)-phenylmenthyl ester 6 A as the major product. The absolute configurations of ( S , S , S ) d Aand ( R , R , R ) d B (R' = (-)phenylmenthyl) were confirmed by X-ray crystallography.[' 'I
Compounds such as 6 A and 6B are interesting synthetic CD ring intermediates for the biologically active cardiac glyIn order to obtain the equally important trans-annuc~sides.['~]
lated C-D building blocks necessary for the synthesis of
vitamin Dj,[l4lthe tertiary OH group must be eliminated. The
transformation of ( S , S , S ) d Ain pyridine (Py) and thionyl chloride affords exclusively the P,y-unsaturated compound (S,S)-8,
which can then be hydrogenated to furnish the desired trans-hydrindan~ne.[~]
R''OAO
(S,S)-8,
R'
=
63%
(+)-phenylmenthyl
Keywords: aldol reactions
esters
-~
- enolates
~
~~
hydrindanones
-
silyl
[l] D. J. Ager, Org. Reucl. NY 1990, 38, 1.
121 Y. Kita, J. Sekihashi, Y. Hayashi. Y - Z . Da, M. Yamamoto, S. Akai, J. Org.
Chem. 1990, 55, 1108.
131 G. Bartolini, M. Bosko. R. Dalpozzo. P. E. Todeso, J Chem. Sor. Chem.
Commw. 1988, 807.
[4] U. Eder, G. Sauer, R. Wiechert. AnKen. Chem. 1971,83,492; Angew. Chem.
Inl. Ed. Engl. 1971.10.496; Z. G. Hajos. D. R. Parrish,J Org. Chem. 1974.39,
1615.
IS] T. Mandai, Y Kaihara, J. Tsuji, J Org. Chem. 1994, 59, 5847.
[6] All new compounds were fully characterized by standard methods.
171 L. Birkhofer. A. Ritter, Angew. Chem. 1965, 77. 414.
IS] M. S. Newman. 3 H. Manhart, J Org. Chem. 1961. 26, 2113.
19) G. L. Larson, V. C. Maldonado. L. M. Fuentes. L. E. Torres, J Org. Chem.
1988, 53, 633.
1101 K. Maruoka, H. Banno. H. Yamamoto, Synleti 1991. 253.
1111 E. J. Corey, H. E. Ensley, J. Am. Chem. Soc. 1975, 97, 6908.
[12] Crystal structure data for (S.S.S)-CA (R' = (-)-phenylmenthyl): C,,H,,O,,
monoclinic, space group P2,. u = 863.91(10). h =1311.3(2), c =1072.90(10)
pm: /1=94.194(8)', V=l.2122nm3. Z = 2 . / ~ = 0 . 0 8 m m - ' . T = -100°C.
Colorless prism (0.85 x 0.45 x 0.4 mm), Siemens R3 diffractometer, 5781 reflections with 2OmaX
= 55' (Mo,, radiation), of which 2895 were independent.
The structure was solved by direct methods and refined anisotropically against
FZ(SHELXL-93 program system, G. M. Sheldrick, Universitiit Gottingen); H
atoms were refined with a ridingmodel with rigid methyl groups; the absolute
configuration of the phenylmenthyl substituent is known, wR(F2) = 0.087.
with conventional R(F) = 0.034. 285 parameters and 292 restraints (for the
components of the displacement factors); S = 1.00; max. A p = 141 enm-'.
Crystal structure data for (R,R,R)-6B (R' = (-)-phenylmenthyl): C,,H,,O,,
orthorhombic. space group P2,2,2,, u =70947(12), h =1299.2(2), c =
2632.2(4) pm, V = 2.4263 nm3. Z = 4, p = 0.08 mm-I. 7 = - 130°C. Colorless prism (0.8 x 0.35 x 0.2 mm), Stoe-STADI-4 diffractometer, 3412 intensities
= 55", of whtch 3375 were independent. Structure elucidation and
with 2OmZx
refinement as for (S.S.S)-6A; nR(F2) = 0.141, R(F) = 0.056, 285 parameters
and 291 restraints; S =1.08; max. Ap = 200enm-'. Crystallographic data
(excluding structure factors) for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC-179-102. C o p m of the data can be obtained free of
Charge on application to The Director. CCDC. 12 Union Road, Cambridge
CB2 lEZ, UK (fax: Int. code +(1223)336-033; e-mail: teched(achemcrys.
cdm.aC.uk).
