Mechanism and Unusual Fragmentation Selectivities of Aryltrialkylstannane Cation Radicals

Article abstract of DOI:10.1021/acs.joc.5b01690

Aryltrialkylstannane cation radicals were generated and characterized by nanosecond transient absorption spectroscopy. Kinetics show the fragmentations of the stannane cation radicals occur by a bimolecular, nucleophile-assisted mechanism (S_N2). Consistent with this hypothesis, steric effects on both the nucleophile and the stannane cation radicals were observed. Steady-state, preparative photooxidation experiments show that aryltrimethylstannane cation radicals have an unusual preference for loss of aryl radicals over methyl radicals and that the selectivity for aryl vs methyl radical loss is dependent on the identity of the nucleophile. The preference for loss of aryl radicals is rationalized by a hypothesis based on Bents rules.

Full text of DOI:10.1021/acs.joc.5b01690

Article
pubs.acs.org/joc
Mechanism and Unusual Fragmentation Selectivities of
Aryltrialkylstannane Cation Radicals
Pu Luo and Joseph P. Dinnocenzo*
Department of Chemistry, University of Rochester, Rochester, New York 14627, United States
S
* Supporting Information
ABSTRACT: Aryltrialkylstannane cation radicals were generated and
characterized by nanosecond transient absorption spectroscopy. Kinetics
show the fragmentations of the stannane cation radicals occur by a bimolecular,
nucleophile-assisted mechanism (SN2). Consistent with this hypothesis, steric
effects on both the nucleophile and the stannane cation radicals were observed.
Steady-state, preparative photooxidation experiments show that aryltrimethyl-
stannane cation radicals have an unusual preference for loss of aryl radicals over methyl radicals and that the selectivity for aryl vs
methyl radical loss is dependent on the identity of the nucleophile. The preference for loss of aryl radicals is rationalized by a
hypothesis based on Bent’s rules.
generating the cation radical and examining its reaction kinetics
in the presence and absence of nucleophiles. Generation of 1+•
was attempted by nanosecond transient absorption spectros-
copy (NTAS) using cosensitized photooxidation of 1 in
acetonitrile or dichloromethane with N-methylquinolinium
INTRODUCTION
■
Organostannanes have long been known to undergo bond
fragmentation upon one-electron oxidation to give an organic
radical and a stannylium cation fragment.1,2 Not surprisingly,
when the stannanes are unsymmetrically substituted, these
oxidative fragmentation reactions generally lead to formation of
the more stable organic radical. For example, the fragmentation
of alkyltrimethylstannane cation radicals (RSnMe3+•) generated
in the gas phase by electron impact uniformly showed
preferential formation of the more substituted (and more
stable) alkyl radicals.1 Fragmentation selectivities for oxidation
of unsymmetrically substituted tetraalkylstannanes determined
in solution similarly show that losses of the more stable alkyl
radicals are strongly preferred.2 In contrast to these trends,
Shine and co-workers reported that the reaction of phenyl-
trimethylstannane (1) with thianthrene perchlorate (2) in
acetonitrile led to oxidative fragmentation with predominant
loss of the less stable phenyl radical over the methyl radical.3
The reaction of 1 with 2 was proposed to proceed by initial
one-electron oxidation and subsequent unimolecular fragmen-
tation of 1+• to primarily give phenyl radical. This seemingly
contrathermodynamic outcome was rationalized by proposing
preferential solvent stabilization of the Me3Sn+ fragment,
resulting from Ph• loss, over the larger PhMe2Sn+, resulting
from Me• loss. An alternative mechanistic hypothesis is that the
fragmentation of 1+• in acetonitrile is nucleophile-assisted,
presumably with the solvent acting as the nucleophile. This
latter mechanism is well precedented in the fragmentation
reactions of organosilane cation radicals.4 Described herein are
our experiments to generate aryltrialkylstannane cation radicals,
elucidate their fragmentation mechanism, and determine their
fragmentation selectivities.
hexafluorophosphate (NMQ+PF6 ) as a photooxidant and
−
toluene (PhMe, 0.5 M) as a codonor.5,6 Under these
conditions, PhMe+• (λmax 430 nm) is generated within the
laser pulse (10 ns). Addition of 1 (up to 0.01 M) results in a
diffusion-controlled reaction with PhMe+•; however, no
transient spectrum could be subsequently detected in the
350−700 nm spectral range. Because 1+• is expected to absorb
in this region, the simplest way to explain this result is that 1+•
is formed by reaction of 1 with PhMe+• but the resulting
stannane cation radical decays more rapidly than it is formed.
Attempted generation of 1+• in hexafluoroisopropyl alcohol
(HFIP), which often stabilizes cation radicals,7 was foiled by
the rapid, thermal destannylation of 1 in HFIP. Unable to
directly observe 1+• by NTAS, we next sought to examine the
chemistry of similar stannane cation radicals that might be
longer lived. (4-Biphenyl)trimethylstannane (3) turned out to
be a suitable candidate.
Preparative photooxidation of 3 was first carried out to
determine the aryl/Me loss selectivity for comparison to that
from reaction of 1 with 2. Using 1,2,4,5-tetracyanobenzene
(TCB) as a photooxidant in CD3CN and a variety of codonors
(0.1−0.5 M PhMe, PhBut, PhCl, and o-PhF2), the aryl/Me loss
ratios were determined at partial conversion (<30%) after
irradiation at 313 nm. Following the method of Wong and
Kochi,2b treatment of the crude reaction mixtures with LiCl
gave stannyl chlorides Me3SnCl and 4-BPMe2SnCl, whose
ratios were determined by 1H NMR. Mass balance, determined
by addition of an internal standard, was >95% in all cases. The
RESULTS AND DISCUSSION
■
In principle, it is possible to distinguish between the
Received: July 21, 2015
mechanisms discussed above for the fragmentation of 1+• by
© XXXX American Chemical Society
A
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average, statistically corrected aryl/Me ratio determined from
the Me3SnCl/4-BPMe2SnCl ratios was found to be 11.8 0.3
and was independent of the codonor used. It is interesting to
note that this ratio is remarkably similar to that reported by
Shine for the oxidation of 1 by 2 (Ph/Me = 14).3 Encouraged
by these results, we next sought to generate 3+• by NTAS.
