J. Org. C h e m . 1992,57,671-677
67 1
Further Studies in Pentacycloundecan-&onePhotochemistry Ronald R. Sauers* and Tyler A. Stevenson Department of Chemistry, Rutgers, The State University of New Jersey, New Brumwick, New Jersey 08904
Received J u l y 23, 1991 Three new syn-11-substitutedpentacycloundecan-&ones la-c (X= CN, C02CH3,C a s ) have been synthesized and evaluated for photochemical reactivity in the context of the development of a force field methodology for evaluation of Norrish Type 11 reactions. None of these compounds underwent significant intramolecular hydrogen abstraction reactions or showed evidence for interactionsbetween the excited allranone and the proximate methylene hydrogens. The absence of isotope effect in the fluorescence of the deuteriated analogue of la provides strong evidence that there are no radiationless processes involving the proximate hydrogens that deactivate the singlet state of this ketone. A novel photostimulated interaction between alkanone singlet states and a "remote"cyano group was observed and rationalized in terms of dynamic processes during the excited state lifetime. Evidence is preaented for competing long-range and contact interactions between singlet aryl rings and ground-state carbonyl groups. syn-11-tert-Butylpentacycloundecanone(2) was prepared and found to be photochemically unreactive as a consequence of a large increase in steric energy associated with formation of the transition structure.
Introduction Previous studies designed to investigate the importance of stereoelectroniceffects in hydrogen abstraction reactions by excited carbonyl groups, i.e., Nonish Type I1 reactions? led to an investigation of the photochemistry of pentacycloundecan-&ones (1). Because of the well-defined geometry of this framework, these substrates appeared to be ideally suited to reveal structural and stereochemical details of reactivity. In the first series that was studied (1, X = H,CH3, Br, Cl), none of the compounds were pho-
2
and fluorescence quantum yields toreactive [&K < were essentially independent of substituent despite the proximity of the methylenic hydrogen atoms to the carbonyl group (ground state O-H* separation -2.6 A). It was concluded that there was a stereoelectronic barrier to bonding interactions between the alkanone excited states and the proximate hydrogen atoms. This concept has been discussed by previous workers.2 Turro and Weiss in 1971 suggested that good overlap between the half-filled n orbital on oxygen with the hydrogen being abstracted was an important factor.2a Wagner in 1976 proposed a cos2 dependency of abstraction rate vs the dihedral angle associated with the developing 0-H bond and nodal plane of the carbonyl ?r system.2e A similar, but more detailed approach has been proposed by Scheffer and Trotter, et al. (see be lo^).^^+^ More recently, the results of semiempirical and ab initio computations4have provided a f m theoretical basis for the preferred angular relationships (1) Sauers, R. R.; Scimone, A.; Shams, H. J. Org. Chem. 1988, 53, 6084-6089. (2) (a) Turro, N. J.; Weiss; D. S. J . Am. Chem. SOC.1971, 90, 2185-2186. (b) Scheffer, J. R. in Organic Solid State Chemistry; Desiraju, C . R., Ed.; Elsevier: New York, 1987; Chapter 1. (c) Chang, H. C.; Popovitz-Biro, R.; Lahav, M.; Leiserowitz, L. J. Am. Chem. SOC.1978,109, 3883-3893. (d) Wagner, P. J.; Park, B . 4 . Org. Photochem. 1991, 11, 227-366. (e) Wagner, P. J. Top. Curr. Chem. 1976, 66, 1-52. (f) Sugi-
yama, N.; Nishio, T.; Yamada, K.; Aoyama, H. Bull. SOC.Chem. Jpn. 1970,43,1879-1880.
(3) Scheffer, J. R.; Trotter, J.; Omkaram, N.; Evans, S. V.; Ariel, S. Mol. Cryst. Liq. Cryst. 1986, 134, 169-196. (4) (a) Dorigo, A. E.; McCarrick, M. A.; Loncharich, R. J.; Houk, K. N. J. Am. Chem. SOC.1990,112,7508-7514. (b) Severance, D.; Pandey, B.; Morrison, H. J. Am. Chem. SOC.1987,109,3231-3233. (c) Severance, D.; Morrison, H. Chem. Phys. Lett. 1989, 163,545-548. (d) Dewar, M. J. S.; Doubleday, C. J . Am. Chem. SOC.1978,100, 4935-4941.
