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Extension of Free Energy Correlations to Gas-Phase Ionic Reactions. Competitive Alkylation of Substituted Benzonitriles by (CH3)2Cl+ Ions

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quest from the Fachinformationszentrum Karlsruhe, Gesellschaft fur
wissenschaftlich-technische Information mbH, W-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD-320276, the
names of the authors. and the journal citation.
S. Harder, J. Boersma, L. Brandsma, J. A. Kanters, W. Bauer, R. Pi, P. von
R. Schleyer, H. Schollhorn, U. Thewalt, OrganometaNics 8 (1989) 1688.
On the basis of model calculations on intramolecular solvation in monomeric 2-furanyllithium, Harder concluded that the 0-C bridging through
Li is considerably overestimated and is overcome by solvent molecules
(three OH, on Li); I thank Dr. S. Harder, University of Utrecht, for
sending us his dissertation (1990). This result, however, is not directly
comparable with the crystal structures of the dimeric [ l a . OiPr,l, and
[2-lithiobenzofuran . TMEDA], [IS], since in the calculated monomer Li
lies in the plane of the furan ring, while in the crystalline dimers the Li
atoms lie above and below the planes of the benzofuran rings (private
communication, A . Harder, September 7, 1991).
The average value of the bond lengths corresponding to 01-C1 (01-C8) is
138.5 (1 37.1) pm for 27 benzofurans (Cambridge Structural Database).
M. Marsch. K. Harms, 0. Zschage, D. Hoppe, G. Boche, Angew. Chem.
103 (1991) 338; Angew. Chem. Int. Ed. Engl. 30 (1991) 321.
T. Clark, P. von R. Schleyer, K . N. Houk, N. G. Rondan, J. Chem. SOC.
Chrm. Commun. 1981,579; P. von R. Schleyer, T. Clark, A. J. Kos, G. W.
Spitznagel, C. Rohde, D. Arad, K. N. Houk, N. G. Rondan, J. Am. Chem.
Soc. 106 (1984) 6467.
C. Riche. Acta Crystallogr. Sect. B29 (1973) 756.
Average value of C-Br for 1121 bromarenes: 189.7 pm (Cambridge Structural Database).
Preparation of 3 PMDETA: 3-(fluorophenyl)phenylsulfone (150 mg,
0.63 mmol) was dissolved in 0.5 mL tetrahydrofuran, deprotonated at
- 75 “C with 0.4 mL (0.64 mmol) of a 1.6 M solution of nBuLi in hexane,
and then treated immediately with 0.2 mL of PMDETA. After 30 min, the
reaction solution was warmed to 20°C and 1 mL of hexane was added.
After 6 h at O’C, crystals suitable for X-ray structural analysis were obtained. ‘H NMR ([DJTHF, 258 K) 6 =7.92 (d, J = 7 Hz, 2H), 7.50 (m,
3H). 7.34 (dd, J(’H, ‘H) = 7 Hz, J(’H, I9F) = 2.6 Hz), 6.98 (dt, J(’H,
‘H) = 7H z,J ( ‘ H , 19F) = 7H~),6.72(d,J=7Hz,lH),2.53(~,4H;CH,PMDETA), 2.52 (s, 4 H ; CH2-PMDETA), 2.44 (s, 3H, CH3-PMDETA),
2.23 (s. 12H, CH,-PMDETA). ‘,C-NMR ([DJTHF, 253 K) 6 = 176.2 (d,
’J(C,F) = 133.3 Hz; C2), 172.2 (d, ‘J(C,F) = 217.5 Hz; C3), 153.9 (d,
,J(C.F) = 39.0 Hz;Cl), 145.0 (C7), 132.6 (CIO), 129.4 (C9-Cll), 128.0
(C8-C12), 126.2 (d, 3J(C,F) = 5.0 Hz; C5), 122.4 (d, 4J(C,F) = 3.8 Hz;
C6). 114.2 (d, ’J(C,F) = 42.7 Hz; C4), 57.7, 55.1, 46.1, 44.4 (PMDETA).
