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Gas-Phase Activation of Fluorocarbons by УBareФ and Coordinated Praseodymium Cations.

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the investigation of a wide variety of hydrogen-bonded
supramolecular assemblies and more generally of self-assembly
through any type of intermolecular interaction. Such work is
currently in progress in our laboratory.
Esprrimiw I ril Pr o c ~ d u r e
Positive-tori ES m a s 5pectr:i were obtained on a VG BioQ triple quadrupole apparatus xvith ;I m;i\s-to-charge (n!:) range up to 4000 (VG Bio Tech Ltd. Altrincham. U K ) . The clectrospray source was heated to 50 C. The sampling cone
volkige ( 1 . ) 161 *:is a t 10 V to avoid fragmentation of the assemblies. Sample
solutions were introduced into the mass spectrometer source ujith a syringe pump
(Harvard type 55 I I 1 1 ;Harvard Apparatus Inc.. South Natick. MA. USA) at a flow
rate o f 6 yI.min- I . Cdibration was performed using protonated horse myoglobin.
The resolution bit\ usu;illy about 500 at in:: 1000 (10% height). Scanning was
performed from m :200 to 2200 in 15 s. The data system was operated as a multichannel analyier. and several scans were summed to obtain the final spectrum.
Simples lor E S M S wcre prepared by dissolving equimolar amounts of the component\ in the appropriate solvent (dichloromethane. benzene. pentane) to achieve the
concentrationa listed in Table 1 . Solutions containing salts were prepared by adding
0.5 mg of salt (KPF,,. NaCIO,, CsCI) iii 0.5 mL of solution and sonicating briefly.
The soluti(iii was removed from the excess salt uith a pipette and directly analyzed.
Labeling expel-imenb uere performed by adding the desired ratio of equimolar
solutions of the iiiiictive label (A without KPF,) and the assembly of interest, and
tlicn ;idding KPF,, 'is previously described.
Received: July 9. 1994
Revised version: September 21, 1994 [Z 71 16 IE]
German version: A n g w . Chern. 1995, 107. 244
Keywords: crown ethers mass spectrometry . supramolecular
[21] For studies at the rather high concentration (lo-' M) used i n this work. ionmolecule reactions in the gas phase might occur in the ES source. To be certain
that peaks observed for assemblies A,B,. A,B,, and A$, s e r e not an artifact
of ESMS. we studied a solution in which A was replaced by henLo[lX]crown-h
(18C6). a compound unable to form hydrogen bonds. The only peaks observed
in the ES mass spectrum of this solution corresponded to ions lXCh:K',
(18C6),;Ki and (18C6),:2K'. N o peaks for possible as~cmbliecof B and
macrocycle were observed.
1221 The ionization of the complexes in S , was also examined by adding NaCIO,
and C K I , instead of KPF,. With NaCIO, the results were similar to those
obtained with KPF; adducts of KPF,,'K+ were replaced by adducts of
NaCIO,.";i+. In contrast. with CsCI, A$, was the on14 complex detected. but
in two charge states: [A,B, + 3Cs+]and [A,B, + 2 C s t ] . The difference in the
ionization might be explained by the specific complexation of the alkali metal
ions by the crown ether. Indeed, Cs+ is known to form thc wndwich complex
(18C6),:Cst. while Na' and K' give thecomplexes 18C6 Na' and 18C6:K ' .
[23] Interestingly. A,B, was present only for large amounts of silt iind A,B, only
for smaller quantities of salt.
Gas-Phase Activation of Fluorocarbons by "Bare"
and Coordinated Praseodymium Cations**
Christoph Heinemann, Norman Goldberg,
Inis C. Tornieporth-Oetting, Thomas M. Klapotke,
and Helmut Schwarz*
Dedicated to Projessor J o ~ q Seibl
on the occasion of his 70th i5irthdu.v
[ I ] JM
Lehn. Aw,qw C h m . 1990. 102. 1347: Angew. Chem. 1/11,Ed. Dig/. 1990.
ZY. 1304.