Photolysis (343 nm; 10 ns) of a solution containing 1 mM
NMQ, 0.5 M PhMe, and 0.01 M 3 in dioxygen-saturated
CH3CN gave the transient spectrum shown in Figure 1a after
Figure 2. Transient spectra assigned to (a) the 4-methoxyphenyl
peroxyl radical in CH3CN and (b) 4+• in CH2Cl2.
that of the 4-methoxyphenyl peroxyl radical, which was
independently generated by photolysis of 4-iodoanisiole in
O2-saturated CH3CN.10 The transient spectrum generated by
photooxidation in CH2Cl2 is shown in Figure 2b. This species
reacts rapidly (k = 1 × 1010 M−1 s−1) with the good electron
donor 4,4′-dimethoxybiphenyl (DMBP) to produce DMBP+•
(λmax ∼425 nm).11 We accordingly assign the transient species
to 4+•. As with 3+•, 4+• reacts rapidly with added nucleophiles.
The rate constants for reaction with CH3CN, HOMe, and
HOBut in CH2Cl2 are 1.1 × 107, 6.4 × 107, and 1.5 × 107 M−1
s−1, respectively. The HOMe/HOBut rate constant ratio again
shows a clear steric effect on the nucleophile. As shown in
Figure 2b, the absorption spectrum of 4+• shows two bands in
the visible region (λmax ∼430 and 550 nm). Rate constants
measured at both 430 and 550 nm were indistinguishable
( 10%), consistent with both absorption bands in Figure 2b
belonging to 4+•. We note that the short-wavelength band is
similar to that found in anisole cation radical (λmax ∼425 nm).12
The long-wavelength band is presumably associated with an
electronic transition involving the trimethylstannyl group.
Although the assignment of the spectrum for 4+• seems secure,
further work will be required to determine the precise nature of
these spectral transitions.
Figure 1. Transient spectra assigned to (a) the 4-biphenyl peroxyl
radical in CH3CN and (b) 3+• in CH2Cl2.
100 ns. This spectrum is not due to 3+•, as the transient does
not react with good electron donors such as 1,2,4,5-
tetramethoxybenzene (TMB). Importantly, the transient is
not observed in the absence of O2. The transient spectrum is in
good agreement with that of the 4-biphenyl peroxyl radical (4-
BPOO•), which was independently generated by photolysis of
4-iodobiphenyl in O2-saturated CH3CN.8 The formation of 4-
BPOO• is consistent with initial generation of 3+• and its
subsequent fragmentation in CH3CN to give 4-BP•, which
rapidly reacts with O2. If this is correct, we reasoned that 3+•
might be directly observed by NTAS in a less nucleophilic
solvent. This was indeed the case. Laser flash photolysis as
described above but in CH2Cl2 produced the transient
spectrum shown in Figure 1b. This transient species reacted
rapidly with TMB (k = 2.1 × 1010 M−1 s−1) to produce TMB•+
(λmax 450 nm). On the basis of this result and the similarity of
the transient spectrum to that of biphenyl+• (λmax ∼380 and
670 nm),9 we assign the transient species to 3+•. Importantly,
the transient was found to rapidly react with added
nucleophiles in reactions that were first order in 3+• and first
order in nucleophile. For example, 3+• reacts with CH3CN with
a rate constant of 1.5 × 108 M−1 s−1 (see the Supporting
Information for representative, pseudo-first-order rate plots).
This large rate constant readily explains why 3+• was not
observed in CH3CN by NTAS. The stannane cation radical 3+•
also reacts with alcohols. The rate constants for reaction with
HOMe and HOBut are 1.9 × 108 and 5.9 × 107 M−1 s−1,
respectively, consistent with a steric effect on the nucleophile.
Results similar to those with 3 were obtained with (4-
methoxyphenyl)trimethylstannane (4). Preparative photoox-
idation of 4 with TCB in CH3CN using the same four codonors
used with 3 gave an average, statistically corrected aryl/Me loss
ratio of 1.1 0.1, with mass balances of >95%. Although this
ratio is lower than that observed for 3, it is still noteworthy that
loss of the less stable aryl radical is competitive with methyl
radical loss. Photooxidation of 4 by NTAS using NMQ/PhMe
in dioxygen-saturated CH3CN gave the transient spectrum
shown in Figure 2a after 100 ns. This spectrum matches well
Although the kinetic experiments clearly show that 3+• and
4+• react with added nucleophiles, it is worth noting that the
experiments do not unequivocally demonstrate that the
reactions lead to fragmentation of the stannane cation radicals.
If a nucleophile-assisted mechanism is operative, the aryl/Me
loss ratio is predicted to be nucleophile-dependent. This
prediction was tested experimentally by performing preparative
photooxidations of 3 and 4 in CH2Cl2/NMQ/PhCH3 with
relatively high concentrations (∼1 M) of added nucleophiles to
ensure that >95% of 3+• and 4+• react with the targeted
nucleophiles instead of adventitious nucleophiles in the solvent.
The Ar/Me loss ratios were determined after photooxidation
with HOMe, HOBut, and CH3CN as nucleophiles from the
stannyl chloride products after treatment with LiCl, as
described above. The statistically corrected fragmentation
selectivities are shown in Table 1 and demonstrate that the
aryl/Me loss ratios are indeed dependent on the nucleophile for
both 3 and 4. It is noteworthy that the Ar/Me loss ratios
measured for CH3CN in CH2Cl2 for 3 and 4 (14.4 and 0.5) are
reasonably similar to those found in CH3CN (11.8 and 1.1).
Interestingly, the fragmentation selectivities for 4+• with HOMe
and HOBut as nucleophiles show that, as with 3+•, aryl radical
loss can also predominate in the fragmentation of 4+•. Finally,
we note that the fragmentation selectivities for 4 show variation
greater than that for 3. The origin of this subtle effect is, as yet,
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cation radicals. A plausible and simple explanation is based on
Bent’s rules,13 one of which states that trigonal-bipyramidal
species are generally more stable when the more electro-
negative substituents are located in the apical positions. Because
an aryl group is significantly more electronegative than a methyl
group, one might plausibly expect a transition state where the
aryl group is in the apical position would be energetically
favored,14 leading to aryl radical loss, despite this transition
state leading to loss of the less stable radical. This could explain
the nucleophile-assisted fragmentation selectivities for 3+• and
4+•. However, the fragmentation reactions of 5+• and 6+• clearly
demonstrate that radical stability also plays an important role in
determining the fragmentation selectivities of stannane cation
radicals.
