Diverging effects of steric congestion on the reaction of tributylstannyl radicals with areneselenols and aryl bromides and their mechanistic implications

Article abstract of DOI:10.1021/jo982514a

The effects of bulky ortho,ortho' groups on the reactions of aryl bromides and areneselenols with tributylstannane have been studied. Bulky ortho,ortho' groups accelerate the reaction of the bromides with the stannane but retard the reactions of the selenols. On the other hand, ab initio and force field calculations show that introducing bulky ortho substituents into selenols causes a greater increase in strain than in the corresponding bromides. Two possible explanations for the divergent reactivity patterns are advanced. On one hand, it is possible that bromine abstraction by stannyl radicals from aryl bromides proceeds in a single step through a linear transition state whereas the abstraction of sell from the selenols involves a T-shaped, hypervalent intermediate. Alternatively, it may be that both reactions are concerted with the bromine abstraction having a late transition state and the sell abstraction an early one. Approximate second-order rate constants for the reaction of tributylstannane with a range of hindered aryl bromides are derived from competition reactions. 2,4,6-Tri-tert- butylbenzeneselenol is able to function moderately well as a catalyst for the stannane-mediated reactions of vinyl bromides. The X-ray crystal structure of bis(2,4,6-triisopropylphenyl) diselenide is presented.

Full text of DOI:10.1021/jo982514a

J . Org. Chem. 1999, 64, 2877-2882
2877
Diver gin g Effects of Ster ic Con gestion on th e Rea ction of
Tr ibu tylsta n n yl Ra d ica ls w ith Ar en eselen ols a n d Ar yl Br om id es
a n d Th eir Mech a n istic Im p lica tion s
David Crich,* J ae-Taeg Hwang, Ste´phane Gastaldi, Francesco Recupero, and Donald J . Wink
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street,
Chicago, Illinois 60607-7061
Received December 28, 1998
The effects of bulky ortho,ortho′ groups on the reactions of aryl bromides and areneselenols with
tributylstannane have been studied. Bulky ortho,ortho′ groups accelerate the reaction of the
bromides with the stannane but retard the reactions of the selenols. On the other hand, ab initio
and force field calculations show that introducing bulky ortho substituents into selenols causes a
greater increase in strain than in the corresponding bromides. Two possible explanations for the
divergent reactivity patterns are advanced. On one hand, it is possible that bromine abstraction
by stannyl radicals from aryl bromides proceeds in a single step through a linear transition state
whereas the abstraction of SeH from the selenols involves a T-shaped, hypervalent intermediate.
Alternatively, it may be that both reactions are concerted with the bromine abstraction having a
late transition state and the SeH abstraction an early one. Approximate second-order rate constants
for the reaction of tributylstannane with a range of hindered aryl bromides are derived from
competition reactions. 2,4,6-Tri-tert-butylbenzeneselenol is able to function moderately well as a
catalyst for the stannane-mediated reactions of vinyl bromides. The X-ray crystal structure of bis-
(2,4,6-triisopropylphenyl) diselenide is presented.
In tr od u ction
smaller as the rate constant for their direct reaction with
stannanes (eq 5) already approaches the diffusion con-
For several years now we have been studying the
catalytic effect of diphenyl diselenide (1) on the reactions
of alkyl and aryl halides with stannanes.1-6 The effect,
which parallels Roberts’ catalysis of silane-mediated
reductions by thiols,7 arises because of the in situ
reduction of the diselenide to the selenol according to eq
1,
trolled limit.10
Ar• + Bu3SnH f ArH + Bu3Sn•
(5)
This difference in catalytic efficiency means that aryl
radical cyclizations are not significantly inhibited by the
presence of catalytic PhSeH (4) while alkyl radical
rearrangements may be effectively prevented under the
same conditions. Thus, the catalytic selenol method can
be used to significantly improve the 5-exo/6-endo ratios
of aryl and vinyl radical cyclizations1,3 by preventing the
rearrangement of the kinetic 5-exo closed alkyl radi-
cal,11,12 with no appreciable loss of overall yield. In
practice, this sequence is effective when aryl or vinyl
iodides are used as radical precursors but fails with the
corresponding aryl or vinyl bromides.1,3 This discrepancy
arises because the catalytic species, PhSeH, itself is
cleaved by stannanes more easily than aryl bromides and
so is rapidly destroyed, whereas aryl iodides, with the
weaker aryl-halogen bond, react more rapidly with the
stannane, permitting the selenol to persist until all of
the iodide has been consumed. We therefore began a
study aimed at the design and synthesis of selenols more
resistant to cleavage by stannanes, which in turn would
enable the extension of the catalytic chemistry to the
more readily available aryl and vinyl bromides. In the
course of this project we had occasion to investigate the
effects of steric congestion on the reaction of stannyl
radicals with both areneselenols and aryl bromides. This
R3SnH + PhSeSePh f R3SnSePh + PhSeH (1)
the subsequent participation of the selenol in the chain
sequence of eqs 2-4, and the 1000-fold difference in the
rate of trapping of alkyl radicals by PhSeH and Bu3-
SnH.8,9
R• + PhSeH f RH + PhSe•
(2)
(3)
PhSe• + Bu3SnH f PhSeH + Bu3Sn•
Bu3Sn• + RX f Bu3SnX + R•
(4)
The catalytic effect of the selenol on the quenching of
aryl radicals by stannanes, on the other hand, is much
(1) Crich, D.; Yao, Q. J . Org. Chem. 1995, 60, 84-88.
