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Flash Photolytic Generation and Study of Ynamines. First Observation of Primary and Secondary Ynamines in Solution

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ii (AE negative). A conceivable reaction at the inversion
center i2, on the other hand, leads to a destabilization (AE
positive) of the products in the cavity. The energy differences
between the two routes indicate that the dimerization to the
symmetrical product should proceed at il, which is consistent with the results of the topochemical consideration. In
the case of the dimer 3a, both formations at il as well as at
i2 are associated with a slight destabilization. Since an energetically much more favorable reaction route exists with
the formation of 2 a at il, 3 a should not be formed, or only
in very small amounts.
For the formation of the dimer 3 b at the inversion center
i2, however, we found a stabilization by the crystal lattice.
Hence, the reaction leading to the unsymmetrical dimer 3 b
should also be possible in the case of the butyrate 1 b, but,
due to the greater stabilization by the cavity, 2b should be
formed preferentially. Thus, a qualitatively correct prediction of the photoproducts is possible with the modified
Cohen model also in the case of the topochemically forbidden reactions leading to 3 a and 3b.
The limited application of this purely energetic approach
is manifested in the photodimerization of crystalline 2,5dibenzylidenecyclopentanone 4 (Scheme 3).["9 ''I Whereas
38 % I101
551151% Ilil
0
Ph-
Ph
8 % 1101
44% 1111
I
Ph
bility of predicting products, is together with the simulation
of molecular packings, an important step on the way to a
rational design of organic solid-state reactions and thus also
to their application in synthetic chemistry.
Received: April 27, 1991 [Z 4591 IE]
German version: Angew. Chem. I03 (1991) 1379
[ll General overviews of organic solid-state reactions: G. R. Desiraju (Ed.):
Organic Solid State Chemistry, Elsevier, New York 1987; M. D. Cohen,
Tetrahedron 43 (1987) 1211; V. Ramamurthy, K. Venkatesan, Chem. Rev.
87 (1987) 433; V. E. Shklover, T. V. Timofeeva, Yu. T. Struchkov, Russ.
Chem. Rev. 55 (1986) 721; V. Ramamurthy, Tetrahedron 42 (1986) 5753.
121 A summary of the works by G. M. J. Schmidt are to be found in D. Ginsburg (Ed.): Solid State Photochemistry, Verlag Chemie, Weinheim 1976.
[3] S. Mohr, W. R. Baker, D. Lawrenz in H. 3x1, J. Friedrich, C. Brauchle
(Eds.): Photoreaktive Festkiirper, M. Wahl, Karlsruhe 1984, p. 455; S.
Mohr, Tetrahedron Lett. 1979,2461,3139;ibid. 21 (1980) 593; Fresenms 2.
Anal. Chem. 304 (1980) 280; C. Kruger, S. Mohr, 1. Ortmann, K.
Schaffner, unpublished; G. Kaupp, H. Frey, G. Behmann, Chem. Eer. 121
(1988) 2135, and references cited therein.
141 M. D. Cohen, Angew. Chem. 8 7 (1975) 439; Angew. Chem. Int. Ed. Engl.
14 (1975) 386.
[5] H. Fruhbeis, R. Klein, H. Wallmeier, Angew. Chem. 99 (1987) 413; Angew.
Chem. Int. Ed. Engl. 26 (1987) 403, and references cited therein.
[6] I. Klopp. Dissermtion, Universitat Wuppertal 1991.
[7] W. 1. Awad, S . M. A. Omran, A. 1. Hashem, J. Chem. U . A . R . 10 (1967)
287; Chem. Abstr. 69 (1968) 86677k.
[8] a) Crystallographic data of l a : P2,/n, a = 6.542(1), b = 11.479(1), c =
22.263(2) A, fi = 97.66(1)", V = 1656.9 A3, 2 = 4, eCalcd
= 1.25 gcm-',
1 = 1.54178 A, p = 6.72 cm-', R = 0.058, R , = 0.063; 3593measured reflections, 3334 independent reflections, 2228 observed ( I > 2u(f)), 209 refined parameters, residual electron density 0.48 e k 3 . b) Crystallographic
data of 1 b: P2,/c, a = 6.512(1), b = 29.347(3), c = 9.534(1) A, =
109.29(1)", V = 1719.7 A', Z = 4, ecz,cd
= 1.26 gcm-', d = 1.54178 A,
p = 6.67 cm-', R = 0.086, R, = 0.097; 3598 measured reflections, 3324
independent reflections, 2507 observed ( I > 2u(f)), 217 refined parameters, residual electron density 0.58 ek'.-Further
details of the crystal
structure investigations are available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Information mbH, W-7514 Eggenstein-Leopoldshafen2 (FRG), on quoting
the depository number CSD-55331, the names of the authors, and the
journal citation.
