Peroxides of Sterically Hindered Derivatives of Schlenk Hydrocarbon Diradical

Article abstract of DOI:10.1021/jo961879x

Reaction of oxygen with three sterically hindered derivatives of Schlenk hydrocarbon diradical gives predominantly cyclic peroxides with cross-conjugated π-systems. For mesitylene-based diradical 3, significantly different yields of the cyclic peroxide fo

Full text of DOI:10.1021/jo961879x

6524
J . Org. Chem. 1997, 62, 6524-6528
P er oxid es of Ster ica lly Hin d er ed Der iva tives of Sch len k
Hyd r oca r bon Dir a d ica l
Andrzej Rajca,* Suchada Rajca, Shailesh R. Desai, and Victor W. Day
Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588
Received October 4, 1996X
Reaction of oxygen with three sterically hindered derivatives of Schlenk hydrocarbon diradical gives
predominantly cyclic peroxides with cross-conjugated π-systems. For mesitylene-based diradical
3, significantly different yields of the cyclic peroxide for the reactions in the solid state vs solution
were obtained. In the absence of crystallographic studies for Schlenk hydrocarbon and its
derivatives, X-ray crystallography for peroxide 1A-O2 supports the structural assignment for the
corresponding diradicals.
In tr od u ction
Reaction of Schlenk hydrocarbon with oxygen was re-
ported by Schlenk and Brauns in their original article,
but no products were isolated.1,9 For other related
diradicals, such as the parent m-xylylene and its alkyl-
substituted derivatives, no corresponding peroxides were
reported.8 Isolation and characterization of peroxides
corresponding to derivatives of Schlenk hydrocarbon
would not only provide additional evidence for the
structure of Schlenk hydrocarbon but also lead to unusual
peroxides.
Schlenk hydrocarbon is one of the most famous organic
diradicals.1 It provides an archetype for spin coupling
between “unpaired” electrons through the 1,3-phenylene
unitsthe most ubiquitous spin coupling unit in high-spin
di- and polyradicals.2 Triplet state for Schlenk hydrocar-
Now we report the characterization of the main prod-
ucts for reaction of diradicals 1-3 with O2 in the solid
state and in solution (Scheme 1).
Resu lts a n d Discu ssion
Three types of products for the reaction of diradicals
1-3 with O2 are considered. These types of products
involve formation of C-O bonds at the sites of the
expected highest spin density in the diradicals: i.e.,
triarylmethyl sites and/or ortho/para positions in the
center benzene ring.
Products of type A and B correspond to 1 + 1 conden-
sation of diradical with O2, leading to formation of a
5-membered ring. Product of type C corresponds to 2 +
2 condensation providing a macrocyclic ring; a similar
[4.4]metacyclophane ring system, where O-O linkers are
replaced with S-S linkers, was prepared by oxidation of
the corresponding dithiol under high-dilution conditions
(Ar ) H and R ) H).10,11
bon was detected by ESR spectroscopy many years ago.3
Recently, sterically hindered derivatives of Schlenk
hydrocarbon,1-5, which are stable at ambient tempera-
ture, were prepared.4,5 Their magnetic studies allowed
for assignment of the ground states.2d,4,5
Important structural evidence for reactive organic
radicals and diradicals is typically obtained from analysis
of their quenching products.6-8 Reaction with oxygen,
providing peroxides, is one of the most established.6,7
When solid diradicals 1-3 are exposed to air (method
1
I), H NMR spectra of the crude mixtures indicate the
corresponding peroxides 1A-O2, 2A-O2, and 3A-O2, re-
spectively, as predominant products. 1A-O2 and 3A-O2
are isolated in 50-70% yields, but 2A-O2 could not be
obtained in an analytically pure form (Scheme 1).
X Abstract published in Advance ACS Abstracts, September 1, 1997.
(1) Schlenk, W.; Brauns, M. Ber. Dtsch. Chem. Ges. 1915, 48, 661.
(2) (a) Buchachenko, A. L. Russ. Chem. Rev. 1990, 59, 307. (b)
Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88. (c) Iwamura, H. Adv.
Phys. Org. Chem. 1991, 26, 179. (d) Rajca, A. Chem. Rev. 1994, 94,
871. (e) Borden, W. T.; Iwamura, H.; Berson, J . A. Acc. Chem. Res.
1994, 27, 109.
(3) Kothe, G.; Denkel, K.-H.; Summermann, W. Angew. Chem., Int.
Ed. Engl. 1970, 9, 906. Lockhurst, G. R.; Pedulli, G. F. J . Chem. Soc.
B 1971, 329.
(8) Goodman, J . L.; Berson, J . A. J . Am. Chem. Soc. 1985, 107, 5409.
Gajewski, J . J .; Paul, G. C.; Chang, M. J .; Gortva, A. M. J . Am. Chem.