(2) Crich, D.; J iao, X.-Y.; Yao, Q.; Harwood, J . S. J . Org. Chem. 1996,
61, 2368-2373.
(3) Crich, D.; Hwang, J .-T.; Liu, H. Tetrahedron Lett. 1996, 37,
3105-3108.
(4) Crich, D.; Hwang, J .-T. J . Org. Chem. 1998, 63, 2765-2770.
(5) Crich, D.; Mo, X.-S. J . Am. Chem. Soc. 1998, 120, 8298-8304.
(6) Crich, D.; Mo, X.-S. J . Org. Chem. 1997, 62, 8624-8625.
(7) Roberts, B. P. Chem. Soc. Rev. 1999, 28, 25-35.
(8) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1981, 103, 7739-7742.
(10) Garden, S. J .; Avila, D. V.; Beckwith, A. L. J .; Bowry, V. W.;
Ingold, K. U.; Lusztyk, J . J . Org. Chem. 1996, 61, 805-809.
(11) Stork, G.; Mook, R. Tetrahedron Lett. 1986, 27, 4529-4532.
(12) Beckwith, A. L. J .; O’Shea, D. M. Tetrahedron Lett. 1986, 27,
4525-4528.
(9) Newcomb, M.; Varick, T. R.; Ha, C.; Manek, M. B.; Yue, X. J .
Am. Chem. Soc. 1992, 114, 8158-8163.
10.1021/jo982514a CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/30/1999

2878 J . Org. Chem., Vol. 64, No. 8, 1999
Crich et al.
Ta ble 1. 77Se NMR Ch em ica l Sh ifts
and 8, respectively. With 3, decolorization was noticeably
slower, indicating that steric hindrance was certainly
retarding attack of the stannane on the diselenide.
Nevertheless, 3 was cleanly converted to two new signals
at δ 119.1 and -19.5 in the 77Se NMR spectrum over the
course of the acquisition. Addition of 1 equiv of Bu3SnH
did not result in loss of either signal over a period of
several hours at room temperature. However, brief
heating of 3 with 2.2 equiv of Bu3SnH to 80 °C in C6D6
followed by inspection by 77Se NMR revealed that the
signal at δ 119.1 was lost under these conditions.
Thus, it appears clear that all three diselenides (1-3)
react stoichiometrically with Bu3SnH according to eq 1
and that the selenols 4 and 5 are decomposed signifi-
cantly more rapidly by further stannane than the more
hindered 6, which, nevertheless, reacts further on brief
heating with Bu3SnH.
Unfortunately, the above experiments are not quanti-
tative, and the acquisition times required, typically
several hours at the concentrations employed, for the 77Se
NMR experiments effectively preclude the use of this
method for the determination of kinetics. Equally unfor-
tunately, monitoring of reaction mixtures by GC was
unsatisfactory owing to the very long retention times and/
or decomposition of the tin selenides (7-9). Thus, we
turned to competition experiments in order to glean some
information on the relative rates of 1-3 and of 4-6 with
Bu3SnH. A 1:1 mixture of 1 and 3 was allowed to react
with 1 equiv of Bu3SnH when the 77Se NMR spectrum
revealed signals corresponding to the two diselenides (1
and 3), one selenol (6), and one stannyl selenide (7). With
2.2 equiv of stannane, the only two signals in the 77Se
NMR were those of 6 and 7. These observations are best
interpreted in terms of rapid reduction of the less
hindered diselenide (1) according to eq 1, followed by
attack of PhSeH (4) or PhSe• on the more hindered
diselenide (3) with formation of a mixed diselenide (10)
and selenol (6) or the corresponding selenyl radical. The
lack of any signals corresponding to 10 in the NMR
spectrum evidently requires it to further react with
PhSeH or PhSe• to give the symmetrical 1 and 6 or the
corresponding selenyl radical. Evidently, an equilibration
is taking place that is driven by the release of the steric
strain inherent in the hindered diselenides 3 and 10. This
equilibration effectively excludes any possibility of ac-
cessing relative rate data by this type of competition
experiment. An alternative mechanism for equilibration
involving reaction of Bu3SnH with 10 is considered less
likely as this would require the contrasteric attack of the
stannane on the more hindered of the two selenium
atoms.
entry
compd
δa
lit. δa
1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9
460
360.6
515.7
142.1
-9.6
119.1
-26.8
-198.1
-19.5
4602,20
359;20 569b,18
515;20 516;19 521b,21
142.22
9.8b,18
24.8b,18
-27.72
a
Unless otherwise stated all chemicals shifts are for C6D6
solutions referenced to external Me2Se. b CDCl3 solution referenced
to Me2Se.
led to the surprising discovery that bulky ortho substit-
uents actually accelerate the cleavage of the bromides,13,14
whereas, as anticipated, they retard that of the selenols.