191 SYBYL 5.3, Tripos Associates, Inc., St. Louis, USA; Force Field: Tripos
(Version 5.2)"; minimization algorithm: conjugate gradient; convergence
criterion: gradient of standard deviation i0.01 k c a l m o l - ' k ' ; nonbonded cutoff 8 A.
[lo] G. Kaupp, 1. Zimmermann, Angew. Chem. 93 (1981) 1107; Angew. Chem.
I n t . Ed. Engl. 20 (1981) 1018.
[ I l l C. R. Theocharis, W. Jones, J. M. Thomas, M. Motevalli, M. B. Hursthouse, J. Chem. SOC.Perkin Trans. II 1984, 71.
26% 1101
-(a)%
[111
Scheme 3. Photoreactions of crystalline 4. The yields quoted in brackets refer
to 25% conversion.
the results of the simulation correlate with the experimental
findings of the [271 + 2 4 cycloaddition leading to 5 and
6,''O. 7 formed by radical [ 2 0 + 2x1
is significantly stabilized by the crystal assembly (see Table 1). Its
high AE value, together with the experimental finding that 7
could only be isolated at incomplete (25 YO)conversion,['
points to a kinetically controlled reaction for which the modified Cohen model is of limited application. Reversal of the
product distribution upon carrying out the reaction in solution[". "]-a
kinetic inhibition of the formation of 7 presupposed[lO1-follows from a comparison of the &-values
of 5 and 6,in agreement with the experiment (Table 1).
That only two energy values are necessary per potential
product molecule in the method presented here enables a
simpler calculation, with generally accessible force-field programs, compared to the classical Cohen model. This possi-
1356
0 VCH
Verlagsgesellschaft mbH. W-6940 Weinheim. 1991
Flash Photolytic Generation and Study
of Ynamines. First Observation of Primary and
Secondary Ynamines in Solution **
By Yvonne Chiang, Andrew S . Grant, A . Jerry Kresge,*
Przemyslaw Pruszynski, Norman P . Schepp, and Jakob Wirz
Although tertiary ynamines 1 are well-known stable substances,"] primary and secondary ynamines 2 and 3, because
['I
Prof. Dr. A. J. Kresge, Y. Chiang, Dr. A. S. Grant, Dr. P. Pruszynski
Department of Chemistry, University of Toronto
Toronto, Ontario M5S 1Al (Canada)
Dr. N. P. Schepp, Prof. Dr. J. Wirz
Institute fur Physikalische Chemie der Universitat
CH-4056 Basel (Switzerland)
[**I This work was supported by the Natural Sciences and Engineering Research Council of Canada, the Donors of the Petroleum Research Fund,
administered by the American Chemical Society, the National Institutes of
Health, and the Swiss National Science Foundation (Project No. 20005.515)
OS70-0833/91/1010-1356$3.S0+.2S/O
Angew. Chem. Int. Ed. Engl. 30 (1991) N o . 10
of their facile isomerization to ketenimines, have heretofore
been observed only in the gas phase or in low-temperature
We wish to report that we have now developed
a method for generating all three classes of ynamines in
aqueous solution and have begun to study their chemistry in
that medium.