Soc. 1994, 116, 5150.
(9) Stark and co-workers reported their apparently unsuccessful
attempts to obtain exocyclic tetraphenyl-substituted m-xylylene and
its products of reaction with oxygen: Stark, O.; Garben, O. Ber. Dtsch.
Chem. Ges. 1913, 46, 659, 2252. Stark, O.; Garben, O.; Klebahn, L.
Ber. Dtsch. Chem. Ges. 1913, 46, 2542. Stark, O.; Klebahn, L. Ber.
Dtsch. Chem. Ges. 1914, 47, 125.
(4) (a) Rajca, A.; Utamapanya, S.; Xu, J . J . Am. Chem. Soc. 1991,
113, 9235. (b) Rajca, A.; Utamapanya, S. J . Org. Chem. 1992, 57, 1760.
(5) Veciana, J .; Rovira, C.; Crespo, M. I.; Armet, O.; Domingo, V.
M.; Palacio, F. J . Am. Chem. Soc. 1991, 113, 2552.
(6) (a) Gomberg, M. J . Am. Chem. Soc. 1900, 22, 757; Ber. Dtsch.
Chem. Ges. 1900, 33, 3150. (b) Glidewell, C.; Liles, D. C.; Walton, D.
J .; Sheldrick, G. M. Acta Crystallogr. Sect. B 1979, 35, 500. (c) J ang,
S.-H.; Gopalan, P.; J ackson, J . E.; Kahr, B. Angew. Chem., Int. Ed.
Engl. 1994, 33, 775. (d) McBride, J . M. Tetrahedron 1974, 30, 2009.
(7) Adam, W.; Grabowski, S.; Wilson, R. M. Acc. Chem. Res. 1990,
23, 165.
(10) (a) Sato, T.; Wakabayashi, M.; Hata, K.; Kainosho, M. Tetra-
hedron 1971, 27, 2737. (b) Dixon, K. R.; Mitchell, R. H. Can. J . Chem.
1983, 61, 1598. (c) Beveridge, K. A.; Bushnell, G. W.; Mitchell, R. H.
Can. J . Chem. 1983, 61, 1603. (d) Houk, J .; Whitesides, G. M.
Tetrahedron 1989, 45, 91.
(11) For a recent collection of reviews on cyclophanes, see: Weber,
E., Ed. Top. Curr. Chem. 1994, 172, 1-201.
S0022-3263(96)01879-8 CCC: $14.00 © 1997 American Chemical Society

Peroxides of Schlenk Hydrocarbon Diradical
J . Org. Chem., Vol. 62, No. 19, 1997 6525
Sch em e 1. Rea ction s of Dir a d ica ls 1-3 w ith Oxygen
The spectroscopic evidence for the predominant prod-
ucts, consistent with structure type A or B but excluding
type C, is as follows. The 1H and 13C NMR spectra
indicate four nonequivalent 4-tert-butylphenyl groups,
consistent with cis/trans isomerism with respect to the
exocyclic CdC bond and the presence of an stereogenic
center. Two nonequivalent i-Pr groups, two nonequiva-
lent Me groups, and three nonequivalent Me groups are
found in 1A-O2, 2A-O2, and 3A-O2, respectively; further-
more, the methyls of the i-Pr groups in 1A-O2 are
diastereotopic. The NMR spectral patterns (chemical
shifts) and the expected intense (M + H)+ clusters in
FABMS are consistent with all three peroxides having
similar structures.
The spectroscopic evidence for the predominant prod-
ucts, indicating that type A is the actual structure, is as
1
follows. In H NMR spectra, allylic coupling (J ) 1 Hz)
is observed only for one Me group in 2A-O2 and 3A-O2,
as indicated by the resolved multiplet splittings, and in
3A-O2, a COSY cross-peak; in 1A-O2, the allylic coupling
gives rise to the ∼0.5 Hz splitting of the vinylic resonance
and broadening of the corresponding septet of the i-Pr
group. In 13C NMR/DEPT spectra for 1A-O2 and 3A-O2,
the two 13C resonances in the 80-100 ppm region, which
are assigned to the carbons in the peroxide moiety (C-
O-O-C), are both detected as quarternary. In FABMS,
the most intense clusters in the m/ z 200-1700 range are
compatible with the following fragment ions: (M-C3H7)+
for 1A-O2 and (M-O2)+ for both 2A-O2 and 3A-O2.
F igu r e 1. Molecular structure and predominant conformation
observed for perioxide 1A-O2 in the solid state. The molecule
belongs to point group C1.
Oxidations of diradical 1 in THF at 0 °C, using either
pure diradical (method II, Experimental Section) or
diradical prepared in situ from dianion (method III,
Experimental Section), give 1A-O2 as a major product.