[13] J. A. Bristol, D. B. Evans, Annu. Rep. Med. Chrm. 1981, 16, 93.
[14]G:D. Zhu, W. H. Okamura. Chem. Rev. 1995, 95, 1877.
Experimental Procedure
A solution of TBAF in T H F (1.1 M, 0.289 mmol, 0.26 mL) was stirred in anhydrous
T H F (2 mL) for 20 min at room temperature with activated molecular sieves to bind
any residual water in the TBAF solution. The resulting anhydrous TBAF solution
Was then added dropwise to a solution of diketone 4d (120 mg, 0.241 mmol) in THF
( 3 mL) cooled to -105°C. The solution was stirred for 20 min at -105°C and
subsequently hydrolyzed with NH,CI. The mixture was extracted twice with diethyl
ether, the combined organic layers were washed with NaCl solution and dried over
MgSO,, and the solvent was removed. Purification by flash chromatography with
pentanejether ( S j l ) yielded 62 mg of (R,R,R)-6B (R' = (-)-phenylmenthyl; 60%)
and 24mg of (S.S,5')-6A (R' = (-)-phenylmenthyl; 23%).
Model Calculations on the Stereoselectivity
of the Triplet Photoreaction of 1,2-Dimethyltrimethylene
Marcus Bockmann and Martin Klessinger*
In a nonadiabatic photoreaction, excitation from the ground
state (So) into the lowest singlet state (S,) or triplet state (T,)
'J=6.5Hz,3H,C-27),0.92(m,1H,C-17),0.98(s,3H.C-10),1.02(m.1H,C-7),
brings about geometrical changes followed by return from S, to
1.09 (m. 1H, C-15),1 .I9 ( s , 3 H. C-19 or C-20). 1.24 (m, 1 H. CX), 1.28 (s. 3 H, C-19
So by a conical intersection or from T, to So by intersystem
or C-201, 1.39 (m, 1 H, C-6), 1.40 (m, 1 H, C-7). 1.46 (m, 1 H. C-16). 1.51 (m. 1 H,
crossing (ISC), and thermal equilibration in the ground state."]
C-6).1.57(dd.3J=10.7,3.3 Hz.1H.C-5), 1.62(m. tH,C-14),1.64(m,l H,C-lS).
1.72(m,fH,C-8),1.84(m, 1H,C-17),1.89(ddd,2J=12.9,3J=9.4,3.5Hz,1H. Although the structures of the products and the efficiencies of
C-3),2.03(m.2H.C-3/13).2.15(ddd,2J=l9.3,3J=9.4.9.1H~.1H.C-2),2.49
their formation depend on the nature of the motions in the
(ddd, ' 5 = 19.3, 'J = 10.3. 3.5 HZ. 1 H, C-2). 4.79 (ddd. ' J = 10.7, 10.7.4.3 Hz. 1 H.
excited state, on the geometry of return to the So state, and the
C-12),4.83(s, 1 H30H),7.12(m.1 H,Ar), 7.24(m.4H,Ar); I3C NMR(100 MHz)final steepest descent motion on the So surface, the geometry of
IR.S(q,C-lO), 21.7(q,C-27), 21.8 (t.C-7).25.1 (q,C-19orC-20),25.2(t,C-6). 26.5
(t. C-lS), 27.8 (4, C-19 or C-20). 29.11 (t. C-8), 31.2 (d, C-16), 31.3 (t. C - 3 ) , 34.2
return is of a paramount importance in solution reactions. For
(t. C-2). 34.4 (t, C-l4), 39.6 (s, C-l8), 41.2 (t, C-17). 47.2 (d, C - 5 ) .49.8 (d. C-l3),
a nonadiabatic singlet reaction, this is the conical intersection,
53.6(s,C-9),75.4(d,C-12),76.9(s,C-4),t25.2(3xd,Ar),127.9(2xd,Ar),~~1.3
which from a mechanistic point of view plays a similar role as
(s,C-21), 175.4(s,C-ll),218(s, C-I); MS (EI):m/;(%):426(Mt,2).408(2).
