Thermolysis and photolysis of a γ-azoperester. Cyclization of γ-azo and γ-perester radicals

Article abstract of DOI:10.1021/ja970143u

Thermolysis of azoperester 5 affords γ-azo radical 14, which cyclizes to hydrazyl radical 15 at a rate of 1.3 x 10⁶ M^-1s^-1 at 11°C. Our experimental results are consistent with loss of a methyl radical from 15 to afford 2-pyrazoline 9, which is oxidized in situ by the starting S to pyrazole 6. This unusual and endothermic β-scission can be rationalized if the odd electron 9n 15 is better aligned with the CH₃-C bond than with the weaker t-Bu-N bond. The fact that 5-endo cyclization of 14 in 5 x 10⁷ faster than that of the analogous olefinic radical 30 led us to carry out ab initio calculation on simplified structures. ΔH((+)) for methyl radical addition to diimide is only 0.84 local/mol lower than for addition to ethylene and the exothermicity is only 3.5 kcal/mol greater. However, the smaller C-N=N than C-C=C bond angle leads to a ΔH((+)) 13.3 cal/lower for 5-endo cylclization of 4,5-diazapenten-1-yl than for the analogous 4-penteryl radical. Photolysis of 5 selectively cleaves for azo group, producing γ-perester radical 20. This species undergoes intramolecular attack on the peroxide linkage to form lactone 23 at a rate of 1.5 x 10⁴ s^-1 at 22°C. The cyclization rate of 20 is slow enough that 5 could be used as a photochemical bifunctional initiator, but cyclization of 14 to the azo group is so rapid that this radical would only rarely attack a monomer, initiator, but cyclization of 14 to the azo group is so rapid that this radical would only rarely attack a monomer.

Full text of DOI:10.1021/ja970143u

J. Am. Chem. Soc. 1997, 119, 6059-6065
6059
Thermolysis and Photolysis of a γ-Azoperester. Cyclization of
γ-Azo and γ-Perester Radicals
Paul S. Engel,*,† Shu Lin He,† and William B. Smith‡
Contribution from the Department of Chemistry, Rice UniVersity, 6100 Main Street,
Houston, Texas 77005-1892, and Department of Chemistry, Texas Christian UniVersity,
Fort Worth, Texas 76129
ReceiVed January 16, 1997X
Abstract: Thermolysis of azoperester 5 affords γ-azo radical 14, which cyclizes to hydrazyl radical 15 at a rate of
1.3 × 109 M-1 s-1 at 110 °C. Our experimental results are consistent with loss of a methyl radical from 15 to afford
2-pyrazoline 9, which is oxidized in situ by the starting 5 to pyrazole 6. This unusual and endothermic â-scission
can be rationalized if the odd electron in 15 is better aligned with the CH3-C bond than with the weaker t-Bu-N
bond. The fact that 5-endo cyclization of 14 is 5 × 107 faster than that of the analogous olefinic radical 30 led us
to carry out ab initio calculations on simplified structures. ∆Hq for methyl radical addition to diimide is only 0.84
kcal/mol lower than for addition to ethylene and the exothermicity is only 3.5 kcal/mol greater. However, the smaller
CsNdN than CsCdC bond angle leads to a ∆Hq 13.3 kcal/mol lower for 5-endo cyclization of 4,5-diazapenten-
1-yl than for the analogous 4-pentenyl radical. Photolysis of 5 selectively cleaves the azo group, producing γ-perester
radical 20. This species undergoes intramolecular attack on the peroxide linkage to form lactone 23 at a rate of 1.5
× 104 s-1 at 22 °C. The cyclization rate of 20 is slow enough that 5 could be used as a photochemical bifunctional
initiator, but cyclization of 14 to the azo group is so rapid that this radical would only rarely attack a monomer.
Introduction
Bifunctional free radical initiators contain two groups capable
of forming radicals thermally or photochemically.1 Before such
initiators can be put to practical use, their chemistry in the
absence of monomers must be understood. Moreover, study
of these compounds can lead to fundamental knowledge about
fragmentation and intramolecular reactions of radicals.
Azoperester 1 contains the two most common free radical
initiators: an aliphatic azo group and a peroxy linkage.2 Three
years ago, we reported that long wavelength irradiation of 1
leads to a â-perester radical, 2, which loses CO2 at a rate kâ of
2.9 × 106 s-1 at 25 °C.3 On the other hand, heating 1 to 110
°C caused initial homolysis of the perester moiety to yield a
â-azo radical 3,4,5 which lost N2 at a rate of about 2.5 × 109
synthesized in 48% overall yield from acetone tert-butylhydra-
zone by the route shown below.7
s-1 6
These fragmentations are so fast that radicals 2 and 3
.
would rarely attack a monomer. In the present work we have
studied azoperester 5, a homolog of 1 that cannot undergo such
multiple fragmentations. Photolysis of 5 produces a radical (20)
that lives long enough to become a polymer end group, but
thermolysis of 5 does not. This new bifunctional initiator was
Thermolysis of 5. A degassed, sealed solution of 5 in
benzene was thermolyzed at 110 °C for 1.5 h. NMR analysis
showed that 95% of 5 had reacted while GC revealed six
products, whose structures were tentatively assigned by GC/
MS and then proven by isolation or by coinjection of authentic
samples. The product yields (cf. Table 1) were quantified using
experimentally determined response factors, except for 7.
Pyrazole 6 and azoether 7 were isolated by preparative TLC.
† Rice University.
‡ Texas Christian University.
X Abstract published in AdVance ACS Abstracts, June 1, 1997.
(1) Su, J. S. N.; Piirma, I. J. Appl. Polym. Sci. 1987, 33, 727. Comanita,
E.; Popa, A. A. ReV. Roum. Chim. 1995, 40, 765. Gonzales, I. M.; Meira,
G. R.; Oliva, H. M. J. Appl. Poly. Sci. 1996, 59, 1015. Narita, Y.; Kinoshita,
H.; Araki, T. J. Polym. Sci., Part A 1992, 30, 333. Hepuzer, Y.; Bektas,
M.; Denizligil, S.; Onen, A.; Yagci, Y. J. Macromol. Sci., Pure Appl. Chem.
1993, A30, 111.
(2) Koenig, T. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York,
1973; Vol. 1, Chapter 3. Moad, G.; Solomon, D. H. In ComprehensiVe
Polymer Science; Allen, G. A., Bevington, J. C., Eds.; Pergamon Press:
Oxford, 1989; Vol. 3, Chapter 8.
(3) Engel, P. S.; Wu, A.-Y. J. Org. Chem. 1994, 59, 3969.
(4) Engel, P. S.; Wang, C.-R.; Chen, Y.-Q.; Ru¨chardt, C.; Beckhaus,
H.-D. J. Am. Chem. Soc. 1993, 115, 65.
(5) Engel, P. S.; Chen, Y.-C.; Wang, C. J. Am. Chem. Soc. 1991, 113,
4355.
(6) Wu, A.-Y. Ph.D. Dissertation, Rice University, 1994, p 65.