121 G. M. Whiteaides. J. P. Mathias. C. T. Seto. Science 1991. 254. 1312.
[3] C. Fouquey. J:M. Lehn, A. M. Levelut. A h . Mute?. 1990. 2. 254.
[4] T. Culik-Krcrywicki. C. Fouquey. J.-M. Lehn. Proc. N u t / . Acud. Sci. USA
1993. 90. 163.
[5] J.-M. Lehn. :Wrrkrornnl. Chcwi. A4act-omol. Svnip. 1993, 69. 1
[6] a ) A. P. Bruins. T. R . Covey. J. D. Henion. Anul. Che~n.1987,59,2642:b) J. B.
Fenn. M Mann, C . K. Meng. S. F. Wong, C. M. Whitehouse, MQXS
K r r . 1990. 9. 37; c ) A. P. Bruins. ibid. 1991, 10. 53.
171 J. B. Fenn, M. Mann. C. K . Meng. S. F. Wong. C. M. Whitehouse. Scwnce
1989. 146. 64.
[XI J. T. Stults. J. c'. Masters, Rupid Cornmiin. Muss Spectroin. 1991. 5 , 359.
191 a ) V. Katta. S. K. Chowdhury. B. T. Chait, L A m Che~n.Soc. 1990, 112. 5348:
b) F. Bitsch. C. 0. Dietrich-Buchecker, A. K. Khemiss. J:P. Sauvage. A. Van
Dorsselaer. i h d 1991. 113. 4023; c) F. Bitsch. G . Hegy. C. 0 . DietrichBuchecker. E . Leize. J.-P. Sauvage. A. Van Dorsselaer. N w J. Chen?.1994. in
a ) B. Ganeni. Y.T. Li. J. D. Henion. J An?. Cheni. Soc. 1991. 113. 6294; b) ihid.
1991. 113. 781X. c) V. Katta. B. T. Chait, ihid 1991. /13, 8534: d ) M. Baca.
S. B. H Kent. h d . 1992, 114. 3992; e) A. K. Ganguly. 9. N. Pramanik, A .
Twrbopoulos, T. R. Covey. E. Huang, S. A. Fuhrman. ihid. 1992, /14.6559: f)
B. Gaiiein. Y. T. Li. J. D. Henion. Tefruhedrm L e f t . 1993. 34, 1445. g) M.
Jaquinod. E. Lei=. N. Potier. A. M. Albrecht, A. Shanzer. A. Van Dorsselaer.
hid. 1993, 34, 2771 ; h) M . Przybylski. M. 0. Clocker. Angel,. Chen?. 1995, in
E. Leire. A. Van Dorsselaer, R. Krimer. J.-M. Lehn. J Chem. So<. Chm,.
~ ' i m m i i i n 1993.
S. R. Wilson. Q. Lu. M . L. Tulchinsky. Y. Wu. J Cheni. So(. Chrm. Cornnirm.
1993, 664.
S K.Wilson. M. L. Tulchinsky. Y.Wu. Biowg. M r d . Cheni. Lett. 1993.3, 1805.
J.-M. Lehn. M Mascal. A. DeCian. J. Flscher. J Chcwi. SOC.Cham. Coinrnun.
1990. 479.
J. A. Zerkowski. C. T. Seto. D. A. Wierda. G. M. Whitesides. J. Am. Cl7rm.
S J ~ .
1990. 112. 902.5.
J. A. Zerkowski. C. T. Seto. G . M. Whitesides, J. Am. Cheni. Sor. 1992, 114.
0 . Diels. Bvr. Drsch. Chcvn. Ges. 1899. 32. 691
E. Fischer. A. Dilthey. Ju,\tri,s Liehigs Ann. Chmi. 1904. 335. 334.
Compound B also gave satisfactory microanalytical data. Triazines A and C
were not crystalline. and it was not possible to obtain satisfactory microanaly\es
R. D. Smith. J. A. Loo. C . J. Barinaga, G . G . Edmonds. H. R. Udseth. J A m .
~ ' h ( w zS
. o( Mu.~.s.Spc<rron7.1990. 1, 53; V. Katta. S. K. Chowdhury. B. T.
