5828
J . Org. Chem. 1985,50, 5828-5833
Functionalized Ethylene Oligomers as Phase-Transfer Catalysts David E. Bergbreiter* and J. R. Blanton Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received July 30, 1985 Ethylene oligomers with M , in the range 1000-3000 containing terminal groups such as crown ethers, azacrown ethers, and phosphonium salts have been prepared and used as phase-transfer catalysts. These oligomeric catalysts have solubility properties similar to polyethylene in that they are insoluble at 25 "C but dissolve in organic solutions a t elevated (90-110 "C) temperatures. Such oligomeric phase-transfer catalysts have activities comparable to their low molecular weight congeners and to activities reported for insoluble polystyrene-bound phase-transfer catalysts. After these oligomeric catalysts have been used in a reaction at 100 "C, they can be readily separated from the reaction products by cooling to form a precipitate, which c?an then be isolated by filtration. Such catalysts may be reused without loss of activity. These catalysts are also effective in "solid-liquid" phase-transfer reactions in which an inorganic solid is suspended in a hot organic solution containing catalyst and substrate.
Phase-transfer catalysis is typically carried out with functionalized ethylene oligomers act as phase-transfer catalysts that are soluble in organic solvents and that catalysts under both "liquid-liquid'' and "solid-liquid'' possess one or several sites which can bind a water-soluble conditions. salt either by coordination of the cation by a neutral ligand In the course of other work in our group directed toward or by ion exchange.' Within the last ten years, insoluble development of new selective methods for functionalization polymeric ammonium salts, phosphonium salts, and crown of polymers,7 we have found that linear ethylene oligomers ethers have been described which can substitute for the whose M , is greater than 1200 have a useful temperaoriginal low molecular weight species used as phaseture-dependent solubility. That is, such oligomers are transfer catalyst^.^-^ Such polymeric species have an insoluble in common organic solvents at 25 "C but do advantage of ready recyclability. A disadvantage of indissolve on heating. When a solution of these oligomers soluble polymeric phase-transfer catalysts is that diffusion is cooled to room temperature, the oligomer reprecipitates of reagents and substrates into and out of an insoluble as a fine powdery precipitate which can be separated from polymer matrix can affect the catalyst's r e a ~ t i v i t y . ~ , ~ the solvent by centrifugation and decantation or by filWhile such diffusional restrictions can lead to novel setration. If a mixture of polyethylene and these oligomers lectivity due to size differences of otherwise similar organic is prepared and heated to effect dissolution in a solvent substrates,6 this limitation on these catalysts' activity is like toluene and the resulting solution is cooled, a coarser more often a matter of concern either synthetically or in precipitate of polyethylene containing these oligomers mechanism studies. Insoluble polymer-bound phaseforms which is readily isolated by fill&ion. We have recently described examples of chemi 'ry in which such transfer catalysts can also be used in "solid+olid reactions recovery procedures can be used with both polyethylenewhere the inorganic material is not dissolved in water but is instead present as a suspension in an organic solvent. bound reagents and transition-metal catalysts.s-10 This paper describes the extension of these methods to include Phase-transfer catalysis has been noted under such solidnonmetallic phase-transfer catalysts. solid conditions but the way or ways in which an insoluble catalyst functions under such conditions is unclear. Here Several methods can be used to prepare ethylene oliwe describe alternative soluble, recyclable polymer-bound gomers such that functional groups capable of forming or phase transfer catalysts derived from functionalized acting as phase-transfer catalysts are incorporated at a ethylene oligomers. These catalysts which we have preterminal position. Anionic oligomerization has proven to pared have the solubility characteristics of polyethylene. be the most useful of these methods." In particular, Namely, they are soluble in many organic solvents at 100 anionic oligomerization of ethylene using n-butyllithium "C but are insoluble in the same solvents at 25 O C S We as an initiator (eq 1) is the most useful method for the have found that this large temperature-dependent solupreparation of terminally functionalized phase-transfer bility difference can be used to recover and separate these catalysts because the oligomers produced in this oligomoligomeric phase-transfer catalysts from the reaction erization reaction are strictly linear and can be recovered products or starting materials of a given reaction by the by precipitation with or without added polyethylene. By simple expedient of cooling a hot solution of the funcRLi E+ CHz=CHz TMEDA- R f€HZCHzf;;CH2CH2Li tionalized ethylene oligomer and then separating the re1 covered solid oligomer by centrifugation or filtration. As n-ClH16 is shown in the work described below, these terminally R eH,CHzh$HzCHzE (1) using a range of different electrophiles (E+),various end groups can be introduced a t the terminus of the alkene (1) Weber, W. P.; Gokel, G. W. "Phase-Transfer Catalysis in Organic oligomer. For example, CIPPhz or COSfollowed by H + / Synthesis"; Springer-Verlag: Berlin, 1977. Starks, C. M.; Liotta, C.