307
the transition state in a thermal reaction.'21 In triplet photoreacjS),214(10), 167 (lo),119(100), 105 (31), 91 (30);1R: V = 3451 (m). 2957 (m). 2927
(R,R,R)dB (R' = (-)-phenylmenthyl): colorless crystals: m.p 96 "C; [XI? =
- 13.37 (c = 0.8 in CHCI,); 'H-NMR (600 MHz): 6 = 0.85 (m, 1H. C-14). 0.86 (d.
(m), 1742 (s), 1698 (s). 1428 (m). 1213 (m). 1120(m), 865 (m). 700cm-'(s): uv:
j.,,, = 194 nm; elemental analysis ("A):calcd (C2,Hje04): C 76.02, H 8.98: found:
C 76.01, H 9.06.
Received: May 30. 1996 [Z9175IE]
German version: Angeu,. Chem. 1996. IO8.2669-2671
2502
0 VCH
Verlugsgesellscha/t mbH, 0-69451 Weinheim. 1996
[*] Prof. Dr. M . Klessinger, Dr. M. Bockmann
Organisch-chemisches Institut der Universitiit
Corrensstrasse 40, D-48149 Munster (Germany)
Fax: In1 code +(251)839772.
e-mail: klessim(i~:uni-muenster.de
0570-083319413521-2502$ lS.OO+ .2S/O
Angew. Chem. I n / .
Ed. Engl. 1996. 35, No. 21
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tions the geometries most favorable for ISC may be considered
86
86
to be of similar relevance.*31
Thus, the knowledge of the "reactive structure", that is the
transition structure of a thermal reaction, the structure of the
conical intersection accessible on the excited state potential enEl
l
ergy surface (PES) of a singlet photoreaction, o r the geometries kcalErnol-'
kcal rnol-'
favorable for ISC in a triplet photoreaction, is decisive for the
discussion and prediction of the stereochemical outcome of the
71
71
0
0
reaction. Triplet stereoselectivity has recently been exploited
mainly in synthetic applications of the Paterno-Buchi reaction.I4]
Herein we present a general procedure for determining the
reactive structure and discussing the stereoselectivity of triplet
photoreactions. The ring-closure reaction of 1,2-dimethyltri8.0
8.0
methylene (DMTM) will be used as an example to show that o n
the basis of the PES of the ground state So and the lowest triplet
state Ti in combination with SOC surfaces, where the spin-orbit
coupling value SOC rather than the energy E is plotted as a
soc/
soc/
function of the reaction coordinates, it is possible to locate the
cm-'
em-'
regions of large SOC and small singlet-triplet separation EST
that are most favorable for ISC.I5IAll calculations are based o n
0
0
0
recently developed methods to treat SOC within the framework
of the semiempirical configuration interaction (CI) approach.[']
Computational results for unsubstituted trimethylene (TM)
Fig. 1. The nng-closure reaction of 1,2-dimethyItrimethyIene Lowest singlet and
have been reported,''. 71 and the structural dependence of SOC
triplet PES for conrotatory motion of the radical centers toward a) the svn (I >O")
has been discussed previously[', 7a, b1 and rationalized on the
and b) the anti conformation ( a < V ) ,as well as SOC surfaces c) for z > O and d)
basis of the two-electron two-orbital model and symmetry confor z < O .
siderations.['] It was shown that conrotatory (p = a) and disrotatory (B = 180" - a ) motions of the terminal methylene groups
those of TM['] it is evident that to a first approximation the
yield similar results. On the basis of a combined analysis of PES
methyl substituents d o not at all affect the SOC values, while
and SOC surfaces, ISC and the ring-closure reaction could be
steric effects appreciably change the appearance of the valley on
discussed : the minimum energy valley of the triplet PES that
the
TI PES. For the syn mode of rotation a barrier of approxicorresponds to nearly free rolation of the radical centers is unfamately 3.4 kcalmol-I is found to separate the local minimum
vorable for ISC since the So state lies energetically above the TI
Mcis at a = 90" from the planar structure ( a = Oc) that is
state. However, in the region of large SOC, in which the terminal
1.0 kcalmol-I lower in energy than the barrier. For the anti
methylene groups are rotated by a >45" toward a face-tomode of rotation, however, the valley descends practically withface arrangement (Scheme I), decreasing the C-C-C valence
out a barrier toward the minimum M,,,,
at a = - 90°,
5.0 kcalmol- lower in energy than Mcis.Some data for selected
geometries including the singlet- triplet intersection line
(EST= O), are collected in Table 1. By decreasing the bond angle
y, geometries are accessible both from Mcisand M,,,,, at which
Soand T, are degenerate (EST= 0) and SOC is appreciable (SOC
> 2 cm- '); the necessary energies are less than 1 kcal mol- I .