Ch'iit. .41t(1/. ( . / i c w i . 1991. 63. 174.
The activation of the strong carbon-fluorine bond by transition metal complexes is a formidable task in organometallic
chemistry.['] Immense industrial interest in this field is due to the
widespread use of fluorocarbons and mixed fluorohalocarbons,
which takes advantage of their high thermal and chemical stability. Evidence for the involvement of these substances in the
mechanisms of stratospheric ozone depletion has made their
future use questionable, and there is now a need for selective and
efficient chemical conversion. Similar to the growing body of
information on C-H and C-C activation of hydrocarbons,
gas-phase studies on the intrinsic reactivities of "bare" and ligated transition metal cations['] are expected to help in the understanding of the molecular mechanisms in the transition metal
assisted cleavage of a C-F bond. In this vein, we report on the
activation of various aliphatic and aromatic C-F bonds by
"bare" Pr' ions and small praseodymium complexes PrL+
(L = 0, F) as observed by Fourier transform ion cyclotron
resonance mass spectrometry (FT-ICR-MS) . Furthermore, we
present the first observation of neutral praseodymium monofluoride PrF and praseodymium difluoride PrF, in the gas phase
by means of neutralization-reionization mass spectrometry
In solution-phase chemistry, the homolytic cleavage of C - F
bonds in perfluorinated olefins has been achieved with electrophilic divalent lanthanide reagents of the type LnCpT
(Ln = Eu, Sm, Yb; Cp* = C,Me,).[31 As a first example for
[*I Prof. Dr. H. Schwarz. DipLChem. C. Heinemann, Dr. N . Goldberg
lnstitut fur Organische Chemie der Technischen Unlverbitiit
Strasse des 17. Juni 135, D-10623 Berlin (Germany)
Telefax: Int. code +(30) 31421102
Dr. 1. C Tornieporth-Oetting, Priv.-Doz Dr. T. M. Klepbtke
Institut fur Anorganische und Analytische Chemie
der Technischen Universitit Berlin
This work was supported by the Deutsche Forschungsgeineinschaft. the Volkswagen-Stiftung. and the Fonds der Chemischen Industrie. We thank Dr. D.
Schroder and DipILChem. N. Raahe for helpful discus~ionsand technical
intermolecular activation of saturated fluorocarbons, Bergman et al. reported recently that the actinide complex
[(MeC,H,),U(tBu)] can abstract a fluorine radical from a variety of fl~oroalkanes.[~]
In the gas phase the anionic tricarbonylmanganese complex [Mn(CO),]- was shown to activate C- F
bonds,['] and cationic fluorolanthanide complexes LnF'
(Ln = La, Ce, Pr) as well as UF' have been observed in nuclear
fission experiments with 235Uin the presence of CF4.I6' In line
with these observations we find that in ion/molecule reactions of
Pr' and a variety of fluorocarbons, homolytic cleavage of C - F
bonds is a quite general phenomenon (Scheme 1 ) .
Pr+ + CH3F
Pr" + F3C-CF3
PrF+ +
14 x l o - " cm3s-'molecule-'). For comparison, this lanthanide cation is unreactive towards benzene. The reaction with
C,H,F provides a lower limit for the bond strength of PrF',
from which we derive an experimental bracketing value of
127 4 kcalmol-'. In contrast to Pr', the less "fluorophilic"
Fe+ ion promotes the loss of hydrogen fluoride from fluorobenzene, resulting in the formation of the cationic iron- benzyne complex [FeC6H4]'191. In the reaction of Pr' with
hexafluorobenzene (E,(C,F,-F) = 116.6 F 16 kcalmol-'), the
double defluorination pathway resulting in the formation of
PrF; and C,F, predominates over the abstraction of a single
fluorine atom. This experiment supports earlier conjectures
from solution chemistry". 'I that tetrafluorobenzyne, the existence of which is still unproved to the best of our knowledge, is
an intermediate in the activation of hexafluorobenzene by electron-deficient lanthanide complexes.
