168
J . Am. Chem. SOC.1984, 106, 168-174
to a greater withdrawal of electron density from the metal center, B u ) , ( P P ~ , ) ~ ]could + be accomplished. This was proven by cara subsequent lowering in energy of the HOMO, and thus a n rying out the reaction between [ReZ(p-H),H2(PPh,),(CN-tincrease in the Ell' values; however, differences in cr-donor abilities B u ) ~ ] ( P F and ~ ) ~tert-butyl isocyanide in a CV cell a t a potential are also a likely contributing factor. of +OS0 V; this prevented reduction back to [Re2(p-H),H2Reactions of [Re2(p-H),H3( PPh,) (CNt -Bu)]( PF6) and (PPh,),(CN-t-Bu),]+ while a t the same time ensuring that [Rez(cr-H)3Hz(PPh3)4(CN-t-Bu)2](PF6)2 with tert-Butyl Iso[ R ~ ( C N - ~ - B U ) , ( P P ~ ,was ) ~ ] +not oxidized ( E l l 2= +0.81 V vs. cyanide. Whereas neither of the diamagnetic congeners of these SCE)." By monitoring this reaction by CV we were able to show two complexes reacts with excess tert-butyl isocyanide over reathat rapid conversion to [Re(CN-t-Bu),(PPh,),]+ occurred and sonable periods of time, both the paramagnetic dications react that no significant quantities of [Re,(p-H),H2(PPh3),(CN-tquite rapidly (Scheme I). The dirhenium heptahydrido species B u ) ~n+] remained. reacts in dichloromethane t o afford a 3:l mixture of [Re2(pAcknowledgment. Support from the National Science FounH)3Hz(PPh3),(CN-t-Bu)z]PF6 and [R~(CN-~-BU),(PP~,)~]PF~ dation (Grant No. CHE82-06117) is gratefully acknowledged. (as monitored by CV). In this reaction H2was identified by GC The Varian XL-200 spectrometer was purchased with funds from analysis as the only volatile gaseous product. When [Re?(pthe National Science Foundation (Grant No. CHE80-04246 to H)3Hz(PPh3),(CN-t-Bu)2](PF6)2 is reacted in this same fashion, Purdue). the major product was the reduced monocation plus some [Re( C N - ~ - B U ) , ( P P ~ , ) ~ ](ca. P F ~4:l ratio of products).27 Thus the Registry No. Re2Hs(PPh,),, 66984-37-0; Re2Hs(PEtPhz),, 66984-38formation of the diamagnetic monocationic dirhenium polyhydride 1; Re2H8(PEt2Ph),,63313-85-9; Re2H8(AsPh3),,87901- 15-3; [Re& species serves as a hindrance to the rapid and complete evolution (PPh3)4]PF6, 86664-86-0; [ReZH8(PEtPhz),]*, 87882-98-2; [Re2&,of Hz and the formation of the thermodynamic product [Re(PEt,Ph),]+, 87882-99-3; [Re2H8(AsPh,),]+, 87883-00-9; [Re2H7(PPh,),(NCMe)]PF,, 86664-85-9; [Re2H7(PPh,),(NCEt)]PF,, 87883( C N - ~ - B U ) ~ ( P P ~ ~ )This ~ ] P being F ~ . the case, we reasoned that 02-1; [Re2H7(PPh3),(NCPh)]PF6, 87883-04-3; [Re2H7(PPh3),(CN-fin its oxidized by keeping [Rez(p-H)3Hz(PPh3)4(CN't.BU)2]z+ 86664-88-2; Bu)]PF6, 86664-91-7; [Re2H7(PPh3)4(NCMe)](PF,)2, state, Le., preventing back reaction to the much more kinetically [Re2H7(PPh3),(NCEt)J(PF,),, 87883-06-5; [Re2H7(PPhJ4(NCPh)Jinert monocation, complete and rapid conversion to [Re(CN-t(PF,),, 87883-08-6; [Re2H7(PPh3),(CN-t-Bu)](PF,),,86664-93-9; [Re2HS(PPh3),(CN-r-Bu),lPF,, 86676-29-1; [Re2Hs(PPh3),(CN-t[R~(CN-~-BU),(PP~~)~]PF~, 80006-29-7; B U ) ~ ] ( P F , )86676-31-5; ~, (27) The mechanism by which the reactive dicationic species [Re(p ReOCI,(AsPh,),, 87863-04-5; Ph,C+PF,-, 437-17-2; C7H7+PF6-, H)3H2(PPh3)4(CN-t-Bu)2](PF6)z is reduced back to its relatively unreactive monocationic congener is at present unknown. 29630-1 1-3; NO+PF6-, 16921-91-8.