We now present in full the results of this investigation
together with the mechanistic implications of the diver-
gent effects of steric congestion on the two reactions,
which are relevant to the current widespread interest in
homolytic substitutions.15-17
Resu lts a n d Discu ssion
We began by investigating the reaction of tributyltin
hydride with bis(2,4,6-tri-iso-propyl)phenyl diselenide
(2)18 and bis(2,4,6-tri-tert-butyl)phenyl diselenide (3).19 As
described previously,2 a C6D6 solution of 1 (0.32 M) was
treated under Ar at room temperature with 1.1 equiv of
Bu3SnH resulting in rapid decolorization and the forma-
tion of two new peaks in the 77Se NMR spectrum at δ
142.1 and -26.8 (Table 1). These chemical shifts agreed
well with values previously observed in this laboratory
and were assigned to 4 and 7, respectively. When 2.2
equiv of Bu3SnH was used, only the signal at δ -26.8
was observed, which indicates that PhSeH is indeed
rapidly cleaved by excess Bu3SnH. In the case of reduc-
tion of 2 with 1.1 equiv of Bu3SnH, signals were observed
at δ -9.6 and -198.1, which, as the former disappeared
on treatment with further Bu3SnH, were assigned as 5
Based on the, in retrospect, naive assumption that
steric hindrance would affect aryl bromides and arene
selenols in the same way, we therefore turned to the
reaction of a series of aryl bromides with stannanes.
These reactions, giving Bu3SnBr as byproduct, could be
monitored readily by GC. Preliminary experiments in
which bromobenzene and 2,4,6-tri-tert-butylbromoben-
zene were allowed to compete for reaction with tributyl-
stannane revealed the initial assumption to be flawed,
with the more hindered bromide reacting more quickly.
Following this preliminary observation, we conducted a
series of competition reactions in order to determine the
relative rates of reductive debromination of the aryl
bromides 11-16 and of decyl bromide (18). 2-Bromoaceto-
phenone (15) and decyl bromide (18) were included in
(13) Crich, D.; Recupero, F. J . Chem. Soc., Chem. Commun. 1998,
189-190.
(14) Galli, C.; Pau, T. Tetrahedron 1998, 54, 2893-2904.
(15) Schiesser, C. H.; Wild, L. M. Tetrahedron 1996, 52, 13265-
13314.
(16) Walton, J . C. Acc. Chem. Res. 1998, 31, 99-107.
(17) Ingold, K. U.; Roberts, B. P. Free Radical Substitution Reac-
tions: Bimolecular Homolytic Substitutions at Saturated Multivalent
Atoms; Wiley: New York, 1971.
(18) Bochmann, M.; Webb, K. J .; Hursthouse, M. B.; Mazid, M. J .
Chem. Soc., Dalton Trans. 1991, 2317-2323.
(19) du Mont, W. W.; Kubiniok, S.; Lange, L.; Phol, S.; Saak, W.;
Wagner, I. Chem. Ber. 1991, 124, 1315-1320.
(20) Martens-von Salzen, A.; Meyer, H.-U.; du Mont, W.-W. Phos-
phorus, Sulfur Silicon 1992, 67, 67-71.
(21) Ishii, A.; Okazaki, R.; Inamoto, N. Bull. Chem. Soc. J pn. 1986,
59, 2529-2535.

Diverging Effects of Steric Congestion
J . Org. Chem., Vol. 64, No. 8, 1999 2879
Ta ble 2. Ap p r oxim a te Rela tive a n d Absolu te Ra tes for
Sch em e 1
Br om in e Abstr a ction by Bu 3Sn • a t 80 °C
entry
bromide
krel k (M-1 s-1
)
1
2
3
4
5
6
7
8
pentafluorobromobenzene (11)
1-bromodecane (18)
2,4,6-triphenylbromobenzene (12)
2,4,6-tri-tert-butylbromobenzene (13)
2-acetylbromobenzene (15)
>50
>1 × 108
15.2
15.2
9.5
6.7
2.6
2.6
1
5.8 × 107
5.8 × 107
2.9 × 107
1.6 × 107
6.2 × 106
6.2 × 106
2.4 × 106
2-phenylbromobenzene (16)
2,4,6-tri-iso-propylbromobenzene (14)
4-tert-butylbromobenzene (17)a
Taken from Curran.22
a
Sch em e 2
electron-withdrawing effect of the combined fluorine
atoms stabilizing the polarized transition state, but we
cannot rule out an electron-transfer mechanism in this
case. 2,4,6-Triphenylbromobenzene (12) also showed
enhanced reactivity in its reaction with stannyl radicals
(Table 2, entry 3). This may be attributed to the combi-
nation of three factors: (i) the stabilization of the polar
transition state by the three phenyl groups, (ii) relief of
steric strain, and (iii) relief of steric inhibition of reso-
nance. However, as with 11, we cannot rule out a single
electron-transfer mechanism in this case.