In the reactions of mixed fluorohalocarbons with Pr'
(Scheme 2), we observed exclusive activation of the weaker
+ CF;
___ --*
P ~ F ++ FHC-CH;
Scheme 2.
PrF+ +
Scheme I . Pr+* represents a Pr+ ion that is kinetically or electronically excited
prior 10 reaction. Thermally excited Pr+ does not react. I ) Because this reaction is
endothermic no relative intensities can be given.
In the simplest example Pr' formally abstracts a
fluorine atom from methyl fluoride (bond dissociation
energy, E,(H,C-F) = I 1 2 2 k 0.3 kcalmol-')['] to form the
PrF' ion. The reaction rate is 16% of the theoretical collision
rate kco,,[81
(k,,, = 3 x 1 0 ~ ' o c m 3 s - ' m o l e c u l e - ' ; k,,,, = 1 9 x
10- l o cm3s-' molecule- '). In the corresponding reaction
with 1,l-difluoroethane (E,(H,CFHC-F) = 113 kcalmol-')
an additional double defluorination pathway gives rise
to PrF: (PrF+:PrF: = 4 : l ) . However, fluorine abstraction
from CF,H
(E,(F,HC-F) = I 2 8 +_ 3 kcalmol-'),
(E,(F,C-F) = 132.4 kcalmol-'), and C,F, (E,(F,CF,C-F) =
127 kcalmol-I) is observed only if the Pr' ions are not properly
thermalized prior to reaction, that is, if they are kinetically or
electronically excited. Since a reliable value for the bond dissocation energy of PrF' is not available. it remains unclear
whether cleavage at the C - F bond is endothermic, whether the
initially formed ion/molecule complex has too short a lifetime
(this may especially apply for the unpolar and unpolarizable
CF, and C,F, molecules), o r whether a substantial activation
barrier is operative. However, assuming that for cleavage of a
simple single bond the barrier should be at most very small, one
may tentatively derive an upper value of 131 kcalmol-' for
E,(Pr+ -F) from the nonobservation of C - F bond activation in
CHF, .
Similar to its reactions with aliphatic fluorocarbons Pr'
cleaves the aromatic C - F bond in fluorobenzene (E,(C,H, F) = 125.3 k 2.3 kcalmol-') at nearly 80% of the theoretical
collision rate (krxp=11 x 1 0 - ' o c m ~ s s - ' m o l e c u l e - i ; k,,,,, =
C-CI or C-Br bonds of the Substrate (e.g. E,(CI,FCF ) =122.7 kcalmol-', E,(CIF,C-CI) = 88.7 kcalmol-I). although C- F bond activation is thermochemically possible.
These experiments provide lower limits of 88.7 and
56.8 kcalmol-* for the bond dissociation energies of Pr'-Cl
and Pr+ -Br, respectively.
Finally, the unpolar and highly unreactive sulfur hexafluoride
molecule SF, is also activated by Pr' in a kinetically efficient process ( k e x p= 7.8 x 10- cm3 s- molecule- ; k,,,, =
7.1 x lo-" cm3s-' molecule-') (Scheme 3). Here we observe,
Scheme 3
in addition to PrF' and PrF;, the sulfur fluoride ions SF: and
SF: as primary reaction products, similar to the results recently
reported for the corresponding reaction of Sc+.['ll The exothermic formation of the SF: species is in accord with their experimental heats of formation['] and appears t o be driven by the
stability of neutral PrF,.
If one follows the commonly accepted view["] that the 4f
electrons of the lanthanide elements are usually chemically inert,
then it is indeed quite remarkable that Pr' shows the observed
reactivity towards fluorocarbons. In particular, the electronic
ground state configuration of this cation" (4f35d06s') should
not be suitable for insertion into a single bond, a process for
which a minimum of two valence electrons (i.e. non-f electrons)
Formally, one may think of a two-electron donation from the metal cation to the o* orbital of the
C-X bond to be activated. In accord with this argument, Pr'
has been shown to be unreactive with most hydrocarbon subs t r a t e ~ . ~'']' ~Since
we have no indication that excited states of
Pr' were not efficiently quenched in our experiments, we propose two possible mechanistic scenarios (Fig. 1) to explain the
observed ground-state reactivity of Pr+ with fluorocarbons.