The relative molar amounts of 1A-O2, tentative 1B-O2,
and 1C-O2 are 100:(3-6):3; also, there is a small admix-
ture of unidentified products.19 Notably, 1C-O2 and the
other product, which might tentatively be assigned to 1B-
O2, are formed in significantly greater and smaller yields,
respectively, compared to the oxidation in the solid state.
Furthermore, oxidations in THF at low temperature (-78
°C) give very complex mixtures, in which only small
amounts of 1A-O2 are detectable.
Spectroscopic evidence for 1C-O2 is as follows. FABMS
gives the expected (M + H)+ cluster centered at m/ z )
1499 and somewhat less intense fragment ion at m/ z )
1466, compatible with the loss of O2. Notably, the most
intense cluster in the m/ z ) 200-1700 range (ca. 30
times more intense than (M + H)+) is at m/ z ) 733.5,
compatible with the loss of O2 and breakage of the other
O-O bond. The cluster at m/ z ) 705.5, which is
dominant in FABMS for 1A-O2, has a negligible intensity.
1H and 13C NMR spectra for 1C-O2 at ambient temper-
ature (∼20 °C) contain many broadened resonances.
Only one type of i-Pr group (with diastereotopic methyls)
and two types of t-Bu groups are found. Relative
integration of the aromatic protons is 1:6:2:3:4:2. The
peak with integration of “3” consists of two singlets with
different line widths: sharp (integration of “1”) and broad
(integration of “2”). Two sharp singlets, which have
relative integration of “1”, have chemical shifts of 9.67
and 6.98 ppm. The 1H NMR spectrum at 10 °C is
considerably less broadened. In 1H NMR spectra at
higher temperature (58 °C), all resonances, except for the
Structure of 1A-O2 is confirmed by X-ray crystal-
lography. The C-O-O-C dihedral angle of 26.8° is
comparable to the angle of 20° found in a derivative of
1,2-dioxolan.12 The C-O bond distances (1.47 and 1.46
Å) are essentially identical to those found in triarylm-
ethyl peroxides;6 the apparent O-O bond distance (1.43
Å) is a few hundredths of an Ångstrom shorter than
expected for triarylmethyl peroxide, presumably due to
disorder in the crystal of 1A-O2 (Figure 1).
1
Analysis of H NMR spectra for the crude mixtures,
obtained by oxidation of solid diradical 1 (method I),
reveals three products, with the 100:30:1 relative
amounts: (1) a major product 1A-O2, discussed above,
(2) a minor product, and (3) a trace amount of 1C-O2.19
The minor product could not be isolated because of its
1
decomposition on silica gel; its H NMR spectral features,
such as two nonequivalent i-Pr groups with diaste-
reotopic methyls, four nonequivalent t-Bu groups, and
two 1-proton resonances in the 5.5-6.5 ppm region, are
compatible with structure 1B-O2, though this evidence
does not establish the structure. (These NMR features
are analogous to those found in 1A-O2.)
(12) Hitchcock, P. B.; Beheshti, I. J . Chem. Soc., Perkin Trans. 2
1979, 126.

6526 J . Org. Chem., Vol. 62, No. 19, 1997
Rajca et al.
two sharp singlets, assigned to the protons of i-Pr-
substituted benzene rings in the aromatic region, and the
CH protons of the i-Pr groups, show even greater
broadening.19 Apparent diastereotopicity is reduced,
compared to the spectra at ambient temperature, e.g.,
methyls of the t-Bu groups and i-Pr groups appear as
broad singlets at 1.3 and 0.5 ppm. Low-temperature
structure is compatible with two nonequivalent sets of
four 4-tert-butylphenyls (e.g., “equatorial” and “axial”);
and in the solid state are significantly different.17 Whether
the different conformations of 3 in solution and in the
solid state give rise to significantly different strengths
of spin coupling (energy gaps between the lowest singlet
and triplet states) is an interesting question from the
viewpoint of organic magnetism.18
Con clu sion
1
Sterically hindered derivatives of Schlenk hydrocarbon,
diradicals 1-3, react with oxygen to form cyclic peroxides
1A-O2, 2A-O2, and 3A-O2 as major products. In the
absence of crystallographic studies for Schlenk hydro-
carbon and its derivatives, X-ray crystallography for
peroxide 1A-O2 supports structural assignment for the
corresponding diradicals.
one of the sets shows slow rotation on the H NMR time
scale, presumably due to steric encumbrance by the i-Pr
groups. At high temperature, both rotation of 4-tert-
butylphenyls and “equatorial”/“axial” exchange lead to
line broadening. The extreme downfield chemical shift
(9.67 ppm) for one of the protons at the i-Pr-substituted
benzene ring may be associated with a particular orien-
tation of the i-Pr-substituted benzene ring, analogous to
that observed in a [3.3]metacyclophane-based tube-shape
molecule;13 however, additional contributions to shield-
ing/deshielding from both peroxide moieties and benzene
rings of 4-tert-butylphenyls in many possible conforma-
tions of 1C-O2 may be significant.10bc,14
Exp er im en ta l Section .
