Domino Fragmentations in Traceless Directing Groups of Radical Cascades: Evidence for the Formation of Alkoxy Radicals via C-O Scission

Article abstract of DOI:10.1021/acs.joc.6b01052

Direct evidence for the formation of alkoxy radicals is reported in radical cascades using traceless directing groups. Despite the possibility of hydrogen abstraction in the fragmenting step, followed by loss of R-OH, β-scission is preferred for the formation of alkoxy radicals. For the first time, the C-O radical was intermolecularly trapped using a silyl enol ether. Various C-X fragmenting groups were explored as possible traceless directing groups for the preparation of extended polyaromatics. Computational evidence shows that a combination of aromatization, steric and stereoelectronic effects assists the fragmentation to alkoxy radicals. Additionally, a new through-space interaction was discovered between O and Sn in the fragmentation as a specific transition state stabilizing effect.

Full text of DOI:10.1021/acs.joc.6b01052

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Article
Domino Fragmentations in Traceless Directing Groups of Radical
Cascades: Evidence for the Formation of Alkoxy Radicals via C-O Scission
Trevor Harris, Gabriel dos Passos Gomes, Ronald J Clark, and Igor V. Alabugin
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b01052 • Publication Date (Web): 15 Jun 2016
Downloaded from http://pubs.acs.org on June 16, 2016
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Domino Fragmentations in Traceless Directing Groups of
Radical Cascades: Evidence for the Formation of Alkoxy Rad-
icals via C-O Scission
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Trevor Harris, Gabriel dos Passos Gomes, Ronald J. Clark, Igor V. Alabugin*
Department of Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306, United States
Corresponding Author
* alabugin@chem.fsu.edu
Direct evidence for the formation of alkoxy radicals is reported in radical cascades using traceless directing groups. Despite the pos-
sibility of hydrogen abstraction in the fragmenting step followed by loss of R-OH, β-scission is preferred for the formation of alkoxy
radicals. For the first time, the C-O radical was intermolecularly trapped using a silyl enol ether. Various C-X fragmenting groups
were explored as possible traceless directing groups for the preparation of extended polyaromatics. Computational evidence show
that a combination of aromatization, steric and stereoelectronic effects assists the fragmentation to alkoxy radicals. Additionally, a
new through-space interaction was discovered between O and Sn in the fragmentation as a specific transition state stabilizing effect.
Introduction.
Reaction cascades that combine cyclizations with fragmentations offer unique advantages for the synthesis of precisely functional-
1
ized cyclic compounds. In particular, the final fragmentation step can adjust the oxidation state, eliminate a directing group,
and/or provide entropic driving force that renders the overall cascade irreversible. Consequently, fragmentations play an important
role in the arsenal of fundamental chemical steps available to a synthetic chemist.
In our recent work, we disclosed the conversion of skipped oligoalkynes into functionalized polyaromatic ribbons assisted by the
1b
traceless OR directing groups (Scheme 1). The key mechanistic feature of this cascade is the “boomerang” transposition of the
radical center through the sequence of bond forming steps. In the first step, the presence of directing OR group ensures chemo- and
regioselective attack of the Sn-radical on the multifunctional substrate and formation of reactive vinyl radical. This radical has σ-
symmetry where the radical center is orthogonal to the π-system. As a result, it is not delocalized and highly reactive. The sequence
of subsequent cyclizations involves fast and selective exo-dig ring closures, each of which produces an analogous vinyl radical that
reacts at the next appropriately positioned alkyne moiety. However, once the last alkyne has reacted, the final cyclization has to
target a different functionality: the terminal aryl group.
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Scheme 1. The directing groups X are “traceless” if cyclization cascade is terminated by C-X fragmentation
This step produces a delocalized π-radical with significant radical density that “comes back” to the locus of the initial attack, the
carbon atom adjacent to the directing OR group. Our mechanistic hypothesis was that radical density at this carbon would facilitate
OR fragmentation (radical β-scission) with the assistance from the bulky SnR3 moiety that pushes the breaking C-O bond closer to
the optimal alignment with the radical center. The resulting departure of the directing OR group makes this structural element
traceless in the overall cascade and provides a reactive O-centered radical for the propagation of the cascade.
Utilization of an alkoxy radical as a leaving group on carbon is unusual. These species are generally produced from cleavage of
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much weaker O-N, O-O or O-I bonds (Scheme 2).
Scheme 2. Representative examples of alkoxy radical fragmentation from weak O-N, O-O, O-I bonds
Perhaps, one of the reasons why homolytic C-O scission is not commonly used in organic radical chemistry is the relatively high
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typical C-O bond dissociation energy, leading to the expectation of the OR moiety to be a poor leaving group in a radical process.
Considering the scarcity of homolytic C-O fragmentations, an alternative mechanistic hypothesis seems plausible. In this scenario,
the C-centered radical is quenched via intermolecular H-abstraction to give an intermediate which is converted into the final aro-
matic product via the loss of methanol (Scheme 3). The last step occurs after the radical cascade is terminated.
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Scheme 3. Alternative aromatization routes: the indirect route through intermolecular H-abstraction and
MeOH elimination (top) and direct radical C-O scission (bottom)
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In order to resolve this mechanistic ambiguity and to test whether the C-O radical fragmentation can be confidently added to the
arsenal of tools for synthetic radical chemistry, we have attempted to trap O-centered radicals with appropriate reagents. Further-
more, we have designed OR leaving groups with a “weak link”, a bond that can undergo scission in the presence of a β-radical. In
our design described in Scheme 4, the driving force for this scission would come from the combination of thermodynamic stability
of the carbonyl group and electronic effects in the translocated radical.
Scheme 4. Formation of stabilized radical and formaldehyde through a weak link design
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We have recently used a similar sequence for driving an otherwise thermodynamically unfavorable process to completion. In the
present work, we illustrate the utility of such fragmentations as a mechanistic tool.
Computational Methods.
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Potential energy profiles for this cascade transformation were evaluated with Gaussian 09 using the (U)M06-2X functional, capable
of providing a relatively accurate description of reaction and activation energies for a variety of chemical processes including radical
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reactions. The LanL2DZ basis set was used for Sn-substituted molecules. Molecules and orbitals were rendered with Chemcraft
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1.7 and CYLView . Frequencies were calculated to confirm each stationary point as either a minimum or a first-order saddle
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point. The electronic structures of reactive intermediates were analyzed with NBO 3.0. The ΔG values for all reactions were calcu-
lated at 110˚C. A truncated version of the attacking radical (SnMe3) was used instead of SnBu3.
Results and Discussion.
In our first attempt to trap the putatively formed methoxy radicals, we relied on the earlier findings of Sammis and coworkers9 who
reported the increased intramolecular reactivity of O-radicals towards vinyl silyl ethers. We extended the use of this electron rich π-
functionality for an intermolecular trapping using the vinyl silyl ether 8. This substrate was prepared by deprotonation of aldehyde
S9 followed by quenching of the resulting enolate with tert-butyldimethylsilyl triflate (TBSOTf). Interestingly, the E:Z ratio (15:85)
in the product showed cis-preference for the bulky TBS group (Scheme 5A). Computational analysis into this selectivity uncovered
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H-bonding between the oxygen and the ortho Ar hydrogen in the product (Scheme 5B). It is possible that similar H-bonding inter-
action can lead to stabilization of the Z-enolate derived from S9. Due to the instability of the trapped product towards hydrolysis,
we found it more convenient to convert it to a previously synthesized10 aldehyde S9 with TBAF prior to the analysis.
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Scheme 5. A: Synthesis of silyl enol ether B: Computed cis-preference for silyl enol ether
The alkoxy radical trapping experiment was performed using 7 and 8 in a 1:1 ratio under radical forming conditions in refluxing
toluene. Although the trapping experiment was successful, the mixed TBS-methyl acetal product 9 was produced in low yield. The
regioselectivity of this intermolecular radical addition to the Z-alkene is different from that reported for the intramolecular version of
this process, but fully consistent with the computational predictions of 7 kcal/mol difference in the two addition barriers (Scheme 6,
bottom). The regioselective preference for the radical addition is also consistent with the gain of benzylic and anomeric stabilization
for the radical that leads to the observed product which is 11.1 kcal/mol more stable than its regioisomer.
Scheme 6. Top: Alkoxy radical trapping using a vinyl silyl ether11; Bottom: Regioselective preference for alkoxy
radical attack. Calculated Gibbs Free Energies at 110˚C in kcal/mol
th
The ratio of two products 3 and 9 suggests that only 1/4 of the formed OMe radical finds the vinyl silyl ether before deactivation.