Table 1. Statistically Corrected Aryl/Me Loss Ratios for
Photooxidations of 3 and 4 in CH2Cl2 with Methanol, tert-
Butyl Alcohol, and Acetonitrile as Nucleophiles
a
aryl/Me loss ratios
nucleophile
methanol
3
4
8.9 0.2
7.0 0.2
14.4 0.1
13.6 0.2
6.8 0.1
0.5 0.1
tert-butyl alcohol
acetonitrile
a
Average of at least two determinations.
unclear. Most important, however, is that the combined data
show that the Ar/Me loss ratios for these stannane cation
radical fragmentations clearly depend on the identity of the
nucleophile and the departing radical, consistent with the
nucleophile-assisted fragmentation mechanism.
Reaction of Phenyltrimethylstannane with Thian-
threne Perchlorate. As mentioned in the Introduction, our
investigation of the mechanism and fragmentation selectivities
of aryltrialkylstannane cation radicals was originally motivated
by the report of Shine et al. on the oxidation of phenyl-
trimethylstannane (1) by thianthrene perchlorate (2).3 While
examining this reaction further, we made an interesting and
mechanistically revealing discovery. The reaction of 1 with 2 in
Shine’s study was done by adding 1 to a solution of 2 in
CH3CN and monitoring the reaction by the disappearance of
the intense color of 2, which we found took >24 h at 22 °C.
Interestingly, when several drops of a solution containing 2
were instead added to a stirred solution containing excess 1
(0.056 M), the color of 2 disappeared immediately (<1 s),
demonstrating that the reaction of 1 with 2 is, in fact, quite
rapid. As explained below, on the basis of the reactivity data and
an estimate of the free energy for electron transfer from 1 to 2,
an electron transfer mechanism for the reaction of 1 and 2 can
be confidently excluded by a simple kinetic competence test.
The free energy for electron transfer from 1 to 2 can be
determined from the difference in the oxidation potentials of 1
and thianthrene. Literature oxidation potentials for thianthrene
in CH3CN vary from 1.22 to 1.30 V vs SCE.15 Although the
oxidation potential of 1 has not been determined electro-
The nucleophile-assisted fragmentation mechanism also
predicts that the rate constants for reaction of aryltrialkyl-
stannane cation radicals with nucleophiles should be dependent
on the steric environment around the tin atom. To test this
prediction, we sought aryltrialkylstannanes with alkyl groups
that had comparable leaving group ability (i.e., radical stability)
but that differed sterically. Stannanes 5 and 6 (Np = neopentyl)
were found to be suitable substrates. These stannanes had the
additional virtue that preparative photooxidation experiments
demonstrated that they both led to exclusive loss of the alkyl
groups, showing that the aryl/alkyl loss selectivity is quite
sensitive to the relative stabilities of the aryl and alkyl groups.
Photooxidation of 5 and 6 with NMQ/PhMe in CH2Cl2 led to
similar transients (Figure 3). Both species rapidly reacted with
chemically, its ionization potential (8.83
0.05 eV)16 is
indistinguishable from that of toluene (8.8228 0.0001 eV),17
whose oxidation potential has recently been accurately
determined to be 2.26 0.02 V vs SCE in CH3CN.9 Given
their comparable ionization potentials and structural similar-
ities, the oxidation potential of toluene should be a reasonable
estimate for that of 1.
A conservative way to estimate the maximum rate constant
for electron transfer from 1 to 2 is to equate the free energy of
electron transfer (ΔGet) to the activation free energy (ΔGet⧧).
Using the largest, literature oxidation potential for 2 (1.30 V vs
SCE)15a and 2.26 V for the oxidation potential of 1, ΔGet is
estimated to be 22 kcal/mol. Using the Eyring equation and
Figure 3. Transient spectra assigned to (a) 5+• and (b) 6+• in CH2Cl2.
TMB (k ≈ 4 × 109 M−1 s−1) to give TMB+•, consistent with
their assignment to 5+• and 6+•. Both cation radicals reacted
with added nucleophiles, but with significantly different rate
constants. For example, with HOMe as nucleophile, the rate
constant for reaction with 5+• is ∼15 times greater than that
with 6+• (k = 6.1 × 106 vs 4.0 × 105 M−1 s−1). The rate
constant ratio is ∼30 with acetonitrile as nucleophile (k = 2.2 ×
106 vs 7.5 × 104 M−1 s−1). These experiments provide a final
piece of evidence consistent with a nucleophile-assisted
fragmentation mechanism.
⧧
substituting ΔGet for ΔGet gives a maximum rate constant for
electron transfer of 2.4 × 10−4 s−1 at 22 °C for the species in
contact. Using the rate constants for diffusional encounter and
separation of 1 and 2 in CH3CN,18 and the concentration of 1
in the experiment described above (0.056 M), the rate constant
for electron transfer from 1 to 2 is calculated to be ≲8 × 10−6
s−1, which corresponds to a half-life of ≳90000 s: i.e., ∼105
times greater than that observed experimentally! Given that this
conservative model does not take into account the reorganiza-
tion energies for electron transfer, which will increase in the
estimated reaction half-life, one can confidently exclude an
electron transfer mechanism for the reaction of 1 with 2. A
Having established that the fragmentation mechanism of
aryltrialkylstannanes is nucleophile-assisted, one can now
speculate with more confidence on the origin of the unusual
selectivity for aryl radical loss from the aryltrimethylstannane
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Reactions utilizing microwave heating were conducted in sealed
Teflon-capped pressure tubes in an Explore 48 model 909480
microwave reactor.
Purifications by column chromatography were performed using
230−400 mesh silica gel.
Solvents. Reagent grade solvents were used for all extraction and
workup procedures. Hexanes and acetone were distilled before use.
Diethyl ether, tetrahydrofuran, and acetonitrile were purified by
passing through a column of activated alumina from a solvent
purification system.21 Dichloromethane was distilled from phosphorus
pentoxide before use.
more likely hypothesis is that 2 reacts with 1 by a polar
mechanism, in analogy with examples of stannane oxidations
with other electron-deficient species.2b,19,20 With some irony,
now two things are clear. First, the similar Ar/Me
fragmentation selectivities observed in the reaction of 1 with
2 and for the authentic stannane cation radicals reported herein
must now be considered fortuitous. Second, the reaction of 1
with 2, which was the original motivation for the present work
on aryltrialkylstannane cation radical fragmentations, does not
proceed via stannane cation radical 1+• as an intermediate.