An authentic sample of 6 was prepared from butenone and tert-
butylhydrazine,8 followed by oxidation of the 2-pyrazoline 9
(7) Baldwin, J. E.; Adlington, R. M.; Bottaro, J. C.; Kohle, J. N.; Perry,
M. W. D.; Jain, A. U. Tetrahedron 1986, 42, 4223.
(8) Auwers, K.; Brocha, H. Chem. Ber. 1922, 55, 3880. Elguero, J.;
Jacquier, R.; Tarrago, G. Bull. Soc. Chim. Fr. 1966, 293.
S0002-7863(97)00143-1 CCC: $14.00 © 1997 American Chemical Society

6060 J. Am. Chem. Soc., Vol. 119, No. 26, 1997
Engel et al.
Table 1. Products from Thermolysis of 5 in Benzene at 110 °C
Table 2. Methyl Radical Products of 5-d6 in 1:1
Benzene-Nitrobenzenea,b
retn time,a
min
response
factorb
yield,c
%
yield,d
%
compound
toluene
precursor
(T)
o-nitro-T m-nitro-T p-nitro-T
tert-butyl alcohol
acetone
toluene
pyrazole 6
azoether 7
azoacid 8
2.44
2.32
5.69
9.92
13.89
15.71
0.74
0.49
1.07
0.58
0.79e
0.30
53
46
22
26
4.5
66
91
0
0
12
3.5
60
di-tert-butyl hyponitrite
tert-butyl peracetatec
5-d6
(1.00)
(1.00)
(1.00)
2.65
3.22
5.22
0.12
0.296
0.32
0.599
d
1.02
1.01
1.83
0.11
5-d6 - ra (ArCD3/ArCH3) 0.079
a Samples thermolyzed at 110 °C. The first three rows are GC FID
area ratios, and the last row is GC/CI/MS abundance ratios of D3 to
H3 isotopes. For toluene the m/e 96 to m/e 93 ratio is listed while for
nitrotoluenes the m/e 141 to m/e 138 ratio is listed. b AT-1701 column,
GC condition A. c See ref. 12. d Not measured because the peak was
too weak.
a DB-5 column, condition A (see the Experimental Section). b Rela-
tive to decane. c No added scavenger. d With 0.61 M t-BuSH. e As-
sumed to equal that of the ether from 1.
with tert-butyl pervalerate (10). Azoether 7 was identified by
benzene with model perester tert-butyl pervalerate (10). Under
these conditions, pyrazoline 9 was converted to 6 while 10 was
reduced to valeric acid. Heating 9 alone in benzene led to no
reaction while heating 10 similarly led to no carboxylic acid.
Thus, the high carboxylic acid yields from 5 (Table 1) are caused
at least in part by perester oxidation of 9.
NMR and by CI/GC/MS, which has proven to be a powerful
tool for structure determination of azoalkanes.9 Whereas azo-
tert-butane (ATB; the systematic name for this compound is
2,2′-azobis[2-methylpropane]) gave no molecular ion by EI, its
CI/MS exhibited only three peaks of relative abundance greater
than 5%, namely, protonated ATB, m/e ) 143 (100), protonated
tert-butyldiazene, m/e 87 (13), and t-Bu+, m/e 57 (10).
Likewise, every peak of 7 over 5% was easily rationalized, as
shown below.
An alternate pathway to 6 is the oxidation of 15 to diazenium
+ 11
cation 16 followed by loss of CH3
.
We sought to distinguish
•
+
CH3 from CH3 by thermolyzing 5 in a mixture of benzene
and nitrobenzene. The methyl radical attacks nitrobenzene
preferentially12 while the cation should behave very much the
opposite. To confirm the behavior of methyl radicals, a solution
of tert-butyl hyponitrite13 in 1:1 benzene-nitrobenzene was
thermolyzed at 110 °C.14 In accord with an earlier study,12
nitrobenzene is at least 4 times more reactive than benzene,
considering the presumably lower GC response factor of the
nitrotoluenes than toluene (cf. Table 2). The analogous experi-
ment with 5 required 5-d6 to distinguish methyl groups arising
from 15 from those of tert-butoxy radical â-scission. Again,
the nitrotoluenes dominated over toluene (>7.6:1, Table 2), not
the expected behavior of methyl cations. CI/GC/MS revealed
that the relative yield of nitrotoluenes-d3 was 10.5 times that of
toluene-d3 (0.12 × 5.22 + 0.11 × 1.83)/0.079. If the CD3
groups from 5-d6 turned up as CD3+, one would expect this
figure to be far below unity. Furthermore, if much CD3+ were
formed and were somehow able to methylate nitrobenzene, the
percent meta from 5-d6 should greatly exceed that from di-tert-
butyl hyponitrite, but the actual figures were 7.8% and 7.5%,
respectively (Table 2). Thus, the thermolysis of 5-d6 in the
mixed solvent does not yield methyl cations, contradicting the
alternate mechanism.
Origin of Pyrazole 6. This unusual product is postulated to
arise by cyclization of 14 to hydrazyl radical 15. Loss of methyl
radical followed by perester oxidation of pyrazoline 9 affords
6. To support this mechanism, the d6 analog of 5 was
synthesized starting from acetone-d6. Thermolysis was carried
out exactly as with 5, and the products were analyzed by GC/
MS. The toluene peak contained 22% PhCD3, strongly sug-
Thermolysis kinetics of 5 and 10 were monitored by NMR,
giving good first-order plots. The rate constants at 110 °C were
2.05 × 10-4 and 1.03 × 10-4 s-1, respectively, showing that
the bifunctional compound decomposed twice as fast as the
simple perester. Interestingly, when an equimolar amount of 9
was included with 10, the rate increased to 1.87 × 10-4 s-1
.
We attribute the increase to oxidation of 9 by 10, indicating
that oxidation of 9 is also responsible for the faster thermolysis
•
gesting that the cleaved CD3 group attacked the benzene
solvent.10 Fragmentation of the tert-butoxy group to acetone
•
(11) If the starting azoperester serves as oxidizer both here and in the 9
f 6 conversion, this alternate mechanism would explain the 2.5-fold greater
yield of 8 than 6. However, we have seen high carboxylic acid yields from
many bifunctional initiators.
(12) Pryor, W. A.; Davis, W. H.; Gleaton, J. H. J. Org. Chem. 1975, 40,
2099.
plus CH3 accounts for the PhCH3. The pyrazole 6 was now
found to be trideuterated, again consistent with the proposed
mechanism. To confirm that 9 was an allowed reaction
intermediate, the authentic compound was heated at 110 °C in
(9) A CAS Online search of chemical ionization mass spec? gave 1616
hits, azo? gave 80385 hits, and the intersect of those sets gave only two
hits, neither of which concerned aliphatic azo compounds.
(13) Mendenhall, G. D. Tetrahedron Lett. 1983, 24, 451.
(14) Since the half-life is 12 s at this temperature,15 most of the
thermolysis took place below 110 °C.
(10) Levy, M.; Szwarc, M. J. Am. Chem. Soc. 1955, 77, 1949.
(15) Kiefer, H.; Traylor, T. G. Tetrahedron Lett. 1966, 6163.