Because such deactivation can occur through hydrogen abstraction and/or β-scission to formaldehyde, we tested if H-abstraction
from toluene is the main reason for the low yield by repeating the trapping experiment in benzene. The C-H BDE of benzene is
~113 kcal/mol, which is 23 kcal/mol greater than the C-H BDE of toluene. The experiment again was successful but the relative
ratio of products remained the same.
Although this experiment provided strong evidence for the alkoxy radical formation, the yields of the trapped products were rather
low. In the following section, we describe how we solved this problem by designing a new mechanistic tool for unambiguous detec-
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tion of O-centered radical formation. This design couples the initial radical C-O scission with another fast fragmentation through a
weak-link shown in Scheme 7. 12
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The tethered naphthyl group was attached to a skipped enediynol 4 through FeCl3-promoted nucleophilic substitution to give the
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cyclization/fragmentation precursor 5 in 96% yield (Scheme 7, top). We found this method to be more convenient than the earlier
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used base-assisted reaction of the alcohol with MeI. With substrate 5 in hand, the radical cyclization was performed with Bu3SnH
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and AIBN in refluxing toluene. The radical cascade gave stannyl-11-phenyl-11H-benzo[a]fluorene (3) in 66% yield along with 52%
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of 1-methylnaphthalene (Scheme 7, middle). Support for the formation of an alkoxy radical from this reaction is two-fold: 1) Evi-
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dence for the β-scission comes from the presence of 1-methylnaphthalene and aldehyde peaks in the H-NMR spectrum of the reac-
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tion mixture (See SI). The appearance of a peak at δ 9.79 ppm is consistent with the presence of formaldehyde. 2) Evidence against
the indirect pathway comes from the absence of 2-naphthylethanol.
Scheme 7. Top: FeCl3 catalyzed substitution to generate the fragmentation precursor 5. Middle: Radical cycliza-
tion and domino fragmentation of 5. Bottom: Control experiment with 2-naphthylethanol under radical forming
conditions
To make sure that 2-naphthylethanol 6 was not present under the reaction conditions, we have also performed the control experi-
ment to eliminate the unlikely scenario formation of an O-centered radical based on the abstraction of a hydrogen atom from the
alcohol by the SnBu3 radical (Scheme 7, bottom). One could expect such process to be thermodynamically uphill based on the dif-
3,16
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ferences in the BDEs of O-H and Sn-H bonds (>100 and 78 kcal/mol, respectively). Indeed, exposure of 2-naphthylethanol to
the combination of Bu3SnH and AIBN in refluxing toluene did not lead to the formation of the fragmented 1-methylnaphthalene
product. Stability of 6 under the cascade conditions confirms that 6 was not formed transiently to react further. In summary, this
result clearly shows that H-abstraction/E2 elimination does not operate because it would give different products (Scheme 8).
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Scheme 8. Direct pathway makes 1-methylnaphthalene and formaldehyde whereas indirect pathway makes on-
ly the alcohol
In the next step, we have evaluated potential energy profile for the observed double fragmentation cascade computationally
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(Scheme 9). The calculated barrier for the 1 (C-O) fragmentation is 2.4 kcal/mol lower than the barrier for the 2 (C-C) fragmen-
tation but both fragmentations are calculated to be sufficiently fast at the reaction conditions. The overall thermodynamic driving
force for domino process is -3.1 kcal/mol with each of the fragmentation steps exergonic by ~1.5 kcal/mol.
Scheme 9. Formation of formaldehyde through fragmentation of O-centered radical and benzylic radical. Cal-
culated Gibbs Free Energies at 110˚C in kcal/mol
We have compared the rates of the intermolecular (addition) and intramolecular (fragmentation) approaches for trapping of alkoxy
radical that we have described earlier by performing radical cyclization/fragmentation cascade of naphthyl-substituted enyne 5 in
the presence of the silyl enol ether 8 (Scheme 10). Only the fragmentation products were observed in yields similar to experiments
in the absence of the intramolecular trap 8. These results demonstrate that fragmentation is so fast that it occurs before the alkoxy
radical
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Scheme 10. Fragmentation of an alkoxy radical is faster than attack of an alkene. Calculated Gibbs Free Ener-
gies at 110˚C in kcal/mol
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Note that both fragmentation steps were designed to be weakly exergonic to render the fragmentation thermodynamically feasible
but without imposing a large additional preference that could promote the radical pathway. The computed barrier for the
OCH2CH2Np fragmentation is ~4 kcal/mol higher than the calculated barrier for OMe fragmentation (vide infra). These data indi-
cate that the C-C scission is not coupled to (and does not assist in) the initial C-O bond rupture.
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A priori, one can suggest two factors contributing to the favorable fragmentation kinetics for the usually strong C-O bond: a) the ste-
reoelectronic assistance via the favorable alignment of the breaking bond with the adjacent radical center and b) the thermodynam-
ic assistance via developing aromaticity.
The former feature arises from an interplay of sterics and stereoelectronics. One can expect that due to its bulkiness, the SnR3 group
can force the OR group closer to the nearly perpendicular geometry of the fragmentation TS. This steric component can enhance
stereoelectronic interaction of the breaking C-O bond with the radical center that ultimately leads to the departure of the OR radi-
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cal by increasing population of the σ*C-O orbital in the TS and assisting aromatization.
It is interesting to compare the magnitude of this radical assistance to C-O scission with the magnitude of hyperconjugation associ-
ated with the classic anomeric effect, i.e., the other system where C-O bond is weakened by the interaction of a p-donor and a vici-
nal σ*C-O acceptor. 19 NBO analysis indicates that the magnitudes of these two effects are remarkably similar: 14.6 kcal/mol of sta-
bilization due to nO → σ*C-O interaction20 vs. 16.7 kcal/mol for the pC → σ*C-O interaction (Figure 1).
Figure 1. Comparison of anomeric effect vs the β-radical C-O scission using NBO analysis
Furthermore, NBO analysis of the intermediate indicates the presence of symmetry enhanced interaction between the cyclic π-
system and the C-O bond that has the key features of hyperaromaticity (Figure 2B.) Such symmetry effects on the efficiency of
radical hyperconjugation are known as the Whiffen effect (Figure 2A).21 The β-scission step was evaluated using NICS(1) values22
shown in Figure 2C. The NICS values suggest that hyperaromaticity is rather small in the reactant but gets progressively large in
the TS before evolving in the fully developed product aromaticity.
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Figure 2. A: Symmetry can enhance or diminish conjugative effect; B: Symmetry-enhanced π /C-O hyperconju-
gation. C: NICS evaluation of aromaticity in the β-scission process
In order to explore the possible role of steric assistance in the fragmentation step, we have varied the size of substituents at the pro-
pargylic position and tested reactivity of ethoxy, isopropoxy, and tert-butoxy analogues of substrate 5 (Scheme 11, entries 10-12).
Whereas the yields of the polycyclic target 3 from the ethoxy, methoxy23, and isopropoxy precursors were similar (64-74%), reaction
yield drops slightly (53%) for the t-Bu substrate 12. One has to note, however, that the yield cannot provide direct information
about the fragmentation, which is unlikely to be the rate-determining step here.24
Scheme 11. Scope of X-directing/fragmenting groups
We have also explored additional variations in the nature of the traceless directing group. The radical reaction of propargylic ace-
tate 13 gave 52% yield of 3, suggesting that this cascade can be used as a new route to the formation of acyloxy radicals. Acyloxy
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radicals are useful in radical chemistry, but their preparation via C-O bond scission is generally difficult (BDE of C-O bond is ~70
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In addition, we have investigated the possibility to employ C-C β-scission for terminating this radical cascade. Indeed, the reaction
of alcohol 14 led to the formation of 3, albeit in low yield (13%). The observed loss of CH2OR moiety offers strong evidence for the
radical cascade termination - E2 elimination via the C-C bond scission is highly unlikely. The low yield is consistent with the calcu-
lated barrier for radical fragmentation (~21 kcal/mol) which seems to be the upper limit for this fragmentation process in refluxing
toluene.
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Finally, we also used acetals 15 and 16 to test whether it is possible to have additional functionality in the polyaromatic product.
Unfortunately, the O-functionalized polyaromatics (17 and 18) were produced in low yield (12% and 28%, respectively) because
this substitution pattern accelerated the competing direct thermal cycloaromatization to the products 19 and 20 (55% and 60%),
respectively (Table 1).27 This observation is consistent with the earlier reports of thermal intramolecular dehydro Diels-Alder reac-
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tions for activated skipped enediynes by Dominguez and Saa.