Gen er a l P r oced u r es. Ether and tetrahydrofuran (THF)
were distilled from sodium/benzophenone in a nitrogen atmo-
sphere. Major chemicals were obtained from Aldrich. Vacuum
line and glovebox techniques are described elsewhere.16
NMR spectra were obtained using Omega spectrometers (1H,
500 and 300 MHz) in either CDCl3 or Me2SO-d6; the chemical
shift references were as follows: 1H, TMS, 0.000 ppm (CDCl3),
and Me2SO-d5, 2.490 ppm (Me2SO-d6); 13C, CDCl3, 77.0 ppm.
J values are given in hertz. For those selected NMR spectra,
where the spectral resolution was digitally improved, the
exponent values for exponential multiplication (EM) and
Gaussian broadening (GB) are given. IR spectra were obtained
using the FT instrument Analect RFX-30 operating in the ATR
mode and equipped with a ZnSe ATR plate parallelogram (45°,
Wilmad); 2-cm-1 resolution was employed.
1C-O2 is recovered unchanged from treatment with
LiAlH4 in THF at ambient temperature; it has been
reported that reduction of analogous, but less sterically
hindered, peroxide linkage in Ph3COOCPh3 required
reflux in benzene/ether.15 Reduction of 1A-O2 with
LiAlH4 in ether at ambient temperature gives alcohol
1
1-OH; its H NMR spectra are identical to those reported
for 1-OH, which was prepared via addition of aryllithium
to ketone.16
Elemental analyses were carried out by Dr. G. M. Dabkowski,
Director-Microlytics, P.O. Box 199, S. Deerfield, MA 01373.
X-r a y Cr ysta llogr a p h y 1A-O2. A single crystal of peroxide
1A-O2 was obtained as described in the synthetic procedure
for reaction of diradicals with O2. Single crystals of C54H68O2
(1A-O2) are, at 20 ( 1 °C, triclinic, space group P1h-C1 (No. 2)
i
with a ) 11.293(3) Å, b ) 11.705(3) Å, c ) 19.608(5) Å, R )
108.41(2)° , â ) 96.17(2)°, γ ) 103.70(2)°, V ) 2342(1) Å3, and
Z ) 2 (dcalcd ) 1.062 g·cm-3; µa(Mo KR) ) 0.06 mm-1). A total
of 5392 independent reflections having 2Θ(MoKR) < 43.0° (the
equivalent of 0.5 limiting Cu KR spheres) were collected on a
computer-controlled Nicolet autodiffractometer using full (0.90°
wide) ω scans and graphite-monochromated Mo KR radiation.
The structure was solved using direct methods techniques with
the Siemens SHELXTL-PC software package as modified at
Crystalytics Co. The resulting structural parameters have
been refined to convergence (R1 (unweighted, based on F) )
When pure diradical 2 in THF at 0 °C is exposed to O2
(method II), 2A-O2 is the predominant product, analo-
gously to the result obtained for diradical 1. However,
when pure diradical 3 in THF at 0 °C is exposed to O2
(method II), 3A-O2 is formed and isolated only in small
amount. 1H NMR spectra of crude mixtures consist of
mostly broad unresolved peaks. What is the origin of
such drastically different reactivity for 3 in solution vs
the solid state? ESR studies suggested that among
diradicals 1-3, diradical 3 is the most out-of-plane
twisted in frozen 2-MeTHF/THF solutions.4 Furthermore,
in the series of related dianions, charge density in the
dianion corresponding to 3 was delocalized to the outer
benzene ring to the greatest degree in the series.4a The
implication is that nonplanarity of the π-conjugated
system of 3 may lead to relatively small spin densities
in the center benzene ring, and consequently, formation
of cyclic peroxides of types A and B may be less favorable.
Thus, it is plausible that conformations of 3 in solution
(17) Two propeller diastereomers could be isolated for the exceed-
ingly hindered perchlorocarbon derivative of the Schlenk hydrocarbon
(ref 5).
(18) Whether spin multiplicity will have a significant direct effect
on reactivity of 3 toward oxygen is not clear. Recent reports indicate
that the rates for the reaction with oxygen are only slightly lower for
singlet state diradicals, compared to many triplet state diradicals (with
different structures), e.g., Reynolds, J . H.; Berson, J . A.; Kumashiro,
K. K.; Duchamp, J . C.; Zilm, K. W.; Scaiano, J . C.; Berinstain, A. B.;
Rubello, A.; Vogel, P. J . Am. Chem. Soc. 1993, 115, 8073. Heath, R.