Thermolysis and Photolysis of a γ-Azoperester
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6061
Table 3. Product Yieldsa from Thermolysisb of 0.235 M 5 in the
Presence of Tempo
[Tempo], M
6, %
19, %c
7, %
8, %
0.051
0.231
0.282
0.359
22.9
13.9
14.2
10.9
1.40
3.71
4.16
4.39
2.9
2.4
2.3
2.0
46.2d
76.1
77.1
80.7
a AT-1701 column, condition A. b Benzene at 110 °C. c Assumed
response factor taken as the average of that for t-BuNdNCMe2CH2O-
t-Bu (0.79) and N-tert-butoxy-2,2,6,6-tetramethylpiperidine (0.43).
d DB-5 column.
products detected were of low molecular weight: isobutene and
isobutane from disproportionation, and acetone and tert-butyl
alcohol from t-BuO•. The identity of 21-23 was proven by
coinjection of authentic samples.
rate of 5. The reason why 5 thermolyzes twice as fast as 10
and appears first-order during the 1.5 half-lives that were
monitored is that the second-order rate constant for oxidation
of 9 is at least 100 times greater than that of the unimolecular
thermolysis of 5.16
Trapping of γ-Azo Radical 14. Thermolysis of 5 was
expected to give γ-azo radical 14, but attempts to trap this
radical using t-BuSH did not produce detectable amounts of
18, an authentic sample of which was synthesized independently.
However, use of Tempo, a much faster radical trap,17,18 led to
a new GC peak at 18.2 min whose CI/GC/MS was beautifully
consistent with structure 19. (See the Experimental Section for
All of these products gave symmetrical, reproducible peaks
on capillary GC (AT-1701 column). In the absence of a
scavenger, the yields of isobutane, isobutene, 21, and 22 were
each in the region of 30%, suggesting that there is no selectivity
in the disproportionation of the unsymmetrical radical pair t-Bu•/
20 (cf. Table 4). As 1,4-cyclohexadiene (CHD) was added,
the yield of 21 rose, that of 23 fell, and 22 remained unchanged
(cf. Table 5). These observations indicate that CHD does not
interfere with the normal behavior of t-Bu•/20 radical pairs,
except to divert some of 20 to 21. Although we previously3
avoided using a H•-donating scavenger because the trapped
product was identical to the cage product, we find here that the
amount of 21 in excess of that produced by cage dispropor-
tionation is proportional to the CHD concentration. Thus, a
plot of the yield ratio of (21-32%):23 versus [CHD] (Table 5)
was linear with slope kt/kc ) 0.618 where kc is the rate constant
of 20 f 23. The trapping rate kt of tert-butyl radicals by CHD
is 9.4 × 103 M-1 s-1 at 22 °C,22 leading to kc ) 1.5 × 104 s-1
.
rationalization of all ions of relative abundance greater than 2%.)
In order to estimate the lifetime of 14, the yield of 6 and 19
was determined as a function of Tempo concentration (cf. Table
3). A plot of the yield of 19 over that of 6 versus [Tempo]
was linear with a slope kt/kr of 1.09 where kr is the rate constant
of 14 f 15. Because so little Tempo was consumed, we simply
used its initial concentration in this plot. On the basis of the
value kt ) 1.4 × 109 M-1 s-1 for Tempo with primary radicals
at 110 °C,17,19 we obtain kr ) 1.3 × 109 M-1 s-1. Since trapping
rates decrease at high concentrations of Tempo,20 the proper
graph21 is ([6]/[9]) [Tempo] versus [Tempo]. In this case we
obtained an intercept kr/kt of 0.83, corresponding to kr ) 1.2 ×
109 M-1 s-1. The discrepancy between these numbers is much
less than the uncertainty in the GC response factor of 19,
estimated to be (20%.
In an attempt to trap 15-d6, a sample of 5-d6 was thermolyzed
in benzene at 110 °C with 0.494 M t-BuSH. GC analysis (AT-
1701, condition A) revealed a new peak 1.24 min earlier than
6-d3 and of very similar size. The CI mass spectrum of this
peak was consistent with 1-tert-butyl-3,3-di(trideuteromethyl)-
pyrazolidine (17-d6), but the compound was neither isolated nor
synthesized independently.
Discussion
Thermolysis of aliphatic tert-butyl peresters normally yields
an acycloxy radical plus a tert-butoxy radical.23 Because
decarboxylation is very rapid (for EtCOO•, k ) 2 × 109 at 20
°C),24 the final products do not include the carboxylic acid. In
the case of 5, we have four unusual results: a high yield of
acid 8, an enhanced thermolysis rate relative to the model
compound tert-butyl pervalerate (10), exceedingly fast 5-endo
•
cyclization of â-azo radical 14, and loss of CH3 from pyra-
zolinyl radical 15. The first two observations are readily
explained by the facts that perester 10 indeed oxidizes 9 to 6,
that the disappearance rate of 10 is nearly doubled by inclusion
of 9, and that 9 increases the yield of valeric acid from 0 to
45%.
Intermolecular25 and intramolecular26,27 radical additions to
the azo group are known, though these reactions are not nearly
as common as addition to olefins.28,29 On the other hand, 5-endo
(22) Hawari, J. A.; Engel, P. S.; Griller, D. Int. J. Chem. Kinet. 1985,
17, 1215.
(23) Ru¨chardt, Ch. Fort. Chem. Forsch. 1966, 6, 251. Sawaki, Y. In
Organic Peroxides; Ando, W., Ed.; Wiley: New York, 1993.
(24) Edge, D. J.; Kochi, J. K. J. Am. Chem. Soc. 1973, 95, 2635. Hilborn,
J. W.; Pincock, J. A. J. Am. Chem. Soc. 1991, 113, 2683.
Photolysis of 5. Since 366 nm irradiation of 1 gave
â-perester radical 2, it was no surprise that similar treatment of
5 afforded γ-perester radical 20. This species then underwent
an intramolecular SH2 reaction to yield lactone 23 in competition
with disproportionation to peresters 21 and 22. The only other
(25) Gray, P.; Thynne, J. C. J. Trans. Faraday Soc. 1963, 59, 2273.
Dacey, J. R.; Mann, R. F.; Pritchard, G. O. Can. J. Chem. 1965, 43, 3215.
Knoll, H.; Scherzer, K.; Geiseler, G. Z. Phys. Chem. 1972, 249, 359. Roberts,
B. P.; Winter, J. N.; Tetrahedron Lett. 1979, 3575. Strausz, O. P.; Berkley,
R. E.; Gunning, H. E. Can. J. Chem. 1987, 109, 5289. Gorgenyi, M.;
Kortvelyesi, T.; Seres, L. J. Chem. Soc., Faraday Trans. 1993, 89, 447.
(26) Benati, L.; Placucci, G.; Spagnolo, P.; Tundo, A.; Zanardi, G. J.