Table 1. Thermal cycloaromatization side products under Bu3SnH/AIBN/toluene/reflux conditions
The calculated barriers and reaction energies for selected C-X scissions are shown in Figure 3. The calculated trends reflect the
competition between several factors such as intrinsic stability of the departing radical and its interaction with the vicinal R3Sn moie-
ty. If one concentrates on the enthalpy by removing the entropy term, every single fragmentation reaction is endothermic (Figure
3B). Entropy plays a major role in the C-X scission by increasing the exergonicity of the reaction and lowering the activation barri-
er in most of the cases. For the analyses of the fragmentations, we will discuss these reactions in terms of their free energies (Figure
3A).
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Figure 3. A: Calculated ΔG‡ and ΔG of C-X scission. B: Calculated ΔH‡ and ΔH of C-X scission
Computations find the fragmentation of acetals 15 and 16 to be endergonic because it leads to the loss of stabilizing anomeric effect
20
present in these reactants. This unfavorable effect is reflected by the 6-10 kcal/mol increase in the activation barriers in compari-
son to the –OMe substrate. For the cyclic acetal, the counterproductive effect of the loss of anomeric reactant stabilization augments
the decrease in the entropic gain.
β-Scission of C-C bonds in the radical derived from substrate 14 shows relatively high ΔG‡ despite the 2c,3e stabilizing assistance in
1a
the fragmented radical. Fragmentation of alkoxy radicals have significantly lower barriers. It is tempting to associate this observa-
tion with the relative efficiencies of pC → σ*C-O and pC → σ*C-C hyperconjugation in the TS. X = OMe shows the best combination
of exergonicity with a low activation barrier. The relative instability of the OH radical renders its formation 12.4 kcal/mol ender-
gonic. On the other hand, the exergonicity decreases for OEt and increases afterwards as expected from steric decompression in the
fragmentation step. However, barriers for the OEt, OiPr and OtBu fragmentations reveal an unexpected trend – the barrier in-
creases for OEt, slightly decreases for OiPr, and then increases again for OtBu. This trend is a violation of the generally observed
correlation between kinetics and thermodynamics for a family of similar reactions.29 Although the loss of t-BuO radical is the most
exothermic of the four processes, it proceeds through the activation barrier that is higher than that for the loss of OMe and OiPr
radicals.
Analysis of the fragmentation TS geometries gives a glimpse into the origin of this anomalous behavior (Figure 4). Only for the
tBuO substituent, the two “bulky” groups (OR and SnMe3) are pushed to the opposite directions. In the MeO, EtO, and iPrO
fragmentation transition states, the SnMe3 group is tilted toward the OR group, indicating the presence of an attractive O•••Sn in-
teraction in these systems. This interaction provides a previously unidentified stabilizing force that assists the OR radical formation.
NBO analyses are consistent with the presence of such through-space interactions, as it decreases as the bulkiness of the R group in-
creases, pushing the Sn-group away.
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Figure 4: Geometries of the alkoxy radical elimination TSs support the importance of O•••Sn interactions
Comparison of the TS and reactant geometries (Figure 5) suggests that this interaction is specific to the TS. The reactant geometries
for both OiPr and OtBu radicals is dominated by sterics (the CCCSn dihedral is negative). The geometry of OMe and OEt sub-
strates reveal positive CCCSn dihedrals but the deviations from planarity is much smaller than they are in the TS.
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Figure 5: Geometries of the alkoxy radical elimination SMs
In summary, this work provides clear evidence for the formation of O-centered radicals via C-O fragmentation and offers the first
glimpses in the interplay of steric and stereoelectronic effects in this process. It also identified the new O…Sn through-space interac-
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tion that selectively stabilizes transition states for the fragmentation of OR bonds adjacent to a stannyl moiety. We plan to further
explore the generality of such new supramolecular forces in future work.
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Experimental Section.
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Toluene (anhydrous), 2-bromobenzaldehyde, 2-(Naphthalen-1-yl)ethanol, tert-butyl alcohol, ethanol (Anhydrous), tributyltin hy-
dride and AIBN were purchased and used for experiments with no additional purification. THF or tetrahydrofuran (No Stabilizer,
Chromatography Grade) and acetonitrile (Super Gradient for HPLC) was used on a Glass Contour SPS-4 Solvent Purification Sys-
tem. Ethyl acetate and hexanes (ACS Reagent Grade) were purchased and used for column chromatography. DCM or dichloro-
methane and DMAP or dimethylaminopyridine were distilled over CaH2 before use. Column chromatography was performed
using silica gel Kieselgel 60 (70-230 mesh) or Kieselgel 60 (230-400 mesh) and preparatory thin layer chromatography was per-
formed using a 1000 μm glass backed plate containing UV dye. Melting points were obtained using a melting point apparatus. In-
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frared spectra (IR) were obtained using a Michelson-type IR instrument; absorptions are reported in reciprocal centimeters. Ac-
1
cuTOF mass spectrometer analyzer was used. H NMRs were run on 400 MHz and 600 MHz spectrometer in CDCl3 and all 13C
NMRs were run on 100 MHz and 150 MHz spectrometers in CDCl3. Proton chemical shifts are given relative to the residual pro-
ton signal of CDCl3 (7.26 ppm). Carbon chemical shifts were internally referenced to CDCl3 (77.00 ppm) signal. All J-coupling val-
ues are reported in Hertz (Hz). δ is in ppm. The following abbreviations were used for multiplicities: s = singlet, d = doublet, t =
triplet, q = quartet, dd = doublet of doublet, dt = doublet of triplet, dq = doublet of quartet, m = multiplet, quin = quintuplet, sext
= sextet, sep = septet, sepd = septet of doublets, br = broad.
Procedure for synthesis of compound (4).
Synthesis of 2-(phenylethynyl)benzaldehyde (S1). To a flame dried round bottom flask purged with argon was added 2-
bromobenzaldehyde (0.216 g, 1.17 mmol), Pd(PhCN)2Cl2 (0.022 g, 0.0583 mmol), and CuI (0.011g, 0.0583 mmol) in 15 mL tri-
ethylamine then P(t-Bu)3 (1M in toluene) (1.2 mL, 0.117 mmol) was added. The solution was outgassed with argon for 30 minutes
followed by dropwise addition of neat phenylacetylene (0.15 mL, 1.40 mmol). The reaction was stirred overnight and was worked
up by washing with saturated NH4Cl solution (20 mL), extracted with EtOAc twice (2 x 15 mL) and dried with Na2SO4 and solvents
removed in vacuo. The crude reaction mixture was purified by column chromatography in 5 % ethyl acetate/hexanes to afford S1
as an orange oil (92 % yield, 0.227 g) which matches spectral data previously reported in the literature.30
Synthesis of 3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-ol (4). To a flame dried round bottom flask purged with argon was added
phenylacetylene (0.33 mL, 3.06 mmol) and 10 mL THF. The round bottom flask was cooled to -78 °C using acetone/dry ice bath
followed by dropwise addition of n-BuLi (1.3 mL, 2.3M in hexane) (3.06 mmol) and then stirred for 45 minutes. S1 (0.526 g, 2.55
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mmol) dissolved in 5 mL THF was added slowly and brought to room temperature after 2 hours and allowed to stir overnight. The
reaction was worked up by washing with saturated NH4Cl solution (30 mL), extracted with EtOAc (3 x 15 mL) and dried with
Na2SO4 and solvents removed in vacuo. The crude reaction mixture was purified by column chromatography in 3 % ethyl ace-
tate/hexanes to afford 4 as an orange oil (93 % yield, 0.731 g) which matches spectral data previously reported in the literature.31
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General procedure for synthesis of compounds (5), (7), and (10).
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1-(2-((3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-yl)oxy)ethyl)naphthalene (5). To 3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-ol
4 (0.100 g, 0.324 mmol) dissolved in acetonitrile (1 mL) was added (6) (0.163 g, 3 equiv) under no exclusion of air or moisture. FeCl3
(0.003 g, 0.05 equiv) was added in one portion. Reaction was stirred for 5 hours and monitored by TLC until full consumption of
starting material. Crude reaction mixture was washed with 1M HCl (10 mL) and extracted with EtOAc (3 x 10 mL). Collected
organics were washed with brine solution then passed through a plug of neutral alumina and solvent were removed in vacuo. The
crude reaction mixture was purified by column chromatography in 10 % ethyl acetate/hexanes to afford 5 (96% yield, 0.144 g) as a
1
-1
clear oil. IR (neat, cm ) 3058, 2863, 2218, 1490, 1069, 753, 689; H NMR (400 MHz, CDCl ) δ 8.05 (d, J = 8.4 Hz, 1H), 7.88 –
3
7.80 (m, 2H), 7.71 (dd, J = 6.1, 3.4 Hz, 1H), 7.58 (dd, J = 7.6, 1.5 Hz, 1H), 7.55 – 7.49 (m, 2H), 7.47 – 7.28 (m, 14H), 5.98 (s, 1H),
13
4.22 (dt, J = 8.9, 7.5 Hz, 1H), 3.99 (dt, J = 9.3, 7.7 Hz, 1H), 3.50 (t, J = 7.8 Hz, 2H); C NMR (100 MHz, CDCl3) δ 140.3, 134.5,
133.7, 132.2, 132.1, 131.8, 131.5, 128.8, 128.6, 128.4, 128.4, 128.3, 128.2, 127.5, 127.0, 126.8, 125.9, 125.5, 125.4, 123.7, 123.1,
122.6, 122.4, 94.3, 87.2, 87.0, 86.9, 70.3, 69.6, 33.3; HR-MS-ESI(+) was performed, calculated as C35H26NaO [M+Na]+= 485.1881,
found 485.1872.