We had anticipated that areneselenols would react
with stannyl radicals by a process involving SeH abstrac-
tion giving an aryl radical, analogously to the situation
with the bromides. However, the differing effect of
sterically bulky ortho groups in the two cases, accelerat-
ing the one and decelerating the other, caused us to bring
this assumption into question. For example, ipso attack
of the stannyl radical on the selenol with formation of
Bu3SnPh and HSe• was now entertained as a possible
alternative mechanism for the selenols. To differentiate
between the two possibilities, diselenide 2 was exposed
under Ar to 1.25 equiv of Bu3SnH in benzene resulting
in complete decolorization within 15 min at room tem-
perature. A further 1.25 equiv of Bu3SnH was then added
and the solution refluxed for 1 h, after which time GC
analysis of the reaction showed the formation of triiso-
propylbenzene in 95% yield. Thus, one of the two aryl
groups of the diselenide is cleanly converted to the arene
on addition of the second portion of Bu3SnH. This is in
accord with an overall reaction scheme (Scheme 2) in
which the selenol reacts with the stannane analogously
to the bromide.
this series in order for us to be able to overlap and
interface with a similar series of aryl bromides previously
reported by Curran.22 Furthermore, Curran deduced,
from the known activation energy and rate constant at
25 °C, the rate constant for abstraction of bromine from
a primary alkyl bromide by tributyltin radicals at 80 °C
to be 5.8 × 107 M-1 s-1. This therefore enables the relative
rate data from the competition experiments to be con-
verted into the approximate absolute values given in
Table 2.
In his earlier study using a wider range of nonhinder-
ing, electron-withdrawing and -donating substituents
Curran concluded that the transition state for bromine
abstraction from aryl bromides by the tributylstannyl
radical is slightly polarized with a partial negative charge
located on the σ framework of the arene (Scheme 1).22
We see no reason to disagree with this general conclusion
and simply highlight the anomalies of entries 4 and 7 in
Table 2.
In effect, bromides 13 and 14 with their electron-
donating substituents are out of the expected sequence.
We attribute these exceptions to the sterically bulky
nature of the isopropyl and tert-butyl groups and ac-
celeration due to the relief of steric strain. Examples of
such steric acceleration of bimolecular radical reac-
tions23,24 are much less common than their unimolecular
counterparts.25 A closely related example of the same
phenomenon was reported recently, however, in which
bromine abstraction from 3-bromodurene was accelerated
by the butressing effect of the methyl groups.14
We also note the very high rate constant for reaction
of pentafluorobromobenzene (11) with tributylstannyl
radicals (Table 2, entry 1). With a lower limit of 108 M-1
s-1 this must qualify as one of the most rapid bromine
abstractions known. Certainly, it is comparable to the
rates of halogen abstraction from alkyl bromides and aryl
iodides, if not with the perfluoroalkyl bromides.26 The
extreme reactivity of 11 possibly stems from the strongly
The X-ray crystal structure of 2,4,6-triphenylbromoben-
zene (12) shows it to be a perfectly normal aryl bromide
with the bromine atom fully in the plane of the central
arene ring.27 Similarly, the structure of 2,4,6-tri-tert-
butylbromobenzene (13), as contained within a crystal
structure of {Li(n-Bu)}2(LiMes*)2‚Mes*Br (where Mes*
is tri-tert-butylphenyl) appears perfectly normal.28 With
both 12 and 13 the C-Br bond length is typical for an
aryl bromide (Table 3). X-ray data are not available for
the selenols, but the structure of diphenyl diselenide (1)
has been previously determined.29 Bis(2,4,6-triisopropyl)-
phenyl diselenide (2) gave crystals suitable for X-ray
(22) Curran, D. P.; J asperse, C. P.; Totleben, M. J . J . Org. Chem.
1991, 56, 7169-7172.
(23) Russell, G. A. In Free Radicals; Kochi, J . K., Ed.; Wiley: New
York, 1973; Vol. 1, pp 275-331.
(27) Haaland, A.; Rypdal, K.; Verne, H. P.; Scherer, W.; Thiel, W.
R. Angew. Chem., Int. Ed. Engl. 1994, 33, 2443-2445.
(28) Ruhlandt-Senge, K.; Ellison, J . J .; Wehmschulte, R. J .; Pauer,
F.; Power, P. P. J . Am. Chem. Soc. 1993, 115, 11353-11357.
(29) Marsh, R. E. Acta Crystallogr. 1952, 5, 458-462.
(24) Brady, P. A.; Carnduff, J . J . Chem. Soc., Chem. Commun. 1974,
816-817.
(25) Ruchardt, C. H. Top. Curr. Chem. 1980, 88, 1-32.
(26) Dolbier, W. R. Chem. Rev. 1996, 96, 1557-1584.

2880 J . Org. Chem., Vol. 64, No. 8, 1999
Crich et al.