Chem. Soc., P1 1977, 1684. Leardini, R.; Nanni, D.; Tundo, A.; Zanardi,
G. J. Chem. Soc., Chem. Commun. 1989, 757. Alberti, A.; Bedogni, N.;
Benaglia, M.; Leardini, R.; Nanni, D.; Pedulli, G. F.; Tundo, A.; Zanardi,
G. J. Org. Chem. 1992, 57, 607. Leardini, R.; Lucarini, M.; Nanni, D.;
Pedulli, G. F.; Tundo, A.; Zanardi, G. J. Org. Chem. 1993, 8, 2419. Christl,
M.; Henneberger, H.; Freund, S. Chem. Ber. 1988, 121, 1675.
(27) (a) Kunka, C. P. A.; Warkentin, J. Can. J. Chem. 1990, 68, 575.
(b) Beckwith, A. L. J.; Wang, S.; Warkentin, J. J. Am. Chem. Soc. 1987,
109, 5289.
(16) Kinetic modeling of the system A f B, A + B f C was carried
out using the Chemical Kinetics Simulation package from IBM.
(17) Bowry, V. W.; Ingold, K. U. J. Am. Chem. Soc. 1992, 114, 4992.
(18) Newcomb, M. Tetrahedron 1993, 49, 1151.
(19) A solvent correction from isooctane to benzene reduces kt by a factor
of 0.61 while a temperature increase from 18 to 110°C raises kt by a factor
of 2.1.
(20) Hollis, R.; Hughes, L.; Bowry, V. W.; Ingold, K. U. J. Org. Chem.
1992, 57, 4284.
(21) Mendenhall, G. D.; Protasiewicz, J. D.; Brown, C. E.; Ingold, K.
U. Lusztyk, J. J. Am. Chem. Soc. 1994, 116, 1718.
(28) Giese, B. Angew. Chem., Int. Ed. Engl. 1983, 22, 753.

6062 J. Am. Chem. Soc., Vol. 119, No. 26, 1997
Engel et al.
Table 4. Products from Photolysis of 5 in Benzene at 22 °C
Table 6. Calculated Enthalpies for Radical Addition
compound
isobutane
isobutene
acetone
tert-butyl alcohol
21
22
23
retn time,a min response factorb yield, %
1.44
1.47
1.0
1.0
0.49
0.74
0.36
0.33
0.55
29.6
32.1
trace
trace
32.0
28.2
34.7
2.19
2.36
19.45
21.27
13.20
a AT-1701 column, condition B (see the Experimental Section).
b Relative to decane.
a kcal/mol using UQCISD/6-31G*//UHF/6-31*. b Activation en-
thalpy. c Reaction exothermicity.
Table 5. Product Yields from Photolysis of 5 with
1,4-Cyclohexadienea,b
generated by Bu3SnD reduction of 29. If we assume a 1%
detection limit and employ a value of 1.0 × 106 M-1 s-1 for
n-Bu3SnD with primary alkyl at 110 °C, the 5-endo cyclization
rate of 30 would fall below 25 s-1, which is 5 × 107 slower
than that of the structurally analogous 14 f 15.
[CHD], M
21
22
23
0.244
0.95
1.95
2.85
34.3
45.9
53.7
56.0
27.0
27.2
28.1
27.1
30.6
21.7
17.1
13.8
The relatively large amount of 6-endo product from 24 has
been attributed to an early transition state caused by resonance
stabilization of hydrazyl radicals and to the shorter NdN bond
and smaller CsNdN angle relative to olefins.27 Which of these
factors are responsible for the enormous rate enhancement of
14 versus the carbon analog 30? The degree to which “hydrazyl
resonance” accelerates cyclization of 14 might be evaluated if
one knew the thermochemistry of simple intermolecular addi-
tions like methyl radical to ethylene and to diimide. Experi-
mental values for the ethylene case are readily available, but
the heats of formation (∆Hf) of both diimide35-37 and hydrazyl
radicals38-40 are not well established. Even if ∆Hf of hydrazyl
itself were well known, it is possible that methyl substitution
a 0.01 M 5 in benzene, 22 °C. b AT-1701 column, condition B.
cyclization is rare in olefins,30,31 but cyclization to the azo group
occurs in radicals 24 and 25 to give both 5-exo and 6-endo
products.27 5- Endo cyclization of aryl radical 26 occurs at a
rate of 9.2 × 108 s-1 at 80 °C while 5-exo cyclization of aryl
radical 24 proceeds at 1.5 × 109 s-1 at 82 °C. Although no
rate was stated for the analogous alkyl radical 25, one can use
the product ratios and the value 6.4 × 106 M-1 s-1 for Bu3SnH
trapping of primary alkyl at 80 °C18 to calculate a 5-exo
cyclization rate of 1.2 × 107 s-1 at 80 °C. This value is over
100 times slower than that for 24, in accord with the higher
reactivity of aryl radicals than alkyl. Although no directly
comparable olefin cyclizations (e.g., o-allylphenyl vs 5-pentenyl
radical) are known to help quantify further the reactivity of aryl
versus alkyl radicals, 27 undergoes 5-exo cyclization 600 times
faster than 28 at 110 °C and 2200 times faster at 25 °C.32-34
•
would stabilize this radical, similar to MeNHCH2 versus H2-
• 41
NCH2 . The thermodynamic approach is the most direct way
to compare olefins and azoalkanes; however, rotation barriers
provide another estimate of the hydrazyl resonance energy.
These barriers are about 8 kcal/mol in highly substituted
hydrazyls while AM1-UHF calculations for H2N-NH• predict
a barrier of 12.6 kcal/mol.42
Ab Initio Calculations of Radical Addition. To help
understand the rapid cyclization rate of 14, we calculated the
enthalpy of activation (∆Hq) and the exothermicity (∆Hr) of
the two intermolecular and two intramolecular reactions shown
in Table 6. Many theoretical treatments have dealt with the
addition of radicals to olefins,43 but the only calculation on azo
compounds44 was semiempirical and is seriously at odds with
the ab initio results presented below. In their detailed studies
of the addition of methyl and related radicals to a series of
These comparisons suggest that 5-endo cyclization of an alkyl
radical to the azo group should be hundreds of times slower
than the 109 range seen for 26; hence, it is remarkable that 14
undergoes exclusive 5-endo cyclization at a rate exceeding 109
(35) Engel, P. S.; Owens, W. H.; Wang, C.-R. J. Phys. Chem. 1993, 97,
10486.
(36) Foner, S. N.; Hudson, R. L. J. Chem. Phys. 1978, 68, 1.
(37) Ruscic, B.; Berkowitz, J. J. Chem. Phys. 1991, 95, 4378.
(38) Grela, M. A.; Colussi, A. J. Int. J. Chem. Kinet. 1988, 20, 713.
(39) Ingemann, S.; Fokkens, R. H.; Nibbering, N. M. M. J. Org. Chem.
1991, 56, 607.
s-1
.
Beckwith et al.30 found no cyclic products from 30
(40) Foner, S. N.; Hudson, R. L. J. Chem. Phys. 1958, 29, 442.
(41) Griller, D.; Lossing, F. P. J. Am. Chem. Soc. 1981, 103, 1586.
Theoretical calculations do not reproduce the alkyl group effect. Kysel,
O.; Mach, P. THEOCHEM 1986, 139, 333.