1-(1-methoxy-3-phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (7). Compound 7 was synthesized using general procedure. Purified by
column chromatography using 10 % ethyl acetate/hexanes (71 % yield, 0.067 g) as a yellow oil which matches spectral data previ-
27
ously reported in the literature.
1-(1-ethoxy-3-phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (10). Compound 10 was synthesized using general procedure. Purified by
-1
column chromatography using 10 % ethyl acetate/hexanes (73 % yield, 0.055 g) as a pale yellow oil. IR (neat, cm ) 3027, 2921,
1
2216, 1494, 1076, 728, 690; H NMR (400 MHz, CDCl3) δ 7.85 (dd, J = 7.8, 1.3 Hz, 1H), 7.60 – 7.54 (m, 3H), 7.50 – 7.45 (m,
13
2H), 7.44 – 7.28 (m, 8H), 5.89 (s, 1H), 3.94 (dq, J = 9.1, 7.0 Hz, 1H), 3.69 (dq, J = 9.2, 7.0 Hz, 1H), 1.32 (t, J = 7.0 Hz, 3H);
C
NMR (100 MHz, CDCl3) δ 140.5, 132.2, 131.8, 131.5, 128.8, 128.4, 128.4, 128.4, 128.2, 128.2, 127.4, 123.1, 122.7, 122.4, 94.1,
+
87.3, 86.9, 86.8, 69.9, 64.9, 15.2; HR-MS-ESI(+) was performed, calculated as C25H20OK [M+K] = 375.1151, found 375.1162.
Procedure for synthesis of compound (13).
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3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-yl acetate (13). A flame dried round bottom flask was purged with argon. 3-phenyl-1-(2-
(phenylethynyl)phenyl)prop-2-yn-1-ol 4 (0.325 g, 1.05 mmol) was added to the round bottom flask using 5 mL of DCM. Pyridine
(0.87 mL, 10.7 mmol) and catalytic amount of DMAP (0.006 g, 0.053 mmol) was added. The resulting solution was cooled on an ice
bath to 0 °C. Acetyl chloride (0.15 mL, 2.1 mmol), previously distilled, was added neat into the reaction mixture slowly over 5
minutes. A white precipitate formed immediately. After full consumption of starting material based on TLC analysis, the reaction
mixture was pushed through a plug of silica gel using diethyl ether (dichloromethane dissolves the precipitate). The solvents were
concentrated down and then the crude mixture was purified by column chromatography using 5 % ethyl acetate/hexanes to give
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13 (0.339 g, 92 % yield) as a pale yellow oil. IR (neat, cm ) 3061, 2921, 2228, 1793, 1491, 1217, 753, 689; H NMR (400 MHz,
CDCl3) δ 7.92 (dd, J = 7.5, 1.6 Hz, 1H), 7.64 – 7.58 (m, 3H), 7.52 (dd, J = 7.4, 2.2 Hz, 2H), 7.46 – 7.32 (m, 8H), 7.20 (s, 1H), 2.13
13
(s, 3H); C NMR (100 MHz, CDCl3) δ 169.6, 138.1, 132.3, 131.9, 131.6, 128.9, 128.7, 128.6, 128.5, 128.3, 128.2, 128.0, 122.8,
+
122.7, 122.1, 95.0, 87.2, 86.1, 85.1, 64.4, 20.9; HR-MS-ESI(+) was performed, calculated as C50H36O4Na [2M+Na] = 723.2511,
found 723.2505.
Procedure for synthesis of compounds (11) and (12).
1-(1-isopropoxy-3-phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (11). To 3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-yl acetate 13
(0.100 g, 0.285 mmol) dissolved in acetonitrile (1 mL) was added isopropanol (65 µL, 0.856 mmol) under no exclusion of air or
moisture. FeCl3 (0.002 g, 0.0142 mmol) was added in one portion. The reaction was stirred at 40 °C for 24 hours. The crude reac-
tion mixture was washed with 1M HCl (10 mL) and extracted with EtOAc (3 x 10 mL). Collected organics were washed with brine
solution, then passed through a plug of neutral alumina and solvent were removed in vacuo. The crude reaction mixture was puri-
-1
fied by column chromatography in 5 % ethyl acetate/hexanes to afford 11 (0.084 g, 84 % yield) as a light orange oil. IR (neat, cm )
1
3060, 2971, 2930, 2219, 1491, 1121, 1038, 754, 690; H NMR (400 MHz, CDCl3) δ 7.88 (d, J = 7.9 Hz, 1H), 7.58 (dt, J = 6.6, 1.6
Hz, 3H), 7.50 – 7.29 (m, 10H), 6.01 (d, J = 2.1 Hz, 1H), 4.11 (sepd, J = 6.2, 1.9 Hz, 1H), 1.36 (d, J = 6.3, 1.7 Hz, 3H), 1.28 (d, J =
13
6.2, 1.7 Hz, 3H); C NMR (100 MHz, CDCl3) δ 141.1, 132.2, 131.7, 131.5, 128.9, 128.4, 128.2, 128.1, 128.1, 127.7, 123.1, 122.8,
+
122.1, 93.9, 88.1, 87.1, 86.2, 70.1, 67.3, 22.8, 21.9; HR-MS-ESI(+) was performed, calculated as C26H22ONa [M+Na] =
373.1568, found 373.1564.
1-(1-(tert-butoxy)-3-phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (12). To 3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-yl acetate 13
(0.100 g, 0.285 mmol) dissolved in acetonitrile (1 mL) was added tert-butanol (82 µL, 0.856 mmol) under no exclusion of air or mois-
ture. FeCl3 (0.002 g, 0.0142 mmol) was added in one portion. The reaction was stirred at 40 °C for 28 hours. The crude reaction
mixture was washed with 1M HCl (10 mL) and extracted with EtOAc (3 x 10 mL). Collected organics were washed with brine
solution, then passed through a plug of neutral alumina and solvent were removed in vacuo. The crude reaction mixture was puri-
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-1
fied by column chromatography in 5 % ethyl acetate/hexanes to afford 12 (0.042 g, 40 % yield) as an orange oil. IR (neat, cm )
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1
3061, 2973, 2930, 2219, 1491, 1181, 1045, 752, 688; H NMR (400 MHz, CDCl3) δ 7.84 (dd, J = 7.9, 1.3 Hz, 1H), 7.61 – 7.56 (m,
3
4
13
2H), 7.53 (dd, J = 7.6, 1.4 Hz, 1H), 7.43 – 7.35 (m, 6H), 7.32 – 7.25 (m, 4H), 6.03 (s, 1H), 1.38 (s, 9H); C NMR (150 MHz,
5
6
CDCl3) δ 143.2, 132.0, 131.6, 131.5, 128.9, 128.4, 128.4, 128.0, 128.0, 127.5, 127.5, 123.2, 123.2, 120.8, 94.2, 90.5, 87.3, 85.2,
7
8
+
75.7, 62.8, 28.6; HR-MS-ESI(+) was performed, calculated as C27H24ONa [M+Na] = 387.1725, found 387.1720.
9
Procedure for synthesis of compound (14).