Materials. Toluene, 1,2-difluorobenzene, tert-butylbenzene, and
chlorobenzene were distilled before use. Spectral grade methanol,
ethanol, and tert-butyl alcohol were distilled before use. 1,2,4,5-
Tetracyanobenzene (TCB) was recrystallized from chloroform.
1,2,4,5-Tetramethoxybenzene was generously provided by the Kodak
Co. Trimethylphenylstannane,22 bis(4-methoxyphenyl)-
dimethylstannane,23 (4-methoxyphenyl)trimethylstannane,24 tri-n-
butyl(3,4-dimethoxyphenyl)stannane,25 and dineopentyltin dichlor-
ide26 were prepared by literature procedures.
The following materials were purchased from commercial sources
and used as received: trimethyltin chloride (1.0 M solution in THF),
4-bromoanisole, 4-bromoveratrole, lithium chloride, tri-n-butyltin
chloride, di-n-butyltin dichloride, iodobenzene, 4-iodoanisole, 4-
bromobiphenyl, phenyltin trichloride, 4-iodobiphenyl, neopentyl
chloride, dichlorodimethyltin, and di-n-butyltin dibromide.
Nanosecond Transient Absorption Spectrosopy. A XeCl
excimer laser (308 nm) was used to pump a dye laser containing p-
terphenyl laser dye for 343 nm excitation. Transient spectral
absorptions were monitored at a right angle to the laser excitation
by using a home-built xenon flashlamp system equipped with a
PerkinElmer FX-193 flashlamp to generate the analyzing light. The
analyzing light was focused into the end of a fiber optic cable and onto
the entrance slit of a monochromator equipped with an intensified
CCD. A pulsed xenon arc lamp was used as the monitoring light
source for kinetics analyses. The monitoring light was passed through a
monochromator and detected using a photomultiplier tube. The signal
from the PMT was directed into a digitizing oscilloscope and then to a
computer for viewing, storage, and data analysis.
CONCLUSIONS
■
In conclusion, several aryltrialkylstannane cation radicals have
been generated and spectroscopically characterized in solution
at room temperature for the first time. Nanosecond transient
absorption experiments demonstrated that the stannane cation
radicals undergo rapid fragmentation by a bimolecular,
nucleophile-assisted mechanism. Consistent with this hypoth-
esis, steric effects on both the nucleophile and the stannane
cation radicals were observed. Preparative photooxidation
experiments show that aryltrimethylstannane cation radicals
have an unusual preference for loss of aryl radicals over methyl
radicals and that the aryl/methyl selectivities are dependent on
the identity of the nucleophile. Finally, we note that the
selective generation of aryl radicals from stannane cation
radicals may find utility in organic synthesis.
EXPERIMENTAL SECTION
■
General Experimental Procedures and Techniques. Unless
otherwise noted, the following conditions were used for all
nonaqueous reactions. Reactions were conducted at room temperature
in oven-dried glassware (125 °C) under a nitrogen atmosphere, and
solutions were stirred magnetically using Teflon-coated magnetic stir
bars. Air- and moisture-sensitive reagents and solutions were
transferred via syringe or cannula and were introduced to the
apparatus through rubber septa or three-way stopcocks under a
vigorous nitrogen purge.
Preparation of (4-Biphenyl)trimethylstannane (3). A 25 mL
flask was charged with 0.130 g (5.4 mmol) of magnesium turnings, 3
mL of THF, and a small crystal of iodine. After gentle warming with a
heat gun, 2 mL of THF was added, followed by the addition of 1.16 g
(4.98 mmol) of 4-bromobiphenyl in 8 mL of THF at a rate sufficient
to maintain reflux. After reflux for 1 h, a trimethyltin chloride solution
(5 mmol, 5 mL of 1 M in THF) was added to the reaction mixture.
The resulting mixture was refluxed for 1 h and stirred at room
temperature for 10 h. After treatment with aqueous saturated
ammonium chloride, the reaction mixture was extracted with diethyl
ether (30 mL × 4). The combined organic layers were dried over
MgSO4 and concentrated to give a yellow solid. Purification by column
chromatography (hexanes, SiO2) gave white crystals (1.43 g, 90%).
Further recrystallization from methanol gave colorless plates.
1H NMR (CDCl3 400 MHz): δ 7.66−7.53 (m, 6.0 H), 7.45 (t, J =
Routine 1H NMR spectra were recorded at either 400 or 500 MHz.
Chemical shifts (δ) are given in ppm relative to tetramethylsilane using
the residual proton in the solvent as an internal standard (CHD2CN, δ
1.95; CHDCl2, δ 5.35; CHCl3, δ 7.27). Proton−proton coupling
constants reflect assumed first-order behavior. The following
abbreviations were used: s (singlet), d (doublet), t (triplet), q
(quartet), pent (pentet), dd (doublet of doublets), dt (doublet of
triplets), m (multiplet), and br (broad). Peak integrations were
normalized by multiplying the fractional peak area (area peak/sum of
all peak areas) by the total number of protons in the proposed
structure. 13C NMR spectra were recorded at either 100 or 125 MHz;
chemical shifts are referenced to internal chloroform-d (CDCl3, δ 78.0
ppm) unless otherwise stated. 119Sn NMR spectra were recorded at
186.5 MHz on a 500 MHz NMR spectrometer; chemical shifts (δ) in
ppm are relative to tetramethyltin.
117
7.6 Hz, 2.0 H), 7.36 (t, J = 7.6 Hz, 1.0 H), 0.33 (s + d, J Sn−H = 53.2
Hz, J
= 55.2 Hz, 9.0 H). 13C NMR (CDCl3 125 MHz): 142.2,
119Sn−H
142.12, 142.09, 137.3 (JSn−C = 36.4 Hz), 129.7, 128.2, 128.1, 127.8
High-resolution electron impact mass spectra were obtained on a
double-focusing magnetic sector instrument.
(JSn−C = 45.6 Hz), −8.5 (J
= 334.2, J
= 349.6 Hz). 119Sn
119Sn−C
117Sn−C
NMR (CDCl3 186.5 MHz): δ −27.50. EI-HSMS: m/z 318.0427 [M]+,
calcd for C15H18Sn 318.0430.