For comparison: ”C NMR spectrum of 3-(fluorophenyl)phenylsulfone
([DJTHF, 253 K): 6 = 163.2 ( d , ‘J(C,F) = 250.3 Hz; C3), 145.2 (d,
’4C.F) = 6.3 Hz; Cl), 142.3 (C7), 134.4 (ClO), 132.5 (d,
’J(C,F) =7.8 Hz; C5), 130.3 (C9-Cll), 128.6 (C8-C12), 124.6 (d,
4J(C.F) = 3.4Hz; C6), 121.2 (d, ’J(C,F) = 21.4 Hz; C4), 115.5 (d,
’J(C,F) = 23.9 Hz, C2). Particularly noteworthy is the difference in the
’J(C2.F) coupling in the “anion” 3.PMDETA (133.3 Hz) and in the
neutral compound 3-(fluorophenyl)phenylsulfone (23.9 Hz). Harder, in
his dissertation [19], described this phenomenon for the first time in 1990
for several other fluorinated pairs of compounds and explained it in terms
of a through-space interaction of fluorine with the “lone pair” of the C-Li
bond. Related couplings have been explained in a similar way by E B.
Mullorv, C . W. Mallory und W. M . Ricker (J. Org. Chem. SO (1985) 457).
The ’J(Cl,F) coupling in the “anion” is also markedly larger (39.0 Hz)
than in the neutral compound (6.3 Hz); not quite as large is the difference
in the ’J(C4,F) couplings: “anion”, 42.7 Hz; neutral compound: 21.4 Hz.
The C2-Li coupling could not be resolved in the temperature range 198253 K, a result also found by Harder for 2-fluorophenyllithium at lower
temperatures 1191.
Crystallographic data for 3 . PMDETA: C,,H,,FUN,O,S, M = 415.5,
monoclinic, space group P2,/n, a = 889.4(3), b = 1997.2(2), c =
1304.6(4)pm, p = 101.34(1)”, Z = 4, e,,.,. = 1.215 gem-,, @(CU,, radiation) = 14.62cm-’. Measurement carried out an Enraf-Nonius CAD4
= 1.54184 A, graphite monochromator,
diffractometer (Cu,, radiation, i.
T = 190 K ) ; 3137 measured reflections, of which 2807 were unique
(R,,, = 0.0667) and 2579 with F > 4 4 F ) were regarded as observed. Solution by direct methods and refinement with the Semen’s SHELXTLPLUS(VMS) program package, R = 0.0636, wR = 0.0693 (ns= l/a2(F)),
all non-hydrogen atoms refined anisotropically, H atoms refined in a
“riding” model with groupwise common isotropic temperature factors [15,
161. The measurement data were corrected empirically with the program
DIFABS [27]. Further details of the crystal structure investigation are
available on request from the Fachinformationszentrum Karlsruhe,
Gesellschaft fur wissenschaftlich-technische Information mbH, W-7514
Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number
CSD-320305, the names of the authors, and the journal citation..
N. Walker, D. Stuart, Acta Crystallogr. Sect. A39 (1983) 158.
a) W. Hollstein. K. Harms, M. Marsch, G. Boche, Angew. Chem. 99 (1987)
1279; Angew. Chem. Int. Ed. Engl. 26 (1987) 1287. This article contains
comprehensive references o n the directing effects of RSO, groups and on
the complex-induced proximity effect; b) The energy difference between
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 11
conformation 5, which is the global minimum, and 6 ( A E =
38.7 kcalmol-’ ; 3-21G*) provides an indication of the “stabilization” of
Li by the neighboring phenylsulfonyl group in [ l a OzPr,],.
[291 A. Schmuck, P. Pyykko, K. Seppelt, Angew. Chem. 102 (1990) 211 ;Angew.
Chem. Int. Ed. Engl. 29 (1990) 213.
[30] H. B. Biirgi, J. D. Dunitz, Acc. Chem. Res. 16 (1983) 153, and references
cited therein.
Extension of Free Energy Correlations to
Gas-Phase Ionic Reactions. Competitive
Alkylation of Substituted Benzonitriles by
(CH,),Cl+ Ions **
By Marina Attina, Fulvio Cacace,* and Andreina Ricci
The study of structure-reactivity relationships is greatly
complicated in solution by the effects of the reaction medium, which are quantitatively reflected in the Hammett equation and in other free energy correlations by the strong dependence of the reaction constant on the nature of the
solvent.[’] The substantially reduced impact of such complicating factors in gaseous media is a strong incentive to extend the study to the gas phase, especially in the case of
charged species whose reactivity is deeply affected and diversified by the effects of solvation, ion pairing, viscosity, etc.
