Radical clock reactions under Pseudo-first-order conditions using catalytic quantities of dipnenyl diselenide. A 77Se- and 119Sn-NMR study of the reaction of tributylstannane and diphenyl diselenide

Article abstract of DOI:10.1021/jo950857s

A method for the trapping of alkyl radicals by constant, catalytic quantities of PhSeH as a clock reaction in radical kinetics is presented. PhSeH is introduced in the form of PhSeSePh and regenerated by slow addition of a stoichiometric quantity of Bu₃SnH. Using this method the rate constant for cyclization of the 6,6-diphenyl-5-hexenyl radical was found to be 6.8 × 10⁷ s^-1 at 20 °C, in fair agreement with the literature value of 4 × 10⁷ s^-1. An extension of the method was used to determine the rate of quenching of the 2,3,4,6-tetra-O-acetoxy-1-glucosyl radical by PhSeH as 3.6 × 10⁶ s^-1 at 78°C. The reaction of Bu₃nH and PhSeSePh was studied by a combination of ⁷⁷Se- and ¹¹⁹Sn-NMR spectroscopy.

Full text of DOI:10.1021/jo950857s

2368
J . Org. Chem. 1996, 61, 2368-2373
Ra d ica l Clock Rea ction s u n d er P seu d o-F ir st-Or d er Con d ition s
Usin g Ca ta lytic Qu a n tities of Dip h en yl Diselen id e. A 77Se- a n d
119Sn -NMR Stu d y of th e Rea ction of Tr ibu tylsta n n a n e a n d
Dip h en yl Diselen id e
David Crich,* Xian-Yun J iao, Qingwei Yao, and J ohn S. Harwood
Department of Chemistry, University of Illinois at Chicago, 845 W. Taylor Street, Room 4500,
Chicago, Illinois 60607-7061
Received May 8, 1995X
A method for the trapping of alkyl radicals by constant, catalytic quantities of PhSeH as a clock
reaction in radical kinetics is presented. PhSeH is introduced in the form of PhSeSePh and
regenerated by slow addition of a stoichiometric quantity of Bu3SnH. Using this method the rate
constant for cyclization of the 6,6-diphenyl-5-hexenyl radical was found to be 6.8 × 107 s-1 at 20
°C, in fair agreement with the literature value of 4 × 107 s-1. An extension of the method was
used to determine the rate of quenching of the 2,3,4,6-tetra-O-acetoxy-1-glucosyl radical by PhSeH
as 3.6 × 106 s-1 at 78 °C. The reaction of Bu3SnH and PhSeSePh was studied by a combination of
77Se- and 119Sn-NMR spectroscopy.
In tr od u ction
prompted Newcomb to introduce the use of hydrogen
atom abstraction from PhSeH (k25 ) 2.1 × 109 M-1 s-1
for a primary alkyl radical).4 Here, we introduce a new
method for the use of the PhSeH radical clock which
operates under pseudo-first-order conditions, without an
excess of reagent, and in which reactions are run to
completion. Problems due to analysis of small quantities
of product in the presence of excess reagent or substrate
are therefore avoided. Furthermore, PhSeH is intro-
duced in a catalytic quantity in the form of PhSeSePh
which obviates the need to prepare and manipulate this
highly air sensitive, noxious compound.
The very considerable success1 seen in recent years in
the application of free radical chain reactions, of ever
increasing complexity, is underpinned by the painstaking
work of physical organic chemists involved in establishing
a data base of rate constants2 for individual radical
reaction steps. Appreciation of this data base is invalu-
able as it enables synthetic chemists to predict with some
confidence whether a particular chain sequence will
provide the desired product or not. The most common
approach for the determination of a radical rate constant
involves the use of a calibrated radical clock whereby the
reaction under investigation is pitted against a rear-
rangement of known rate in a competition reaction. The
desired rate is then obtained from the ratio of products
formed. When the competition is intermolecular it is
usual to use one of the reagents in large excess such that
pseudo-first-order kinetics may be assumed and the
experiment and calculations considerably simplified.
Alternatively, in order to make the same assumption, the
reaction is only run to low conversion. This approach
has been widely and successfully applied but problems
can arise through the necessity of determining ratios of
products accurately in the presence of a large excess of
reagent or of substrate depending on which protocol is
employed. Much of the work in the literature has relied
Resu lts a n d Discu ssion
Recently, we described the catalysis of Bu3SnH reduc-
tions of alkyl halides by catalytic quantities of PhSeH,
introduced as PhSeSePh, and its application to the
prevention of unwanted radical rearrangments.5 Key
steps in this chemistry are the rapid reduction of Ph-
SeSePh by Bu3SnH (eq 1).
R3SnH + PhSeSePh f R3SnSePh + PhSeH (1)
and the three propagation step chain sequence (eqs 2-4).
Bu3Sn• + RX f Bu3SnX + R•
(2)
on the use of the abstraction of hydrogen atoms from Bu3-
SnH by primary alkyl radicals (k25 ) 2.4 × 106 M-1 s-1
)
3
R• + PhSeH f RH + PhSe•
(3)
(4)
as a clock reaction, but the need for faster clock reactions
in the determination of picosecond radical kinetics has
PhSe• + Bu3SnH f PhSeH + Bu3Sn•
X Abstract published in Advance ACS Abstracts, March 1, 1996.