DMTM
For rotational angles ( a (< 45", however, much larger energies ( > 10 kcal mol - ') are required and geometries with
t
SOC < 1 cm- are reached.
t
t
t
t
I
'
H
H
'CH,
Table 1. 1,2-Dimethyltrimethylene.
Stateenergies and SOC values for selected geometries from MNDOC-CI calculations [al.
trans
Scheme 1. Definition of the rotational angles 3 and /I, and of the bond angle 7 .
z = /.' = 0 corresponds to a side-to-side arrangement of the radical centers, and
1x1 = i/jI = 90 to a face-to-face arrangement.
angle 7 to a value of 105" raises the energy only slightly
(I - 2 kcal mol ' ) and leads to singlet - triplet degeneracy
(EST= 0) and therefore to geometries very favorable to ISC.'61
In D M T M there are two different modes of conrotatory motion (b = z) of the radical centers, leading to stereoisomeric
cyclization products: rotation by positive (a > 0) and negative
(a<O) values of the rotational angle yields cis- and [vansdimethylcyclopropane, respectively (see Scheme 1). So and T,
PES together with SOC surfaces are shown for both modes of
rotation in Figure 1.19] From a comparison of these results with
Geometry
b. a1 [bl
[I11, - 901
[112. OJ
M,,,,
[109.601
[110. 901
[105, 90J
[105, 751
[103. 601
196. 45)
[106,- 901
B
M,,,
[105, -751
[ 103,- 601
[96, - 451
Energy [kcal mol
Esr [dl
' E [cl
0
1.5
2.6
6.0
5.0
5.4
5.8
6.6
06
0.6
0.7
1.5
6.8
1.1
[a] Singlet open-shell 3-3' CI. [b]
73.4 kcal mol-'. [d] EST= ' E - 'E.
1 .o
1.3
0
0
0
0
0
0
0
0
/.' = z.
'1
SOC [crn 'J
1.778
0.002
1.090
1912
2.631
2.315
1.613
0.923
2.612
2.316
1.627
0.930
[c] Relative to Afff(3Mlmec)=
COMMUNICATIONS
85"
85"
Y
Y
L
120"
-90"
a
CI
+90"
1200
-90"
120"
U
+90"
SCF
MMX
85"
85"
Y
Y
12
-90"
a
+90"
120"
-90"
Fig. 2. Contour diagrams of the So and T, PES
for the ring-closure reaction of 1,2-drmethyltnmethylene. as calculated by the CI approach
and estimated by SCF and force-field (MMX)
methods. Triplet-singlet intersection EST= 0 IS
indicated by heavy lines.
120"
U
-90"
+90"
From these results, it can be concluded that, as in the unsubstituted TM, the reactive structure of optimal ISC is characterized by a face-to-face orientation of the radical centers and a
C-C-C angle y slightly smaller than for the triplet minima. As
the singlet PES drops steeply toward the cyclopropane minimum for small values of y. the triplet reaction yields preferably
cyclic products. The conditions for optimal ISC are similar for
both minima Mcisand M,,a,,; therefore, the stereochemical differentiation between cis- and trans-substituted products is due,
in this special case, to the energy difference of the two minima
M,,, and M,,,,
.,
This explains the experimental observation that
the triplet-sensitized photoreaction of cis- as well as trans-3,4dimethyl-1-pyrazoline yields preferably trans-1,2-dimethylcycIopropane, and negligible amounts of acyclic products, in
contrast to the singlet photoreaction, which occurs preferably
with retention of the configuration and yields appreciable
amounts of acyclic products.["l
Thus, the reactive structure of the triplet photoreaction of
trimethylene that corresponds to optimal ISC is hardly affected
by methyl substitution, and the product ratio reflects the energy
difference between M,, and Mi,,,, on the T, surface. Since this
energy difference is due mainly to steric effects, it should be
possible to estimate it on the basis of simple models. For this
purpose we calculated the difference PES E(DMTM) - E(TM)
for DMTM and the unsubstituted TM by SCF-MO as well as by
force-field methods1' and added this difference to the PES of
the unsubstituted TM.[61The resulting contour diagrams together with the singlet-triplet intersection line (& = 0) are
shown in Figure 2 for the T, as well as the So state. For comparison, results of the complete CI calculations are also given. The
rotational angle is varied from s( = - 90" to c( = 90", thus including the syn and anti motions that are shown separately in
Figure 1 (cf. the contour diagrams of the triplet PES given at the
bottom of Figure 1). It is quite apparent that both methods
describe the main features of steric effects on the PES quite
correctly; the SCF results slightly overestimate and the forcefield results slightly underestimate the nuclear repulsions at
small values of the C-C-C angle .