F 1 < ] --.+ Prod.
- R'
Fig. 1. Mechaiiirms lor C F bond actwation by ground-state Pr' ions. a) Curvecrossing model involving the low-lying 4fzSd2 electronic configuration of Pr'
(Prod. = productr). h ) Single electron transfer (SET) followed by fluorine transfer
in the initially formed ion:molecule complex.
1) A curve-crossing model, favored by the strong spin-orbit
coupling present in the lanthanide series (Fig. 1 a):1131Here.
transitions from the potential energy surface of the Pr+ ground
state to a potential energy surface of an appropriate low-lying
excited state of Pr+ (lowest state with 4f25d2 configuration:
0.70 eV; lowest state with 4f25d16s' configuration:
+0.94 eV1131)can occur due to avoided crossings. Essentially,
this results in a lowering of the activation barrier for the insertion of the ground state metal cation into the activated bond.["]
2) Single-electron transfer (SET) from Pr' to the fluorine atom
of the substrate in the initial ion/molecule complex (Fig. 1 b):
This step is followed by C-F bond cleavage, as postulated in
similar cases in solution chemistry.["
In addition to the reactions of "bare" P r + ions, small
praseodymium complexes PrL' (L = 0, F ) also induce C - F
bond activation in various substrates (Scheme 4). PrF' reacts
P r o + F,HC-CH?4
with C,H,F by abstraction of a fluorine atom, indicating that
the second fluorine atom in PrF: has a bond dissociation energy comparable to the first one (E,(FPr+-F > I 3 kcalmol-I).
However, the secondary processes are often significantly slower
than the primary reactions (e.g. for Pr+ + SF, k =7.8 x
1 0 - * o ~ m 3 ~ - 1 m o l e c u l e - ' ; for
PrF' + SF,
k = Sx
10- ' cm3s- molecule- '). Since PrF' has a open-shell 4f3
electronic configuration," 1' its reactivity is even more surprising
than that of Pr' and indicates that the 4f electrons of lanthanide
ions can in fact be chemically active if the driving force for the
reaction is sufficiently large and solvation effects are absent. We
note in passing that PrF' can be oxidized by N 2 0 to form the
mixed fluorooxo complex PrOF+ with praseodymium in a formal oxidation state of +IV.
Even more interesting is the reactivity of P r o + (generated
from Pr' and N,O), a species with an extremely strong P r + - 0
bond (E,(Pr+-O) = 191 kcalmol-1[71),which is generally unreactive with hydrocarbons. For example, we only observed
formation of an adduct complex when this cationic oxide was
exposed to benzene, while for the less oxophilic FeO' oxygen
transfer to the unsaturated substrate has been reported.["]
However, upon reaction of P r o ' with 1,I-difluoroethane, very
slow (relative reaction rate 1 % compared to the "bare" Pr'
cation) addition of hydrogen fluoride to P r o + takes place
(Scheme 4). In kinetically more efficient subsequent reaction
steps an additional molecule of H F is added to this complex,
and H,O is replaced by one molecule of the substrate. These
observations suggest that the overall reaction proceeds via a
mixed cationic fluorohydroxo complex and the cationic difluoropraseodymium complex, whereby the formerly strongly
bound oxygen ligand becomes part of the loosely bound water
molecule. Elimination of H F from the substrate proceeds presumably by a 1,2 mechanism, since the most promising substrate
for 1,l elimination of HF, trifluoromethane, is not activated
by Pro'. With reference to the theoretical analysis of other
surprisingly slow ion/molecule reactions involving transition
metal 0x0 cations.['61 the very small reaction efficiency in the
PrO+/l,l-difluoroethane system suggests that a spin-orbitmediated curve-crossing mechanism (Fig. 1 ) may be operative
in this case as well.