F igu r e 1. ORTEP drawing of the molecular structure of 2
showing the full molecule with thermal ellipsoids at the 50%
probability level. Numbering of atoms within the phenyl rings
proceeds according to the scheme C(x1)‚‚‚C(x6) according to
the pattern shown. Isopropyl carbons are numbered with the
C(xy1)‚‚‚C(xy3) pattern.
F igu r e 2. Total energy of 2,6-disubstituted selenols vs
bromides.31
The greater increase in strain with substitution found
in the selenols, as compared to the bromides, coupled
with the absence of unusual structural features in either
case, is at apparent odds with the differing effects of ortho
substitution on the reactions of the two series with
tributyltin hydride. We suggest that this dichotomy can
best be reconciled through the operation of different
mechanisms in the two series. The most recent high-level
ab initio calculations by the Schiesser group on homolytic
substitution at bromine and selenium indicate that these
reactions proceed smoothly in a single step via essentially
linear transition states, as opposed to via hypervalent
intermediates, at least for the simple systems studied.
Thus, Schiesser found computationally that hydrogen
atoms abstract Br from HBr and MeBr via linear transi-
tion states.33 Similarly, Me• and H3Sn• both were calcu-
lated to react with MeBr via backside attack on the
bromine without the intervention of any intermediates.33
Comparable calculations also indicated that the reaction
of both Me• and H3Sn• on MeSeH takes place via backside
attack leading to T-shaped transition states, not hyper-
valent intermediates.34 At the highest levels of theory
Ta ble 3. Obser ved a n d Ca lcu la ted Ar -X Bon d Len gth s
(Å)
Ar-SeH
Ar-SeSeAr
(calcd)
Ar-Br
(obsd)
Ar
(obsd)
STO-3G/MM
Ph
1.9329
1.86
1.93/1.93
1.93/1.95
1.93/1.94
1.8532
2,4,6-tri-iso-propyl-Ph
2,4,6-tri-tert-butyl-Ph
2,4,6-triphenyl-Ph
1.8728
1.8927
diffraction from pentane. Its structure was subsequently
revealed to be perfectly normal (Figure 1) even though
the C-Se bond length was somewhat shorter than in 1.
Unfortunately, crystals of 3 suitable for diffraction could
not be obtained. To probe the selenols themselves we
turned to computational methods. Ab initio calculations
at the STO-3G level and molecular mechanics calcula-
tions using the universal force field were conducted
(Table 3). Both methods revealed the C-Se bond length
to be the same as in diselenide 1 and unaffected by
substitution in the ortho position.30 A plot of total energy,
as determined by molecular mechanics calculations, in
a series of increasingly hindered 2,6-disubstituted aryl
bromides against the same function for the comparable
selenols (Figure 2)31 reveals that strain increases more
severely in the selenols than in the bromides. This
appears to be a function of the Se-H bond by virtue of
which the SeH group is significantly larger than a
bromine atom. Both the ab initio and molecular mechan-
ics calculations show the C-Se-H bond angle to be 94
( 4°, somewhat smaller than the C-Se-Se bond angle
found crystallographically in 129 and 2. The dihedral
angle C-C-Se-H increases from 0°, in the case of the
unsubstituted selenol 1 through 45° in selenol 5 to 89°
in 6. This change in dihedral angle is related to that seen
crystallographically in the diselenides; in 1 the C-C-
Se-Se angle is ∼0°, whereas in 2 it is 76°. Clearly, the
rotation about the C-Se bond reduces steric strain but
does so at the cost of reduced orbital overlap.
•
used SeH3 was also found to be a T-shaped transition
state.35 The results obtained here are most consistent
with the tributylstannyl radicals abstracting bromine
from the aryl bromides in a single step, as shown in
Scheme 1, with a lengthening of the C-Br bond and a
concomitant reduction in strain at the transition state.
On the other hand, the retarding effect of increasing
steric bulk on the reaction of the stannyl radicals with
the selenols is most consistent with a stepwise process
involving reversible formation of a T-shaped intermediate
selenanyl radical that shows increased strain over and
above the ground-state selenol (Scheme 3). This is not to
say that the calculations of Schiesser et al. are incorrect
for the simple reactions studied, only that substituents
can have a significant effect on the mechanism of the
homolytic substitution especially when capable of stabi-
lizing a hypervalent intermediate. Alternatively, it may
be that both sets of reactions proceed along concerted
(30) Dynamic NMR of hindered diselenides: Pinto, B. M.; Leung,
R. Y. N.; Sharma, R. D. Magn. Reson. Chem. 1988, 26, 729-734.
(31) The relationship indicated in Figure 2 is not intended to suggest
any linear correlation, but simply to help convey the idea of the
differing effects of strain on the two classes of molecule. A similar plot
is obtained using the energies obtained from the MO calculations.
(32) International Tables for X-ray Crystallography; Kynock Press:
Birmingham, 1962; Vol. 3.
(33) Schiesser, C. H.; Smart, B. A.; Tran, T.-A. Tetrahedron 1995,
51, 3327-3338, 10651.
(34) Schiesser, C. H.; Smart, B. A. Tetrahedron 1995, 51, 6051-
6060.