In summary, the combined analysis of PES and S o C Surfaces
is very well suited for determining the reactive geometries that
are most favorable for ISC. The knowledge of these geometries
together with the T, excited-state PES then allows for a detailed
elucidation of the mechanism of a triplet photoreaction and a
2504
m
Q VCH VerlugsgeseiischuJi tnbH. 0.69451 Weinheim, 1996
U
+90"
rationalization of the stereoselectivity. Since purely steric substituents like methyl groups do not affect the SOC values, it is
possible to evaluate the structural dependence of SOC for the
unsubstituted system only, and to use simple models to estimate
steric effects on the PES. In the case of DMTM different reactive geometries leading to cis- and truns-l,2-dimethylcyclopropane may be reached from the minima M,, and M,,,,,, and
the necessary energies are less than 1 kcalmol-I. Thus, stereodifferentiation is due to energetic behavoir of the T, PES. Since
this is, to our knowledge, the first triplet photoreaction for
which the mechanism has been elucidated in detail, future investigations have to show whether this is a general feature of
stereodifferentiation in triplet reactions.
Received: May 20, 1996 [291291E]
German version: Angew. Chem. 1996, 108. 2681 -2683
Keywords: photochemistry
methylene
.
theoretical chemistry
-
tri-
M Klessinger. J. Mlchl. E.wited Slates and Photochemistry of Organic
Molecules, VCH, New York, 1995
M. Olivucci, F. Bernardi, S. Ottani, M. Robb, J. Am. Chem. Soc. 1994, 116,
2034.
M. Klessinger. M. Bockrnann, J. Mihlmann, J. Inf. Rec. Mats. 1994. 21,
549.
a) H. Buschmann. H.-D. Scharf. N. Hoffmdnn, M. Plath, 1. Runsink. J Am.
Chem. SOC.1989, "1, 5367; b) A. G. Griesbeck. H. Mauder, S. Stadtmiiller.
Acc' Chrm. Res. 1994.27 70; c) T. Bach, Liebrgs Ann. 1995,855; Angew. Chem.
1996. 108. 976; Angew. Chem. In!. Ed. Engi. 1996,3S, 884.
S. P. McGlynn. T Azumi, M. Kinoshltd, Molecular Spectroscopy ofrhe Triplet
Sfate. Prentic Hall. Englewood Cliffs, NJ, 1969.
M. Bockmann. M. Klessinger. J Phvs. Cheni. 1996. 100, 10570.
a ) T R. Furlani. H. F. King, J Chcm. P/i.vs. 1985.82, 5577; b) L. Carlacci, C .
Doubleday Jr., T. R. Furlani, H. King, 1 M. Mclver Jr., J: Am. Chem. SOC.
1987,llIP. 5323: c) H. E. Zimmerman, A. G. Kutateladze, [bid. 1996,118,249.
J. Michl. J. Am. Chem. Sor. 1996, I I X , 3568.
Geometry taken from ref. [6] with idealized methyl groups; radical centers were
rotated by a in steps of 15". while the C-C-C y angle was varied in steps of 5".
The C1 wavefunctions include all configuration state functions (CFS) within an
3-3' active soace. For further details see Ref. 161
[lo1 R. Moore, A. Mishrd. R. J. Crawford, Cund J. Chem. 1968. 4 6 , 3305.
[ I l l MMX force field of PCMODEL V 5.0, Serena Software, Bloomington, IN,
1993.
0J70-0833/96/352l-2504 $ 15.00i ,2510
Angen. Chem. Inl. Ed. E n d . 1996, 35, No. 21
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