So far, experimental approaches to the problem have met
with considerable problems, since in the low-pressure range
accessible to mass spectrometry operation of a unique “electrostatic activation” mechanism prevents ion-molecule reactions from obeying thermal kinetics, a necessary prerequisite
for the construction of free energy correlations. In fact, without efficient collisional thermalization, the energy released
by the electrostatic interaction of the reactants remains
stored in the ion-molecule complex formed and is available
to overcome the activation barrier of the reaction.[2.31 This
explains why most exothermic ion-molecule reactions have
been found to proceed at the collision rate, often with negafive temperature coefficients, when investigated by massspectrometric techniques.
Here we report the construction of a free energy correlation in a typical gas-phase ionic reaction by a different approach, based on the combination of mass-spectrometric
and high-pressure radiolytic techniques successfully exploited in the study of thermal ion-molecule kinetics.l4]
Since it was deemed important to follow in the gas phase
exactly the same procedure adopted in solution for the
derivation of the original Hammettt5] equation, a suitable
ionic process was sought, involving a side-chain reaction
center bound to a meta- or para-substituted aromatic ring.
The reaction chosen [Eq. (a)], the counterpart of the Ritter
reaction long known in solution[6]and previously investigat-
Prof. Dr. F. Cacace, Dr. M. Attina, Dr. A. Ricci
Dipartimento di Studi di Chimica e Tecnologia delle Sostanze
Biologicamente Attive
Universita di Roma “La Sapienza”
P.le A. Moro, 5, 1-00185 Rome (Italy)
[**I This work was supported by the Italian Minister0 dell’universita e della
Ricerca Scientifica e Tecnologica (MURST) and the National Research
Council (CNR).
Verlagsgesellschafi mbH, W-6940 Weinheim, 1991
0570-0833~9ljllll-14573 3.50+.25/0
ed in the gas phase,['] involves N-alkylation of substituted
benzonitriles by (CH,),CI+ ions, obtained upon ionization
of gaseous CH,CI according to a well-established[*]reaction
+ (CH,),CI+
Even in the absence of specific thermochemical data for all
the substrates involved, it can be asserted with confidence
that reaction (a) is generally exothermic (e.g., a AH' value of
- 163 kJmol- can be estimated for X=H[91).The nitrilium
ions 1 can subsequently be captured by a gaseous nucleophile (e.g., water), eventually yielding the corresponding N methylbenzamides [Eq. (b)].
yields upon addition of a gaseous base which intercepts the
(CH,),CI+ ions (see Table 1).
The results of the competition experiments conform to a
Hammett-type equation, lg k/ko = pa+, characterized by a
reaction constant p = - 1.9, a correlation of 0.98, and a zero
intercept (Fig. 1). The quality of the correlation is quite reasonable if one considers that the (T' constants used are the
ones derived from conventional solution kinetics['] and their
values may be somewhat different in a gaseous reaction
medium. Furthermore, the largest deviation from linearity,
observed for the OCF, substituent, may reflect, at least in
part, the uncertainty of its (T+ constant measured in solution.
+H 0
1 4XC,H,C(~H,)NCH,
lg -
- BH
Chemical ionization (CI) mass spectrometry of appropriate gaseous mixtures demonstrates the occurrence of processes (a) and (b), in particular the CH,CI/CI spectra of
benzonitriles diplay ions 1 as major charged adducts.
Whereas the CI experiments allow the mechanistically informative detection of the charged reactants and intermediates, they are not equally useful from the kinetic standpoint;
that is, no meaningful selectivity data could be obtained
from the competitive alkylation of nitriles under the conditions (p(CH,CI) = 67 Pa, T = 330 K) prevailing in the CI
ion source.
The radiolytic experiments, representative examples of
which are illustrated in Table 1, involved y irradiation of
CH,CI gas (100 kPa, 320 K) containing traces of the nitriles
and of water, followed by analysis of the neutral end products by GC and GC/MS. The results show that indeed Nmethylbenzamides are obtained in high yields, accounting
for up to 30% of the radiolytically formed (CH,),CI+ ions,
depending on the composition of the irradiated systems,
where the nitriles compete with other nucleophiles (e.g., the
water vapor added to trap the nitrilium ions) for the
(CH,),C1+ ions. The ionic character of the alkylation is ensured by a large excess of an effective radical scavenger (0,)
and independently demonstrated by the depression of the
Fig. I . Hammett-type plot of the relative reactivity ofsubstituted benzonitriles,
XC,H,CN, toward gaseous (CH,),Clt ions in CH,CI gas at 100 kPa, 320 K.
The selectivity of (CH,),CI characterizes the gaseous reagent as a typical, if rather indiscriminate, electrophile, as
shown by the small and negative value of its p constant.