(1) (a) Motherwell, W. B.; Crich, D. Free Radical Chain Reactions
in Organic Synthesis; Academic Press: London, 1992. (b) Giese, B.
Radicals in Organic Synthesis: Formation of Carbon-Carbon Bonds;
Pergamon Press: Oxford, 1986. (c) Curran, D. P. In Comprehensive
Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford,
1992; Vol. 4, pp 715, 779. (d) Curran, D. P. Synthesis 1988, 417, 489.
(e) J asperse, C. P.; Curran, D. P.; Fevig, T. L. Chem. Rev. 1991, 91,
1237. (f) Ramaiah, M. Tetrahedron 1987, 43, 3541 and references
therein.
(2) (a) Lanholt-Bornstein New Series, Vol. 13abc, Fischer, H., Ed.,
Springer Verlag: Berlin, 1983, 1984, 1985. (b) Ingold, K. U.; Griller,
D. Acc. Chem. Res. 1980, 13, 317. (c) Newcomb, M. Tetrahedron 1993,
49, 1151.
(3) Chatgilialoglu, C.; Ingold, K. U.; Scaiano, J . C. J . Am. Chem.
Soc. 1981, 103, 7739.
This sequence, which enabled us to take advantage of
the very superior properties of PhSeH as a trap for
nucleophilic alkyl radicals, can be thought of as an
example of polarity reversal catalysis as enunciated by
Roberts.6,7 Thus, the abstraction of a hydrogen atom
from Bu3SnH by a nucleophilic alkyl radical is unfavor-
able due to the hydridic nature of the tin hydride,
(4) (a) Newcomb, M.; Varick, T. R.; Ha, C.; Manek, M. B.; Yue, X. J .
Am. Chem. Soc. 1992, 114, 8158. (b) Martin-Esker, A. A.; J ohnson, C.
C.; Horner, J . H.; Newcomb, M. J . Am. Chem. Soc. 1994, 116, 9174.
(5) Crich, D.; Yao, Q. J . Org. Chem. 1995, 60, 84.
0022-3263/96/1961-2368$12.00/0 © 1996 American Chemical Society

Radical Clock Reactions Under Pseudo-First-Order Conditions
J . Org. Chem., Vol. 61, No. 7, 1996 2369
Ta ble 1. Releva n t 77Se-NMR Ch em ica l Sh ifts
whereas that from PhSeH is favorable due to its acidic
nature. The cycle is completed (eq 4) when the electro-
philic PhSe• radical abstracts the electron rich hydrogen
from the stannane, a reaction in which the polarities are
nicely matched.8
compound
δ (C6D6)
PhSeSePh
Bu3SnSePh
PhSeH
460
-27.7
142.2
We reasoned that, by application of the above catalytic
cycle, the PhSeH clock4 could be applied to the measure-
ment of a wide range of radical kinetics. The clock
sequence to be applied mirrors closely that in Newcomb’s
excellent work; however, the PhSeH is used in substo-
ichiometric quantities, rather than in excess, and is
regenerated by slow addition of Bu3SnH. Consider the
general case for determination of the rate constant for
the rearrangement of a radical A• to a radical B•.
Dropwise addition of 1 mol equiv of Bu3SnH to a mixture
of radical precursor A-X, wherein X is Br, I, or PhSe
and a precisely controlled catalytic amount of PhSeH will
result in the formation of the reduction product A-H and
the rearranged product B-H. The thousandfold differ-
ence in rates for the quenching of A• (and B•) by PhSeH
and Bu3SnH, coupled with the rapid recycling of PhSeH
and the dropwise addition of Bu3SnH, renders the
contribution of Bu3SnH to the trapping of A• negligible
and ensures that the concentration of PhSeH is kept
constant. The conditions for pseudo-first-order kinetics
are therefore established. Moreover, it should be possible
to run the reaction to complete consumption of A-X,
without the need for an excess of PhSeH or Bu3SnH: the
analytical problems alluded to above are therefore avoided.
Conducting several runs in the presence of different
concentrations of PhSeH will enable a linear plot of A-H/
B-H against [PhSeH] to be traced for which the slope is
the ratio of kH/kR where kR is the rate of the rearrange-
ment and kH the known rate of quenching of A• by PhSeH
(Scheme 1). In this manner it should be possible to
determine the rates of a wide variety of radical rear-
rangements. Alternatively, using a rearrangement of
known rate, it should be possible to determine the rate
of hydrogen abstraction from PhSeH by a variety of
differently substituted alkyl radicals.
Ta ble 2. Releva n t 119Sn -NMR Ch em ica l Sh ifts
compound
δ (C6D6)
Bu3SnH
Bu3SnSePh
Bu3SnPh
Bu3SnSnBu3
Bu3SnBr
-88.4
60.2
-42.7
-83.4
134
PhSeSePh and to the actual concentration of PhSeH
used, an uncertainty which caused Newcomb and co-
workers to analyze all samples of PhSeH by GLC im-
mediately before use.10 A similar concept for radical
kinetics has previously been put forward by Newcomb,2c
based on Roberts original work,6 in which a catalytic
quantity of thiol, as hydrogen atom donor, is recycled by
reaction with a silane. Competing addition of the thiol
and silane to alkenes, however, caused Newcomb to
abandon this approach.