around the reaction center. Unfortunately, because it is not feasible to routinely evaluate the structures and energetics of complex systems by this methodology, an alternative methodology for quantitative analysis of these reactions is clearly desirable. We have been developing a force field protocol for analysis of intramolecular hydrogen abstraction reactions in an effort to devise a quantitative computational methodology to analyze Norrish Type I1 photochemical react i o n ~ . ~ *The method utilizes molecular mechanics to model the structures and steric energies of excited states and the corresponding transition structures and compares the differences with those of known reactive systems, e.g. 2-pentanone. Our thesis is that by examining examples of reactive and unreactive systems, it should be possible to develop force field parameters that will correlate their reactivities and provide a basis for understanidng the interplay of the various structural factors that control these reactions. Our initial studies led to a set of parameters that indicate that hydrogen abstraction in 1 and its derivatives would proceed via strained transition states relative to 2-pentanone. Although earlier experimental tests with 1 supported this prediction, it was especially important to evaluate the reactivity of 1 more rigorously since it was to serve as a prototype of an unreactive ketone. In this study we test further the reactivity of the methylene-X hydrogen atoms of 1by use of substituents designed to stabilize an incipient odd electron species a t this site by inductive and/or resonance effects. To this end, syntheses of the cyano derivative la, the carbomethoxy system lb, and the benzyl system IC are reported. In addition, the tert-butyl derivative 2, a unique example of a flagpole-substituted boat form of cyclohexanone, was designed as an alternative probe of carbonyl reactivity in this framework. Overall photoreactivity and various photophysical properties were examined for these molecules in an effort to reveal singlet-state and triplet-state hydrogen abstraction processes.5b
Results Syntheses. The carbomethoxy system lb (X = C02CH,) and the nitrile la (X = CN) were prepared by the procedures of Marchand et al. starting from the known ketal 3 (eq 1).6 The assignment of syn stereochemistry in both syntheses is based on stereochemical precedents (5) (a) Sauers, R. R. Krogh-Jespersen, K. Tetrahedron Lett. 1989, 527-530. (b) Sauers, R. R.; Huang, S.-Y. Tetrahedron Lett. 1990, 5709-5712. (6)Marchand, A. P.; Deshpande, M. N. J. Org. Chem. 1989, 54, 3226-3229.
0022-326319211957-0671$03.00/0 0 1992 American Chemical Society
672 J. Org. Chem., Vol. 57, No. 2, 1992
Sauers and Stevenson
-Wo
Scheme I CHZCOZCZHS 1. (EtO)~P(O)CHpCOzEt
1. LDA, CH,I
2. Hz, Pd
2, LDA. CH,I
~~
4
3
8
8
for catalytic hydrogenations in this system that have been established by these workers. The benzyl derivative IC (X = C,H,) was prepared by two independent routes. Reaction of phenyl cuprate with the known' iodo compound 5 gave IC in poor yield (eq 2). A more reliable route involved addition of benzylmagnesium chloride to 8,ll-pentacycloundecanedione followed by catalytic homodebenzylation (eq 3). In addition to the desired syn-benzyl ketone a small amount of an alcohol was produced that corresponded to the antibenzyl system.
g
O
y
g
H
(2)
w Howcyz 5
1c
PhCH,MgCI
6
Table 1. Absorption and Fluorescence Data for Ketones in Acetonitrile absorption compd
H Ph,C"Li
Ph
Raney Ni
10
(3)
7
Synthesis of ketone 2 required a less bulky protecting group for the incipient carbonyl center. Attempts to use 3 led to neighboring group rearrangements in later stages. For this reason, the methoxymethyl8 proved to be a more versatile substrate. A Wadsworth-Emmons carboxyvinylation reaction on 8 followed by Catalytic reduction led to a 87:13 mixture of esters 9. The major product in this sequence was assigned the syn configuration in accord with precedent for the catalytic hydrogenation step established by Marchand et The mixture of esters was subjected to bis-methylation in two separate stages to form 10. Upon reduction with lithium aluminum hydride the resulting mixture of neopentyl alcohols was separated by flash chromatography to yield the pure alcohol 11, X = OH (Scheme I). The stereochemical assignment was confirmed by examination of the coupling constants for the hydrogen H-11. In all derivatives of syn isomers in this framework, this proton appears as a triplet as a result of coupling with the two vicinal protons on the adjacent five-membered ring. The major alcohol 11 (X = OH) formed in the reduction showed a 1-H triplet a t 6 1.46 (J = 3.2 Hz) for this proton in the lH NMR spectrum. In the minor product (anti isomer) this absorption appeared as a singlet at 6 1.78. The neopentyl alcohol was converted to the tert-butyl ether 11 (X = H) via reduction of the p-toluenesulfonate derivative 11 (X = OTs) with SuperHydride. The tert-butyl ketone 2 was smoothly generated in one stage by treatment of the methoxymethyl ether 11 (X = H) with acidic aqueous sodium dichromate. Spectral Data. Absorption and fluorescence spectral data for the new compounds referenced to s y n - l l methylpentacycloundecan-8-one(le, R = H) are shown in Table I. Photochemistry. Irradiations of la-c and 2 a t 300 nm were carried out for extended periods in acetonitrile, CD,OD, tert-butyl alcohol, and acetone. Monitoring by 'H NMR and thin-layer chromatography did not reveal significant accumulation of new compounds or loss of starting material. Maximum quantum yields for ketone
2. Jones
11
10
Ph
/I
O
MJ
le lb la la-dp lab IC
2
fluorescence"
A(h2nm) 296 296 299
(&5%) 17 20 21
299 298 261 295
17 22 219 26
-
-
Amax
(13nm) 441 444 439 439 454 440 440 443
@re1
7f
(&lo%) (ns, ilO%) 1.00 9.6 0.93 0.23 2.3,11 0.22 0.68 2.0,6.2 1.02 10.5 0.67'~~ 0.97 6.8
'Excitation a t 310-320 nm. *In cyclohexane. 'Excitation at 262 nm. dRelative to l e a t A,, 262 nm.