(42) Nelsen, S. F. In Acyclic Organonitrogen Stereodynamics, Lambert,
J. B., Takeuchi, Y., Eds.; VCH Publishers: New York, 1992; p 256.
(43) Basilevsky, M. V.; Chlenov, I. E. Theor. Chim. Acta 1969, 15, 174.
Hoyland, J. R. Theor. Chim. Acta 1971, 22, 229. Fujimoto, H.; Yamabe,
S.; Minato, T.; Fukui, K. J. Am. Chem. Soc. 1972, 94, 9205. Dewar, M. J.
S.; Olivella, S. J. Am. Chem. Soc. 1978, 100, 5290. Clark, D. T.; Scanlan,
I. W.; Walton, J. C. Chem. Phys. Lett. 1978, 55, 102. Fueno, T.; Kamachi,
M. Macromolecules 1988, 21, 908. Gonzales, C.; Sosa, C.; Schlegel, H. B.
J. Phys. Chem. 1989, 93, 2435, 8388. Zipse, H.; He, J.; Houk, K. N.; Giese,
B. J. Am. Chem. Soc. 1991, 113, 4324. Wong, M. W.; Pross, A.; Radom,
L. J. Am. Chem. Soc. 1993, 115, 11050. Wong, M. W.; Pross, A.; Radom,
L. Isr. J. Chem. 1993, 33, 415. Wong, M. W.; Pross, A.; Radom, L. J. Am.
Chem. Soc. 1994, 116, 6284.
(29) Walbiner, M.; Wu, J. Q.; Fischer, H. HelV. Chim. Acta 1995, 78,
910. Tedder, J. M.; Walton, J. C. Tetrahedron 1980, 36, 701. Tedder, J. M.
Angew. Chem., Int. Ed. Engl. 1982, 21, 401, 433. Tedder, J. M.; Walton, J.
C. Acc. Chem. Res. 1976, 9, 183.
(30) Beckwith A. L. J.; Easton, C. J.; Lawrence, T.; Serelis, A. K. Aust.
J. Chem. 1983, 36, 545.
(31) Bogen, S.; Malacria, M. J. Am. Chem. Soc. 1996, 118, 3992.
(32) Beckwith, A. L. J.; Schiesser, C. H. Tetrahedron 1985, 41, 3925.
(33) Johnson, L. J.; Lusztyk, J.; Wayner, D. D. M.; Abeywickrema, A.
N.; Beckwith, A. L. J., Scaiano, J. C., Lee, K. H.; Chin, K. L.; Brumby, S.
J. Am. Chem. Soc. 1985, 107, 4594.
(34) Beckwith, A. L. J. Landolt-Bornstein, New Series Vol. 13a, Radical
Reaction Rates in Liquids, Springer-Verlag: Berlin 1984; p 276.
(44) Kortvelyesi, T.; Seres, L. React. Kinet. Catal. Lett. 1995, 56, 371.

Thermolysis and Photolysis of a γ-Azoperester
Table 7. Geometry of Transition States
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6063
In accord with the thermochemistry (Table 6), the calculated
TS geometries (Table 7) show that bond making is slightly more
advanced in the diazapentenyl than the hydrocarbon analog,
judging from its shorter 1,2-bond distance and the greater
deviation of its 5-2-3-4 torsion angle from 180°. A molec-
ular model of the 5-pentenyl TS reveals a great deal of strain if
the radical attacks perpendicular to the nodal plane of the double
bond and at the ideal 1-2-3 angle of 109°.32 These require-
ments are compromised in the actual TS, where the 1-2-3
angle is 88.8° and the 1-2-3-4 torsion angle is 41.8°. To
rationalize the facile cyclization of diazapentenyl (Table 6), one
can say simply that the azo group is more tolerant of nonideal
attack angles than an olefin, but we did not investigate the
possibility computationally. Alternately, one can recognize, as
did earlier workers27b that the CsNdN angle of azoalkanes is
about 10° smaller and the double bond is about 0.1 Å shorter46
than in olefins. Reducing the 2-3-4 angle has two beneficial
effects: it moves the carbon radical farther behind the site of
attack, closer to the ideal 109°, ( 1-2-3 ) 97.4°), and it
increases the 1-2-3-4 torsion angle to 44.4°, placing the
attacking radical farther above the 2,3,4 plane than in the
5-pentenyl radical. As reflected in the activation energies (Table
6), far less strain is encountered to achieve this acceptable
geometry in the diaza than in the hydrocarbon case. To estimate
the increase in ring strain during cyclization, each acyclic radical
was first minimized as a linear zigzag structure using the MMX
force field of PCMODEL (Mac version 4.51, Serena Software,
Bloomington, IN). The difference in strain energy was 1.1 kcal/
mol, favoring the diaza radical. Minimization of the cyclization
TSs was then carried out with a fixed 1-2 atom distance (see
atom numbering in Table 7). The diazapentenyl TS was now
5.0 kcal/mol below that of the hydrocarbon radical, offering a
partial explanation for the cyclization rate enhancement. An
additional but minor contribution may arise from the absence
of a syn hydrogen on the azo nitrogen being attacked, on the
basis of the observations that a 5-methyl group slows 5-endo
cyclization of 5-hexenyl radical by a factor of 4032 and that
hydrogen-methyl repulsion in diimide reduces the torsion angle
during attack (-77.9°, Table 7).
olefins, Wong and Radom45 found that QCISD/6-31G* energies
based on UHF/6-31G* geometries offered an adequate descrip-
tion of these reactions. As a test of our methodology, the
transition structure (TS) for the addition of methyl radical to
ethylene was calculated starting from their geometry (UQCISD/
6-31G*//UHF/6-31G*). The resulting geometry and energy
(∆Hq ) 8.49 and ∆Hr ) -27.61 kcal/mol) were in exact
agreement with those of Wong and Radom (cf. Tables 6 and
7).
The starting point for modeling the TS for the addition of
the methyl radical to trans-diimide was the corresponding
structure for ethylene. Initial calculations were carried out at
the 3-21G* level; then cases of interest were reoptimized at the
UHF/6-31G* level before energy determinations were made.
In view of the possible interaction of the attacking methyl with
the nitrogen lone pair electrons, a wide range of C-N-N-H
torsion angles was explored as well as various attack angles
(C-N-N). Several TSs were found by this procedure, indicat-
ing that the methyl-diimide potential energy surface is con-
siderably more complex than for the methyl-ethylene system.
However, the TS given in Table 7 was found to be significantly
below all others and was taken as representing the optimum
energy pathway for the reaction forming the 1-methyl-2-hydrazyl
radical.
The calculated ∆Hq for methyl plus diimide is about 0.8 kcal/
mol below that for methyl plus ethylene, and the ∆Hr is 3.5
kcal/mol more exothermic. These small differences and there-
fore “hydrazyl resonance” cannot account for the observed rate
acceleration of 14 versus 30. The transition state geometries
(Table 7) show that the attack angle (1-2-3) is nearly identical
for ethylene and diimide but the torsion angle is about 9° smaller
for diimide. This deviation from perpendicular could be simply
due to steric interaction between CH3• and the hydrogen on the
N being attacked.