Scheme 1 would be complicated if PhSeSnBu3 were to
react with Bu3SnH (eq 5) as this would double the
concentration of PhSeH in the reaction mixture and lead
to an error of a factor of two in any rate constants
determined.
PhSeSnBu3 + Bu3SnH f
PhSeH + Bu3SnSnBu3 (5)
This is clearly not the case. Alkyl phenyl selenides1
and acyl phenyl selenides11 are excellent precursors to
alkyl and acyl radicals, respectively, for radical cycliza-
tions, when used in conjunction with Bu3SnH. If eq 5
were to operate, not only would 2 mol equiv of stannane
be required to consume the substrate in these reactions,
but also the stoichiometric quantites of PhSeH generated
would be more than sufficient to prevent any rearrange-
ments from occurring.5 Firm evidence was provided by
a combination of 77Se-NMR12 and 119Sn-NMR13 experi-
ments with relevant chemical shifts grouped in Tables 1
and 2. A 0.4 M solution of PhSeSePh in C6D6 was treated
at room temperature under Ar with 1 mol equiv of Bu3-
SnH resulting in immediate decolorization. In the 77Se-
NMR spectrum two new signals in a 1:1 ratio at δ 142.2
and -27.7 ppm were formed and attributed with the aid
Sch em e 1
(10) Newcomb, M.; Manek, M. B.; Glenn, A. G. J . Am. Chem. Soc.
1991, 113, 949.
A distinct advantage of this method lies in the intro-
duction of PhSeH in the form of PhSeSePh. This
eliminates the need to prepare and handle PhSeH which
is extremely air sensitive and a vesicant.9 Aside from
avoiding the use of a very unpleasant substance, this
removes any doubt as to the contamination of PhSeH by
(11) (a) Crich, D.; Fortt, S. M. Tetrahedron Lett. 1988, 29, 2585. (b)
Crich, D.; Fortt, S. M. Tetrahedron 1989, 45, 6581. (c) Crich, D.;
Eustace, K. A.; Ritchie, T. J . Heterocycles 1989, 28, 67. (d) Crich, D.;
Eustace, K. A.; Fortt, S. M.; Ritchie, T. J . Tetrahedron 1990, 46, 2135.
(e) Batty, D.; Crich, D.; Fortt, S. M. J . Chem. Soc., Chem. Commun.
1989, 1366. (f) Batty, D.; Crich, D. Synthesis 1990, 273. (g) Batty, D.;
Crich, D.; Fortt, S. M. J . Chem. Soc., Perkin Trans. 1 1990, 2875. (h)
Batty, D.; Crich, D. Tetrahedron Lett. 1992, 33, 875. (i) Batty, D.; Crich,
D. J . Chem. Soc., Perkin Trans. 1 1992, 3193. (j) Batty, D.; Crich, D.
J . Chem. Soc., Perkin Trans. 1 1992, 3205. (k) Boger, D. L.; Mathvink,
R. J . J . Org. Chem. 1988, 53, 3377. (l) Boger, D. L.; Mathvink, R. J . J .
Org. Chem. 1989, 54, 1777. (m) Boger, D. L.; Mathvink, R. J . J . Am.
Chem. Soc. 1990, 112, 4003. (n) Boger, D. L.; Mathvink, R. J . J . Org.
Chem. 1990, 55, 5442. (o) Boger, D. L.; Mathvink, R. J . J . Am. Chem.
Soc. 1990, 112, 4008. (p) Curran, D. P.; Liu, H. J . Org. Chem. 1991,
56, 3463.
(6) (a) Allen, R. P.; Roberts, B. P.; Willis, C.; R. J . Chem. Soc., Chem.
Commun. 1989, 1387. (b) Cole, S. J .; Kirwan, J . N.; Roberts, B. P.;
Willis, C. R. J . Chem. Soc., Perkin Trans. 1 1991, 103. (c) Dang, H.-S.;
Roberts, B. P. Tetrahedron Lett. 1995, 36, 2875.
(7) For early examples of this principle involving decarbonylation
of aldehydes catalyzed by thiols see: (a) Harris, E. F. P.; Waters, W.
A. Nature 1952, 170, 212. (b) Harris, E. F. P.; Waters, W. A. Discussions
Farad. Soc. 1953, 14, 221. (c) Berman, J . D.; Stanley, J . H.; Sherman,
W. V.; Cohen, S. G. J . Am. Chem. Soc. 1963, 85, 4010.
(8) An alternative explanation for this type of catalysis has recently
been advanced: Zavitsas, A. A.; Chatgilialoglu, C. J . Am. Chem. Soc.
1995, 117, 10645.
(9) Paulmier, C. Selenium Reagents and Intermediates in Organic
Synthesis; Pergamon Press: Oxford, 1986; p 26.
(12) McFarlane, H. C. E.; McFarlane, W In NMR of Newly Accessible
Nuclei; Laszlo, P., Ed.; Academic: New York, 1983; Vol. 2, p 275.
