42
J . A m . Chem. SOC.1988,110, 42-46
Gas-Phase Acid-Induced Nucleophilic Displacement Reactions. 8.' Structural Analysis of Intermediate Ions Formed by the Decomposition of P-Arylethyl Onium Ions Simonetta Fornarini,*ZaCinzia Sparapani,2band Maurizio Speranza*2c Contribution from the Istituto di Chimica Farmaceutica, Universitci di Roma "La Sapienza", 00185 Rome, Italy, Istituto di Chimica Nucleare del CNR, Monterotondo Stazione, 00016 Rome,
Italy, and Dipartimento di Scienze Chimiche, Universitd di Camerino, 62032 Camerino, Macerata. Italy. Receiued March 30, 1987 Abstract: The existence of the stable unsubstituted ethylenebenzenium ion 3 as a stable gaseous species is inferred as occurring in the reaction pathway induced by the attack of radiolytically formed gaseous acids GA+ (GA+ = D3+, C,H5+ ( n = 1, 2). and CH3FCH3+).on P-phenyl-Y-ethanes (Y = F, C1, and OH). Neutral product analysis allows use to define the detailed reaction mechanism and to extend previous conclusions concerning adjacent phenyl group participation in the nucleophilic displacement process, which takes place in competition with 1,2-H migration. Occurrence of alternative cyclic structures, e.g., 7, in the participation step is ruled out on the grounds of results of specificallydesigned radiolytic experiments. The mechanistic picture that characterizes neighboring phenyl-group participation in cationic nucleophilic substitutions occurring in the gas phase presents interesting analogies with related processes occurring under solvolytic conditions and in superacidic media. The results from the present gas-phase radiolytic approach are discussed and contrasted with pertinent mass spectrometric data and theoretical predictions.
In the preceding papers of this series, the dependence of the structural and stereochemical features of gas-phase acid-induced nucleophilic displacement at saturated carbon upon the presence and the nature of nucleophilic groups adjacent to the reaction center has been thoroughly evaluated by a radiolytic method.' In particular, the almost complete stereospecificity characterizing these processes when occurring on erythro- ((ret/inv) 1 160)and threo-3-phenyl-2-chlorobutane (5 < (ret/inv) < 48)suggests very efficient anchimeric assistance of the vicinal phenyl moiety, in qualitative analogy with related substitutions in ~ o l u t i o n . Ex~ tensive phenyl-group participation has been inferred as well in cationic nucleophilic substitutions occurring at primary carbon, based upon structural analysis of the neutral isomeric products formed in these processes from isomeric P-phenyl-Y-propanes (Y = CI, OH). The intermediates involved in such anchimerically assisted nucleophilic displacements, namely the alkylenebenzenium ions 1 and 2, are found to be relatively stable species in the gas phase (1 55-760 Torr), displaying a scarce tendency for isomerization to the corresponding most stable open-chain structure^.^ The same methodology is now extended to the assessment of a long-standing structural problem in gas-phase ion c h e m i ~ t r y , ~ concerning the occurrence of the ethylenearenium ions 3 as well as of other conceivable bridged isomers as stable species in the diluted gas state and the evaluation of structural effects, if any, on their stability. To this end, we selected a set of gaseous Brernsted (GA' = D3+,C,H5+( n = 1, 2)) and Lewis acids (GA' = C2H5+,CH3FCH3+),obtained in known yields by y-radiolysis of their appropriate precursors (D2, CH4, and CH3F).6 In the (1) Part 7: Fornarini, S.; Sparapani,C.; Speranza, M. J . Am. Chem. SOC., preceding paper in this issue. (2) (a) University of Rome. (b) Istituto di Chimica Nucleare del C.N.R., Monterotondo Stazione. (c) University of Camerino. (3) Lancelot, C. J.; Cram. D. J.; Schleyer, P. v. R. Carbonium Ions; Olah, G. A., Schleyer, P. v. R., Eds.; Wiley-Interscience: New York, 1972; Vol. 111, Chapter 27. (4) Griengl, H.; Schuster, P. Tetrahedron 1974, 30, 117. (5) (a) Nibbering, N. M. M.; de Boer, T. J.; Org. Mass Spectrom. 1969, 2, 157. (b) Venema, A,: Nibberina. N. M. M.: de Boer, T. J. Orp. Mass Speczrom. 1970, 3, 1589. (c) Nibbiring, N. M. M.; Nishishita, T.yVan de Sande,C. C.; McLafferty, F. W . J . Chem. SOC.,Chem. Commun. 1976,810. (d) Koppel, C.; McLafferty, F. W. J . Am. Chem. SOC.1976, 98, 8293. (e) Koppel, C.; Van de Sande, C . C.; Nibbering, N. M. M.; Nishishita, T.; McLafferty, F. W. J . Am. Chem. SOC.1977, 99, 2883. (6) (a) Aquilanti, V.; Galli, A.: Giardini-Guidoni, A,; Volpi, G. J . Chem. Phys. 1968,48,4310. (b) Ausloos, P.; Lias, S . G., Scala, A. A. Ado. Chem. Ser. 1966,58, 264. (c) Ausloos, P.; Lias, S. G.; Gorden, R., Jr. J. Chem. Phys. 1963, 39, 3341. (d) Ausloos, P.; Lias, S G . ;Gorden, R., Jr. J . Chem. Phys. 1964,40, 1854. ( e ) Colosimo, M.; Bucci, R. J . Phys. Chem. 1979,83, 1952. (f) Speranza, M.; Pep, N.; Cipollini, R. J . Chem. Soc.,Perkin Trans. 2 1979, 1179.