(35) Smart, B. A.; Schiesser, C. H. J . Comput. Chem. 1995, 16,
1055-1066.

Diverging Effects of Steric Congestion
J . Org. Chem., Vol. 64, No. 8, 1999 2881
Ta ble 4. Selected Bon d Dista n ces, An gles, a n d Tor sion a l
An gles for 2
Sch em e 3
atoms
distance
atoms
distance
Se1-Se2
Se1-C11
C11-C12
C11-C16
C12-C13
C13-C14
C14-C15
C15-C16
2.336(3)
1.86(2)
1.36(3)
1.43(3)
1.38(3)
1.44(3)
1.37(4)
1.37(3)
Se2-C21
C21-C22
C21-C26
C22-C23
C23-C24
C24-C25
C25-C26
1.87(2)
1.40(3)
1.47(4)
1.35(4)
1.42(4)
1.30(3)
1.37(4)
atoms
angle
atoms
angle
Sch em e 4
Se2-Se1-C11
Se1-C11-C12
Se1-C11-C16
101.5(6)
121(2)
120(2)
Se1-Se2-C21
Se2-C21-C22
Se2-C21-C26
100.1(7)
123(2)
121(2)
atoms
angle
C11-Se1-Se2-C21
Se2-Se1-C11-C12
Se2-Se1-C11-C16
Se1-Se2-C21-C22
Se1-Se2-C21-C26
76(1)
-108(2)
76(2)
75(2)
-104(2)
Sch em e 5
Exp er im en ta l Section
Gen er a l P r oced u r es.36 Bis(2,4,6-tri-iso-propyl)phenyl di-
selenide (2)18 and bis(2,4,6-tri-tert-butyl)phenyl diselenide (3)19
were prepared according to literature methods. The aryl
bromides with the exceptions of 2,4,6-tri-iso-propylbromoben-
zene (14)37 and 2,4,6-triphenylbromobenzene (12),38 which
were prepared according to literature methods, were com-
mercial and used as purchased. N-(2-Bromo-2-propenyl)-N-(2-
propenyl)benzenesulfonamide (19)39 and diethyl 2-(2-bromo-
2-propenyl)-2-(2-propenyl)-propanedioate (22)40 were prepared
according to the literature.
pathways with backside attack but with transition states
located at different points on the reaction coordinate.
Thus, the Br abstraction could be taking place via a late
transition state with a long C-Br bond, resulting in the
observed steric acceleration, whereas the transition state
for SeH abstraction may be early, with a short C-Se
bond, leading to steric hindrance of the approach of the
stannyl radical. However, the computations of Schiesser
mitigate against this possibility.33,34 At the highest level
of theory employed (QCISD/MP2) these authors find the
abstraction of Br from MeBr by H3Sn• to be substantially
exothermic, i.e., with an early transition state, whereas
that of SeH from MeSeH by H3Sn• is marginally endo-
thermic, i.e., with a later transition state.33,34 Although
these calculations are for abstractions from sp3 C-X
bonds, and not the sp2 C-X bonds studied here, it is
reasonable to suppose that the trend will be similar.
Finally, returning to the initial objective, we briefly
investigated the ability of the hindered selenol 6, intro-
duced as 3, to inhibit radical rearrangements in reactions
in which the original radical is derived from sp2 C-Br
bonds. Thus, a refluxing 0.05 M benzene solution of vinyl
bromide 19 was treated dropwise with Bu3SnH and
catalytic AIBN over 3 h, which resulted in the formation
of the kinetic and thermodynamic products 20 and 21 in
the ratio of 1.8/1. When the reaction was repeated in the
presence of 10 mol % 3, i.e., 0.005 M 6, the ratio was 3/1
(Scheme 4). In a similar experiment, vinyl bromide 22
gave a ratio of 23/24 ) 7/3 without the catalyst and 89/
11 in the presence of 0.005 M 6 (Scheme 5). Thus, it is
seen that although 6 is eventually destroyed by Bu3SnH
in benzene at reflux, reaction of the stannane with the
bromide is competitive and catalysis can be achieved.
Work is in progress on the further development of stable
selenol catalysts and will be reported in due course.
Rea ction of Diselen id es w ith Bu 3Sn H w ith Mon itor in g
by NMR Sp ectr oscop y. The diselenide 1, 2, or 3 (0.032
mmol) was dissolved in C6D6 (1 mL) under Ar in a 5 mm NMR
tube, and then Bu3SnH (9.2 µL, 0.035 mmol) was added. After
decolorization of the yellow solution, the 77Se NMR spectrum
was measured. In a second set of experiments, Bu3SnH (18.4
µL, 0.07 mmol) was added to a solution of diselenide (0.032
mmol) and then the 77Se NMR spectrum was recorded.