The TS for the cyclization of 4-penten-1-yl was initiated using
the C-C bond lengths found for the ethylene case while the
corresponding cyclization of the 4,5-diaza-4-penten-1-yl radical
was modeled upon the TS for the all-carbon analog. The
calculated difference in ∆Hq for 4-pentenyl versus the diaza
analog (Table 6) is 19.54-6.27 ) 13.27 kcal/mol, corresponding
to a slower cyclization of 4-pentenyl by a factor of 3.7 × 107
at 110 °C. Since our observed rate for 14 is 1.2 × 109 s-1, the
estimated rate for 30 is 32 s-1, an expectedly slow value. We
have assumed that the rate enhancement due to the Thorpe
Ingold effect is reflected only in ∆Hq and is the same for the
hydrocarbon and diaza systems. On the basis of the data for
radical cyclization to olefins,18 the first assumption appears quite
good. The self-consistency of this calculation suggests that
unlike the 2-formylbenzoyl radical,21 the rate-limiting step is
radical cyclization, not rotation of 14 into the proper conforma-
tion.
An additional result of our calculations is the N-H bond
dissociation energy (BDE) of methylhydrazine, 75.0 kcal/mol
(cf. Table 8). This figure can be compared with the experi-
mental N-H BDE for hydrazine itself (80.5 kcal/mol47 ) and
with a recently determined BDE for hydrazobenzene (73.1 kcal/
mol).39
FMO theory has been widely discussed for free radical
addition to olefins,28,48 but is unable to rationalize the results
in Table 6. The SOMO of a primary radical (IP ) 8.7 eV)49 is
closer to the LUMO (π*) orbital of ATB (EA ) 0.63 eV)50
than to the π* orbital of propene (EA ) 1.99 eV),51 and the
smaller SOMO-LUMO energy gap would predict faster radical
attack on the azo linkage. While this notion is consistent with
(46) Experimental olefin and azoalkane bond lengths and angles are 1.34
Å, 124.3° and 1.254 Å, 111.9°, respectively, while the calculated values
(UQCISD/6-31G*, cf. Supporting Information for output files) for 5-pen-
tenyl and the diaza analog case are 1.32 Å, 125.35° and 1.22 Å, 115.35°,
respectively.
(47) This is the most recent N-H BDE of hydrazine. An earlier
experimental value (76 kcal/mol) and estimated value (87.5 kcal/mol) can
be found in refs 40 and 38, respectively.
(48) Fleming, I. Frontier Orbitals and Organic Chemical Reactions;
Wiley: London, 1976; p 182.
(49) Traven, V. F. Frontier Orbitals and Properties of Organic
Molecules; Ellis Horwood, Ltd.: West Sussex, 1992; p 295.
(50) Modelli, A.; Jones, D.; Rossini, S.; Distefano, G. Tetrahedron 1984,
40, 3257.
(51) Franklin J. L.; Dillard, J. G.; Rosenstock, H. M.; Herron, J. T.; Draxl,
K. Ionization Potentials, Appearance Potentials, and Heats of Formation
of Gaseous PositiVe Ions; NSRDS-NBS 26; U.S. Government Printing
Office: Washington, DC, 1969.
(45) Wong, M. W.; Radom, L. J. Phys. Chem. 1995, 99, 8582.

6064 J. Am. Chem. Soc., Vol. 119, No. 26, 1997
Engel et al.
Table 8. UQCISD/6-31G* Energies Based on UHF/6-31G* Geometries
species
methyl radical
ethylene
propyl radical
methyl-ethylene TS
cis-diimide
energy, hartrees
species
energy, hartrees
-39.688 868 5
-78.312 405 2
-118.045 267 4
-117.987 738 4
-110.323 098 7
-110.333 014 7
-150.009 697 6
1-methyl-2-hydrazyl radical
4-penten-1-yl radical
4-penten-1-yl TS
4,5-diazapenten-1-yl radical
4,5-diazapenten-1-yl TS
methylhydrazine
-150.071 561 9
-195.188 837 6
-195.157 692 5
-227.204 953 7
-227.194 960 2
-150.689 353 5
-0.498 232 9
trans-diimide
methyl-trans-diimide TS
hydrogen atom
the rapid cyclization of 14, the ab initio results show only a
0.84 kcal/mol ∆Hq difference between diimide and ethylene
(Table 6). Radical attack on the azo n- orbital (IP ) 8.2 eV)
appears to be a good energetic match, but the third electron
would have to go into a high-lying σ* orbital.
Cyclization of γ-Perester Radical 20. While γ-azo radical
14 cyclizes very rapidly, the related γ-perester radical 20 affords
lactone 23 much more slowly. The attack of radicals on the
peroxy bond is well known, and its synthetic possibilities have
been explored recently.57 Using the reaction of 39 with CCl4
Fragmentation of Hydrazyl Radical 15. Our results
indicate that hydrazyl 15 undergoes an unusual C-C homolysis,
resulting in loss of a methyl radical. C-N homolysis of
hydrazyl radicals is known in such compounds as 3252 and 33,53
but 34 does not lose tert-butyl• because the odd electron is in a
as a radical clock, Maillard and co-workers obtained a cycliza-
tion rate constant of 4 × 104 s-1 at 80 °C.58 Despite the lower
temperature and the tertiary nature of the attacking radical in
20, our measured kc of 1.5 × 104 s-1 at 22 °C is slower than
this value by only a factor of 2.6. It appears from these results
that more methyl groups on the attacking radical center have
little effect on the lactonization rate, much like the 5-hexenyl
radical.18
p orbital perpendicular to the C-N bond to be cleaved.54 The
same situation must obtain for 15. Even though loss of t-Bu•
In connection with our interest in bifunctional free radical
initiators,1 it is useful to compare the kc of 20 with the â-scission
rate of 2, where kâ/kr of a cyclopropyl analog was 0.16. The
•
would be thermodynamically favored over loss of CH3 , the five-
membered ring prevents the necessary torsion about the N-N
bond that would allow overlap of the odd electron p orbital
with the N-t-Bu bond. On the other hand, the CH3-C bond
of 15 is sufficiently well aligned to exhibit the first case of
hydrazyl C-C homolysis. In 32, the flexibility of the six-
membered ring permits the requisite overlap so only the
thermodynamic process takes place, despite the loss of hydrazyl
resonance in the transition state.55 Although the CR-Câ bond
of bicyclic hydrazyl 35 is well aligned with the nitrogen p orbital
for â-scission, this radical actually persists for months.54 The
greater entropy for fragmentation of 15 than 35 and the high
temperature of our study apparently facilitate â-scission of 15.