Analytical gas chromatography was performed on a chromatograph
equipped with a flame ionization detector and a Restek Rtx-1 column
(60 m × 0.25 mm column with a 0.5 μm film thickness). Preparative
gas chromatography separations were performed on a chromatograph
equipped with a thermal conductivity detector using helium as the
carrier gas.
Steady-state photolysis reactions were performed using a 200 or 500
W mercury arc lamp equipped with a liquid filter filled with deionized
water to absorb IR light. For 334 nm irradiation the light was filtered
by an Oreil No. 56521 cutoff filter and a No. 56421 334 nm
interference filter. For 313 nm irradiation the light was filtered by an
Oreil No. 59450 cutoff filter and No. 56511 313 nm interference filter.
Preparation of Bis(4-biphenyl)dimethylstannane. A 50 mL
flask was charged with 0.2 g (8 mmol) of magnesium turnings, 5 mL of
THF, and a small crystal of iodine. After gentle warming with a heat
gun, 5 mL more of THF was added, followed by the addition of 1.8 g
(7.7 mmol) of 4-bromobiphenyl in 8 mL of THF at a rate sufficient to
maintain reflux. After reflux for 1 h, 0.845 g (3.85 mmol) of
dimethyltin dichloride in 5 mL of THF was added to the reaction
mixture. The resulting mixture was refluxed for 1 h and stirred at room
temperature for 10 h. After treatment with aqueous saturated
ammonium chloride, the reaction mixture was extracted with diethyl
D
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ether (50 mL × 4). The combined organic layers were dried over
MgSO4 and concentrated to give a white solid. Purification by
recrystallization (charcoal) from ethyl acetate gave white needles (1.1
g, 63%).
51.5 Hz, 6.0 H), 1.07 (s, 27.2 H). 13C NMR (CDCl3 125 MHz): δ
117
119
146.0 (J
= 360.9 Hz, J
= 378.5), 137.6 (JSn−C = 31.1 Hz),
Sn−C
117
Sn−C
128.8 (JSn−C = 40.0 Hz), 128.6, 34.7 (JSn−C = 32.5 Hz), 33.8 (J
314.8 Hz, J
=
Sn−C
= 329.2 Hz), 33.2 (JSn−C = 19.4 Hz). 119Sn NMR
119Sn−C
1H NMR (CDCl3 500 MHz): δ 7.69−7.60 (m, 12.0 H), 7.45 (t, J =
7.5 Hz, 4.0 H), 7.36 (tt, J = 1.0 Hz, 7.5 Hz, 1.9 H), 0.58 (s + d + d,
(CDCl3 186.5 MHz): δ −79.49.
Preparation of (3,4-Dimethoxyphenyl)trineopentylstannane
(6). In a 100 mL round-bottom flask containing 4.26 g (10.4 mmol) of
trineopentylphenylstannane and 10 mL of diethyl ether was slowly
placed 17 mL (11.3 mmol) of a 0.67 M HCl solution in diethyl ether
over 1 h. The volatiles were removed under reduced pressure to give
white crystals (trineopentyltin chloride),30 which were used without
purification. The crystals were transferred into a 50 mL three-necked
flask and dissolved in 15 mL of THF. A (3,4-dimethoxyphenyl)
magnesium bromide solution (14 mmol, 23 mL of a 0.6 M in THF)
was added dropwise. The resulting red solution was heated to reflux
for 10 h and then treated with a saturated aqueous ammonium
chloride solution. The reaction mixture was extracted with diethyl
ether (100 mL × 4). The combined organic layers were dried over
MgSO4 and concentrated in vacuo to give a green oil that was purified
twice by column chromatography (20/1 hexanes/EtOAc, SiO2) to give
a colorless oil (4.5 g, 92%). A small portion of the purified product was
distilled under reduced pressure to give a colorless oil (bp 117 °C, 0.05
Torr).
J
= 53.5, J
= 56.0 Hz, 6.0 H). 13C NMR (CDCl3 100
119Sn−H
117Sn−H
MHz): 142.5, 142.1, 140.4, 137.7 (JSn−C = 38.0 Hz), 129.8, 128.3,
128.1, 128.0 (JSn−C = 48.0 Hz), −9.0 (JSn−C = 348.9 Hz). 119Sn NMR
(CDCl3 186.5 MHz): δ −57.39. EI-HSMS: m/z 456.0891 [M]+, calcd
for C26H24Sn 456.0900.
Preparation of (4-Biphenyl)dimethyltin Chloride. A 100 mL
flask was charged with 0.42 g (0.92 mmol) of bis(4-biphenyl)-
dimethylstannane and 30 mL of diethyl ether. To the resulting white
suspension was added dropwise 2.4 mL of a 0.38 M HCl solution
(0.92 mmol) in diethyl ether to give a clear solution. The volatiles
were removed under reduced pressure to give a yellow powder that
was recrystallized from hexanes to give colorless crystals (0.11 g, 35%).
1H NMR (CDCl3 400 MHz): δ 7.76−7.65 (m, 4.1 H), 7.61 (d, J =
7.2 Hz, 1.9 H), 7.48 (t, J = 7.2 Hz, 2.0 H), 7.39 (t, J = 7.2, 0.9 H), 0.90
(s + d, JSn−H = 56.8 Hz, 6.0 H). 1H NMR (CD3CN 400 MHz): δ 7.77
(d + d + d, J = 8.4 Hz, JSn−H = 55.2 Hz, 2.0 H), 7.74−7.65 (m, 4.0 H),
7.49 (t, J = 7.2 Hz, 2.1 H), 7.40 (t, J = 7.6, 1.0 H), 0.88 (s + d, JSn−H
=
1
65.8 Hz, 5.9 H). H NMR (CD2Cl2 400 MHz): δ 7.79−7.64 (m, 6.0
1H NMR (CDCl3 400 MHz) δ 7.17−7.03 (m, 2.0 H), 6.88 (d, 7.6
Hz, 1.0 H), 3.90 (s, 3.0 H), 3.88 (s, 3.0 H), 1.35 (s + d, JSn−H = 50.4
Hz, 5.9 H), 1.02 (s, 27.2 H). 13C NMR (CDCl3 125 MHz): δ 149.8,
H), 7.49 (t, J = 7.2 Hz, 2.1 H), 7.41 (t, J = 7.2 Hz, 1.0 H), 0.92 (s + d
+ d, J Sn−H = 58.8, J
= 60.0 Hz, 5.9 H). 13C NMR (CDCl3 125
117
119
Sn−H
MHz): 143.9, 141.6, 140.1, 136.5 (JSn−C = 49.9 Hz), 129.9, 128.7,
149.4, 136.4, 130.3 (JSn−C = 32.9 Hz), 120.2 (JSn−C = 40.1 Hz), 112.0
= 395.0 Hz). 119Sn NMR
119Sn−C
117Sn−C
117
128.5, 128.2, −1.1 (J
= 377.5, J
(JSn−C = 49.8 Hz), 56.8, 56.5, 34.6 (JSn−C = 32.3 Hz), 33.9 (J
=
Sn−C
(CDCl3 186.5 MHz): δ 99.60. EI-HSMS: m/z 337.9872 [M]+, calcd
for C14H15ClSn: 337.9884.