In a competition experiment, 1 (0.032 mmol) and 3 (0.032
mmol) were dissolved in C6D6 (1 mL) under Ar in a 5 mm NMR
tube and then Bu3SnH (9.2 µL, 0.035 mmol) or Bu3SnH (18.4
µL, 0.07 mmol) was added. After standing at room temperature
for 3 h, the 77Se NMR spectrum was measured. A similar
experiment was conducted with 1 (0.032 mmol) and 2 (0.032
mmol) and Bu3SnH (18.4 µL, 0.07 mmol) and the 77Se NMR
spectrum was measured.
Com p etition of Br om id es for Rea ction w ith Tr ibu tyl-
tin Hyd r id e. To a solution of the competing bromides (0.5-1
M in dry benzene) in the ratios reported in the Supporting
Information were added Bu3SnH (0.5 equiv based on the sum
of the two substrates) and a catalytic amount of AIBN. The
solution was heated at 80 °C for 3 h under an inert atmosphere
and cooled to room temperature, and p-diethoxybenzene was
then added as standard and an aliquot of the solution was used
for GC and NMR analysis. The relative rate constants were
determined using the following equation for competitive bi-
molecular reactions.22,41
(36) For general experimental details see ref 1.
(37) Miller, A. R.; Curtin, D. Y. J . Am. Chem. Soc. 1976, 98, 1860-
1865.
(38) Kohler, E. P.; Blanchard, L. W. J . Am. Chem. Soc. 1935, 57,
367-371.
(39) Padwa, A.; Nimmesgern, H.; Wong, G. S. K. J . Org. Chem. 1985,
50, 5620-5627.
(40) Grigg, R.; Stevenson, P.; Worakun, T. Tetrahedron 1988, 44,
2033-2048.
(41) Newcomb, M. Tetrahedron 1993, 49, 1151-1176.

2882 J . Org. Chem., Vol. 64, No. 8, 1999
k1/k2 ) (ln([Ar1Br]i/([Ar1Br]i -
Crich et al.
To a 0.05 M solution of 19 (102 mg, 0.32 mmol) in benzene
(3.2 mL) at reflux under Ar was added, by means of a syringe
pump, a solution of Bu3SnH (192 µL, 0.7 mmol) and AIBN (3
mg, 0.02 mmol) in benzene (3.2 mL) dropwise over 3 h. After
a further 0.5 h at reflux, the reaction mixture was cooled to
room temperature and the solvent removed under reduced
pressure. The crude mixture was taken up in acetonitrile,
washed with petroleum ether, and evaporated to give a
mixture of 20 and 21 in the ratio of 1.8 and 1 as assigned by
1H NMR spectroscopy. The experiment was repeated with 10
mol % of 6 added to the initial benzene solution of 19 when
the ratio of 20 and 21 was found to be 3:1. 20:39 1H NMR δ
1.02 (3H, d J ) 6.5 Hz), 2.55-2.74 (2H, m), 3.58 (1H, t, J )
6.5 Hz), 3.76 (1H, m), 3.95 (1H, m), 4.81-4.89 (2H, m), 7.44-
7.62 (3H, m), and 7.80 (2H, m). 21:39 1H NMR δ 1.64-1.69
(2H, m), 2.08 (2H, t J ) 6.1 Hz), 3.07 (2H, t J ) 5.6 Hz), 3.50
(2H, s), 4.80-4.89 (2H, m), 7.44-7.62 (3H, m), and 7.80 (2H,
m).
[Ar1H])))/(ln([Ar2Br]i/([Ar2Br]i - [Ar2H])))
In almost all cases the amounts of the two residual compet-
ing substrates were determined. In few cases it was
a
experimentally more convenient to determine the products.
The full set of experimental data is given in the Supporting
Information.
Rea ction of Diselen id e 2 w ith Tr ibu tylsta n n a n e: F or -
m a tion of Tr iisop r op ylben zen e. Bu3SnH (72.7 mg, 0.25
mmol) and 2 (0.2 mmol) were dissolved in benzene (3 mL)
under Ar. The solution became colorless after being stirred at
room temperature for 15 min. There was no visible change in
the solution after it was refluxed for 1 h under Ar, a further
portion of Bu3SnH (0.25 mmol) was then added, and the
solution was refluxed for another hour after which GC analysis
showed the formation of 1,3,5-triisopropylbenzene in 95% yield
(i.e., one, not both, of the two arene rings), as determined with
the aid of 1,3,5-tri-tert-butylbenzene as an internal standard.
X-r a y Cr ysta llogr a p h ic Str u ctu r e Deter m in a tion of
Bis(2,4,6-tr iisop r op yl)p h en yl Diselen id e (2). Details of
data collection and the results of structure refinement are
given in the Supporting Information. Important bond distance
and angle information is given in Table 4. Data were collected
using a Mo X-ray source. An analytical absorption correction
was applied to the data. Data were processed, and the
structure was solved and refined on F using the XTAL 3.542,43
suite of programs. The selenium atoms and the carbon atoms
in the aromatic ring were refined with anisotropic thermal
parameters. Isopropyl group carbons were refined with iso-
tropic thermal parameters. Hydrogen atoms were located in
idealized positions based on the carbon backbone and were
given a fixed isotropic thermal parameter U ) 0.120. Analysis
of the final Fourier difference revealed the presence of
significant electron density in the vicinity of the selenium
atoms and near the isopropyl group carbons. Other methods
for absorption correction (Ψ scans) and inclusion of anistropic
thermal parameters for the isopropyl group carbons did not
remove these peaks.