To enhance our understanding of hydrazyl fragmentation, we
calculated the enthalpy change of a simple model reaction. MP2
calculations on species 36-38 gave total energies of
most recent value59 of kr is 1.8 × 107 s-1, so that kâ is 2.9 ×
106 s-1 at 25 °C. Since cyclization of 20 is 200 times slower
than â-scission of 2, irradiation of 5 should produce tertiary
radicals that initiate sytrene polymerization much faster than
they cyclize. The addition rate of 20 to styrene is 1.3 × 105
M-1 s-1 × 8.7 M ) 1.1 × 106 s-1,1 which is 73 times faster
than cyclization of 20. While 34% of 2 would be incorporated
into polystyrene, the figure is nearly 99% for 20. Production
of an initiator-containing polymer from 5 by photochemical
initiation is therefore feasible, but due to the rapid cyclization
of 14, thermal initiation is not. To achieve that goal, the azo
and perester groups will have to be separated by a rigid spacer.
In summary, we have found that γ-azo radical 14 produced
by thermolysis of 5 cyclizes at least 5 × 107 faster than the
analogous olefinic radical 30. Ab initio calculations of the 4,5-
diazapenten-1-yl radical, methyl plus diimide, and the related
olefins suggest that the normally “forbidden” 5-endo cyclization
is facilitated by the smaller CsNdN than CsCdC angle. We
propose that following cyclization, hydrazyl radical 15 under-
goes an unprecedented cleavage of a methyl radical to afford a
pyrazoline 9 which is finally oxidized by the starting perester
to pyrazole 6. Since the azo chromophore absorbs at much
longer wavelength than the perester, near UV irradiation of 5
produces γ-perester radical 20 which cyclizes to lactone 23 at
a rate of 1.5 × 104 s-1 at 22 °C. This reaction is slow enough
that 5 could be used as a bifunctional radical initiator. However,
thermolysis of 5 would not produce an azo-containing polymer
because cyclization of 14 is too rapid.
-189.208 641, -149.481 871 8, and -39.670 749 9 hartrees,
respectively. Fragmentation of 36 is therefore endothermic by
35.2 kcal/mol. If we neglect the barrier to the reverse reaction
and use the highest reasonable log A value (17)56 in the
Arrhenius equation, the half-life of 36 comes out to be 32 min
at 110 °C. Fragmentation of 15 should be more favorable than
that of 36 on account of the fully substituted CdN bond in 9.
However, this large endothermicity and our tentative success
in trapping 15 suggest that fragmentation occurs only when no
other pathway is available. Independent generation and study
of 15 would be a worthwhile endeavor.
(52) Wang, S. F.; Mathew, L.; Warkentin, J. J. Am. Chem. Soc. 1988,
110, 7235.
(53) Kaba, R. A.; Lunazzi, L.; Lindsay, D.; Ingold, K. U. J. Am. Chem.
Soc. 1975, 97, 6762.
(54) Nelsen, S. F.; Landis, R. T. J. Am. Chem. Soc. 1973, 95, 6454.
(55) Raban, M.; Aviram, K.; Kost, D. Tetrahedron Lett. 1985, 26, 3591.
(56) Benson, S. W.; O’Neal, H. E. Kinetic Data on Gas Phase
Unimolecular Reactions; NSRDS-NBS-21; U.S. Government Printing
Office: Washington, D.C., 1970; p 28.
(57) Degueil-Castaing, M.; Navarro, C.; Ramon, F.; Maillard, B. Aust.
J. Chem. 1995, 48, 233. Ramon, F.; Degueilcastaing, M.; Maillard, B. J.
Org. Chem. 1996, 61, 2071.
(58) Maillard, B.; Kharrat, A.; Gardrat, C. NouV. J. Chim. 1987, 11, 7.
(59) Engel, P. S.; He, S.-L.; Banks, J. T.; Ingold, K. U.; Lusztyk, J. J.
Org. Chem. 1997, 62, 1210.

Thermolysis and Photolysis of a γ-Azoperester
J. Am. Chem. Soc., Vol. 119, No. 26, 1997 6065
-20 °C was added MeLi (8 mL, 11.2 mmol, 1.4 M in hexane). The
solution was stirred for 30 min at -20 °C and then cooled to -60 °C.
A solution of ethyl acrylate (1.3 g, 13 mmol) in THF (10 mL) was
added, and the color of the solution changed from orange to light yellow.
After stirring for 2 h at -60 °C, acetic acid (1 mL) in hexane (20 mL)
was added and the mixture was slowly warmed to room temperature.
Water (30 mL) was added, and the organic layer was separated. The
aqueous layer was extracted with ether (2 × 30 mL). The combined
organic solution was dried over MgSO4 and filtered, and the solvent
was rotary evaporated. The product 4 (1.6 g, 70%) was obtained by
flash column chromatography on silica gel (ethyl acetate-hexane, 1:8).
1H NMR: δ 1.09 (s, 6H), 1.14 (s, 9H), 1.24 (t, 3H, J ) 7.1 Hz), 1.99
(m, 2H), 2.25 (m, 2H), 4.11 (q, 2H, J ) 7.1 Hz). 13C NMR: δ 12.17,
22.64, 25.11, 27.74, 33.94, 59.73, 66.79, 67.01, 177.1.
4-Methyl-4-(tert-butylazo)pentanoic Acid (8). NaOH (aqueous
20%, 4 mL) was added to a solution of 4 (1.14 g, 5 mmol) in 30 mL
of ethanol at room temperature. The reaction mixture was stirred for
4 h. Ethanol was rotary evaporated, and the residue was dissolved in
10 mL of water. The aqueous solution was washed with ether (30
mL) and acidified with aqueous HCl to pH 2. This acidic solution
was extracted with ether (3 × 50 mL). The combined ether layers
were dried over MgSO4. After filtration and solvent evaporation, 8
was obtained as a solid (940 mg, 94%), mp 31-32 °C. 1H NMR: δ
1.10 (s, 6H), 1.15 (s, 9H), 1.99 (m, 2H), 2.33 (m, 2H). 13C NMR: δ
24.38, 26.70, 29.10, 34.96, 66.90, 67.05, 180.20. MS (M + 1): m/e
201 (100), 183 (10). Anal. Calcd: C, 59.97; H, 10.06; N, 13.99.
Found: C, 60.04; H, 9.89; N, 14.11.
tert-Butyl 4-Methyl-4-(tert-butylazo)peroxypentanoate (5). Using
the method of Staab et al.,64 a solution of 8 (710 mg, 3.55 mmol) in
THF (5 mL) was added to a solution of 1,1′-carbonyldiimidazole (737
mg, 4.5 mmol) in 10 mL of THF at room temperature, and the reaction
mixture was stirred for 0.5 h. tert-Butyl hydroperoxide (410 mg, 5.5
mmol) was added at 0 °C, and the reaction was stirred for 4 h. Ether
(25 mL) was added, and this solution was washed with aqueous NaOH
(5%), water, and brine. The ether solution was dried over Na2SO4.