0002-7863/88/1510-0042$01 S O / O
presence of suitable nucleophilic substrates, such as the P-arylethyl halides 4-F, 443, and 4-Me and alcohols 4-OH, the GA+ acids are expected to generate inter alia the corresponding "onium" derivatives 5, wherein the loss of the potential leaving group YA may be assisted by the participation of the vicinal phenyl moiety (k,) to give the corresponding ethylenearenium ions 3, much like analogous processes carried out under solvolytic conditions. The present work is aimed at determining the structural features of the intermediates involved in the decomposition of P-arylethyl onium ions 5 from eq 1, in the presence of an external nucleophile CH30H, in order to gather positive evidence on the existence and the stability of conceivable bridged intermediates (e.g., 3) in the gas phase, where interference from solvation, ion pairing, etc. typical of the condensed phase is excluded.
1 (Rl, R, (or R4) = H; Rz, R4 (or R3) = Me) 2 (R,-R3 = H; R4 = Me)
ill Y,R
F H l ? - F l C HI:-C.1
Oh
il
l-OU)
C8
Me IC-Me)
Experimental Section Materials. Deuterium, methane, methyl fluoride, oxygen, and trimethylamine were high-purity gases from Matheson Co., used without further purification. Methanol, 8-phenylethyl chloride (4-CI), and alcohol 4-OH, as well as styrene, benzyl methyl ether, and benzaldehyde, are commercially available from Fluka A.G. and Aldrich-Chemie GmbH. Those compounds, which are not available from commercial sources, such as P-phenylethyl fluoride (4-F), @-p-tolylethylchloride (4-Me), isomeric methoxyethylbenzenes and -toluenes, and benzocyclobutene and its methylated derivatives, were synthesized by conventional procedures, their identity being checked by mass spectrometric and NMR analyses.' Before irradiation, the starting compounds 4 were repeatedly purified on a preparative gas chromatographic column (2-m 20% DC-200/6% Bentone 34 on 60-80 mesh Chromosorb W-AW) and their purity checked (7) Edgell, N. F.; Parts, L. J . Am. Chem. SOC.1955, 77, 4899.
0 1988 American Chemical Society
J . Am. Chem. SOC.,Vol. 110, No. 1 , 1988 43
Acid- Induced Nucleophilic Displacement Reactions. 8
Table I. Product Yields from the Gas-Phase Attack of GA+ Acids on @-PhenylethylHalides and Alcohols
system composition' bulk gas GA+ (Torr) C,H5+ CH, (680) C,H5+ CH, (700)
substrate (Torr) 4-F (1.0) 4-F (1.0) 4-CI (0.2) 4-CI (0.2) 4-CI (0.9) 4-CI (0.3) 4-CI (0.9) 4 4 1 (0.9) 4-CI (1.0) 4-CI (0.9) 4-C1 (0.3) 4-CI (0.9) 4-Me (0.5) 4-OH (0.3) 4-OH (0.3)
D, (750) D, (700) D, (660)
D3'
DS+ DS+
relative distribution of products (a)
total absolute yield
CHSOH (Torr) 1.3 4.8 1.3 4.0 4.8 0.5 1.2 1.3' 4.1 4.6 0.3 1.2 1.5 1.3 4.2
ph-OMe
54 50 27 20 20 49 61 5
(@/a)ratio
ph
35 27 16 15 15 11 11 1 7 8 4 4 14 6 3
1.5 1.8 1.7 1.4 1.3 4.3 5.4 4.8 7.4 7.3 16.2 19.6 4.1 5.1 6.0
PhCHO PhCH20Me 9 11 33 39 27 26 19 20 24 24 30 20 17 60 79
n.d.d 3 3 n.d. 3 n.d.
PhA
2 9 20 26 35 13 8 74 12 10 2
102G(M{ 88.8 69.2 28.0 29.0 80.5 139.0 96.0 33.3 63.0 86.6 41.6 23.0 43.4 30.1 6.7
%'
32 25 9 10 27 50 34 12 23 31
CH, (175) C,H5+ CHI (730) C,H5+ n.d. n.d. CH4 (700) C,H5+ 1 55 C,H5+ CH4 (740) 1 57 C,H5+ CH, (670) n.d. 64 CH,F (160) CH3FCH,+ n.d. n.d. 76 CH3F (700) CH3FCH,+ n.d. 11 16 57 C,H5+ CH, (700) 3 11 n.d. 31 C,H5+ CH, (700) CH, (730) C.H