While PrF: is unreactive with CF, and SF,. we observed
formation of adduct complexes with water, fluorobenzene. and
1,l -difluoroethane (Scheme 5). Collision-induced dissociation
of the F2Pr+-F,HC-CH, adduct complex at low energies leads
only to PrF: . At higher collision energies the appearance of the
C,H,F+ ion (accompanied by formation of the neutral PrF,)
suggests that in the adduct 1,l-difluoroethane coordinates to the
cationic transition metal center through a fluorine atom. These
observations point to results from solution-phase chemistry,
where precoordination of the substrate molecule to the transi-
Schemc 4
Scheme 5. n
tion metal center prior to activation of the C - F bond has been
deduced from solvation effects.l3I In addition to adduct formation, P r F l also induces loss of hydrogen fluoride from this
substrate, resulting in the formation of the CHF=CH, - P r F l
As a general conclusion we note that the strength of the
praseodymium -fluorine bond is apparently the driving force in
the reactions of the praseodymium cation with different fluorocarbons. Overall, Prf activates exclusively C - F and not C-H
or C-C bonds. However, as already stressed in a recent review,['] the unfavorable thermodynamics of these systems
severely restrict the design of cafa/.)itic cycles['b1 for C- F bond
The stability of rare earth fluorides is not restricted to the
most common +HI oxidation state, as shown by the characterization of various lanthanide subfluorides by spectroscopic techniques["' and their preparative-scale synthesis.[2o] However,
neutral praseodymium(1) fluoride and praseodymium(r1) fluoride are unknown in the condensed phase, and solid lanthanide(1r)fluorides with exact LnF, stoichiometry have only
been characterized for Ln = Sm, Eu, Yb, and presumably
Tm.[201Among the lanthanide fluorides studied in the gas
phase,[17d%191 neutral P r F and PrF, are still missing. In this
context, we have applied the technique of neutralization-reionization mass spectrometry (NRMS)[211to PrF' and P r F l . The
NRMS spectra (Fig. 2) give clear evidence that neutral
Fig. 2. Neutraiization -reionization mass spectra (NRMS) of PrF' (top) and PrF:
(bottom). The signals labeled PrF+ (top) and PrF: (bottom) are recovery signals.
praseodymium mono- and difluoride are in fact stable chemical
species, at least under gas-phase conditions.
Experimen fnl Procedure
lonimolecule reactions under thermal conditions were performed using a Spectrospin CMS-47X Fourier transform ion cyclotron resonance mass spectrometer
equipped with an external ion source (for details see ref. [22]). I4'Pr+ ions were
generated by laser desorptionkuer ionization by focusing the beam of a Nd:YAG
laser (Spectron Systems; i.
= 1064 nm) onto a piece of pure praseodymium metal
(Strem. 3N REO). The ions were transferred to the analyzer cell by a system of
21 6
i"VCH Vi.rlugsgrsell.sch~rfrffm h H , 0-69451 Weinheirn, 1995
electrostatic potentials and lenses, decelerated, trapped in the field of a superconductive magnet (Oxford Instruments. maximum field strength: 7.05 T), and thermalized with pulsed-in argon (maximum pressure cd. 5 x
mbdr). Reactants
were admitted to the cell by means of a leak valve at a stationary pressure of
(1 - 5 ) x
mbar (as measured by an uncalibrated ion gauge. BALZERS
IMG070). Reaction products were analyzed by their high-resolution mass spectra,
and their pathways were investigated by using MSjMS and double-resonance techniques. All rate constants were derived from the pseudo-first-order decay of the
starting ion and are reported with an estimated accur
For the neutralization- reionization (NRMS) experiments a large-scale four-sector
tandem mass spectrometer was used [23]. The P r F + ions were produced
by fast-atom bombardment (FAB) of PrF, (241, whereas PrF: was generated
in a chemical ioniralion source by ionizing tris(hexafluoroacety1acetonato)praseodymium (Strem) in the presence of NF,. The mass-selected ions were neutralimd in the first part of a tandem collision cell upon collisions with xenon (80%
transmission. T). [ons that did not undergo neutralization in this region were detlected from the beam of neutral molecules by an electrode (charged at 1000 V)
situated between the two collision cells. Neutralized molecules were reionized by
colliding them with oxygen (80% T). The minimal life time I , which can be derived
from the transit t h e of the neutral particles to travel from the first to the second
part of the tandem-collision cell, is of the order of 10 ps.