As to the general problem addressed in this study, it seems
fair to conclude that the results are definitely encouraging. In
fact, a typical ion-molecule reaction, whose selectivity is inappreciable under mass-spectrometric conditions, when
studied in a pressure domain where thermal kinetics are
obeyed, lends itself to the construction of a free energy correlation analogous to those established in solution.
Extension of the approach outlined here to other reactants, and to different reference reactions, can be a useful
step toward unified structure-reactivity relationships, unaffected by the complicating effects of the medium that influence ionic processes in solution, and hence amenable to direct correlation with theoretical results.
Table 1. Methylation of benzonitriles, XC,H,CN, by (CH,),Cl+ ions in CH,CI at 320 K, 100 kPa.
Partial pressures of the components [Pa] [a]
H2 0
41 1
Yields [nrnolJ-'] [b]
[a] All systems contained 0, (1.33 kPa) as a radical scavenger. SF, (0.667 kPa) was used in certain irradiations as a thermal-electron scavenger. [b] Standard deviation
f 10%. [c] Not detectable.
Verlagsgesellschaft mbH, W-6940 Weinheim, 1991
0570-0833I9lillll-1458 $3.50+ .2SiO
Angew. Chem. Ini. Ed. Engl. 30 (1991) N o . 1 1
Experimental Procedure
Pyrex ampules containing premixed gaseous systems of the desired composition. preheated for 22 h at 320 K to ensure complete evaporation of relatively
involatilenitriles,wereirradiated(9.6~lo' Gy, 1.9 x lo4 Gyh-')inaGammacell (Nuclear Canada Ltd.), equipped with a thermostatic device, at 320 K,
suitable blanks being run to rule out the occurrence of "dark" reactions. The
products were analyzed by GC and GC/MS, using a HP 5970 B mass-selective
detector and a TRIO l/QMD 1000 (VG Micromass). The C1 measurements
were carried out in CH,CI at 67 Pa, 330 K, using a HP 5982 A quadrupole
instrument or a VG Micromass ZAB-2F magnetic spectrometer.
Received: May 31, 1991 [Z4665 IE]
German version: Angew. Chem. 103 (1991) 1527
CAS Registry numbers:
p-CH,%C,H,CN, 104-85-8; m-CH,C,H,CN,
620-22-4; p-C,H,C,H,CN,
25309-65-3: p-i-C,H,C,H,CN.
13816-33-6; p-CF,C,H,CN, 455-18-5; mCF,C,H,CN. 368-77-4; m-OCF,C,H,CN, 52771-22-9; C,H,CN, 100-47-0;
(CH,),CI'. 24400-15-5.
[l] R. Taylor: Electrophilic Aromafic Subsrirurion, Wiley, Chichester 1990, and
references therein.
[2] W. N. Olmstead. J. L. Braumann. J Am. Chem. Soc. 99 (1977) 4219.
[3] T. F. Magnera, P. Kebarle in A. Ferreira (Ed.): Ionic Processes in rhe Gus
Phuse. Reidel. Dordrech 1983, and references therein.
[4] F. Cacace, Ace. Chem. Res. 21 (1988) 215, and references therein.
[5] L. P. Hammett. J Am. Chem. Soc. 59 (1937) 96.
[6] J. J. Ritter. P. P. Minieri, J Am. Chem. Soc. 70 (1948) 4045.
[7] F. Cacace. G. Ciranni, P. Giacomello, J. Am. Chem. Soc. 104 (1982) 2258.
[S] J. L. Beauchamp, D. Holz. S. D. Woodgate, S. L. Patt. J Am. Chem. Soc. 94
(1972) 2798.
[9] Estimated from the methyl-cation affinity of CH,CN and the proton affinity of CH,CN and C,H,CN, according to C. A. Deakine, M. Meot-Ner
(Mautner). J. Phys. Chem. 94 (1990) 232.
PMDETA] (1; PMDETA = (Me,NCH,CH,),NMe) from
the metalation reaction of Ph,SnH with nBuLi in the presence of PMDETA and its characterization in solution and in
the solid state. In the crystal, 1 is monomeric (n = 1) and
contains a Sn-Li bond between the pseudotetrahedral Ph,Snanion and the (PMDETA)Li+ cation. This is the first simple
triaryl- or trialkylstannyllithium reagent to be structurally
characterized and the first clear identification of a Sn-Li
bond, or indeed, we believe, of any bond between tin and an
alkali metal or alkaline-earth metal. Cryoscopy measurements
show that 1 is similarly monomeric in solution. The maintenance of Sn-Li bonding is proved by the first observation, in
low-temperature 7Li NMR studies, of direct ' ' 17Sw7Li
Addition of nBuLi to a chilled solution of Ph,SnH and
PMDETA in toluene (1 :1 :1 equivalents) leads to the formation of a white precipitate. This dissolves on heating and,
from the clear yellow solution produced, colorless crystals of
1 were grown in good yield (see Experimental Procedure).