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2-(2-(phenylethynyl)phenyl)oxirane (S2). To a flame dried round bottom flask purged with argon was added sodium hydride 60 % in oil
(0.048 g, 1.19 mmol) and trimethylsulfonium iodide (0.304 g, 1.49 mmol) and 5 mL of DMSO. After 15 minutes of stirring at room
temperature, S1 (0.205 g, 0.994 mmol) was added in 5 mL of DMSO and color of the reaction changed to deep red. After an hour
color changed to brown (indication of reaction completion). The reaction was diluted with ether (10 mL), washed with water (20
mL), and extracted with ether (3 x 15 mL). The combined organics were washed with brine, and dried with Na2SO4. Purification by
column chromatography using 5% ethyl acetate/hexanes gave S2 (0.124 g, 57 % yield) as a yellow oil. Matches spectral data previ-
ously reported in the literature.32
4-phenyl-2-(2-(phenylethynyl)phenyl)but-3-yn-1-ol (14). To a Schlenk flask flame dried and purged with argon was added phenylacetylene
(42 µL, 0.381 mmol) and 0.5 mL of THF. The flask was cooled to -50 °C. N-BuLi (0.15 mL, 2.6 M in hexanes) (0.381 mmol) was
added and stirred for 45 minutes. Chlorotitanium triisopropoxide (0.099 g, 0.381 mmol) was dissolved in 0.5 mL of THF and added
to round bottom flask and stirred for 10 minutes. After addition of S2 (0.042 g, 0.191 mmol) to round bottom flask, the reaction was
brought to room temperature and stirred for 14 hours. For workup, the reaction was diluted with ether (5 mL) and washed with
NH4Cl (10 mL). The aqueous layer was washed with ether (3 x 10 mL), then the combined organics were washed with brine (3 x 10
mL) and dried with magnesium sulfate. Purification by column chromatography using 8 % ethyl acetate/hexanes, gave 14 (0.042 g,
-1
1
68 % yield) as a yellow oil. IR (neat, cm ) 3399, 3060, 2925, 2877, 2216, 1492, 1061, 753, 689; H NMR (400 MHz, CDCl3) δ 7.74
(dd, J = 7.8, 1.3 Hz, 1H), 7.60 – 7.54 (m, 3H), 7.52 – 7.47 (m, 2H), 7.42 – 7.27 (m, 8H), 4.78 (dd, J = 7.4, 5.1 Hz, 1H), 4.06 (ddd, J
13
= 10.4, 8.0, 5.2 Hz, 1H), 3.87 (ddd, J = 10.3, 7.4, 4.8 Hz, 1H), 2.01 (t, J = 6.9 Hz, 1H); C NMR (150 MHz, CDCl3) δ 139.5,
132.3, 131.8, 131.6, 128.8, 128.5, 128.4, 128.3, 128.3, 128.2, 127.3, 123.0, 122.9, 122.3, 94.3, 88.3, 86.8, 84.8, 66.7, 40.5; HR-MS-
+
ESI(+) was performed, calculated as C24H18NaO [M+Na] = 345.1255, found 345.1276.
Procedure for synthesis of acetylenic ketals (15) and (16).
1-(2-bromophenyl)-3-phenylprop-2-yn-1-ol (S3). Phenylacetylene (0.71 mL, 6.49 mmol) and THF (30 mL) was added to a flame-dried
round bottom flask purged with argon. The round bottom flask was brought to -78 °C using dry ice/acetone bath. N-BuLi (2.8 mL,
2.3M in hexane) (6.49 mmol) was added dropwise. The resulting mixture was allowed to stir for 45 minutes at same temperature. 2-
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Bromobenzaldehyde (0.63 mL, 5.40 mmol) was added neat over 10 minutes. The resulting yellow-orange mixture was taken off the
dry ice/acetone bath after 2 hours and stirred at room temperature overnight. The crude reaction mixture was worked up by wash-
ing with saturated NH4Cl solution (20 mL), extracted with EtOAc (3 x 15 mL). After drying with Na2SO4, the solvents were re-
moved in vacuo. The crude reaction mixture was purified by column chromatography in 10 % ethyl acetate/hexanes to afford S3
in 95 % yield (1.47 g, 5.13 mmol) as a yellow oil. Matches spectral data previously reported in the literature.33
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1-(2-bromophenyl)-3-phenylprop-2-yn-1-one (S4). 1-(2-bromophenyl)-3-phenylprop-2-yn-1-ol (0.679 g, 2.37 mmol) was added to a flame
dried round bottom flask purged with argon followed by the addition of DCM (10 mL). Activated manganese dioxide (2.06 g, 10
equiv) was added in portions and observed the formation of gas bubbles in the reaction mixture. The heterogeneous mixture was
stirred and monitored by TLC until full consumption of starting material. The reaction mixture was filtered through celite with
DCM and purified by flash chromatography using 10 % ethyl acetate/hexanes to afford S4 in 92 % yield (0.621 g, 2.17 mmol) as
an orange oil. Matches spectral data previously reported in the literature.34
2-(2-bromophenyl)-2-(phenylethynyl)-1,3-dioxolane (S5). To a round bottom flask equipped with a Dean-Stark trap was added 1-(2-
bromophenyl)-3-phenylprop-2-yn-1-one (0.938 g, 3.29 mmol), ethylene glycol (0.22 mL, 3.95 mmol), and p-toluenesulfonic acid
(0.013 g, 0.0657 mmol) and 20 mL of benzene. The reaction mixture was brought to reflux for 1 day and monitored by TLC and
revealed only partial conversion. After no change, the solvents were removed in vacuo and purified by column chromatography (2
% ethyl acetate/hexanes) to give S5 in 36 % isolated yield (0.387 g, 1.17 mmol) as an orange oil; 99% yield based on recovered
-1
1
starting material. IR (neat, cm ) 3063, 2893, 2193, 1489, 1202, 1025, 755, 688; H NMR (400 MHz, CDCl3) δ 7.88 (dd, J = 7.9,
2.1 Hz, 1H), 7.70 – 7.63 (m, 1H), 7.48 (dt, J = 7.8, 2.1 Hz, 2H), 7.37 – 7.29 (m, 4H), 7.23 (dt, J = 7.7, 2.0 Hz, 1H), 4.40 – 4.29 (m,
13
2H), 4.21 – 4.09 (m, 2H); C NMR (100 MHz, CDCl3) δ 137.8, 134.7, 131.8, 130.2, 128.7, 128.1, 127.5, 126.9, 121.8, 121.4,
101.8, 85.7, 85.5, 65.1; HR-MS-ESI(+) was performed, calculated as C17H13BrNaO2 [M+Na]+= 350.9997, found 350.9984.
1-bromo-2-(1,1-dimethoxy-3-phenylprop-2-yn-1-yl)benzene (S6). 1-(2-bromophenyl)-3-phenylprop-2-yn-1-one (0.130 g, 0.459 mmol) was
added to a flame dried round bottom flask and put under argon. 3 mL of MeOH, distilled twice over Mg, was added followed by
trimethylorthoformate (60 µL, 0.551 mmol) and p-toluenesulfonic acid (0.009 g, 0.0459 mmol). The reaction mixture was allowed to
stir for 24 hours at room temperature. Solvents were removed and the crude was purified by column chromatography using silica
gel (pH 6-7) in 5 % ethyl acetate/hexanes which gave S6 in 26 % yield based on recovered starting material (0.040 g, 0.119 mmol)
-1
1
as an amber oil. IR (neat, cm ) 3063, 2893, 2193, 1489, 1202, 1025, 755, 688; H NMR (400 MHz, CDCl3) δ 7.85 (dd, J = 7.9,
1.7 Hz, 1H), 7.62 (dd, J = 7.9, 1.3 Hz, 1H), 7.47 – 7.41 (m, 2H), 7.33 – 7.28 (m, 1H), 7.27 – 7.21 (m, 3H), 7.16 (ddd, J = 7.9, 7.4,
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1.8 Hz, 1H), 3.31 (s, 6H); C NMR (100 MHz, CDCl3) δ 137.4, 134.8, 131.9, 129.9, 129.2, 128.7, 128.1, 126.9, 121.9, 121.6, 96.9,
1
2
85.8, 85.0, 50.0; HR-MS-ESI(+) was performed, calculated as C17H15BrNaO2 [M+Na]+= 353.0153, found 353.0142.
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2-(2-iodophenyl)-2-(phenylethynyl)-1,3-dioxolane (S7). To a flame dried Schlenk flask purged with argon was added 2-(2-bromophenyl)-
2-(phenylethynyl)-1,3-dioxolane (0.178 g, 0.541 mmol) and 5 mL of THF. The flask was brought to -78 °C and n-BuLi (0.26 mL,
2.5M in hexane) (0.649 mmol) was added dropwise and the color of the mixture changed from dark orange to deep purple. After 15
minutes, iodine (0.171 g, 2.5 equiv) was added all at once and color changed back to orange and brought to room temperature and
stirred for 4 hours. Saturated Na2S2O3 solution (15 mL) was added and the aqueous phase extracted twice with diethyl ether (30
mL). The organics were washed with water again and dried with Na2SO4 and solvents were removed in vacuo. Could not be puri-
fied so crude mixture was taken to next step of Sonagashira reaction. Using 1,2-dichloroethane as an internal standard gave S7 in
52 % yield (0.106 g, 0.281 mmol).