119Sn−C
315.5 Hz, J
= 330.2 Hz), 33.2 (JSn−C = 19.4 Hz). 119Sn NMR
(CDCl3 186.5 MHz): δ −75.33. EI-HSMS: m/z 470.2205 [M]+, calcd
for C23H42O2Sn 470.2207.
Preparation of (4-Methoxyphenyl)dimethyltin Chloride. A
100 mL round-bottom flask was charged with 5.4 g (15 mmol) of
bis(4-methoxyphenyl)dimethylstannane and 20 mL of diethyl ether,
followed by dropwise addition of 23 mL of a 0.65 M HCl solution (15
mmol) in diethyl ether over 2 h. The reaction mixture was
concentrated, and the yellow residue was purified by column
chromatography (10/1 hexanes/EtOAc, SiO2) to give a yellow oil
(3.15 g, 73%).
Preparation of Di-n-butylbis(3,4-dimethoxyphenyl)-
stannane. A 50 mL two-necked flask was charged with 25 mL of a
0.73 M (3,4-dimethoxyphenyl)magnesium bromide solution (18.3
mmol) in THF. Di-n-butyltin dibromide (2.0 mL, 3.5 g, 8.9 mmol) in
8 mL of THF was then added dropwise. The resulting reaction
mixture was stirred for 10 h and then treated with aqueous ammonium
chloride solution. After extraction with diethyl ether (100 mL × 4) the
combined organic layers were dried over MgSO4 and concentrated to
give a purple oil. Purification by column chromatography (97/3
hexanes/EtOAc, SiO2) gave a colorless oil (4.0 g, 88%).
1H NMR (CDCl3 400 MHz) 7.51 (d + d + d, J = 8.4 Hz, JSn−H
=
54.8 Hz, 2.0 H), 7.00 (d, J = 8.4 Hz, 2.0 H), 3.84 (s, 6.0 H), 0.84 (s +
d + d, J Sn−H = 56.8 Hz, J Sn−H = 59.2 Hz, 6.0 H). 1H NMR (CD3CN
400 MHz) 7.59 (d + d + d, J = 8.8 Hz, JSn−H = 55.2 Hz, 2.0 H), 7.02
117
119
1H NMR (CDCl3 500 MHz): δ 7.05 (dd, J = 1.0 Hz, 7.5 Hz, 1.9 H),
7.00 (d, J = 1.0 Hz, 1.9 H), 6.92 (d, J = 7.5 Hz, 2.1 H), 3.89, 3.86 (s, s,
11.9 H combined), 1.64 (pent, J ≈ 7.5 Hz, 4.0 H), 1.39 (sext, J ≈ 6.8
Hz, 4.1 H), 1.30−1.27 (m, 4.0 H), 0.91 (t, J = 7.0 Hz, 5.8 H). 13C
NMR (CDCl3 125 MHz): δ 150.4, 149.8 (JSn−C = 53.8 Hz), 132.0,
130.5 (JSn−C = 34.4 Hz), 119.8 (JSn−C = 41.2 Hz), 112.3 (JSn−C = 53.9
Hz), 56.7, 56.5, 29.8 (JSn−C = 20.8 Hz), 28.2 (JSn−C = 58.2 Hz), 14.6,
117
(d, J = 8.8 Hz, 2.0 H), 3.81 (s, 6.0 H), 0.82 (s + d + d, J Sn−H = 63.6
Hz, J Sn−H = 66.4 Hz, 6.0 H). 1H NMR (CD2Cl2 400 MHz) 7.55 (d +
119
d + d, J = 8.4 Hz, JSn−H = 53.6 Hz, 2.0 H), 7.03 (d, J = 8.8 Hz, 2.0 H),
117
119
3.85 (s, 6.0 H), 0.87 (s+ d + d, J
= 57.2 Hz, J
= 60.0 Hz,
Sn−H
Sn−H
6.0 H). 13C NMR (CDCl3 125 MHz): δ 162.0, 137.3 (JSn−C = 55.3
117
Sn−C
119
Hz), 131.9 (J
= 555.0 Hz, J
= 580.9), 115.5 (JSn−C = 63.4
Sn−C
Hz), 56.0, −1.2 (JSn−C = 396.6 Hz). 119Sn NMR (CDCl3 186.5 MHz):
δ 101.67. EI-HRMS m/z: [M]+, 291.9675, calcd for C9H13ClOSn
291.9677.
11.4 (J
= 350.9 Hz, J
= 367.4 Hz). 119Sn NMR (CDCl3
119Sn−C
117Sn−C
186.5 MHz): δ −63.32. EI-HSMS: m/z 508.1628 [M]+, calcd for
C24H36O4Sn 508.1636.
Preparation of Trineopentylphenylstannane. Trineopentyl-
phenylstannane was prepared by modification of a literature
procedure.27 A 100 mL three-necked flask was charged with 1.7 g
(70 mmol) of magnesium powder, 0.27 g (1.51 mmol) of anthracene,
50 mL of THF, and 0.10 mL of methyl iodide (1.6 mmol).28 The
reaction mixture was placed in a sonication bath for 6 h. To the
resulting yellow suspension was added 5.0 g (47 mmol) of neopentyl
chloride in 6 mL of THF over 0.5 h. The reaction mixture was heated
at 70 °C for 12 h. Titration29 showed that the solution contained 0.55
M active Grignard reagent. Phenyltin trichloride (1.65 mL; 10 mmol)
was slowly added to this solution. The resulting mixture was heated to
reflux for 8 h and sequentially treated with 50 mL of water and 50 mL
of saturated aqueous ammonium chloride. The aqueous layer was
extracted with diethyl ether (100 mL × 4). The combined organic
layers were dried over MgSO4 and then concentrated to give a yellow
oil. Purification by column chromatography (hexanes, SiO2) gave
colorless crystals (3.8 g, 60%).