(13) (a) Harris, R. K.; Kennedy, J . D.; McFarlane, W. In NMR and
the Periodic Table; Harris, R. K., Mann, B. E., Eds.; Academic: London,
1978; p 342. (b) Smith, P. J .; Tupciauskas, A. P. Annu. Rep. NMR
Spectrosc. 1978, 8, 291.

2370 J . Org. Chem., Vol. 61, No. 7, 1996
Crich et al.
Ta ble 3. Rea ction of 3 w ith Bu 3Sn H a n d P h SeSeP h
of authentic samples to PhSeH and PhSeSnBu3, respec-
tively, whereas the 119Sn-NMR spectrum showed a single
signal at δ 60.2 due to PhSeSnBu3.14 It is clear that
equimolar quantities of Bu3SnH and PhSeSePh react
very rapidly in a stoichiometric manner according to eq
1. In a second experiment, an authentic sample15 of
PhSeSnBu3 was treated with 1 mol equiv of Bu3SnH and
heated to 80 °C in C6D6 for 1 h. Examination of the
reaction mixture by 77Se- and 119Sn-NMR revealed that
no reaction had taken place. It is therefore clear that
there is no further reaction at 80 °C between PhSeSnBu3
and additional Bu3SnH, i.e. eq 5 is not valid.
a t 20 °C
mol % PhSeSePh
[PhSeH] (M × 103)
ratio 4/5
5
10
15
20
25
30
35
3.1
6.2
9.6
12.4
15.5
18.6
21.7
0.029
0.14
0.21
0.30
0.39
0.51
0.56
double bond present in 3 and 4 was not observed,
presumably owing to the reverse reaction being more
rapid than quenching of the adduct radical under the
dilute conditions used. The data are collected in Table
3. A plot of 4/5 against molar concentration of PhSeSePh
is a straight line of slope 29 ( 3.2,19 passing tolerably
close to the origin with an intercept of -0.058 ( 0.05.
The slope of the plot is equal to kH/kR where kH is the
rate constant for the reaction of 1 with PhSeH and kR
that of cyclization. Substituting 1.98 × 109 for kH at 20
°C20 gives kR ) 6.8 × 107 s-1 at 20 °C. This rate constant
is in fair agreement with that obtained by Newcomb and
serves to validate the proposed method. Nevertheless,
it is appropriate to ask why better agreement with the
literature values for this cyclization is not found? One
possible source of error lies in the position of the
equilibrium described by eq 4, which will depend to some
extent on the relative bond dissociation energies (BDE’s)
of Sn-H and Se-H. The BDE of Me3Sn-H is 74 kcal
In order to test the proposed kinetic scheme we elected
to redetermine the rate constant of an established radical
clock. The rearrangement of the 6,6-diphenyl-5-hexenyl
radical 1 to the cyclopentyl diphenylmethyl radical 2 was
selected for this purpose. The rate constant for this
rearrangement has been determined16 by Newcomb using
his PTOC adaptation2c of the Barton O-acyl thiohydrox-
amate chemistry,17 indirectly by trapping with thiols. The
calculated rate constant at 20 °C, as determined from
7
the Arrhenius function, is 5 × 10 s-1. More recently,
Newcomb has reported an updated Arrhenius function
(eq 6) for this cyclization using the LFP method leading
to a rate constant of 4 × 107 s-1 at 20 °C.18
log(kR) ) 9.8 - 3.0/2.3RT
(6)
Appropriate concentrations of 3 and PhSeSePh for the
obtention of readily measureable ratios of the 4:5 on
treatment with Bu3SnH were established through pre-
mol-1 21 estimates for that of PhSe-H vary between 6722
;
and 7423 kcal mol-1. In order to probe this possibility
parallel cyclizations of 3 were carried out, under the
standard conditions, using on the one hand Bu3SnH and,
on the other, Ph3SnH with its higher rate of trapping of
24
primary alkyl radicals (k25 ) 5 × 106 M-1 s-1
)
and
therefore weaker Sn-H bond. Within the limits of
experimental error, both runs gave the same ratio of 4:5
suggesting that the equilibrium (eq 4) lies fully to the
right and hence that the effective concentration of PhSeH
is the same as that of the PhSeSePh administered
initially. This is readily understood in terms of the
equilibrium (eq 4) being constantly driven to the right
by the removal of Bu3Sn• following its reaction with the
radical precursor RX, in this case 3. Several experiments
using a set concentration of PhSeSePh and in which the
reaction was stopped at different stages of conversion
were also conducted. Within the limits of experimental
error, the ratio 4:5 was found not to vary with conversion,
indicating that the concentration of PhSeH is constant
throughout the course of the reaction. A further plausible
source of error lies in kH, the rate constant for trapping
of a primary alkyl radical by PhSeH,4 determined by the
PTOC/PhSeH method with all the disadvantages of using
stoichiometric PhSeH outlined above. Evidently, it is
possible to take the, obviously accurate, LFP rate con-
stant kR ) 4 × 107 s-1 for the cyclization of 1 and the
slope from the above plot and calculate a new value for
liminary experiments. A stock solution of PhSeSePh in
benzene was used to prepare seven flasks containing a
set amount of 3 and between 5 and 35 mol % of
PhSeSePh, which were then made up to 100 mL.