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1-(1,1-dimethoxy-3-phenylprop-2-yn-1-yl)-2-iodobenzene (S8). To a flame dried Schlenk flask purged with argon was added 1-bromo-2-
(1,1-dimethoxy-3-phenylprop-2-yn-1-yl)benzene (0.130 g, 0.392 mmol) and 3.5 mL of THF. The flask was brought to -78 °C and n-
BuLi (0.18 mL, 2.45M in hexane) (0.431 mmol) was added dropwise and the color of the mixture changed from amber to opaque
purple. After 15 minutes, iodine (0.075 g, 1.5 equiv) was added all at once and color changed to brown, then to green, and back to
orange. The reaction was brought to room temperature and stirred for 2 hours. Saturated Na2S2O3 solution (15 mL) was added and
the aqueous phase extracted twice with diethyl ether (20 mL). The organics were washed with water again and dried with Na2SO4
and solvents were removed in vacuo. Could not be purified so crude mixture was taken to next step of Sonagashira reaction. Using
1,2-dichloroethane as an internal standard gave S8 in 61 % yield (0.091 g, 0.239 mmol).
2-(phenylethynyl)-2-(2-(phenylethynyl)phenyl)-1,3-dioxolane (15). To an oven dried round bottom flask under argon equipped with stir bar
was added 2-(2-iodophenyl)-2-(phenylethynyl)-1,3-dioxolane (0.106 g, 0.281 mmol), Pd(PPh3)2Cl2 (0.010 g, 0.014 mmol), CuI (0.005
g, 0.0281 mmol) and 5 mL of triethylamine. The resulting solution was outgassed with argon for 30 minutes. Neat phenylacetylene
(37 µL, 0.337 mmol) was added over 10 minutes. The reaction was stirred for 18 hours at room temperature and monitored by
TLC. Reaction was run through a plug of celite, and washed with NaHCO3 (2 x 15 mL), extracted with EtOAc (3 x 10 mL), dried
with Na2SO4 and solvents removed. Crude mixture was purified by column chromatography (silica gel pH 6-7) using 10 % ethyl
-1
acetate/hexanes to give 15 in 60 % yield (0.059 g, 0.168 mmol) as a yellow oil. IR (neat, cm ) 3062, 2894, 2224, 1492, 1280, 1071,
1
755, 690; H NMR (600 MHz, CDCl3) δ 7.95 – 7.89 (m, 1H), 7.68 – 7.60 (m, 1H), 7.55 (ddt, J = 5.6, 2.7, 1.3 Hz, 2H), 7.45 – 7.40
13
(m, 2H), 7.39 – 7.34 (m, 2H), 7.34 – 7.28 (m, 5H), 7.25 – 7.21 (m, 2H), 4.46 – 4.31 (m, 2H), 4.28 – 4.17 (m, 2H); C NMR (150
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MHz, CDCl3) δ 139.9, 134.1, 131.8, 131.4, 128.9, 128.6, 128.2, 128.1, 128.1, 127.8, 126.2, 123.7, 121.9, 121.6, 102.3, 94.6, 88.4,
86.3, 85.5, 65.3; HR-MS-ESI(+) was performed, calculated as C25H18NaO2 [M+Na]+ = 373.1204, found 373.1206.
1
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3
4
5
6
7
8
1-(1,1-dimethoxy-3-phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (16). To an oven dried round bottom flask under argon
equipped with stir bar was added 1-(1,1-dimethoxy-3-phenylprop-2-yn-1-yl)-2-iodobenzene (0.090 g, 0.239 mmol), Pd(PPh3)2Cl2
(0.008 g, 0.0119 mmol), CuI (0.004 g, 0.0239 mmol) and 4 mL of triethylamine. The resulting solution was outgassed with argon for
30 minutes. Neat phenylacetylene (31 µL, 0.286 mmol) was added over 10 minutes. The reaction was stirred for 4 hours at room
temperature and monitored by TLC. Reaction was run through a plug of celite, and washed with NaHCO3 (2 x 15 mL), extracted
with EtOAc (3 x 10 mL), dried with Na2SO4 and solvents removed. Crude mixture was purified by column chromatography (silica
9
10
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gel pH 6-7) using 3 % ethyl acetate/hexanes to give 16 in 76 % yield (0.064 g, 0.181 mmol) as a light yellow oil. IR (neat, cm )
1
3059, 2936, 2829, 2196, 1490, 1114, 1064, 753, 688; H NMR (400 MHz, CDCl3) δ 7.87 – 7.81 (m, 1H), 7.66 – 7.61 (m, 1H), 7.58
13
– 7.53 (m, 2H), 7.40 – 7.33 (m, 4H), 7.32 – 7.28 (m, 3H), 7.25 – 7.17 (m, 3H), 3.41 (s, 6H); C NMR (100 MHz, CDCl3) δ 140.5,
133.9, 131.8, 131.5, 128.5, 128.3, 128.1, 128.0, 127.9, 127.7, 127.0, 123.8, 122.1, 121.9, 97.2, 94.7, 88.4, 86.0, 85.7, 50.3; HR-MS-
+
ESI(+) was performed, calculated as C25H20NaO2 [M+Na] = 375.1361, found 375.1358.
Synthetic Scheme A. General procedure for the synthesis of compounds (3), (17), (18).
tributyl(11-phenyl-11H-benzo[a]fluoren-6-yl)stannane (3). A round bottom flask connected to water jacketed condenser was flame dried
and purged with argon. 1-(2-((3-phenyl-1-(2-(phenylethynyl)phenyl)prop-2-yn-1-yl)oxy)ethyl)naphthalene (5) (0.118 g, 0.255 mmol)
was added to round bottom flask with 3 mL of anhydrous toluene and outgassed with argon for 10 minutes followed by heating in
an oil bath at 110 °C. Bu3SnH (96 µL, 0.357 mmol) and AIBN (0.012 g, 0.0765 mmol) was dissolved in 2 mL of toluene and out-
gassed with argon for 10 minutes. Using a syringe pump, Bu3SnH and AIBN were added into the reaction vessel (1 mL / hr). The
reaction was allowed to stir for 14 hours or until full consumption of starting material based on TLC analysis. The reaction was
flushed through a celite plug with DCM and solvents were removed under pressure. 1,2-dichloroethane was added as an NMR
27
internal standard which gave 66 % yield of 3, and matches previously reported spectral data in the literature.
Synthesis of 2-((11-phenyl-6-(tributylstannyl)-11H-benzo[a]fluoren-5-yl)oxy)ethan-1-ol (17) and 2-((11-phenyl-
11H-benzo[a]fluoren-5-yl)oxy)ethan-1-ol (17’).
A round bottom flask connected to water jacketed condenser was flame dried and purged with argon. 2-(phenylethynyl)-2-(2-
(phenylethynyl)phenyl)-1,3-dioxolane (15) (0.047 g, 0.134 mmol) was added to round bottom flask with 1 mL of anhydrous toluene
and outgassed with argon for 10 minutes followed by heating in an oil bath at 110 °C. Bu3SnH (51 µL, 0.187 mmol) and AIBN
(0.007 g, 0.0402 mmol) was dissolved in 2 mL of toluene and outgassed with argon for 10 minutes. Using a syringe pump, Bu3SnH
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and AIBN were added into the reaction vessel (1 mL / hr). The reaction was allowed to stir for 14 hours or until full consumption of
starting material based on TLC analysis. The reaction was flushed through a celite plug with DCM and solvents were removed
under pressure. 1,2-dichloroethane was added as an NMR internal standard which gave 12 % yield of 17. The reaction mixture
was dissolved in DCM (10 mL) and 3M HCl (10 mL) and stirred for 1 hour and monitored by TLC analysis. NaHCO3 was slowly
added until gas formation stopped. The aqueous layer was extracted with DCM (3 x 10 mL), dried with Na2SO4, and solvents re-
moved under pressure. Column chromatography was performed using gradient of 2 % and 4 % ethyl acetate/hexanes and gave 17’
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1
(5 mg, 11 % isolated yield) as a beige solid. (m.p. 161-164 °C); IR (neat, cm ) 3264, 3056, 2921, 1586, 1215, 735, 699; H NMR
(600 MHz, CDCl3) δ 8.39 – 8.26 (m, 1H), 7.78 (d, J = 7.5 Hz, 1H), 7.66 – 7.57 (m, 1H), 7.41 – 7.32 (m, 4H), 7.32 (s, 1H), 7.26 –
13
7.18 (m, 4H), 7.16 – 7.08 (m, 2H), 5.27 (s, 1H), 4.44 (t, J = 4.5 Hz, 2H), 4.27 – 4.06 (m, 2H), 2.22 – 2.09 (m, 1H); C NMR (150
MHz, CDCl3) δ 155.2, 149.6, 141.8, 141.1, 139.3, 135.2, 131.0, 128.8, 127.9, 127.1, 127.0, 126.9, 126.7, 125.7, 124.8, 124.6,
+
124.5, 122.9, 119.3, 97.9, 69.9, 61.7, 53.6; HR-MS-ESI(+) was performed, calculated as C25H20NaO2 [M+Na] = 375.1361, found
375.1360.
