1-Boraadamantane blows its top, sometimes. The mono-and polyhomologation of 1-boraadamantane

Article abstract of DOI:10.1021/ol0159726

(equation presented) 1-Boraadamantane·THF (3) reacts with 1 equiv of dimethylsulfoxonium methylide (4) to afford a monohomologated product. The polyhomologation of 1-boraadamantane·THF by ylide 4 followed by oxidative cleavage generates star polymethylene

Full text of DOI:10.1021/ol0159726

ORGANIC
LETTERS
2001
Vol. 3, No. 20
3063-3066
1-Boraadamantane Blows Its Top,
Sometimes. The Mono- and
Polyhomologation of 1-Boraadamantane
Carl E. Wagner and Kenneth J. Shea*
Department of Chemistry, UniVersity of California IrVine, IrVine, California 92697
kjshea@uci.edu
Received April 10, 2001 (Revised Manuscript Received August 25, 2001)
ABSTRACT
1-Boraadamantane‚THF (3) reacts with 1 equiv of dimethylsulfoxonium methylide (4) to afford a monohomologated product. The polyhomologation
of 1-boraadamantane‚THF by ylide 4 followed by oxidative cleavage generates star polymethylene polymers incorporating a cyclohexane core.
However, only one-third of the initiators lead to product formation, resulting in an observed degree of polymerization three times higher than
expected. The polyhomologation of 3 was found to contain branch points after the fourth and fifth methylene insertions. At the branch points,
the propagating species either terminate in tricyclic trialkylborane cages with collapsed, pyramidal inverted boron centers that are unreactive
toward ylide or they continue in uninterrupted polymerization and eventually result in the formation of giant “tube-like” structures such as 5.
Multiple homologations (polyhomologation) of cyclic and
bicyclic organoboranes provide a novel route to oligomeric
and polymeric boracyclanes and carbocyclics.1 Applying the
polyhomologation reaction2 to polycyclic organoboranes
offers an opportunity for the construction of “tube-like”
organoboranes which may be elaborated to hydrocarbon star
polymer architectures. 1-Boraadamantane‚THF (3) was se-
lected for study as a tricyclic alkylborane initiator to access
a three-armed star polymethylene polymer, with the arms
radiating from a cyclohexane core. In this strategy, trialkyl-
borane 3 would serve as an initiator while dimethylsulfoxo-
nium methylide (4) would serve as a methylene monomer
source. One equivalent of DMSO would be expelled for each
equivalent of ylide 4 consumed. The “tube-like” macropoly-
cyclic organoborane (5) would be formed as the polymeric
product. The living nature of the polyhomologation reaction
would allow control of the length of each wall of the “tube-
like” 5.3 Subsequent oxidation of 5 would give a regular star
polymethylene polymer (6) (Scheme 1).
Thus, 1-boraadamantane‚THF (3) was synthesized by the
Scheme 1
(1) (a) Shea, K. J.; Lee, S. Y.; Busch, B. B. J. Org. Chem. 1998, 63,
5746. (b) Shea, K. J.; Busch, B. B.; Paz, M. Angew. Chem., Int. Ed. 1998,
37, 1391.
(2) Shea, K. J.; Walker, J. W.; Zhu, H.; Paz, M.; Greaves, J. J. Am.
Chem. Soc. 1997, 119, 9049.
10.1021/ol0159726 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/13/2001