A
quantity of Bu3SnH equal to that of PhSeSePh was then
added to each flask such that the yellow solution was
decolorized and PhSeH was generated quantitatively (eq
1). After purging with Ar the flasks were placed, one by
one, in a water bath at 20 °C in a Rayonet photoreactor
and irradiated for 1.5 h during which time Bu3SnH (110
mol % with respect to 3) and AIBN were added dropwise
with the aid of a motor-driven syringe pump. The solvent
was then removed under vacuum and the ratio of 4:5
determined by integration of the olefinic signal of 4
against the benzhydryl signal of 5 in the 1H-NMR
spectrum. In each experiment 3 was completely con-
sumed. Depletion of the catalyst by addition to the
(14) Addition of a further 1 equiv of Bu3SnH results in a second
slow reaction, incomplete after several days at room temperature,
which results in the gradual consumption of PhSeH and Bu3SnH with
slow evolution of a colorless gas and formation of a grey precipitate,
presumed to be Se metal.
(19) Errors are given at the 95% confidence interval (3.2σ for the
results obtained here with
Newcomb with a much larger data set).
a small data set and 2σ for those of
(15) (a) Fukuzawa, S.; Niimoto, Y.; Fujinami, T.; Sakai, S. Heteroat.
Chem. 1990, 1, 491. (b) Macmullin, E. C.; Peach, M. E. J . Organomet.
Chem. 1973, 52, 355.
(16) Ha, C.; Horner, J . M.; Newcomb, M.; Varick, T. R.; Arnold, B.
R.; Lusztyk, J . J . Org. Chem. 1993, 58, 1194.
(20) Calculated from the combined (THF + toluene) Arrhenius
parameters [log(kH) ) 10.87 - 2.10/2.3RT] for trapping of a primary
alkyl radical by PhSeH.4
(21) Griller, D.; Kanabus-Kaminska, J . M.; Maccoll, A. J . Mol.
Struct. (THEOCHEM) 1988, 163, 125.
(17) Barton, D. H. R.; Crich, D.; Motherwell, W. B. Tetrahedron
1985, 41, 3901.
(18) Newcomb, M.; Horner, J . H.; Filipowski, M. A.; Ha, C.; Park,
S.-U. J . Am. Chem. Soc. 1995, 117, 3674.
(22) See footnote 34 in ref 10.
(23) Newcomb, M. Private communication. This value is based on a
BDE for PhS-H of 85 kcal.mol-1 rather than that of 78 used in ref 10.
(24) Carlsson, D. J .; Ingold, K. U. J . Am. Chem. Soc. 1968, 94, 6059.

Radical Clock Reactions Under Pseudo-First-Order Conditions
J . Org. Chem., Vol. 61, No. 7, 1996 2371
Ta ble 4. Deter m in a tion of Ar r h en iu s F u n ction for
Ta ble 5. Rea ction of 8 w ith Bu 3Sn H a n d P h SeSeP h
RCH2• w ith P h SeH in Tolu en e
mol %
PhSeSePh
[PhSeH]
ratio 9/10
(run 1)
ratio 9/10
(run 2)
temp
(°C)
[PhSeH]
ratio
4/5
kH
(M × 104)
(M × 103)
kH/kR
(× 10-8 s-1
)
3
4
5
6
7
2.9
3.9
4.9
5.9
6.8
3.1
4.2
4.9
5.7
6.5
3.4
4.3
5.3
6.2
7.1
20
0
-
-
33.3a
30.4
43.5
54.3
69.6
12.0a
7.6
7.0
5.0
3.4
4.6
4.6
4.6
4.6
0.14
0.20
0.25
0.32
-20
-42
-62
827 was consumed. The data is presented in Table 5.
Plots of 9:10 against molar concentration of PhSeSePh
gave straight lines of gradient 8.5 ( 1.0 × 103 and 9.5 (
0.3 × 103 for the two runs, respectively.19 The weighted
mean gradient28,29 for the two runs is 8.9 ( 1.2 × 103
from which the rate constant for hydrogen atom abstrac-
tion from PhSeH by radical 6 is calculated to be 3.6 (
0.5 × 106 M-1 s-1. The relatively large difference in slope
between the two runs is probably best attributed to
inaccuracies in the preparation of the two stock solutions.
In this respect we note that the rearrangement of 6 to 7
is rather slow compared to other preparatively useful
radical processes, making the ratio of 9:10 and the slope
of the above plots very susceptible to changes in the
concentration of PhSeH used. As such the method
described here can be expected to be less accurate for
determination of the kinetics of slower processes.
a
Calculated using the data from Table 3 (in benzene).
F igu r e 1. log (kH) vs 1000/T.
the trapping of primary alkyl radicals by PhSeH at 20
°C in benzene. Such a calculation gives kH ) 1.2 × 109
M-1 s-1 at 20 °C. Equally, it was possible to conduct the
cyclization of 3 using our catalytic PhSeSePh/Bu3SnH
method over an 80 °C range of temperature (Table 4) and
so determine a relative Arrhenius function (eq 7) for the
cyclization and trapping of 1 with PhSeH in toluene.