Synthesis of tributyl(5-methoxy-11-phenyl-11H-benzo[a]fluoren-6-yl)stannane (18) and 5-methoxy-11-phenyl-
11H-benzo[a]fluorene (18’).
A round bottom flask connected to water jacketed condenser was flame dried and purged with argon. 1-(1,1-dimethoxy-3-
phenylprop-2-yn-1-yl)-2-(phenylethynyl)benzene (16) (0.074 g, 0.209 mmol) was added to round bottom flask with 2 mL of anhy-
drous toluene and outgassed with argon for 10 minutes followed by heating in an oil bath at 110 °C. Bu3SnH (79 µL, 0.294 mmol)
and AIBN (0.010 g, 0.0629 mmol) was dissolved in 2 mL of toluene and outgassed with argon for 10 minutes. Using a syringe
pump, Bu3SnH and AIBN were added into the reaction vessel (1 mL / hr). The reaction was allowed to stir for 14 hours or until full
consumption of starting material based on TLC analysis. The reaction was flushed through a celite plug with DCM and solvents
were removed under pressure. 1,2-dichloroethane was added as an NMR internal standard which gave 28 % yield of 18. The reac-
tion mixture was dissolved in DCM (10 mL) and 3M HCl (10 mL) and stirred for 1 hour and monitored by TLC analysis. NaHCO3
was slowly added until gas formation stopped. The aqueous layer was extracted with DCM (3 x 10 mL), dried with Na2SO4, and
solvents removed under pressure. Column chromatography was performed using 5% ethyl acetate/hexanes and gave 18’ (6 mg, 10
-1
1
% isolated yield) as a beige solid. (m.p. 153-156 °C); IR (neat, cm ) 3063, 2930, 2852, 1587, 1452, 1217, 757, 699; H NMR (400
MHz, CDCl3) δ 8.31 (dt, J = 8.5, 0.9 Hz, 1H), 7.80 (dt, J = 7.5, 0.9 Hz, 1H), 7.59 (dt, J = 8.0, 0.9 Hz, 1H), 7.41 – 7.31 (m, 4H),
13
7.30 (s, 1H), 7.25 – 7.18 (m, 4H), 7.15 – 7.09 (m, 2H), 5.27 (s, 1H), 4.16 (s, 3H); C NMR (150 MHz, CDCl3) δ 156.4, 149.6,
141.9, 141.3, 139.3, 134.6, 130.9, 128.8, 127.9, 127.0, 126.9, 126.8, 126.7, 125.8, 124.8, 124.5, 124.3, 123.1, 119.3, 96.6, 55.7,
+
53.6; HR-MS-ESI(+) was performed, calculated as C24H17O [M] = 321.1279, found 321.1277.
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Thermal cycloaromatization product of (15) to 10-phenylspiro[benzo[b]fluorene-11,2'-[1,3]dioxolane] (19) using conditions from
Synethetic Scheme A. 1,2-dichloroethane was added as NMR internal standard to the reaction mixture containing 19 which gave
19 in 55 % yield. Column chromatography was performed using gradient of 2% and 4% ethyl acetate/hexanes and gave 19 (9 mg,
1
2
3
4
5
6
-1
1
20 % isolated yield) as a yellow solid (m.p. 185-188 °C); IR (neat, cm ) 3056, 2924, 1496, 1252, 1072, 747, 701; H NMR (600
7
8
MHz, CDCl3) δ 8.00 (s, 1H), 7.91 – 7.86 (m, 1H), 7.78 – 7.72 (m, 1H), 7.59 – 7.27 (m, 11H), 4.10 – 3.99 (m, 2H), 3.19 – 3.09 (m,
9
13
2H); C NMR (150 MHz, CDCl3) δ 146.2, 138.5, 138.2, 137.9, 137.4, 137.3, 134.9, 133.8, 130.8, 130.0, 128.8, 128.2, 127.5,
10
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+
127.1, 126.9, 126.5, 125.7, 123.3, 120.3, 117.8, 113.4, 65.7; HR-MS-ESI(+) was performed, calculated as C25H18NaO2 [M+Na] =
373.1204, found 373.1207.
Thermal cycloaromatization product of (16) to 11,11-dimethoxy-10-phenyl-11H-benzo[b]fluorene (20) using conditions from Synthetic
Scheme F. 1,2-dichloroethane was added as NMR internal standard to the reaction mixture containing 20 which gave 20 in 60 %
yield. Column chromatography was employed, however 20 hydrolyzed to the ketone 10-Phenylbenzo[b]fluorenone which ketal
could not be isolated. The cyclized ketone matches previously reported spectra.35 H NMR (600 MHz, CDCl3) δ 7.94 (s, 1H), 7.89 –
1
13
7.87 (d, J = 8 Hz, 1H), 7.78 – 7.76 (d, J = 8 Hz, 1H), 7.64 – 7.52 (m, 7H), 7.39-7.31 (t, J = 8 Hz, 4H); C NMR (150 MHz,
CDCl3) δ 192.2, 144.0, 138.3, 136.6, 136.3, 135.4, 134.6 133.8, 129.6, 129.1, 128.9, 128.7, 128.0, 127.9, 126.7, 124.1, 120.6,
118.6.
Procedure for synthesis of silyl enol ether (8).
2-(naphthalen-1-yl)acetaldehyde (S9). An oven dried flask was put under argon and Dess-Martin reagent (0.219 g, 0.516 mmol, 1.2
equiv) was weighed out and transferred to reaction vessel. Methylene chloride (4 mL) was used to dissolve 2-(naphthalen-1-yl)ethan-
1-ol (0.074 g, 0.43 mmol) and transferred all at once at room temperature. The reaction was monitored by TLC analysis and
showed full consumption of starting material after 2 hours. The reaction mixture was diluted with DCM and washed with Na2S2O3
(10 mL), extracted with DCM (2 x 10 mL), followed by saturated NaHCO3 solution (15 mL) and extracted again with DCM (2 x 10
mL). The combined organics were washed with brine solution (20 mL), dried with Na2SO4, and solvents removed under pressure.
The crude reaction mixture was filtered through silica gel plug (pH 6-7) in 20 % ethyl acetate/hexanes and gave S9 (0.408 mmol,
95 % yield) and matches previously reported spectra.36
tert-butyldimethyl((2-(naphthalen-1-yl)vinyl)oxy)silane (8). To an oven dried flask under argon was added S9 (0.146 g, 0.857 mmol) in 8
mL of methylene chloride. The reaction flask was put on an ice water bath. Diisopropylethylamine (0.3 mL, 1.71 mmol) was added
all at once to reaction flask and stirred for 15 min. Freshly distilled tert-butyldimethylsilyl triflate/TBSOTf (0.24 mL, 1.03 mmol)
was added all at once and color changed from clear to yellow. Reaction was allowed to stir for 15 min. at 0 °C followed by another
15 min. at room temperature and checked by TLC analysis. Once reaction was complete, the reaction was quenched with saturated
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NaHCO3 solution (15 mL), extracted with DCM (3 x 10 mL) and combined organics were washed with brine (15 mL) and dried
with Na2SO4. Once the solvents were removed under pressure, the crude mixture was purified using flash chromatography (silica
gel pH 6-7) with 5 % ethyl acetate/hexanes to afford 8 (0.202 g, 0.712 mmol, 83 % yield, 85 % cis : 15 % trans) as a clear oil. Rf =
1
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4
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6
-1
0.78 in 10 % ethyl acetate/hexanes; IR (neat, cm ) 3048, 2929, 2857, 1635, 1256, 1100, 776; (Both cis and trans reported for
7
8
1
NMR) H NMR (600 MHz, CDCl3) δ 8.16 (dd, J = 7.3, 1.2 Hz, 1H), 8.14 – 8.11 (m, 1H), 8.11 – 8.08 (m, 0.15H), 7.88 – 7.81 (m,
9
1H), 7.72 (dd, J = 13.4, 8.0 Hz, 1H), 7.53 – 7.39 (m, 4H), 6.96 (dd, J = 11.9, 0.9 Hz, 0.12H), 6.74 (d, J = 11.9 Hz, 0.14H), 6.69 (d,
J = 6.7 Hz, 1H), 6.02 (d, J = 6.7 Hz, 1H), 1.03 (d, J = 0.8 Hz, 1H), 0.97 (d, J = 0.7 Hz, 9H), 0.28 (d, J = 0.7 Hz, 1H), 0.23 (d, J =
10
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0.6 Hz, 6H); C NMR (150 MHz, CDCl3) δ 143.6, 141.1, 133.6, 131.7, 131.1, 128.5, 128.3, 126.7, 126.2, 125.6, 125.4, 125.4,
+
125.1, 124.3, 123.9, 122.8, 110.1, 104.9, 25.6, 18.2, -5.1, -5.3; APPI-FT-ICR(+) was performed, calculated as C18H24OSi [M] =
284.1591, found 284.1589.