RCsCNR,
1
RCsCNH,
2
nism is further supported by the correlation illustrated in
Figure 1, which shows that the rate of reaction increases with
increasing basicity, and therefore cation stabilizing ability,
of the amino portion of the substrate. This step in the case of
tertiary ynamines is then followed by hydration of the cation
and amide formation, as shown in [Eq. (e)], and in the case
of primary and secondary ynamines, by loss of a proton
from nitrogen, [Eq. (f)]. The fact that primary, secondary,
RC=CNHR
3
We have found that flash p h o t ~ l y s i sof
~ ~phenylaminocy]
clopropenones 4 in aqueous solution gives carboxylic acid
amides as the ultimate reaction products [Eq. (a)], via shortlived transient intermediates with strong absorbance in the
region 3. = 270-290 nm, which, from their behavior as well
as by analogy with our previous flash photolytic generation
0
II
Ph
hv
PhCECNR'R'
+ CO
-
0
11
H2O
0
II
PhCH,CNHR
HZO
-Ha
+
OH
I
PhCH=CNR,
-
PhCH=C=NR
+ H@
0
11
PhCH,CNR,
(e)
(0
and tertiary ynamines obey the same correlation (Fig. 3 ) implies that this proton loss from nitrogen is not concerted with
proton addition to carbon, for such proton loss cannot occur
in the case of tertiary ynamines. The slope of the correlation
is 0.34, which suggests an early transition state consistent
with the rapid nature of these reactions.
Fig. 1. Relationship between rate constants for the reaction of ynamines with
hydronium ion in aqueous solution at 25°C and acid dissociation constants of
the conjugate acids of amines corresponding to the amino portion of the
ynamines; from left to right: R' = H, RZ= C,F,; R' = H, R2 = C,H,;
R' = R2 = H; R' = H, RZ= C6Hl,. and R'R2 = (CH,),.
In sufficiently acidic solutions, ynamines can be expected
to experience reversible protonation on nitrogen in addition
to rate-detemining proton transfer to carbon [Eq. (g)]. The
N-protonated species will be much less susceptible to elec-
PhC=CNHRf
of phenylketene-N-phenylimineand phenylketene-N-isopropylimine respectively, are in good agreement with our values,
k = 98 and 2190 M-'s-'. Ynamines without amino hydrogen, i.e. tertiary ynamines, cannot isomerize to ketenimines,
but their direct hydration has also been studied before,'' '1
and again the rates we measure are consistent with those
reported, e.g. k = 1.03 s-' for the "uncatalyzed" reaction of
phenylpiperidinoacetylene in 8.6 70 dioxane-water vs. our
k = 1.84 s-' for wholly aqueous solution.
These reactions of primary, secondary, and teriary ynamines are acid catalyzed, and the conversion of primary and
secondary ynamines into ketenimines is base catalyzed, as
well. The acid-catalyzed reactions show general catalysis and
= 5.9
give sizeable kinetic isotope effects, e.g. kH2POl/kD2POP
for phenylpiperidinoacetylene. This is classic evidence for
rate-determining proton transfer to carbon and suggests that
the reaction is initiated by electrophilic addition of a proton
to acetylenic carbon, as shown in [Eq. (d)]. Such a mecha-
-
PhCH=CNHR@
PhCH,CNRIRZ (a)
of ynols from arylhydroxycyclopropenones~sl
[Eq. (b)], we
conclude are ynamines.t6] Ynamines produced in this way
with at least one hydrogen substituent on amine nitrogen are
transformed into still other transient species which we have
identified as ketenimines from their rates of reaction: the
hydration of ketenimines to amides [Eq. (c)], has been studied
and the rate constants reported, e.g. k = 100
and 2200 M-'s-' for the hydrogen-ion catalyzed reactions
PhCH=C=NR
PhCH=CNRf
K.
PhCECNR,
la1
+ H@ --+
PhCH=CNRF
(g)
[a] rate determining
trophilic attack on carbon, and carbon protonation will consequently be inhibited. We detected no such inhibition up to
the most acidic solutions we examined, e.g. [He] = 0.5 M for
phenylcyclohexylaminoacetylene. This sets an upper limit of
pK, < 0.3, i.e. - Ig 0.5, for the conjugate acid of this
ynamine, and that makes the ynamine more than ten orders
of magnitude less basic than cyclohexylamine (pK,(BH@) =
10.6). This strong base-weakening effect of the acetylenic
group parallels the remarkable acid-strengthening effect
which we found in the case of ynols,['- 12] and which is also
evident in the ionization of ynamines as acids (vide infra).