Diet h yl 2-(2-b r om o-2-p r op en yl)-2-(2-p r op en yl)-p r o-
p a n ed ioa te (22):40 1H NMR δ 1.24 (6H, t, J ) 7.1 Hz), 2.76
(2H, d, J ) 7.4 Hz), 3.13 (2H, s), 4.19 (4H, m), 5.10 (2H, m),
5.57-5.69 (3H, m); 13C NMR δ 13.7 (2C), 35.9, 42.8, 56.7, 61.5
(2C), 119.5, 121.9, 127.1, 130.0, 170.0.
Rea ction of Dieth yl 2-(2-Br om o-2-p r op en yl)-2-(2-p r o-
p en yl)-p r op a n ed ioa te (22) w ith Bu 3Sn H a n d Diselen id e
6: 4,4-Bis(eth oxyca r bon yl)-1-m eth yl-2-m eth ylen ecyclo-
p en ta n e (23) a n d 1,1-Bis(eth oxyca r bon yl)-3-m eth ylen e-
cycloh exa n e (24). To a 0.05 M solution of 22 (104 mg, 0.31
mmol) in benzene (3.2 mL) at reflux under Ar was added, by
means of a syringe pump, a solution of Bu3SnH (192 µL, 0.7
mmol) and AIBN (3 mg, 0.02 mmol) in benzene (3.2 mL)
dropwise over 3 h. After a further 0.5 h at reflux, the reaction
mixture was cooled to room temperature and the solvent
removed under reduced pressure. The crude product mixture
was taken up in acetonitrile, washed with petroleum ether,
and evaporated to give a mixture of 23 and 24 in a ratio of
1
70:30 as assigned by H NMR spectroscopy. When the experi-
ment was conducted in the presence of 10 mol % of 6 added to
the initial benzene solution of 22 the ratio of 23:24 was 89:11.
23:40 1H NMR δ 1.08 (3H, d J ) 6.3 Hz), 1.25 (6H, dt, J ) 1.1,
7.0 Hz), 1.71 (1H, m), 2.58 (2H, m), 3.0 (2H, q, J ) 17.0 Hz),
4.18 (4H, m), 4.79 (1H, d, J ) 2.1 Hz), 4.89 (1H, d, J ) 2.0
Hz); 13C NMR δ 13.9 (2C), 17.8, 37.2, 40.4, 42.0, 58.1, 61.3 (2C),
105.3, 153.3, 171.9 (2C). 24:40 1H NMR δ 1.25 (6H, t, J ) 7.1
Hz), 1.65 (2H, m), 2.08 (4H, m), 2.65 (2H, s), 4.18 (4H, m),
4.70 (2H, s).
Com p u ta tion s. Force field calculations were carried out
with the Universal Force Field.44 Ab initio calculations were
carried out by the HF SCF method using the STO 3G basis
set in Gaussian94. All computations were conducted with an
SGI Octane system using the Cerius2 3.0 suite of programs.
N-(2-Br om o-2-pr op en yl)-N-(2-p r op en yl)ben zen esu lfon -
a m id e (19):39 1H NMR δ 3.85 (2H, t, J ) 6.5 Hz), 4.04 (2H, s),
5.15 (2H, m), 5.50-5.65 (2H, m), 5.83 (1H, q, J ) 1.7 Hz), 7.48-
7.60 (3H, m), 7.84 (2H, m); 13C NMR δ 49.9, 53.7, 119.4, 120.0,
127.2 (2C), 127.7, 129.0 (2C), 131.7, 132.7, 140.0.
Ack n ow led gm en t. We are grateful to the NSF
(CHE 9625256) for support of this work and to Drs. J ohn
and Cynthia Harwood for their help with multinuclear
NMR spectroscopy and computations, respectively. F.R.
thanks the Politecnico di Milano for study leave and
support.
Rea ction of N-(2-Br om o-2-p r op en yl)-N-(2-p r op en yl)-
ben zen esu lfon a m id e (19) w ith Bu 3Sn H a n d Diselen id e
6: N-(P h en ylsu lfon yl)-3-m eth yl-4-m eth ylen ep yr r olid in e
(20) a n d N-(P h en ylsu lfon yl)-3-m eth ylen ep ip er id in e (21).
Su p p or tin g In for m a tion Ava ila ble: Table of experimen-
tal data for the competitive reactions of aryl bromides with
tributylstannane and X-ray structural data for 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
(42) In XTAL 3.4 User’s Manual; Hall, S. R., King, G. S. D., Steward,
J . M., Eds.; Lamb: Perth, 1995.
(43) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473.
(44) Rappe´, A. K.; Casewit, C. J .; Colwell, K. S.; Goddard, W. A.,
III; Skiff, W. M. J . Am. Chem. Soc. 1992, 114, 10024-10035.
J O982514A