Supporting Information
See supporting information for 1H NMR, 13C NMR, crystal structure data for compound 19, and computational details. The support-
ing information is available free of charge at http://pubs.acs.org.
Acknowledgement
I.A. is grateful to the NSF (CHE-1465142) for support. We thank Christopher J. Evoniuk and Dr. Nikolay Tsvetkov for helpful discussions,
Dr. Vladislav Lobodin for help with MS analysis, NSF XSEDE (TG-CHE160006) and RCC FSU for computational resources. G. G
thanks IBM for the 2016 IBM Ph.D Scholarship. We appreciate a thought-provoking question from Professors Uta Wille and Carl Schies-
ser at the 2015 Physical Organic GRC that stimulated this work.
1
(a) Mohamed, R.; Mondal, S.; Gold, B.; Evoniuk, C. J.; Banerjee, T.; Hanson, K.; Alabugin, I.V. J. Am. Chem. Soc. 2015, 137, 6335. (b) Pati, K.;
Gomes, G. P.; Harris, T.; Hughes, A.; Phan, H.; Banerjee, T.; Hanson, K.; Alabugin, I.V. J. Am. Chem. Soc. 2015, 137, 1165 (c) Mondal, S.; Gold,
B.; Mohamed, R. K.; Alabugin, I. V. Chem. Eur. J. 2014, 20, 8664. (d) Wille, U. Chem. Rev. 2013, 113, 813. (e) Debien, L; Zard, S. Z. J. Am. Chem.
Soc. 2013, 135, 3808. (f) Evoniuk, C. J.; Ly, M.; Alabugin, I. V. Chem. Commun. 2015, 51, 12831. (g) Mohamed, R. K.; Mondal, S.; Jorner, K.; Faria
Delgado, T.; Lobodin, V. V.; Ottosson, H.; Alabugin, I.V. J. Am. Chem. Soc. 2015, 137, 15441.
2 Selected examples of alkoxy radical formation from cleavage of O-O, O-N bonds and O-I bonds: (a) Wiebe, H. A.; Heicklen, J. J. Am. Chem. Soc.
1973, 95, 1. (b) Mendenhall, G. D.; Sheng, X. C. J. Am. Chem. Soc. 1991, 113, 8976. (c) Lorance, E. D.; Kramer, W. H.; Gould, I. R. J. Am. Chem.
Soc. 2002, 124, 15225. (d) Waldemar, A.; Marquardt, S.; Kemmer, D.; Saha-Moller, C. R.; Schreier, P. Org. Lett. 2002, 4, 225. (e) Headlam, H. A.;
Mortimer, A.; Easton, C. J.; Davies, M. J. Chem. Res. Toxicol. 2000, 13, 1087. (f) Banks, J. T.; Scaiano, J. C. J. Phys. Chem. 1995, 99, 3527. (g) Con-
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1
2
3
4
5
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7
8
9
cepcion, J. I.; Francisco, C. G.; Hernandez, R.; Salazar, J. A.; Suarez, E. Tetrahedron Lett. 1984, 25, 1953. (h) Antunes, C. S. A.; Bietti, M.; Ercolani,
G.; Lanzalunga, O.; Salamone, M. J. Org. Chem. 2005, 70, 3884.
3 CH3-OCH3 has C-O BDE of 83 kcal/mol. Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255.
4
(a) Baroudi, A.; Alicea, J.; Flack, P.; Kirincich, J.; Alabugin, I. V. J. Org. Chem. 2011, 76, 1521. (b) Baroudi, A.; Mauldin, J.; Alabugin, I. V. J.
Am. Chem. Soc., 2010, 132, 967.
5 Zhao, Y.; Truhlar, D. G., Acc. Chem. Res. 2008, 41, 157.
6 Zhao, Y.; Truhlar, D. G., J. Phys. Chem. A, 2008, 112, 1095.
7 http://www.chemcraftprog.com. (b) CYLview, 1.0b; Legault, C. Y., Université de Sherbrooke, 2009 (http://www.cylview.org).
10
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8 Weinhold F. in Schleyer P.V. R. Ed. Encyclopedia of Computational Chemistry: Wiley: New-York, 1998, 3, 1792.
9
For intramolecular trapping of alkoxy radicals with silyl enol ethers, see: (a) Zlotorzynska, M.; Zhai, H.; Sammis, G. M. Org. Lett. 2008, 10, 5083
(b) Rueda-Becerril, M.; Leung, J. C. T.; Dunbar, C. R.; Sammis, G. M. J. Org. Chem. 2011, 76, 7720.
10 See SI for additional 1H NMR data.
11 Yields given based on standard of 1,2-dichloroethane in CDCl3.
12
For precedents of fragmentations of phenethyloxy radicals to benzylic radicals see: (a) Ledwith, A.; Russell, P. J.; Sutcliffe, L. H. Proc. Roy. Soc. A,
1973, 332, 151. (b) Ledwith, A.; Russell, P.J.; Sutcliffe, L. H. J. Chem. Soc., Perkin Trans. 2, 1973, 630 (c) Trahanovsky, W. S.; Macaulay, D. B. J.
Org. Chem. 1973, 38, 1497.
13 Zhan, Z.-P; Yu, J.-L; Liu, H.-J; Cui, Y.-Y; Yang, R.-F; Yang, W.-Z; Li, J.-P. J. Org. Chem. 2006, 71, 8298.
14 Yields given based on standard of 1,2-dichloroethane in CDCl3.
15
1
Haak, S. et. al. reported appearance of formaldehyde in H-NMR in CDCl3 at 9.75 ppm: Haak, S.; Neels, A.; S.-Evans, H.; S.-Fink, G.; Thom-
as, C. M. Chem. Commun. 1999, 1959.
16 The bond energy of an O-H bond is ca. 102 kcal/mol. T. L. Cotrell: “The Strengths of Chemical Bonds.” Butterworths, London 1954.
17 Laarhoven, L.; Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342.
18 For an example of switch in reaction selectivity for C-C bond cleavage when such alignment is impossible, see ref. 1f.
19 (a) Freitas, M. P. Org. Biomol. Chem. 2013, 11, 2885-2890. Graczyk, P. P.; Mikolajczyk, M. Topics in Stereochemistry, 1994, 21, 159. (b) Juaris-
ti, E. Conformational Behavior of Six-Membered Rings; VCH Publishers: New York, 1995. (c) Zefirov, N. S.; Shekhman, N. M. Russ. Chem. Rev.
1971, 40, 315.
20 Gomes, G. P.; Vil’, V.; Terent'ev, A.; Alabugin, I. V. Chem. Sci. 2015, 6, 6783.
21 (a) Whiffen, D. H. Mol. Phys. 1963, 6, 223. (b) Davies, A. G. J. Chem. Soc., Perkin Trans 2, 1999, 11, 2461.
22 Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer, P.v. R.; Chem. Rev. 2005, 105, 3842. See SI for more NICS values.
23 Yield of radical cyclization of methoxy precursor is 69% based on 1,2-dichloroethane standard.
24
The rate limiting step of the radical cascade using the methoxy substrate is the 1,5-hydrogen shift (~18 kcal/mol); see ref 1b for more infor-
mation.
25
(a) Beckwith, A. L. J.; Crich, D.; Duggan, P. J.; Yao, Q. Chem. Rev. 1997, 97, 3273 (b) Barton, D. H.; Dowlatshahi, H. A.; Motherwell, W. B.;
VIllemin, D. J. Chem. Soc., Chem. Commun. 1980, 732.
26 See ref 1a for more examples of C-C scission in radical cascades.
27 X-ray data of 19 are included in the SI.
28 Rodriguez, D.; Navarro, A.; Castedo, L.; Dominguez, D.; Saa, C. Org. Lett., 2000, 2, 1497.
29 For analysis of the kinetics/thermodynamics relationships in radical reactions through the prism of Marcus theory, see: Alabugin, I.V.; Mano-
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