Crystals of 1, mounted directly from the mother liquor,
were investigated by X-ray diffra~tion.'~]
The structure
(Fig. 1) is that of a monomer, [Ph,SnLi . PMDETA]. The
two crystallographically independent molecules within the
asymmetric unit differ essentially only in the stereochemistry
of the coordinated tridentate amine ligand. The main feature
Observation of a Direct Sn-Li Bond;
The Crystal and Molecular Structure of Monomeric
IPh,SnLi PMDETA] and the Detection of
1 1 9 , 1 1 7 ~ 'Li
~ - NMR Coupling in Solution""
By David Reed, Dietmar Stalke, and Dominic S . Wright *
Stannyl-metal reagents (R,SnM, R = alkyl or aryl; M =
Li, Na, K, MgX, X = halogen) have been used widely by
organic chemists for some years.['] Such reagents have been
prepared prior to use by low-temperatye in situ reactions of
organotin halides (R,SnX), hexazrganoditins (R,SnSnR,), or
organotin hydrides (R,SnH) with the metals (M) or organometallics (RM).['I They are particularly useful in the formation of tin-carbon bonds by stannylation of electrophilic substrates (e.g., ketones, epoxides, and vinyl halides) and many
of these reactions occur with control of regio- and stereoisomerism.[2]However, despite such synthetic utility, even the
simple and widely employed trialkyl and triaryl derivatives
have not been systematically studied in solution and the solid
state. Reflecting such lack of data on these species, there has
been some controversy as to whether the lithium ion is bound
to the tin or whether a solvent-separated ion pair is present.[2*31We describe here the synthesis of [Ph,SnLi .
[*] Dr. D. S. Wright, Dr. D. Stalke
University Chemical Laboratory
Lensfield Road, GB-Cambridge CB2 1EW (UK)
Dr. D. Reed
Department of Chemistry,
University of Edinburgh, GB-Edinburgh EH9 335 (UK)
[**I This work was supported by Gonville and Caius College Cambridge (Research Fellowship for D . S . W.) and by the DAAD (NATO Scholarship for
D. S.).
Angew. Chem. lnt. Ed. Engl. 30 (1991) No. 11
0 VCH Verlagsgeseilschafr mbH,
Fig. 1. The molecular structure of both monomer molecules of 1 in the asymmetric unit. Hydrogen atoms have been omitted for clarity. Selected distances
[A] and angles ["I: Li(1)-Sn(1) 2.861(7), Li(2)-Sn(2) 2.882(7), Sn(1)-C(1)
2.221(4), Sn(l)-C(7) 2.210(4), Sn(l)-C(13) 2.212(3), Sn(2)-C(19)2.205(4), Sn(2)C(25) 2.210(4), Sn(2)-C(31) 2.180(4); C-Sn-C 96.1(2) (average): Li-Sn-C
120.7(2) (average).
is the clear occurrence of a Sn-Li bond (Sn-Li, average
2.817(7) A). This bond is only slightly longer than expected
for pure covalent bonding (sum of covalent radii of Sn and
Li, ca. 2.74 A). Although a considerable number of alkali
and alkaline-earth stannate complexes have been structurally
characterized, none of these contain metal-Sn contacts;
rather, these are ion-separated species or else bridging heteroatoms (e.g., 0, P, C1) hold the metal and Sn centers together. The closest analogue to 1 is the only other lithium
triorganostannate thus far characterized in the solid, [Li(dioxane),]' [Sn(furyl), . Li(furyl),Sn]- [furyl = -C(CH),O],
the anion of which consists of two pyramidal Snffuryl); ions
linked via their furyl 0 atoms to a central six-coordinate
Li center; there is no obvious Sn-Li ~ 0 n t a c t . I Also,
within the structure of the trialkoxystannate, [Li(p-2,6-
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reaction, alkylation, energy, benzonitriles, gas, competition, ions, phase, correlation, extension, free, ioni, ch3, substituted, 2cl
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