Addition of eq 6 then leads to the Arrhenius function (eq
8, Figure 1) for trapping of the primary alkyl radical 1
by PhSeH in toluene. Comparison of this function with
that obtained by Newcomb by the PTOC/PhSeH method
in toluene as solvent (eq 9) shows that, at the 95%
confidence interval, the two methods are in excellent
agreement.25
This rate constant is substantially smaller than that
for the reaction of a primary alkyl radical with PhSeH
(vide supra). Hence, it is of some interest to note the
recent work of Newcomb on the rate constants for the
trapping of radical 11 by t-BuSH as well as for its
log(kH/kR) ) 0.55 ((0.56) + 1.24 ((0.64)/2.3RT (7)
log(kH) ) 10.35 ((0.58) - 1.76 ((0.64)/2.3RT (8)
log(kH) ) 10.88 ((0.19) - 2.06 ((0.24)/2.3RT (9)
With the validity of the method established we applied
it to the determination of the rate constant for hydrogen
atom abstraction from PhSeH by a substituted alkyl
radical. The â-(acetoxy)alkyl migration of the tetra-O-
acetylglucopyranos-1-yl radical 6 to 7 was chosen for this
purpose. The rate constant for this migration in benzene
cyclization. Both hydrogen atom abstraction from t-
BuSH and cyclization were found to have essentially the
same rate constants as the equivalent reactions of 1.30
This was attributed to the favorable polar characteristics
for the two reactions overriding any stability provided
to 11 by resonance with the methoxy group. The slower
rate of hydrogen atom abstraction from PhSeH by 6
cannot therefore simply be attributed to stabilization by
the ring oxygen. Radical 6 is known to exist in a boatlike
conformation (Figure 2) in which the single electron is
2
at 75 °C has been determined to be 4.0 × 10 s-1 by
Sustmann, Giese, and co-workers using the kinetic ESR
method.26 The experiment was conducted essentially as
in the above example, except that it was run in benzene
at reflux, and that the slower clock reaction required the
use of lower amounts of PhSeSePh. This experiment was
conducted in duplicate. In both runs all the substrate
(27) (a) Adlington, R. M.; Baldwin, J . E.; Basak, A.; Kozyrod, R. P.
J . Chem. Soc., Chem. Commun. 1983, 944. (b) Bonner, W. A.; Robinson,
A. J . Am. Chem. Soc. 1950, 72, 354.
(28) Smith, E. D.; Mathews, D. M. J . Chem. Ed. 1967, 44, 757.
(29) Errors given for the 95% confidence interval (2.3σ).
(30) J ohnson, C. C.; Horner, J . H.; Tronche, C.; Newcomb, M. J . Am.
Chem. Soc. 1995, 117, 1684.
(25) As noted by Newcomb the activation parameters in eqs 8 and
are such that trapping by PhSeH must be partially diffusion
controlled in low viscosity solvents. For a full discussion of this point
see ref 4a.
(26) Korth, H.-G.; Sustmann, R.; Groninger, K. S.; Leisung, M.;
Giese, B. J . Org. Chem. 1988, 53, 4364.
9

2372 J . Org. Chem., Vol. 61, No. 7, 1996
Crich et al.
dried and distilled by standard techniques. Tetrahydro-
furan (THF) was distilled under nitrogen from sodium
benzophenone ketyl before use.
Rea ction of Bu 3Sn H a n d P h SeSeP h . PhSeSePh (62
mg, 0.2 mmol) was dissolved in C6D6 (0.5 mL) under Ar
in a 5 mm NMR tube and the 77Se-NMR spectrum
recorded. Bu3SnH (54 µL, 0.2 mmol) was added by
syringe, resulting in immediate decolorization of the
yellow solution. The 77Se- and 119Sn-NMR spectra were
recorded. Addition of further Bu3SnH (54 µL, 0.2 mmol)
caused a gradual evolution of a colorless gas and the
appearance of a metallic grey prepicipate. This reaction,
which resulted in slow consumption of PhSeH and of Bu3-
SnH as judged by 77Se- and 119Sn-NMR spectroscopy,
respectively, was incomplete after several days at room
temperature.
Attem p ted Rea ction of P h SeSn Bu 3 a n d Bu 3Sn H.
An authentic sample of PhSeSnBu3 (89 mg, 0.20 mmol)
was dissolved in C6D6 (0.5 mL) under Ar in a 5 mm NMR
tube and the 77Se-NMR spectrum recorded. Bu3SnH (54
µL, 0.20 mmol) was then added and the tube heated to
80 °C for 1 h. After cooling the 77Se- and 119Sn-NMR
spectra were recorded.
F igu r e 2.
F igu r e 3.
periplanar with one lone pair from the ring oxygen and
with the antibonding σ* orbital of the adjacent C-OAc
bond resulting in what is assumed to be a stabilizing
extended anomeric effect (Figure 3).31,32 Presumably, it
is this additional stabilization over and above that in 11
which retards the reaction of 6 with PhSeH. Alterna-
tively, it could be argued that the electron-withdrawing
â-acyloxy group renders the anomeric radical less nu-
cleophilic and in doing so removes the advantage of a
polarity matched reaction with PhSeH.