B.; Bush, L. C.; Feng, X.-W.; Berson, J . A.; Scaiano, J . C.; Berinstain,
A. B. J . Phys. Chem. 1993, 97, 13355. Furthermore, oxygen and other
paramagnetic molecules are known to catalyze intersystem crossing,
e.g., Klessinger, M.; Michl, J . Excited States and Photochemistry of
Organic Molecules; VCH: New York, 1995; pp. 254-256. In organo-
metallics, intersystem crossings are expected to be much faster
compared to typical organics, but still the notion of significant effect
of spin state on reactivity reemerges only to be disproved: Detrich,
J . L.; Reinaud, O. M.; Rheingold, A. L.; Theopold, K. H. J . Am. Chem.
Soc. 1995, 117, 11745 and references therein.
(13) Vo¨gtle, F.; Schro¨der, A.; Karbach, D. Angew. Chem., Int. Ed.
Engl. 1991, 30, 575.
(14) For an overview of calculations of NMR chemical shifts, with
proteins as examples, see: Augspurger, J . D.; Dykstra, C. E. Prog.
NMR Spectrosc. 1995, 30, 1. Ring current contribution: Haigh, C. W.;
Mallion, R. B. Prog. NMR Spectrosc. 1980, 13, 303. Haigh, C. W.;
Mallion, R. B. Org. Magn. Reson. 1972, 4, 203.
(19) See Supporting Information. The author has deposited atomic
coordinates for 1A-O2 with the Cambridge Crystallographic Data
Centre. The coordinates can be obtained, on request, from the Director,
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, U.K.
(15) LiAlH4 + (Ph3CO)2: Mustafa, A. J . Chem. Soc. 1952, 2435.
(16) Rajca, S.; Rajca, A. J . Am. Chem. Soc. 1995, 117, 9172.

Peroxides of Schlenk Hydrocarbon Diradical
J . Org. Chem., Vol. 62, No. 19, 1997 6527
0.051 for 3159 independent reflections having 2Θ(Mo KR) <
43.0° and I > 3σ(I) using counter-weighted full-matrix least-
squares techniques and a structural model which incorporated
anisotropic thermal parameters for all ordered non-hydrogen
atoms and isotropic thermal parameters for all included
hydrogen atoms. Three of the four t-Bu and one of the two
i-Pr groups are disordered with two preferred orientations for
the methyl groups of each in the lattice. The major orientation
for each is specified with nonprimed subscripts to the atomic
symbols of the methyl carbons and the minor orientation with
primed subscripts to the atomic symbols of the methyl carbons.
from ether/methanol to give 26.0 mg of light yellow crystals,
mp 150 °C dec; a single crystal was selected for the structure
determination by X-ray crystallography.
The more polar fraction, peroxide 1A-O2: FABMS (3-NBA)
cluster m/ z (% RA in the m/z 200-1700 range) at (M + H)+
748.5 (10), 749.5 (60), 750.5 (40), 751.5 (10); (M-C3H7)+ 704.5
(10), 705.5 (100), 706.5 (70), 707.5 (30); other weak clusters
1
733.5 (15), 716.5 (25), 689.4 (40); H NMR (500 MHz, CDCl3)
δ (1H-1H COSY cross-peak in the aromatic region) 7.308 (d, J
) 9, 2 H, 7.080), 7.287 (d, J ≈ 9, 2 H, 7.14), 7.26 (d, J . 9,
overlapped with solvent peak, 2 H, 7.177), 7.194 (d, J ) 9, 2
H, 7.014), 7.177 (d, J ) 9, 2 H, 7.26), 7.14 (bd, J ≈ 8, 2 H,
7.287), 7.080 (d, J ) 9, 2 H, 7.308), 7.014 (d, J ) 9, 2 H, 7.194),
6.233 (s, 1 H), 5.883 (d, J ≈ 0.5, 1 H, EM ) -1.5, GB ) +0.6),
2.235 (sep, J ) 7, 1 H), 2.149 (sep, J ) 7, 1 H), 1.325 (s, 9 H),
1.305 (s, 9 H), 1.297 (s, 9 H), 1.272 (s, 9 H), 1.045 (d, J ) 7, 3
H), 0.879 (d, J ) 7, 3 H), 0.851 (d, J ) 7, 3 H), 0.822 (d, J )
7, 3 H); 1H NMR (500 MHz, Me2SO-d6): δ 7.419 (d, J ≈ 9, 2
H), 7.368 (d, J ≈ 9, 2 H), 7.326 (d, J ≈ 9, 2 H), 7.329 (d, J ≈
9, 2 H), 7.150 (d, J ≈ 9, 2 H), 7.104 (d, J ≈ 9, 2 H), 7.034 (d,
J ≈ 9, 2 H), 6.861 (d, J ≈ 9, 2 H), 6.029 (s, 1 H), 5.824 (s, 1 H),
2.182 (sep, J ) 7, 1 H), 2.125 (sep (slightly broadened), J ) 7,
1 H), 1.278 (s, 9 H), 1.254 (s, 9 H), 1.244 (s, 9 H), 1.205 (s, 9