hydroboration of 3-methoxy-7-(methoxymethyl)-3-borabicyclo-
[3.3.1]non-6-ene (2). This compound was prepared by the
reaction of triallylborane (1) with methyl propargyl ether
followed by methanolysis, according to the procedure of
Mikhailov and co-workers (Scheme 2).4 Triallylborane (1)
Monohomologation was achieved by slowly adding 1
equiv of ylide 4 to a solution of 1-boraadamantane‚THF (3)
in toluene at -78 °C. The resulting white crystalline
precipitate was filtered cold, washed with hexanes, and dried
under vaccuum (0.01 Torr, -60 °C). Upon warming to room
temperature under vacuum, the white crystalline solid rapidly
melted, exothermically, forming a colorless liquid. The liquid
crystallized upon cooling. These observations are consistent
with the initial formation and isolation of a 1-boraadamantane‚
ylide complex (8) that subsequently undergoes a [1,2]-
migration upon warming to room temperature. The expelled
DMSO is captured to form 1-borahomoadamantane‚DMSO
(9) (Scheme 4).
Scheme 2
Scheme 4
was synthesized in 60% yield according to the method of
Zakharkin and co-workers.5 Complex 3 was purified by
vacuum sublimation, and its structure was confirmed by
X-ray crystallography.
The polyhomologation reaction involves a two-step process
consisting of addition of the nucleophilic ylide 4 to form an
“ate” complex followed by a [1,2]-migration. Related migra-
tions of bicyclic organoboranes proceed via an antiperiplanar
transition state.6 To ensure the compatibility of the 1-bo-
raadamantane core with addition-migration, we first exam-
ined its oxidation. Compound 3 is readily oxidized by
trimethylene N-oxide dihydrate (TAO),7 giving the triol, cis-
1,3,5-tris(hydroxy-methyl)cyclohexane (7), in 76% yield
(Scheme 3).
Complex 9 sublimes (0.01 mmHg, 75 °C) and was isolated
in 65% yield. The structure of 9 was confirmed by X-ray
crystallography.
The monohomologation of 1-boraadamantane was previ-
ously achieved with trimethylamine methylide. The primary
product, 1-borahomoadamantane‚trimethylamine, proved dif-
ficult to separate from the nonhomologated 1-boraadamantane‚
trimethylamine formed as a byproduct of the reaction.8
Monohomologation of 1-boraadamantane by triphenylphos-
phine methylide could not be accomplished because of
thermal decomposition prior to migratory insertion.9 Thus,
the successful monohomologation of 1-boraadamantane
depends not only on the facility with which the leaving group
departs but also on the degree to which the ylide remains
complexed to boron under a given set of conditions for [1,2]-
migration. The monohomologation of 1-boraadamantane by
dimethylsulfoxonium methylide (4) is a rather unique
example, because it readily transpires from the solid state,
but most importantly it demonstrates the controlled homolo-
gation of a trialkylborane10 by ylide 4.
Scheme 3
1
The H NMR spectrum of triol 7 in deuterium oxide
includes a doublet at 3.36 ppm (J ) 6.3 Hz), assigned to
the hydroxymethylene protons of 7. Also, the equatorial and
axial methylene protons of the cyclohexane ring appear as a
doublet at 1.67 ppm (J ) 12.0 Hz) and a quartet at 0.50
ppm (J ) 12.3 Hz), respectively.
Compound 9 is readily oxidized with TAO, providing cis-
1-hydroxyethyl-3,5-bis(hydroxymethyl)cyclohexane (10) in
87% yield after chromatography (Scheme 5).
(3) Shea, K. J. Chem. Eur. J. 2000, 6, 1113.
(4) (a) Mikhailov, B. M.; Baryshnikova, T. K.; Kiselev, V. G.; Shashkov,
A. S. IzV. Akad. Nauk SSSR, Ser. Khim. 1979, 28, 2544. (b) Mikhailov, B.
M.; Baryshnikova, T. K.; IzV. Akad. Nauk SSSR, Ser. Khim. Nauk, 1979,
28, 2541.
(5) Zakharkin, L. I.; Stanko, V. I. IzV. Akad. Nauk SSSR, Otd. Khim.
Nauk, 1960, 1896.
(8) Mikhailov, B. M.; Govorov, N. N.; Angelyuk, Ya. A.; Kiselev, V.
G.; Struchkova, M. I. IzV. Akad. Nauk SSSR, Ser. Khim., 1980, 29, 1621.
(9) Mikhailov, B. M.; Gurskii, M. E.; Pershin, D. G. J. Organomet.
Chem., 1984, 260, 17.
(6) Soderquist, J. A.; Najafi, M. R. J. Org. Chem. 1986, 51, 1330.
(7) Kabalka, G. W.; Hedgecock, H. C., Jr. J. Org. Chem. 1975, 40, 1776.
(10) (a) Tufariello, J. J.; Lee, L. T. C. J. Am. Chem. Soc. 1966, 88, 4757.
(b) Goddard, J.; Le Gall, T.; Mioskowski, C. Org. Lett. 2000, 2, 1455.
3064
Org. Lett., Vol. 3, No. 20, 2001