The base-catalyzed isomerization of primary and secondary
ynamines to ketenimines shows only specific hydroxide-ion
catalysis. This suggests a mechanism, shown in [Eq. (h)],
PhCECNHR
+ Hoe
K
PhC=CNRe
+ H,O
kbl
PhCH=C=NR
+ Hoe
(h)
[a] rate-determining
Angew. Chem. Inr. Ed. Engl. 30 (1991) No. 10
0 VCH Verlagsgesellschaft
mbH, W-6940 Weinheim, 1991
0570-0833/9l/lO10-13S7 $3.50+.25/0
135'7
consisting of proton transfer from the substrate to base in a
rapid and reversible reaction, whose position of equilibrium
is governed by the hydroxide ion concentration, followed by
an "uncatalyzed" rate-determining protonation at carbon
by water. The hydroxide-ion catalytic coefficient for such a
process is equal to the equilibrium constant for the first step
times the rate constant for the second: k,", = Kk. This equilibrium constant is also equal to the acidity constant of the
ynamine ionizing as an acid divided by the autoprotolysis
constant of water, K = KJK,. Use of k = l o L 1sK1 as an
upper limit for this rate
combined with the experimentally determined value of kHOe= 6.5 x lo6 M-'s-'
for phenylaminoacetylene then gives pK, I18.0 as an upper
limit for the acid dissociation of this ynamine [Eq. (i)]. This
K.
P h C ECN H ,
PhC=CNHe
+ H0
(i)
makes phenylaminoacetylene at least 17 pK units more
acidic than ammonia (pK, = 35['51) and provides another
example of the dramatic acidifying effect of the acetylenic
group.
Received: May 8, 1991 [Z 4618 IE]
German version: Angew. Chem. 103 (1991) 1407
[l] H. G. Viehe in H. G. Viehe (Ed.): Chemistry of' Acetylenes, Marcel
Dekker, New York 1969, Chapter 12; J. M. Z. Gladych, D. Hartley in
D. Barton, W. D. Ollis (Eds.): Comprehensive Organic Chemistry, Perga-
mon, New York 1979, pp. 75-79.
121 H.-W. Winter, C. Wentrup, Angew. Chem. I n ? . Ed. Engl. 19 (1980) 720;
J. M. Buschek, J. L. Holmes, Org. Mass Spectrom. 21 (1986) 729; B. van
Baar, W Koch, C. Lebrilla, J. K. Terlouw, T. Weiske, H. Schwarz; Angew.
Chem. l n t . Ed. Engl. 25 (1986) 827; J. K. Terlouw, P. C. Bergers, B. L. M.
van Baar, T. Weiske, H. Schwarz, Chimia 40 (1986) 357; C. Wentrup, H.
Briehl, P. LorenEak, U. J. Vogelbacher, H.-W. Winter, A. Maquestiau.
R. Flammang, J. Am. Chem. Sot. 110 (1988) 1337.
[3] Ethynamine has also been characterized by high level ab-initio calculations: S . Saebo, L. Farnell, N. Riggs, L. Radom, 1 Am. Chem. Soc. 106
(1984) 5047; B. J. Smith, L. Radom, unpublished results; R. D. Brown,
E. H. N. Rice, M. Rodler, Chem. Phys. 99 (1985) 347; P. von R. Schleyer
(Universitat Erlangen-Niirnberg (FRG)), private communication.
[4] Two different flash systems were used: 1) a conventional apparatus with
excitation flash produced by capacitor discharge through a pair of xenon
flash lamps, 50 ps pulse width, 0.5 to 2.0 kJ per pulse. 2) a KrF eximer laser
system, I.,,, = 248 nm, 25 ns pulse width, 200 mJ per pulse; monitoring
wavelengths varied from I = 265 to 315 nm.
[5] Y. Chiang, A. J. Kresge, R. Hochstrasser, J. Wirz, J. Am. Chem. Soc. 1 I 1
(1989) 2355.
[6] Cyclopropenone substrates were prepared by reaction of phenylchlorocyclopropenone[7] with amines and were characterized by their NMR, IR,
and mass spectra.
171 J. S. Chickos, E. Patton, R. West, 1 Org. Chem. 39 (1974) 1647.
[8] A. F. Hegarty, D. G. McCarthy, J. Am. Chem. SOC. 101 (1979) 1345; J.