H), 0.972 (d, J ) 7, 3 H), 0.832 (bd, J ≈ 7, 3 H), 0.814 (bd, J
≈ 7, 3 H), 0.770 (bd, J ) 7, 3 H); 13C{1H} DEPT (135°) NMR
(125 MHz, CDCl3); (aromatic/olefinic quaternary region)
expected 12 resonances, found 11 resonances at 150.4 (q), 149.9
(q), 149.8 (q), 149.6 (q), 149.4 (q), 142.6 (q), 141.0 (q), 140.3
(q), 139.8 (q), 137.9 (q), 131.7 (q), (aromatic/olefinic nonqua-
ternary region) expected 10 resonances found 10 resonances
at 129.1, 128.9, 126.9, 126.6, 125.2, 125.0, 124.94, 124.85,
124.7, 124.6, 120.0, (aliphatic region) 90.7 (q), 89.0 (q), 34.8,
34.55 (q), 34.52 (q), 34.48 (q), 34.37 (q), 31.33, 31.28, 29.5, 24.7,
21.1, 17.3, 17.2; IR (cm-1) 2962 (CH), 829 (OO). Anal. Calcd
for C54H68O2: C, 86.58; H, 9.15. Found: C, 86.67; H, 9.39.
The less polar fraction, peroxide 1C-O2: FABMS (3-NBA)
cluster m/ z (% RA in the m/z 200-1700 range) at (M + H)+
1497.0 (40/30), 1498.0 (95/30), 1499.0 (100/30), 1500.0 (70/30),
1501.0 (40/30), calcd for C108H137O4 1498.05 (80), 1499.06 (100),
1500.06 (60), 1501.06 (25) (M - O2)+ 1465.0 (70/30), 1466.0
(80/30), 1467.0 (60/30), 1468.0 (35/30), (M - C3H7)+ 1455.0
(<20/30), (M/2)+ 747.5 (30), 748.5 (40), 749.5 (70), 750.5 (35),
((M - O2)/2)+ 731.5 (35), 732.5 (95), 733.5 (100), 734.5 (45),
((M/2) - C3H7)+ 705.4 (<15); 1H NMR (500 MHz, CDCl3) δ
9.672 (s, 2 H), 7.119 (d, J ≈ 8, 12 H), 7.061 (bd, J ≈ 8, 4 H),
6.978 (s, bs, 6 H), 6.833 (d, J ≈ 8, 8 H), 6.271 (bd, J ≈ 7, 4 H),
2.394 (sep, J ) 7, 4 H), 1.374 (s, 36 H), 1.279 (s, 36 H), 0.516
(bd, J ≈ 6, 12 H), 0.455 (bd, J ≈ 6, 12 H), no detectable
exchange with D2O; IR (cm-1) 2961 (CH), 827 (OO).
Disordered methyl carbons C12′, C13′, C14′, C32′, C33′, C34′, C42′
,
C43′, C44′, C52′, and C53′ were included in the structural model
with isotropic thermal parameters; the remaining disordered
methyl carbons were included in the structural model with
anisotropic thermal parameters.
Before determining the extent of disorder for three of the
four t-Bu and one of the two i-Pr moieties, the 16 methyl
groups (C12, C13, C14, C22, C23, C24, C32, C33, C34, C42, C43, C44
,
C52, C53, C62, C63, and their hydrogens) were refined as rigid
rotors with sp3-hybridized geometry and a C-H bond length
of 0.96 Å. The initial orientation of each methyl group was
determined from difference Fourier positions for the hydrogen
atoms. The final orientation of each methyl group was deter-
mined by three rotational parameters. The refined positions
for these rigid-rotor methyl groups gave C-C-H bond angles
which ranged from 95° to 123°. At that point, a difference
Fourier revealed alternate (minor) positions for disordered
methyl carbons. These alternate carbon sites were included
in the structural model with variable occupancy factors which
were normalized over the major and minor sites. Hydrogen
atoms were not included in the structural model for minor sites
of these disordered methyl groups, and the orientations of the
methyl groups for the major sites were no longer allowed to
vary in refinement cycles. The following occupancy factors
were used for methyl groups in the structural model to describe
this disorder: C12-C14 and their hydrogens, 0.80; C32-C34 and
their hydrogens, 0.92; C42-C44 and their hydrogens, 0.88; C52
-
C53 and their hydrogens, 0.81; C12′-C14′, 0.20; C32′-C34′, 0.08;
C42′-C44′, 0.12; C52′-C53′, 0.19. The remaining hydrogen atoms
were fixed at idealized sp3- or sp2-hybridized positions using
a C-H bond length of 0.96 Å and their isotropic thermal
parameters were fixed at values 1.2 times the equivalent
isotropic thermal parameter of the carbon atoms to which they
are covalently bonded.