Preparation of Di-n-butyl(3,4-dimethoxyphenyl)tin Chlor-
ide. In a glovebox, a 10 mL microwave reaction vessel was charged
with 1.2 g (4.0 mmol) of di-n-butyltin dichloride and capped with a
snap-on septum cap. After removal from the glovebox, 1.1 g (2.2
mmol) of di-n-butylbis(3,4-dimethoxyphenyl)stannane was added.
The reaction mixture was heated at 100 °C in a microwave reactor
with stirring for 1 h and then passed through a short, acidic alumina
column with diethyl ether as eluent. Removal of the volatiles in vacuo
gave a colorless oil (1.1 g, 60%), which was contaminated with ∼5%
1,2-dimethoxybenzene. This material was purified before use by
preparative gas chromatography (10% OV-1 8020 GAS-CHROM, 1/4
in. × 6 ft, 250 °C).
1H NMR (CDCl3 400 MHz) 7.15−7.01 (m, 2.0 H), 6.96 (d, J = 8.0
Hz, 1.0 H), 3.92 (s, 3.0 H), 3.90 (s, 2.9 H), 1.82−1.69 (m, 3.9 H),
1.54−1.36 (m, 7.9 H), 0.93 (t, J = 7.6 Hz, 6.2 H). 13C NMR (CDCl3
125 MHz): δ 151.5, 150.4, 132.4, 129.4, 118.5, 112.6, 56.9, 56.7, 28.8
117Sn−C
(JSn−C = 24.5 Hz), 27.8 (JSn−C = 66.2 Hz), 18.8 (J
= 363.4 Hz,
1H NMR27 (CDCl3 500 MHz): δ 7.63 (dd + d + d, J = 1.5 Hz, 7.5
J
= 379.5 Hz), 14.6. 119Sn NMR (CDCl3 186.5 MHz): δ 87.56.
119
Sn−C
Hz, JSn−H = 39.5 Hz, 1.9 H), 7.36−7.27 (m, 2.9 H), 1.42 (s + d, JSn−H
=
EI-HSMS: m/z 406.0707 [M]+, calcd for C16H27ClO2Sn 406.0722.
E
DOI: 10.1021/acs.joc.5b01690
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
P r e p a r a t i o n o f B i s ( 3 , 4 - d i m e t h o x y p h e n y l ) -
dineopentylstannane. A 25 mL three-necked flask was charged
with 0.30 g (0.90 mmol) of dineopentyltin dichloride in 5 mL of THF.
A (3,4-dimethoxyphenyl)magnesium bromide solution (2.28 mmol,
4.0 mL of a 0.57 M solution in THF) was added dropwise. The
resulting red mixture was heated to reflux for 12 h and then stirred at
room temperature for 12 h. After treatment with aqueous ammonium
chloride solution, the reaction mixture was extracted with diethyl ether
(50 mL × 4). The combined organic layers were dried over MgSO4
and concentrated in vacuo to give a yellow oil. Purification by column
chromatography (10/1 hexanes/EtOAc, SiO2) gave a colorless oil
(0.25 g, 52%).
conversions. A stopcocked quartz cuvette containing a Teflon-coated
magnetic stir bar was charged with 1.5 mL of a 0.0164 M solution of 5
in dioxygen-saturated acetonitrile containing 1.6 mM NMQ+PF6− and
0.5 M toluene. The solution was photolyzed at 334 nm with stirring
for 80 min. To the resulting solution was added 100 μL of a 0.362 M
dibenzyl ether (internal standard) solution in CD3CN, which was
stirred over 0.080 g of dry LiCl powder for 3 h. The mixture was then
concentrated under reduced pressure, triturated with CDCl3, filtered
1
through a plug of glass wool, and analyzed by H NMR spectroscopy,
which showed that di-n-butyl(3,4-dimethoxyphenyl)tin chloride was
formed in near-quantitative yield. 119Sn NMR spectroscopy confirmed
that di-n-butyl(3,4-dimethoxyphenyl)tin chloride was the only tin
chloride product formed.
1H NMR (CDCl3 400 MHz): 7.11 (d + d + d, J = 6.8 Hz, JSn−H
=
Proc edure for the Photooxidation of (3, 4-
Dimethoxyphenyl)trineopentylstannane (6) in CH3CN. 6 was
purified by preparative GC (10% OV-1 8020 GAS-CHROM 1/4 in. ×
6 ft, oven temperature 250 °C) immediately before use, because
photooxidations carried out without purification by preparative GC
resulted in low conversions. A stopcocked quartz cuvette was charged
with 0.021 g (0.045 mmol) of 6 and 2 mL of a dioxygen-saturated
43.2 Hz, 2.0 H), 7.04 (s + d, JSn−H = 44.0 Hz, 2.0 H), 6.91 (d + d, J =
7.6 Hz, JSn−H = 11.2 Hz, 2.0 H), 3.89, 3.84 (s, s, combined for 12.0 H),
1.48 (s + d, JSn−H = 53.6 Hz, 4.0 H), 0.97 (s, 18.0 H). 13C NMR
117
(CDCl3 125 MHz): δ 150.2, 149.8 (JSn−C = 54.4 Hz), 133.9, (J
=
=
Sn−C
117
421.5 Hz, J Sn−C = 441.6 Hz), 130.8 (JSn−C = 35.5 Hz), 120.2 (JSn−C
42.1 Hz), 112.3 (JSn−C = 54.4 Hz), 56.9, 56.6, 34.4 (JSn−C = 19.6 Hz),
117
119
33.2 (J Sn−C = 346.5 Hz, J Sn−C = 362.25 Hz), 33.0 (JSn−C = 19.6 Hz).