Received: July 6. 1994 [Z 7097 1E]
German version: .4ngew. Chem. 1995, 107. 225
Keywords: fluorocarbons . lanthanide compounds mass spectrometry
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c) H. H. Cornehl, C. Heinemann, D. Schroder. H. Schwarz, 0rgunumetullrc.s.
in mess.
(161 For a recent analysis of a spin -orbit-niedtated curve-crossing mechanism. see.
a) D. Schroder, A. Fiedler. M. E Ryan, H. Schwarz, J. Phj.s. Chrm. 1994, 98,
68: b) A. Fiedler. D. Schroder. S. Shaik. H. Schwarz, J. Am. Chem. Soc., in
[17] a) 1. Gotkis. J. P h ~ s Cltrm.
1991, 95, 6086; b) C. Heinemann. H. Schwdrz.
unpublished theoretical results.
[lX] a) D. Schroder, H. Schwarz, H e / i S .Clrinz. Actu 1992, 75, 1281; b) H. Becker. D.
Schroder. W. Zummack, H. Schwarz, J. .4m. Cheui. Soc. 1994, 116, 1096.
[19] For a review on comprehensive mass-spectrometric studies, see: K . F. Zmbov.
J. L. Margrave. Muss Specfronierry iw Inorgunic C I w m i ~ ~(Ed.:
r j ~ J. L. Margrave) A d e . Cliern. Ser. 1968, 72, 267.
0570-083319210202-0216 $ 10.00 f ,2510
Angew. Chem. Inf. Ed. Engl. 1995, 34, No. 2
[XI] Gindni Handhod I!/ /norgunk Chmiisry, N o . 39 C , Springer. Berlin 1976.
[21] NRMS has been successful in probing the existence of elusive inorganic
Cheni. 1993. 97,
inolecules: a ) M. Iraqi, N. Goldberg. H. Schwarz. .lP/?J.Y.
11 371: b) D. Schrdder, J. HruSik, I. C. Tornieporth-Oetting. T. M. Klapotke,
H . Schwiirz. ,Angwi~.Chein. 1994, /Oh. 223; Angew. Cliem. / n t . Ed. €nx/. 1994.
33. 212. c) N . Goldberg, M. Iraqi. H. Schwarz. A. Boldyrev. J. Simons. J.
C/IWI./'/I).\ 1994. /(I/. 2871; d j D. Schroder, A. Fiedler, H. Schwarz. h i t . J.
1994. 134.239; e j for a recent review on NRMS,
see N. Coldhcrg. H. Schwarz, At-c. Cheni. Res. 1994. 27. 347.
1221 a ) K Eller. H . Schwai-z, In/.J. M u I . ~Spcr
. r nni. /on Procrssr~1989. 93. 243;
h ) K. Ellrr. W. Zummack. H. S c h w r z . J. A m . Cheni. Soc. 1990. 112. 621.
[23] For ,I description of the apparatus. see: a) R. Srinivas. D. Sulze. T. Weiske. H .
Schwnrz. I n / . .I .Mn.s.s Spwtroni. /on 1990, 107, 369; b) R. Srinivas.
D. S i i l ~ c .W. Koch. C. H. DePuy. H. Schwarz. J. Ani. C/wm Sot-. 1991. 113.
S970. c ) R. Si-iiuvas. D. K . Bohme. D. Sulze. H. Schwarz. J. Piim Cilcw?.1991.
95. 9836.