Chem. Soc. Perkin Trans. 2 1980, 579.
(91 Although ketenimines substituted at nitrogen are hydrated to amides directly[8], we have found that phenylketenimine, whose nitrogen substituent is hydrogen, is first converted into the corresponding nitrile; i.e., the
cationic intermediate formed by protonation of the ketenimine at carbon
loses a proton from nitrogen more rapidly than it reacts with water:
PhCH=C=NH
-A-
PhCH,C=NH"
0 VCH
By Jan Kroon, Jan W Verhoeven,* Michael N . Paddon-Row,*
and Anna M . Oliver
Dedicated to Professor Kurt Schaffner on the occasion
of his 60th birthday
Photoinduced electron transfer constitutes one of the most
versatile and potentially efficient methods for converting
light energy into useful (e1ectro)chemical energy, as unequivocally demonstrated by its successful implementation in the
photosynthetic apparatus. The ongoing extensive investigations['] of such processes have made clear that, e.g., in synthetic D(onor)-bridge-A(cceptor) systems very large charge
separation distances can be realized but at the same time it
has become evident that the success is critically dependent
upon a number of factors relating to the structure of the
system itself as well as that of its environment. We and others
have addressed, via both experimental and theoretical methods, the structural requirements to be met by the "bridge" in
order to sustain sufficient electronic coupling over long distances.[*- During these studies we have also occasionally
noted the remarkable difference in solvent sensitivity that the
rate of charge separation may show in structurally quite
closely related D-bridge-A systems and we have pointed out
the decisive role that the interdependence of solvent stabilization and solvent reorganization may play.[5. In the
present paper this important point is quantified by comparison of the solvent effect on the rate of photoinduced charge
separation in the fully rigid and structurally related systems
1a and 1b (Scheme I), for which kinetic data could be collected in a wide variety of solvents. Furthermore, as will be
shown, consideration of the interplay between solvent stabilization and solvent reorganization leads to the formulation
la X=OMe
lb X=H
+
2a
€0,
1.10 V
€00
3.78 eV
2 PhCH,CN
This is consistent with the generally accepted mechanism for acid catalyzed
hydrolysis of nitriles in which rapid and reversible protonation on nitrogen
is followed by rate-determining reaction of the N-protonated intermediate
with water[l0].
[lo] M. Liler: Reaction Mechanisms in Su[furic Acid and other Strong Acid
Solutions, Academic Press, New York 1971, pp. 205-208.
[ l l ] W. F. Verhelst, W. Drenth, J. Am. Chem. Soc. 96 (1974) 6692.
[12] B. J. Smith. L. Radom, A. J. Kresge, J. Am. Chem. Soc. 1 1 1 (1989) 8297.
[13] This upper limit is for a diffusion-controlled reaction governed by the
rotational correlation time of water[l4].
[I41 D. Eisenberg, W. Kauzmann: The Slructure and Properties of Water, Oxford University Press, New York, 1969, pp. 206-214.
[15] R. P. Bell: The Proton in Chemistry, Second Edition, Cornell University
Press. Ithaca, NY 1973, p. 86.
1358
Solvent Dependence of Photoinduced
Intramolecular Electron Transfer: Criteria
for the Design of Systems with Rapid,
Solvent-Independent Charge Separation**
Verlagsgesellschaft mbH, W-6940 Weinheim. 1991
Scheme 1. Structure and relevant physicochemical properties of the systems
investigated.
['I
[*'I
Prof. Dr. J. W. Verhoeven, Drs. J. Kroon
Laboratory of Organic Chemistry, University of Amsterdam
Nieuwe Achtergracht 129, NL-1018 WS Amsterdam (The Netherlands)
Prof. Dr. M. N. Paddon-Row, Dr. A. M. Oliver
Department of Chemistry, University of New South Wales
P. 0. Box 1, Kensington, N.S.W. 2033 (Australia)
The present investigations were supported in part by the Netherlands
Foundation for Chemical Research (SON) with financial aid from the
Netherlands Organization for the Advancement of Research (NWO). We
also thank the Australian Research Council for support.
$3.50+.25/0
0570-0833/9lj~0lO-1358
Angew.. Chem. lnt. Ed. Engl. 30 (1991) No. 10
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