Finally, we note that attempts at the trapping of the
aryl radical 12, generated from the iodide 13,33 using the
Bu3SnH/PhSeSePh couple were fruitless. Experiments
using as much as 30 mol % of PhSeSePh gave less than
5% of 14 as compared to the cyclization product 15. The
estimated34 rate constant for cyclization of 12 is 5.3 ×
109 s-1 and although this is two orders of magnitude
greater than that for the cyclization of 1 to 2 it is well
within the range initially intended by Newcomb in his
original studies of trapping by PhSeH.4 This failure is
therefore probably best attributed to the unfavorable
polar characteristics of the hydrogen atom abstraction
reaction between an electrophilic aryl radical and the
acidic PhSeH.
6-Br om o-1,1-d ip h en yl-1-h exen e (3). To a solution
of (5-hydroxypentyl)triphenylphosphonium bromide35 (2.54
g, 5.9 mmol) in THF (40 mL) was added n-butyllithium
(6.5 mL, 2M in pentane) with stirring at -78 °C under
Ar. The reaction mixture was allowed to warm to room
temperature, stirred for a further 20 min, and then
treated dropwise with a stirred solution of benzophenone
(1.20 g, 6.58 mmol) in THF (15 mL). After stirring for
10 h at room temperature, water (5 mL) was added and
the solvent removed under vacuum. The residue was
taken up in water (15 mL) and dichloromethane (40 mL),
the organic layer separated, and the aqueous layer
extracted with dichloromethane (2 × 40 mL). The
combined organic layers were dried (MgSO4), filtered, and
concentrated to give a residue, which after chromatog-
raphy on silica gel (eluent: hexane/ethyl acetate 20/1)
gave 1,1-diphenyl-1-hexen-6-ol as a colorless oil (1.39 g,
93%). 1H-NMR, δ: 7.28 (10H, m), 6.08 (1H, t, J ) 7.46),
3.59 (2H, t, J ) 5.57), 2.16 (2H, m), 1.55 (4H, m), 1.25
(1H, bs); 13C-NMR, δ: 142.7, 141.8, 140.1, 129.6, 128.1,
128.0, 127.2, 126.7, 126.6, 62.6, 32.2, 29.4, 26.0. This
alcohol (1.80 g, 7.15 mmol) and pyridine (0.57 g, 7.25
mmol) were dissolved in dichloromethane (10 mL) and
added to a suspension of triphenylphosphine dibromide
in dichloromethane formed by dropwise addition of
bromine in tetrachloromethane (2 M, 3.6 mL, 7.25 mmol)
to a stirred solution of triphenylphosphine (1.90 g, 7.25
mmol) under Ar in dichloromethane (15 mL). After
stirring for 1 h at room temperature the precipitate was
filtered off and the filtrate poured into water (10 mL).
The organic layer was run off and the aqueous layer
further extracted with dichloromethane (2 × 20 mL). The
extracts were dried (MgSO4) and concentrated under
reduced pressure to give a residue, which after chroma-
tography on silica gel (eluent: hexane/ethyl acetate 20/
1) yielded 3 as a colorless oil (1.70 g, 76%). 1H-NMR, δ:
7.27 (10H, m), 6.06 (1H, t, J ) 7.46), 3.34 (2H, t, J )
6.76), 2.14 (2H, dt, J ) 7.43, 7.40), 1.85 (2H, m), 1.59
(2H, m); 13C-NMR, δ: 142.5, 142.2, 140.0, 129.8, 129.0,
128.2, 128.1, 127.1, 126.9, 126.9, 33.6, 32.2, 28.7, 28.3;
Exp er im en ta l Section
Gen er a l. 1H-NMR spectra were recorded at 300 MHz
and 13C-NMR spectra at 75 MHz with chemical shifts (δ)
downfield from tetramethylsilane as internal standard;
J -values are given in Hz. 77Se-NMR and 119Sn-NMR
spectra were recorded at 38.2 and 74.6 MHz, respectively,
with chemical shifts referenced to Me2Se and Me4Sn,
respectively. Microanalyses were performed by Midwest
Microanalytical, Indianapolis, IN. All solvents were
(31) (a) Beckwith, A. L. J .; Brumby, S. J . Chem. Soc., Perkin Trans.
2 1987, 1801. (b) Dobbs, A. J .; Gilbert, B. C.; Norman, R. O. C. J . Chem.
Soc., Perkin Trans. 2 1972, 786. (c) Gilbert, B. C.; Trenwith, M.; Dobbs,
A. J . J . Chem. Soc., Perkin Trans. 2 1974, 1772.
(32) (a) Dupuis, J .; Giese, B.; Ru¨egge, D.; Fischer, H.; Korth, H.-G.;
Sustmann, R. Angew. Chem., Int. Ed. Engl. 1984, 23, 896. (b) Korth,
H. G.; Sustmann, R.; Dupuis, J .; Giese, B. J . Chem. Soc., Perkin Trans.
2 1986, 1453.
(33) Beckwith, A. L. J .; Gara, W. G. J . Chem. Soc., Perkin Trans. 2
1975, 795.
(34) J ohnson, L. J .; Lusztyk, J .; Wayner, D. D. M.; Abeywickrema,
A. N.; Beckwith, A. L. J .; Scaiano, J . C.; Ingold, K. U. J . Am. Chem.
Soc. 1985, 107, 4594.
IR υmax: 3059, 2958, 1598 cm-1. Anal. Calcd for C18H19
-
Br: C, 68.58; C, 6.07. Found: C, 68.61; H, 6.07.