119Sn NMR (CDCl3 186.5 MHz): δ −86.00. EI-HSMS: m/z 536.1929
−
acetonitrile solution containing 1.6 mM NMQ+PF6 and 0.5 M
[M]+, calcd for C26H40O4Sn 536.1949.
toluene. The solution was photolyzed at 334 nm with stirring for 90
min. To the resulting solution was added 100 μL of a 0.208 M
dibenzyl ether (internal standard) solution in CD3CN, which was
stirred over 0.080 g of dry LiCl powder for 3 h. The mixture was then
concentrated under reduced pressure, triturated with CDCl3, filtered
Preparation of (3,4-Dimethoxyphenyl)dineopentyltin Chlor-
ide. A 10 mL flask was charged with 0.15 g (0.4 mmol) of bis(3,4-
dimethoxyphenyl)dineopentylstannane in 2 mL of diethyl ether. An
HCl solution (0.42 mmol, 0.62 mL of a 0.67 M solution in diethyl
ether) was slowly added over 20 min. The resulting mixture was
concentrated in vacuo to give a yellow oil (0.21 g). Purification by
preparative gas chromatography (column 10% OV-1 8020 GAS-
CHROM 1/4 in. × 6 ft, 206 °C) gave a colorless oil (0.050 g, 29%).
1H NMR (CDCl3 400 MHz) 7.18 (s + d, JSn−H = 54.4 Hz, 1.0 H),
7.13 (dd + d + d, J = 0.8 Hz, 8.0 Hz, JSn−H = 54.4 Hz, 1.0 H), 6.95 (d +
d + d, J = 8.0 Hz, JSn−H = 16.0 Hz, 1.0 H), 3.92, 3.90 (s, s, combined
1
through a plug of glass wool, and analyzed by H NMR spectroscopy,
which showed that (3,4-dimethoxyphenyl)dineopentyltin chloride was
formed in near quantitative yield. 119Sn NMR spectroscopy confirmed
that (3,4-dimethoxyphenyl)dineopentyltin chloride was the only tin
chloride product formed.
General Procedure for the Generation and Observation of
Stannane Cation Radicals by Nanosecond Transient Absorp-
tion Spectroscopy. Transient absorption experiments were typically
performed in quartz stopcocked cuvettes containing solutions with ∼1
for 6.0 H), 1.70 (s + d, JSn−H = 52.4 Hz, 4.0 H), 1.07 (s, 18.0 H). 13
C
NMR (CDCl3 100 MHz): δ 151.1, 150.3, 134.8, 129.1 (JSn−C = 54.6
Hz), 118.4 (JSn−C = 53.3 Hz), 112.5 (JSn−C = 69.5 Hz), 57.0, 56.7, 40.6
−
mM NMQ+PF6 (OD ≈ 1 at 343 nm), 0.5 M toluene, and ∼0.01 M
stannane in either dioxygen-saturated acetonitrile or dichloromethane.
Transient spectra were generally recorded after 100 ns so as to avoid
interference from the N-methylquinolyl radical.31 For this reason,
fitting of transient decays or growths for kinetic experiments were only
done after 100 ns.
117
119
(J Sn−C = 352.0 Hz, J Sn−C = 368.5 Hz), 34.2 (JSn−C = 43.0 Hz), 33.2
(JSn−C = 23.2 Hz). 119Sn NMR (CDCl3 186.5 MHz): δ 68.73. EI-
HSMS: m/z 434.1019 [M]+, calcd for C18H31ClO2Sn 434.1035.
Representative Procedure for Photooxidations of Aryltri-
methylstannanes in CD3CN. A stopcocked quartz cuvette
containing a Teflon-coated magnetic stir bar was charged with 0.5
mL of a 0.0225 M 1,2,4,5-tetracyanobenzene solution in CD3CN, 40
μL of a 0.13 M solution of stannane in CD3CN, 0.4 mL of CD3CN,
and an aromatic codonor (0.1−0.5 M). The solution was purged with
argon for ∼15 min and then photolyzed with stirring at 313 nm for 10
min. To the resulting solution was added 30 μL of a 0.17 M dioxane
solution (internal standard) in CD3CN and 20 μL of a 4 M aqueous
LiCl solution. After it was stirred for 10 min, the solution was quickly
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.joc.5b01690.
■
S
NMR spectra (1H, 13C, and 119Sn) and representative
kinetic plots for bimolecular rate constants (PDF)
1
dried over MgSO4, filtered into a NMR tube, and analyzed by H
NMR spectroscopy. Integrations of the methyl group hydrogens of the
tin chlorides were used to calculate the aryl/Me fragmentation ratio
and the mass balance (vs the internal standard).
AUTHOR INFORMATION
Corresponding Author
*E-mail for J.P.D.: jpd@chem.rochester.edu.
Notes
■
Representative Procedure for Photooxidations of Aryltri-
methylstannanes in CH2Cl2. A stopcocked quartz cuvette
containing a Teflon-coated magnetic stir bar was charged with 2 mL
of a 0.0133 M stannane solution in 9/1 CH2Cl2/CD2Cl2 containing 4
The authors declare no competing financial interest.
−
mM NMQ+PF6 , 0.5 M toluene, and 1 M nucleophile. The reaction
ACKNOWLEDGMENTS
The NSF (CHE-1464629) is gratefully acknowledged for
financial support.
■
solution was photolyzed at 334 nm with stirring for ∼60 min, and then
25 μL of a 0.282 M dioxane solution (internal standard) in CH2Cl2
was added. The reaction mixture was stirred over dry LiCl powder for
1
3 h, filtered through a plug of glass wool, and then analyzed by H
REFERENCES
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preparative GC (10% OV-1 8020 GAS-CHROM 1/4 in. × 6 ft, oven
temperature 250 °C) immediately before use, because photooxidations
carried out without purification by preparative GC resulted in low
■
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DOI: 10.1021/acs.joc.5b01690
J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
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(18) (a) The (maximum) rate constant for electron transfer from 1
to 2 is equal to (kd[1]/k−d)ket, where kd is the rate constant for
diffusional encouter, k−d is the rate constant for diffusional separation,
and ket is the maximum rate constant for electron tranfer for 1 and 2 at
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Smoluchowski equation,18b kd in CH3CN is calculated to be ∼2 ×
1010 M−1s−1 (kd = 4πNr1+2D1+2/1000) with r1+2 = 6 Å and D1+2 = 4 ×
G
DOI: 10.1021/acs.joc.5b01690
J. Org. Chem. XXXX, XXX, XXX−XXX