Scheme 5
Table 1. Molecular Weight Data for Star Polymers 6
dead catalystsc
b
trial DPa calcd
MN
DPb DPc
53 73
(%)
PDIb
1
2
3
4
25
50
69
2403
66
66
63
60
1.06
1.09
1.08
1.08
5595 129 148
10 542 246 190
19 613 462 345
138
a The DP refers to n (Scheme 1). b Data refer to GPC in o-xylenes at
100 °C calibrated with linear polyethylene standards. c Data refer to 1H
NMR analysis end group analysis (i.e., the ratio of the terminal hydroxy-
methylene protons to the broad polymethylene peak of 6).
1-Boraadamantane‚THF (3) was then tested as an initiator
for the polyhomologation reaction. Toluene solutions of ylide
4 were preheated to 80 °C and treated with an aliquot of a
toluene solution of 3. The ylide was rapidly consumed (5
min), and TAO was added to affect oxidation under reflux
conditions. Removal of solvents followed by filtering and
washing of the polymeric solids with methanol, water, and
hexanes gave near quatitative crude yields of polymeric triol
6. Purification by reprecipitation in toluene/acetonitrile
afforded a purified white polymer in high yield.
The 1H NMR spectra of the star polymer 6 (C6D6, 78 °C)
produced by these reactions have features consistent with
the proposed assignment of a regular star architecture. There
is a triplet at 3.38 ppm (J ) 6.4 Hz) corresponding to
terminal hydroxymethylene protons of 6. Also, there is an
apparent doublet at 1.85 ppm (J ) 12.2 Hz) and a quartet at
0.59 ppm (J ) 11.8 Hz) corresponding to the equatorial and
axial methylene protons of the cyclohexane ring of 6 (Figure
1).
25, 50, 69, and 138. These reactions should produce regular
star polymers 6 with n ) 25, 50, 69, and 138, respectively
(Scheme 1). In all cases, however, the DP by 1H NMR end
group analysis was consistently three times greater than the
calculated DP.
While the GPC was calibrated with linear polyethylene
standards, corrections for star topology should result in
observed (GPC) molecular weights that are lower than the
calculated molecular weight on the basis of stoichiometry.11
Furthermore, previous examples of regular three-armed star
polymethylene carbinol polymers resulted in polymers whose
theoretical and observed molecular weights by GPC cor-
responded within 10%.3 Importantly, the PDI’s of all trials
in the present study are <1.09, a result consistent with living
polymerization.
Because extreme care was taken to quantitate the sto-
ichiometric ratios of catalyst 3 to monomer 4, it appeared
that a significant fraction of propagating species terminated
and did not result in polymer formation. This presented us
with a mechanistic quandary. Following an apparent smooth
initiation by 3, approximately two-thirds of the propagating
species must terminate (dead catalysts in Table 1) to account
for the molecular weight discrepancy. The demise of these
propagating species must be rather sudden, since competing
termination reactions during the course of polymerization
would result in a higher polydispersity.
Insight into this problem came from a polymerization
conducted with an intial (1/3)ylide:boraadamantane ratio of
27. After oxidation and reprecipitation, polymer 6 was
isolated in 93% yield. The polymer’s DP was observed to
be 100 by GPC with a polydispersity of 1.08. The filtrate
from reprecipitation and washing of the polymer was found
by TLC to contain a distribution of triols with Rf values
similar to that of 10. The triols were acetylated by heating
with excess acetic anhydride at 100 °C, followed by
purification by column chromatography.
1
Figure 1. Representative H NMR spectrum of polymeric triol 6
(C6D6, 78 °C, 500 MHz). This spectrum depicts the polymeric
product of trial 2 in Table 1.
The GC-chromatograph of the acetylated triols produced
by the oxidation of terminated species revealed three
components (Figure 2). The two species appearing at 20.9
and 21.7 min both have masses corresponding to a DP of 5
and account for 14 and 51% of the terminated species,
respectively (MS-CI). The third component appearing at 22.4
min has a mass corresponding to a DP of 6 and accounts for
The clear psuedo-symmetry of the equatorial and axial
protons, as well as the multiplicity of the terminal hydroxy-
methylene groups, supports the conclusion that all three
branches of the propagating species participate in migration
to generate a star polymer.
The degree of polymerization (DP) was established by
GPC and 1H NMR end group analysis (Table 1). Reactions
were run with initial ratios of (1/3)ylide:boraadamantane of
(11) Burchard, W. AdV. Polym. Sci. 1999, 143, 113.
Org. Lett., Vol. 3, No. 20, 2001
3065

of the possible architectures and pathways of polymerization
support this analysis and point to structures 11 and 12 as
likely candidates for two of the terminated organoboranes
with DP 5 (Figure 3).
Figure 3. Possible candidates for two of the terminated organobo-
ranes.
In conclusion, we have synthesized 1-borahomoadamantane‚
DMSO (9) by the controlled monohomologation of 1-bora-
adamantane‚THF (3) with dimethylsulfoxonium methylide
(4). Also, we have demonstrated that 3 can be used as an
initiator for polyhomologation, giving rise to “tube-like”
trialkylborane structures 5. The “tube-like” species 5 may
be further elaborated by oxidation to regular star polymers
6. Discrepancies between the calculated and observed
molecular weight of 6 have led to the discovery of branch
points in the early stages of polymerization in which
approximately two-thirds of the propagating molecules
terminate by the formation of unreactive tricyclic organobo-
ranes. Molecular modeling offers insight into the possible
candidates that result in the termination of the polymerization.
Figure 2. GC-chromatograph of acetylated triols produced by the
oxidation of terminated species.
35% of the terminated species. The total mass of the
acetylated triols accounts for 63% of the initiator used.
We now speculate that following the fourth and fifth
methylene insertions the tricyclic organoboranes are pre-
sented with branch points in the path of polymerization. At
these points, approximately 60-66% of the propagating
species terminate by forming cage structures with collapsed,
inverted pyramidal boron centers that are unreactive toward
ylide. The remaining 34-40% of propagating species
continue to polymerize via cage structures that remain
reactive toward ylide and result in “tube-like” organoborane
structures 5. Preliminary results from molecular modeling
Acknowledgment. This research was supported by a
grant from the National Science Foundation (Grant CHE-
9617475).
OL0159726
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Org. Lett., Vol. 3, No. 20, 2001