After filtration and evaporation of solvent, the residue was chromato-
graphed on Florisil (ethyl acetate-hexane 1:15). A yield of 620 mg
of 5 (64%) was obtained. 1H NMR (C6D6): δ 0.99 (s, 6H), 1.12 (s,
9H), 1.15 (s, 9H), 2.05 (m, 2H), 2.15 (m, 2H). 13C NMR: δ 24.37
(Me2), 26.14 (t-Bu), 26.27 (CH2), 26.75 (t-Bu), 35.17 (CH2), 66.51 (C),
67.00 (C), 83.30 (C), 171.3 (CO). IR (neat): 2975, 1779 cm-1. UV
(hexane): 366 nm (ꢀ 22). The methylene peaks of 5 were unexpectedly
complex.65
Experimental Section
Instrumentation. NMR spectroscopy: Bruker AC-250 with δ
relative to CDCl3 (7.26), C6D6 (7.15) or TMS (0.00). All H NMR
1
spectra were obtained at 250 MHz and all 13C spectra at 62.5 MHz in
CDCl3 unless otherwise specified. IR spectroscopy: Nicolet 205 FT-
IR. Mass spectrometry: Finnigan MAT 95. CI spectra employed
methane as reactant gas. UV spectroscopy: HP8452A diode array
spectrometer. Analytical GC: HP5890A with a DB-5 capillary column
(0.25 mm × 30 m) or a AT-1701 capillary column (0.25 mm × 30 m)
and HP3365 software. GC condition A: injector 210 °C, detector 250
°C, initial temperature 35 °C, initial time 2.5 min, 10 °C /min ramp to
250 °C. GC condition B: injector 170 °C, detector 250 °C, initial
temperature 35 °C, initial time 2.5 min, 10 °C/min ramp to 100 °C,
hold for 15 min, then ramp to 170 °C at 10 °C/min, final time 10 min.
Materials. Ether and THF were distilled from Na/Ph2CO, while
hexane was distilled from sodium. Benzene and methylene chloride
were distilled from CaH2. Decane, used as a GC internal standard,
and 1,4-cyclohexadiene, used as a scavenger, were distilled before use.
Tempo was purified by column chromatography and sublimation. All
other starting materials were used as received.
Thermolysis and Photolysis Techniques. All samples for rate and
product studies were freeze-thaw-degassed (-196 °C) three times
and sealed on a vacuum line. For thermolysis, the tubes were immersed
completely in a DC-200 silicone oil bath contained in a 1.5 gal Dewar
flask with a mechanical stirrer. The temperature was regulated by a
Bayley Model 123 temperature controller and was measured with a
Hewlett-Packard Model 3456A digital voltmeter and a platinum
thermometer. An Oriel 500 W high-pressure mercury lamp with a 366
nm filter solution was employed in photolysis work.
The thermolysis of di-tert-butyl hyponitrite and 5-d6 in benzene-
nitrobenzene is a typical GC experiment. A 9 mg portion of di-tert-
butyl hyponitrite13 in 0.6 mL of a 1:1 v/v solution of benzene and
nitrobenzene was degassed and sealed into a 7 mm × 6 cm Pyrex tube.
After heating at 110 °C for 15 min, the tube was opened and the
contents were analyzed on the AT-1701 capillary column using GC
condition A described under Instrumentation. Toluene, o-nitrotoluene,
and p-nitrotoluene were identified by coinjection with authentic samples
while the small peak at 15.35 min was assumed to be m-nitrotoluene
on the basis of boiling points (ortho 221.7 °C, meta 232.6 °C, and
para 238.3 °C) and expected product ratios (cf. Table 2). Other GC
peaks of size similar to these were observed but were not assigned. A
0.066 M solution of 5-d6 in the same solvent mixture was thermolyzed
at 110 °C for 1.5 h and then analyzed by GC and by CI/GC/MS.
Because the deuterated compounds exhibited a shorter retention time
than the protiated compounds, the mass spectrum was summed over
the GC peak due to both species.
Acknowledgment. We thank the National Science Founda-
tion and the Robert A. Welch Foundation for financial support
and Professors Stephen F. Nelsen, University of Wisconsin, and
Thomas A. Albright, University of Houston, for helpful discus-
sion. The critical and constructive comments of the reviewers
are gratefully acknowledged.
CI Mass Spectrometry. Tempo trapping product 19 was identified
only by this technique, giving ions at m/e (relative abundance)
•
assignment 311 (4) M•+, 312 (5) MH+, 296 (3) M+ - CH3 , 226 (3)
MH+ - t-BuNNH analogous to 12, 155 (100) 11, 140 (22) 2,2,6,6-
tetramethylpiperidinium cation, and 99 (6) 3,3-dimethylpyrazolidinium
cation 13. The assignment of the tempo-derived fragments was
supported by the CI/MS of 1-methoxy-2,2,6,6-tetramethylpiperidine:60
m/e 172 (25) MH+, 171 (40) M•+, 170 (22) M•+ - H•, 156 (41) M•+
Supporting Information Available: Synthesis and spectral
data of 6, 7, 9, 10, 18, 21, 22, and 23, and a table of Gaussian
94 quantum chemical results (11 pages). See any current
masthead page for ordering and Internet access instructions.
•
- CH3 , 140 (100) 2,2,6,6-tetramethylpiperidinium cation, 126 (7) M+•
- CH3 - CH2O. The tentative structural assignment of 17-d6 is based
only on CI/GC/MS: m/e 163 (8) MH+, 162 (15) M•+, 147 (68) M•+
CH3, 131 (32), 119 (56) M+• - CH3 - C2H4, 107 (93) MH+
isobutene, 57 (100) t-Bu+.
-
-
JA970143U
(62) Gaussian 94, Revision B.2: Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.;. Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.,
Gaussian, Inc., Pittsburgh, PA, 1995.
Theoretical Calculations. Ab initio geometry optimizations were
carried out at the UHF/6-31G* level either with SPARTAN61 or with
Gaussian 94.62 A ll transition structures met the requirement of one
imaginary frequency corresponding to the reaction coordinate.
Ethyl 4-Methyl-4-(tert-butylazo)pentanoate (4). Even though
methyl acrylate was said to give negligible yields of C-addition products
with acetone tert-butyl hydrazone (ATBH) anion,7 we experienced no
difficulty preparing 4. ATBH was synthesized in 90% yield according
to the literature.63 1H-NMR: δ 1.17 (s, 9H), 1.71 (s, 3H), 1.92 (s,
3H). To a solution of ATBH (1.28 g, 10 mmol) in THF (40 mL) at
(63) Smith, P. A. S.; Clegg, J. M.; Lakritz, J. J. Org. Chem. 1958, 23,
1595.
(64) Staab, H. A.; Rohr, W.; Graf, F. Chem. Ber. 1965, 98, 1122.
(65) While the spectrum of 5 was the most second-order, the methylenes
of 4 and 8 (but not 7) also appeared distorted. The reason is that
conformationally mobile molecules ZCH2CH2Y are actually AA′XX′
systems that sometimes appear as A2X2. See: McLean, D.; Waugh, J. S. J.
Chem. Phys. 1957, 27, 968. Silverstein, R. M.; LaLonde, R. T. J. Chem.
Educ. 1980, 57, 343. We thank Dr. Lawrence B. Alemany for helpful
discussion on this point.
(60) Whitesides, G. M.; Newirth, T. L. J. Org. Chem. 1975, 40, 3448.
The EI MS of this methyl trapping product exhibits m/e 171 (10.5) and
156 (100) and smaller m/e of ra below 18, but not m/e 140.
(61) Wavefunction, Inc., 18401 Von Karman Ave., Irvine, CA.