Template Effects. 6.' The Effect of Alkali Metal Ions on the Formation of Benzo-3x-crown-x Ethers over a Wide Range of Ring Sizes Luigi Mandohi* and Bernard0 Masci* Contribution from Centro C.N.R. di Studio sui Meccanismi di Reazione, c/o Istituto di Chimica Organica, Universitd "La Sapienza" di Roma, P.le Aldo Moro, 2 00185 Roma, Italy. Received April 12, 1983
Abstract: The rate of formation of benzo-3x-crown-x ethers with x = 4, 5 , 7, 10, and 16 via intramolecular alkylation of o--OC6H4(OCHzCH,),,Br in 99% aqueous Me2S0 was found to be markedly affected by added alkali metal bromides. Catalysis or inhibition was observed, depending on the cation-substrate pair. Combination of the present results with those previously reported for the formation of B18C6 offers a large variety of patterns. The magnitude of the observed effects ranges over four powers of ten. The dependence of the observed rates (koM) on metal ion concentration was expressed in terms of independent contributions from free and cation-paired aryl oxide ions, whose relative weights are ruled by the rate constants ki and kip, respectively, and by the ion-pairing association constants KAro-. A self-consistent analysis was used to derive numerical values of the above parameters. A definite contribution from an additional reaction path involving two metal ions was detected in the case of the K+-catalyzed formation of B30C10. The equilibrium constants Kc for associations between many cation-crown pairs were also determined under the same conditions. A comparative analysis of structure and metal ion effects on the extent of interaction of the alkali metal ions with the reactants, transition states, and reaction products shows that the cation interaction in the transition state is strongly reminiscent of the host-guest interactions found in the cation-B3xCx complexes. The catalytic efficiency of the alkali metal ions (log kip/ki)shows a definite tendency to parallel the strength of interaction with the reaction products (log Kc), thus indicating that a metal ion capable of binding strongly with a crown ether is also a good catalyst for the formation of the crown ether itself.
In a recent paper' on the formation of benzo-18-crown-6 in 99% aqueous MezSO (eq 1, x = 6), we found that the catalytic effi-
- a;)
ay:CH,Cli2] ArO'
x - 1 Br
x- 1 B3xCx
ciency of the alkali metal ions was closely related to the strength of interaction between the alkali metal ions and the reaction product, B18C6. Since associations between alkali metal ions and crown ethers are known to be highly dependent upon both the
(1) (1) Part 5: Ercolani, G.; Mandolini, L.; Masci, B. J . A m . Chem. Soc., 1983, 105, 6146.
(2) Illuminati, G.; Mandolini, L.; Masci, B. J . Am. Chem. Soc. 1983, 105,
555.