Rea ction of d ir a d ica ls 1-3 w ith O2. In all general
procedures, described below, crude products were analyzed by
1H NMR spectroscopy and TLC; typically, purification by PTLC
and further characterization followed. Preparation of diradicals
1-3, corresponding dianions, and diethers was previously
described.4a Solid diradicals 1-3 were stored in a glovebox
refrigerator (-30 °C) prior to use.
Meth od I: Solid diradical was loaded into a vial in a
glovebox and, then, removed from the glovebox. The diradical
was exposed to air for ∼1 day. The color of the solid gradually
changed from green to yellow.
Meth od II: Solid diradical was loaded into a rubber
septum-capped vial or a Schlenk vessel in a glovebox and, then,
attached to the nitrogen/vacuum line outside the glovebox.
THF was added, and the resultant solution was exposed to
dry oxygen at 0 °C. Removal of the solvent in under vacuo
gave a crude reaction mixture.
Meth od III: A solution of diradical, which was prepared
in situ from the corresponding dianion in THF,4a was exposed
to dry oxygen at 0 °C. Usual aqueous workup gave a crude
reaction mixture.
P er oxid e 2A-O2. Methods I and II gave similar results.
Meth od II: Diradical 2 (5.3 mg) in THF (0.5 mL) was
exposed to O2 at 0 °C; color of the reaction mixture changed
from green to yellow. PTLC (silica gel deactivated with 2%
Et3N, 1% ether in hexane) gave 3.4 mg of the product (80+%
peroxide 2A-O2). While peroxide 2A-O2 was reproducibly
obtained in crude mixtures with good yield, its purification
was not well reproducible (some PTLC runs gave a fraction of
the expected amount); peroxide 2A-O2 readily decomposed on
silica gel (even when pretreated with Et3N or NaOH): FABMS
(ONPOE) cluster m/ z (% RA in the m/ z 200-2000 range) at
M+/(M + H)+ 692.5 (10), 693.5 (60), 694.5 (30), 695.5 (<10),
(M - CH3)+ 676.5 (15), 677.5 (45), 678.5 (25), 679.5 (<10), (M
- O2)+ 659.5 (<10), 660.5 (100), 661.5 (65), 662.5 (20); 1H NMR
(300 MHz, CDCl3) δ 7.35-7.20 (m, 10 H), 7.134 (d, J ) 9, 2
H), 7.054 (bd, J ≈ 9, 4 H), 6.269 (s, 1 H), 5.877 (d, J ) 1, 1 H),
1.419 (bs, 3 H), 1.385 (bd, J ≈ 1, 3 H), 1.319 (s, 9 H), 1.306 (s,
18 H), 1.293 (s, 9 H).
P er oxid e 3A-O2. Meth od I: Solid diradical 3 (82.5 mg,
0.122 mmol) was exposed to air for 2 days. Part of the crude
product (80.0 mg) was purified by PTLC (silica gel deactivated
with 1% NaOH in 93% EtOH/H2O followed by activation at
∼160 °C for 2 days; elution with 5% ether in hexane) to give
52.0 mg (65%) of peroxide. A sample of analytical purity was
obtained by recrystallization from ether/MeOH (white solid):
FABMS (3-NBA) cluster m/ z (% RA in the m/ z 200-1700
range) at M+/(M + H)+ 705.4 (20), 706.4 (20), 707.4 (45), 708.4
P er oxid es 1A-O2 a n d 1C-O2. All three methods (I-III)
gave 1A-O2 as a predominant product.
Meth od III: Dianion, prepared from the diether (compound
3-(OMe)2 in ref 4a; 156.7 mg) and Li wire (multimolar excess)
in THF (2.5 mL), was oxidized with iodine (51.3 mg).4a
Exposure of the reaction mixture to dry oxygen gave 157.3 mg
of crude product. PTLC (silica gel deactivated with 2% Et3N,
1% ether in hexane) of a 70.0 mg portion of the crude mixture
gave two fractions: more polar (46.0 mg, 69%) and less polar
(3.3 mg, 5%). The more polar fraction was recrystallized twice

6528 J . Org. Chem., Vol. 62, No. 19, 1997
Rajca et al.