[24] Sqntheais of praaeodyiniuin tritluoride from the elements: 1. C. TornieporthOerting. T. M. Klapotke. unpublished results (for an earlier synthesis. see:
E L Muettri-tier. L. E. Castle. J. h o r x . Nut-/. Cheni. 1961, 18. 118).
Scheme 1. Diels-Alder reactio? between diene 2 and dienophile 3 to give E S O adduct 4 and endo adduct 5. The structures of 4 and 5 were distinguished unamhiguously by NOE difference spectroscopy.
Free-Energy Profile for a Host-Accelerated DielsAlder Reaction: The Sources of e m Selectivity**
Christopher J. Walter and Jeremy K. M. Sanders*
The use of free-energy profiles['] for enzyme-catalyzed reactions has transformed the study of enzyme mechanisms. because
the dependence of the height of the energy barrier on substrate
variation and on enzyme mutation allows one to dissect out
rationally the important contributions to binding and catalysis.[21We have now applied this approach to enzyme mimics as
exemplified by the porphyrin trimer 1I3l and have examined its
em-selective acceleration of the Diels-Alder reaction shown in
Scheme 1.[4. There are as yet few effective enzyme mimics for
bimolecular reactions, because the design rules governing their
operation are not understood;[', 'I in particular the importance
of Factors such as substrate strain. host flexibility, solvation, and
[*] Dr. J. K M kinderr. Dr. C. J. Walter
Cambridge C'entre for- Molecular Recognition
UniVel-Sitf Chemical Laboratory
Lensfield Road. GB-Cambridge CB2 IEW (UK)
Telelhx: In[ code +(1223j 336913
e-mdil : JK M SXI
I**] We thank the Science and Engineering Research Council for financial support
and Dr. R. S. Wylie for valuable advice.
quality of fit is not yet clear. We have begun to uncover the rules
for our hosts by elucidating some of the binding and kinetic
parameters for the reactions in Scheme 1, and in doing so have
acquired some insight into the exo selectivity of 1. We also
demonstrate the accelerated conversion of endo adduct to exo
adduct by 1.
The Diels-Alder reaction has rich stereo- and regiochemistry,
stringent geometrical requirements, and no need for external
reagents, and it has been the subject of related studies using
catalytic antibodies.['] The reaction also offers the possibility of
altering stereo- and regiochemistry through controlling the
transition state within a cavity. We have used a furan-derived
diene and maleimide-derived dienophile, because this combination reacts reversibly,[g1giving the opportunity of studying the
kinetics, and therefore the approach to the transition states, in
both the forward and reverse directions.["] The reaction in
Scheme 1 was inhibited by strongly bound products; this could
have been avoided by engineering a second step,[*] but the
apparent disadvantage of lack of turnover is outweighed by
the direct access to the transition state energies available
from a study of the unimolecular reverse reactions as described
The equilibrium constant for the reaction shown in Scheme 1
is quite large ( k , (e.uo)/k-, (exo) is approximately 5000 M - ~in
C,H,CI, at 30 "C, decreasing to 400 at 60 "C), but the equilibrium at millimolar concentrations lies on the side of the starting
compounds. The relative orientations of the pyridine groups in
the two adducts are very different, and molecular models suggested that the endo adduct would fit less well into the cavity of
the trimer than the exo adduct would. The measured binding
constants confirm this; the exo adduct binds around 15 times
more strongly in C,H,CI, at 30 "C (9 x lo6 M - vs 6 x l o 5 M - ')
and six times more strongly at 60-C (4.6 x l o 5 M - ' vs
8 x lo4 M - I ) . In the absence of porphyrin trimer, the endo adduct obtained by kinetic control is a significant product at low
temperature, while the exo adduct obtained by thermodynamic
control is the only product detected at high temperatures; at
60 "C the cxolendo ratio is roughly 4 initially, but the endo adduct then disappears.
Addition of one equivalent of trimer to the two reactants
(0.9 mM each in C,H,CI,) accelerates the forward Diels-Alder
reaction around 1000-fold at 30 "C and 200-fold at 60 'C, yielding the exo adduct as the only detectable product.[4,51 The initial
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