0002-7863/84/1506-0l68$01.50/00 1984 American Chemical Society
Format ion of Benzo- 3x - crown- x Et hers
J . Am. Chem. Soc., Vol. 106, No. 1, 1984 169 Table I. Rate and Equilibrium Constants for the Alkali Metal Ion Assisted Cronn Ether rorming Reactions 1 in 9 9 8 Me,SO a t 25.0 'Ca
Scheme I
k,Jk,b
oroduct
B12C4
nature of the metal ion and the ring size of the crown ether,j changes in the chain length of the phenoxide reactant ArO- are expected to produce highly structured reactivity patterns exhibiting features other than those observed in the formation of benzo18-crown-6. In this paper we report on a detailed kinetic investigation of the template effect of alkali metal ions on the formation of B3xCx ethers with ring sizes of 12, 15, 21, 30, and 48 atoms (x = 4, 5 , 7, 10, and 16, respectively) in 99% aqueous M e 2 S 0 a t 25.0 OC. In many cases the equilibrium constants Kc for association between the alkali metal ions and the crown ethers have been obtained under the same conditions from independent equilibrium measurements. Combined with our previous results related to the formation of B18C6,2 the data from the present work provide a wide picture of metal ion and ring-size effects on the formation of crown ethers catalyzed by alkali metal ions.
Results Rate Measurements and Treatment of Kinetic Data. The cyclization of the phenoxide reactants to the corresponding crown ethers was followed in 99% M e 2 S 0 at 25.0 "C by monitoring the disappearance of the phenoxide absorption in the region of 300-315 nm in very dilute substrate solutions (ca. 1-2 X lo4 M). These solutions were prepared by neutralizing solutions of the parent phenols with a calculated amount of (CH,),NOH stock solution. The alkali metal ions were added as bromides over a wide concentration range up to 0.1-0.2 M. In many cases the effect of added tetraalkylammonium bromides was also studied. The reactions followed first-order kinetics up to high conversions in all cases. A complete list of observed rate constants kobsdis given in the Experimental Section. Log-log plots of kohd against added salt concentration are shown in Figure 1 for the individual crown ethers. Clearly, either rate enhancement or rate retardation results from the addition of the alkali bromides. In contrast, the effect of tetraalkylammonium salts is very small, which indicates that the koM values obtained when the stoichiometric Me4N+ion is the sole cation present (given by the solid horizontal lines in Figure 1) are to be interpreted as the rate constants ki for reactions of the free ArO- ions. The effect of the alkali metal ions may be interpreted on the hypothesis of 1:l associations with ArOand of independent contributions to the overall reaction from the free ion ArO- and the ion pair ArO-MC (Scheme I). From Scheme I the basic equation
is derived, where KM- is the equilibrium constant for association. As was shown in a previous paper,2 eq 2 is found to closely rep-
resent the influence of alkali metal ions on the formation of Bl8C6 (eq 1, 3x = 18) and other Williamson-type reactions in 99% M e 2 S 0 , and also may be applied to the reactions of the present work. The results are summarized in Table I. The solid lines in Figure 1 are plots which are calculated using eq 2. With the sole exception of the high concentration range of the K+-catalyzed formation of B30C10 (Figure l D ) , eq 2 fits the data remarkably well. For many reactions (Na', K+, Rb', and Cs+ in the formation of B15C5; K', Rb+, and Cs+ in the formation of B21C7 and (3) (a) Lamb, J. D.; Izatt, R. M.; Christensen, J. J.; Eatough, D. J. In "Coordination Chemistry of Macrocyclic Compounds"; Melson, G. A,; Ed.; Plenum Press: New York, 1979. (b) Lamb, J. D.; Izatt, R. M.; Christensen, J. J. In "Progress in Macrocyclic Chemistry"; Izatt, R. M., Christensen, J. J., Eds.; Wiley: New York, 1981; Vol. 2. (c) Poonia, N. S.; Bajaj, A. V. Chem. Rev. 1979, 79, 389.
Li+ Na'
K+ R b+
cs+ B15C5
Li+ Nat
K+ B18C6e
Rb' CS' Li+ Na'
K' Rbt
c S+ B21C7
Li' Na+
K' B30C10
Rb' c s+ Li+ Na+
Kif Rb' CS'
B48C16
Li' Na' K+ Rb' c s+
0.044 (0.78) (0.81) (0.62) (0.50) 0.089 11.5 7.32 4.64 2.75