loss were less than 3 X
Discussion The new molecules prepared in this study represent more stringent tests of the reactivity of the methylene hydrogens in 1 and its derivatives toward intramolecular hydrogen abstraction reactions. The phenyl ring in IC reduces the carbon-hydrogen bond strength to ca. 85 kcal/mol, for example. Similarly, the electron-withdrawing substituents, cyano and carbomethoxy, were expected to enhance reactions originating from the electron-rich ?r region of the excited carbonyl moiety by enhancement of the C-H acidity and/or by providing resonance stabilization of intermediate radicals. Cyano Ketone la. Although the cyano system (la) proved to be photochemically stable, it displayed a ca. 4.5-fold reduction in fluorescence intensity relative to the methyl system. Initially, this observation was believed to be evidence for reversible hydrogen abstraction reactions between the carbonyl singlet and the proximate hydrogen atoms. These notions were dispelled by the observation that the corresponding a-dideuterio compound showed the same reduced fluorescence yield. Evidence that partial charge transfer is involved in the interaction between the two functional groups was provided by the finding that the fluorescence yield was enhanced by a factor of ca. 3 on changing the solvent from acetonitrile to cyclohexane. These observations were surprising in view of the fact that fluorescence measurements of ketones are routinely carried out in neat acetonitrile yet no evidence of solvent/solute quenching have been reported, i.e. neither lifetimes nor quantum yields of fluorescence are significantly affected by solvent changes from acetonitrile to methanol.' Single photon counting measurements on IC carried out in the two solvents revealed dual exponential fluorescence emission profiles: (acetonitrile) 2.3 ns (86%)and 11.0 ns (7) Encinas, M.V.;Liasi, E. A.; Scaiano, J. C. J. Phys. Chem. 1980,84, 948-95 1.
J. Org. Chem., Vol. 57, No. 2, 1992 673
Studies in Pentacycloundecan-&one Photochemistry
Scheme IV
Scheme I1 hv
v5 I1 N’
N’
Scheme 111
ann-1
syn-1
anti’-1
(14%); (cyclohexane) 2.0 ns (12%) and 6.2 ns (88%).For comparison, the methyl derivative le (R = H) showed a monotonic decay curve with T = 9.6 ns in acetonitrile. A deceptively simple interpretation (Scheme 11)of the data would suggest that the carbonyl group in la exists in two distinct environments: “normal” conformations K*that give rise to the longer lived emission, and a conformation K’* in which carbonykcyano interaction (k,)competes with emission and intersystem crossing (kh). Dual fluorescence behavior is expected under two sets of conditions.8 On the one hand, if significant fractions of the conformers are “locked” into specific rotamers, i.e., single bond rotational rates are slower than carbonyl decay rates (k,,k’, k d , k,, and k’d, a single exponential fluorescence decay curve is expected because conformational equilibration is estblished and maintained before decay. By default, the best representation of the case a t hand lies between these extremes and is described by eqs 4 and 5 in which Is,,,, and Imti I~,,,,= (yle-t/rl + a2e-t/rz (4)
:
I ant1. = flle-t/rl
+ f12e-t/r2
.
-1 3.6kcaVmol
-
13
12
HI
HI
62 k d h “
order of magnitude as the typical singlet alkanone decay rate for these systems, i.e. -lo8 s-l. Insight into the nature of the singlet carbonyhitrile interaction was gained by use of semiempirical methodology to examine energies and geometries of model systems. Initial calculations were done on acetonitrile and acetone (SI)using the AM1 Hamiltonian.lo Because the HOMO-LUMO gap for this system is fairly small (ca. 5.0 eV) and the LUMO orbital is associated with the ?r bond of the cyano group, we investigated the possibility that biradicals analogous to 12 might be involved in the radiationless decay of alkanone singlet states. An energy/ geometry minimized structure for this intermediate was not obtained using AM1. Instead, the energy-gradient profile showed a sharp discontinuity a t which point the adduct fragmented into its ground-state components. Repetition of the calculations for 12 using MINDO/3 Hamiltonian’l gave rise to a true minimum-energy structure. Comparison of the heats of formation for acetone singlet and acetonitrile with those of intermediate 12 is shown in Scheme IV and provides support for the contention that formation of 12 could provide a viable pathway for radiationless decay. The isomeric species 13 was not considered further in view of its high energy. Regardless of the nature of the intramolecular quenching mechanism in lc, it is of interest to speculate on the lack of intermolecular quenching of alkanone singlet states by bulk acetonitrile (18.9 M). Apparently k,(hb,)[CH3CN] is considerably leas than 10s s-l (typical b o n e singlet-state decay rate) which would lead to a value of kc(inbr)