(25), 709.4 (5); (M - CH3)+ 689.4 (15), 690.4 (25), 691.4 (40),
692.4 (20), 693.4 (10), (M - O2)+ 673.4 (15), 674.4 (100), 675.4
(70), 676.4 (25); 1H NMR (500 MHz, EM ) -1.0, GB ) +0.65,
CDCl3) δ (1H-1H COSY cross-peak) 7.401 (d, J ) 9, 2 H, 7.329),
7.329 (d, J ) 9, 2 H, 7.401), 7.315 (d, J ) 9, 2 H, 7.249), 7.314
(d, J ) 9, 2 H, 7.178), 7.249 (d, J ) 9, 2 H, 7.315), 7.200 (d, J
) 9, 2 H, 7.151), 7.178 (bd, J ≈ 9, 2 H, 7.314), 7.151 (d, J ) 9,
2 H, 7.200), 5.967 (d, J ) 1, 1 H, 1.365), 1.542 (bs, 3 H), 1.371
(s, 9 H), 1.365 (d, J ) 1, 3 H, 5.967), 1.298 (s, 9 H), 1.296 (s,
9 H), 1.246 (s, 9 H), 0.889 (s, 3 H). 1H NMR (500 MHz Me2SO-
d6) δ 7.463 (d, J ) 9, 2 H), 7.364 (d, J ) 9, 4 H), 7.272 (d, J )
9, 2 H), 7.215 (d, J ) 9, 2 H), 7.198 (d, J ) 9, 4 H), 7.065 (d,
J ) 9, 2 H), 5.955 (d, J ) 1, 1 H), 1.518 (s, 3 H), 1.317 (s, 9 H),
1.301 (d, J ) 1, 3 H), 1.254 (s, 9 H), 1.239 (s, 9 H), 1.218 (s, 9
H), 0.794 (s, 3 H); 13C{1H)} DEPT (135°) NMR (125 MHz,
CDCl3); (aromatic/olefinic quaternary region) expected 13
resonances, found 12 resonances at 151.5 (q), 151.0 (q), 150.4
(q), 150.2 (q), 149.9 (q), 140.8 (q), 140.24 (q), 140.16 (q), 139.7
(q), 138. 4 (q), 137.8 (q), 136.2 (q), (aromatic/olefinic nonqua-
ternary region), expected 9 resonances, found 9 resonances at
129.2, 128.5, 128.4, 128.3, 128.0, 125.1, 124.9, 124.8, 124.4,
(aliphatic region) 91.8 (q), 84.9 (q), 34.55 (q), 34.48 (q), 34.39
(q), 31.35, 31.33, 31.37, 22.9, 22.0, 18.0. IR (cm-1) 2961 (CH),
824 (OO). Anal. Calcd for C51H62O2: C, 86.64; H, 8.84.
Found: C, 86.55; H, 8.75.
usual aqueous workup, crude product (9 mg) was obtained.
1H NMR spectrum indicated 1-OH as predominant product
(>80%). PTLC (silica gel deactivated with 2% Et3N, 3% ether
in hexane) gave pure product as a white solid (5 mg): FABMS
(ONPOE) cluster m/ z (% RA in the m/z 500-1000 range) at
(M - OH)+ 717.5 (100), 718.5 (60), 719.5 (20), calcd for C54H69
717.5 (100), 718.5 (62), 719.5 (19); 1H NMR (500 MHz, CDCl3)
δ 7.23 (d, J ) 8, 4 H), 7.22 (s, 1 H), 7.17 (d, J ) 8, 4 H), 7.03
(d, J ) 8, 4 H), 6.81 (d, J ) 8, 4 H), 6.43 (s, 1 H), 5.61 (s, 1 H),
3.28 (sep, J ) 7, 1 H), 3.16 (sep, J ) 7, 1 H), 2.79 (s, 1 H), 1.29
(s, 18 H), 1.28 (s, 18 H), 1.16 (d, J ) 7, 6 H), 0.89 (d, J ) 7, 6
H).
Ack n ow led gm en t. We gratefully acknowledge the
National Science Foundation, Divisions of Materials
Research and Chemistry, for support of this research
(DMR-9204826 and CHE-9510096). We thank Crysta-
lytics Co. (Lincoln, NE) for the use of its equipment and
supplies for the structure determination of compound
1A-O2. We also thank J irawat Wongsriratanakul for
help with the synthetic studies. Finally, we thank Dr.
Ronald Cerny for mass spectral determinations at the
Nebraska Center for Mass Spectrometry.
Meth od s II a n d III: 1H NMR and TLC analyses of the
crude mixtures revealed a multitude of products and only a
low yield of peroxide 3A-O2. Using method II (21.0 mg of 3 in
1 mL of THF), 1.6 mg (7%) of 3A-O2 was isolated. In a parallel
reaction, using method I (20.4 mg of solid 3), 11.5 mg (54%) of
3A-O2 was isolated.
Su p p or tin g In for m a tion Ava ila ble: 1H NMR spectra for
selected crude reaction mixtures (17 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
1-OH (Rea ction of 1A-O2 w ith LiAlH4). 1A-O2 (9 mg) was
stirred with LiAlH4 (10 mg) in ether (2 mL) for 6 h. Following
J O961879X