(35) Meyers, A. I.; Collington, E. W. Tetrahedron 1971, 27, 5979.

Radical Clock Reactions Under Pseudo-First-Order Conditions
J . Org. Chem., Vol. 61, No. 7, 1996 2373
Kin etics of Rea r r a n gem en t of 3. A stock solution
of 3 (4.00 g) in benzene (40 mL) was prepared and 2 mL
(0.634 mmol) transferred to each of seven 25 mL pear-
shaped Pyrex flasks. A stock solution of PhSeSePh (0.28
g) made up to 28 mL in benzene was then used to add 5,
10, 15, 20, 25, 30, or 35 mol % of PhSeSePh to these
flasks. An amount of Bu3SnH corresponding to that of
PhSeSePh was added to each flask and the volume made
up to 10 mL. A steady stream of Ar was then passed
through each flask for several minutes. Each flask was
then immersed, sequentially, in a circulating water bath
maintained at 20 °C in a Rayonet photoreactor (254 nm)
and irradiated, with stirring under Ar, while 0.5 mL of
a stock solution of Bu3SnH (2.22 g, 7.6 mmol) and AIBN
(0.10 g, 0.63 mmol) made up to 5 mL in benzene was
added dropwise with a motor-driven syringe pump over
50 min. Irradiation was continued at 20 °C for a further
0.5 h before the solvent was removed in vacuo and the
residue examined by 1H-NMR. In each case the substrate
was completely consumed. Integration of the olefinic
signal (4, CdCH) at δ 6.09 and of the benzhydryl signal
(5, Ph2CH) at δ 3.57 gave the ratio of 4/5 as recorded in
Table 3. The spectral data for 4 and 5 were identical to
those recorded in the literature.16 The total volume
change in the course of the reaction, owing to the addition
of Bu3SnH, was essentially negligible at 5%. In calculat-
ing the molar concentration of PhSeSePh and so of
PhSeH a mean volume of 10.25 mL was taken.
Deter m in a tion of th e Ar r h en iu s P a r a m eter s for
Qu en ch in g of Ra d ica l 1 w ith P h SeH. A stock solu-
tion of 3 (1.60 g) in toluene (16 mL) was prepared and 1
mL (0.32 mmol) transferred to each of four 25 mL round-
bottomed flasks. A stock solution of PhSeSePh (0.35 g)
in toluene (35 mL) was made up and 1.5 mL (0.048 mmol)
added to each flask, followed by Bu3SnH (1.67 mL of a
0.03 M solution in toluene, 0.057 mmol). Each flask was
then made up to 10 mL with toluene and purged with a
steady stream of Ar for several minutes. In turn, each
flask was equilibrated at the required temperature (Table
4) and irradiated with 100 W medium pressure Hg lamp
(through Pyrex) while Bu3SnH and AIBN in toluene (0.5
mL of a stock solution containing 2.22 g, 7.6 mmol of Bu3-
SnH and 0.19 g, 0.63 mmol of AIBN in a total of 10 mL)
was added with the syringe pump over 50 min. After
the addition the irradiation was continued for a further
0.5 h before the solvent was removed under vacuum and
the residue examined by 1H-NMR to give the data
recorded in Table 4. The data for 20 °C were taken from
Table 3. In calculating the molar concentration of
PhSeSePh and so of PhSeH a mean volume of 10.25 mL
was taken.
Kin etics of Rea r r a n gem en t of 8. Five flasks were
made up, using stock solutions, containing 8 (97.5 mg,
0.2 mmol), PhSeSePh (3, 4, 5, 6, or 7 mol %) and Bu3-
SnH (3, 4, 5, 6, or 7 mol %) in benzene (20 mL) essentially
as decribed above for the rearrangement of 3. After
purging with Ar, each flask was brought to reflux under
Ar and treated dropwise (syringe pump) with Bu3SnH
(0.070 g) and AIBN (1.6 mg) in benzene (1 mL) over 2.33
h. Reflux was maintained for a further 1 h before the
solvent was removed under vacuum and the residue
1
examined by H-NMR. In each case the substrate was
completely consumed. Integration of the resonances at
δ 3.31 (9, H-1axial) and 6.26 (10, H-1) gave the ratios of
9/10 in Table 5. The spectral data of 9 and 10 were
identical with those in the literature.36 In calculating
the molar concentration of PhSeSePh and so of PhSeH a
mean volume of 20.5 mL was taken.
Ack n ow led gm en t . We thank Professors W. L.
Mock, UIC, and especially Martin Newcomb, Wayne
State University, for several very helpful discussions,
and NSF, NIH, and the Donors of the Petroleum
Research Fund, administered by the American Chemi-
cal Society, for support of our programs in free radical
chemistry. D.C. is a Fellow of the A. P. Sloan Founda-
tion and Q.Y. is a Dean’s Scholar of the University of
Illinois at Chicago.
Su p p or tin g In for m a tion Ava ila ble: Plots of 4:5 versus
[PhSeH] and of 9:10 versus [PhSeH] (2 pages). This material
is contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O950857S
(36) Beckwith, A. L. J .; Duggan, P. J . J . Chem. Soc., Perkin Trans.
1 1993, 1672.