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'Phase Transfer Catalysis"; Academic Press: New York, 1978. (2) Montanari, F.; Landini, D.; Rolla, F. Top. Curr. Chem. 1982, 101, 147-200. (3) Ford, W. T.; Tomoi, M. Adu. Polym. Sci. 1984, 55, 49-104. (4) Regen, S. L. Angew. Chem., Int. Ed. Eng2. 1979, 18, 421-429. (5) Tomoi, M.; Yanai, N.; Shiiki, S.; Kakiuchi, H. J . Polym. Sci., Polym. Chem. Ed. 1984,22,911-926. (6) Regen, S.L.; Nigam, A. J . Am. Chem. SOC.1978, 100, 7773-7775. Tomoi, M.; Ford, W. T. Ibid. 1981, 203, 3828-3832. (7) Bergbreiter, D. E.; Chen, Z.; Hu, H.-P. Macromolecules 1984, 17, 2111-2116.
0022-3263/85/1950-5828$01.50/0
(8) Bergbreiter, D. E.; Blanton, J. R. J . Chem. SOC.,Chem. Commun. 1985, 337-338.
(9) Bergbreiter, D. E.; Chandran, R. J . Am. Chem. Soc. 1985, 107, 4792-4793. (10) Bergbreiter, D. E.; Chandran, R. J . Chem. SOC.,Chem. Commun. 1985, 1396I1397. (11) Electrophilic substitution of living polymers derived from alkyllithium initiated oligomerization is discussed in: Young, R. N.; Quirk, R. P.; Fetters, L. J. Ado. Polym. Sei. 1984, 56, 1-90,
0 1985 American Chemical Society
J. Org. Chem., Vol. 50, No. 26, 1985 5829
Functionalized Ethylene Oligomers HzO can be used to prepare a polyethylenediphenylphosphine or a carboxylated ethylene oligomer, respectively. The living oligomer 1 produced in eq 1 can thus be readily transformed either directly or indirectly into analogues of known phase-transfer catalysts (eq 2-4).
121
functionalized ethylene oligomers derived from oxidized polyethylene. Oxidized polyethylene is commercially available as a derivative of polyethylene and contains both carboxylic and ketone f~nctiona1ity.l~~'~ Although this polyfunctionality and the possibility of multiple functional groups per chain complicate any mechanistic studies and make characterization of the polymer derivative less feasible, we have found that it is possible to prepare phasetransfer catalysts using eq 5 (PE = polyethylene). Both
S C O O H
An advantage of procedures 2-4 over syntheses of divinylbenzene (DVB) cross-linked insoluble polystyrenebound phase-transfer catalysts is that it is much easier to characterize the products of reactions 2-4. As is the case for insoluble polymer-bound catalysts, these oligomers can be characterized by IR as solids. Unlike insoluble polymer-bound catalysts, multinuclear NMR spectroscopy can also be used to both determine the identity of the attached functional groups and to determine the success of any given reaction. IH NMR spectroscopy is particularly useful. While the oligomers 1-7 all contain a large broadened peak due to the CHz groups, the methyl end and the functional group end of the oligomer both contain groups that are separated enough from the methylene envelope to be observed even a t 90 MHz. Integration of the terminal groups' protons at higher fields or relative to an internal standard at 90 MHz directly yields quantitative data about the identities and loading of functional groups on these oligomers.12 The results of such lH NMR studies yield molecular weight information and, more importantly, allow the success of the syntheses in eq 2-4 to be monitored and verified. 13Cor 31PNMR spectroscopy can also be used with these to study these oligomers as well. While it is known that 13C and 31PNMR spectroscopy can be used to study functionalization of insoluble lightly DVB cross-linked poly~tyrene,'~ the solubility of the oligomeric catalysts described in this work facilitates their analysis by 13C or 31PNMR spectroscopy. While most of our work to date has involved use of terminally functionalized ethylene oligomers derived from anionic oligomerization of ethylene, we briefly examined the utility of more readily available and inexpensive
the polyethylenecarboxamide derivative 8 of aza-15crown-5 and the corresponding amine 9 were prepared. While the carboxamide 8 was not a very active phasetransfer catalyst, the amine 9 had activity comparable to that of an aza-15-crown-5 or 15-crown-5 derivative of a linear ethylene oligomer. As is shown by the results described in Tables I and 11, the phase-transfer catalysts derived from eq 2-5 all are active as phase-transfer catalysts in nucleophilic substitution reactions and in reduction of carbonyl compounds with sodium borohydride under liquid-liquid conditions. In a typical procedure under liquid-liquid conditions, the oligomeric crown ether was dissolved in an organic solvent like xylene at 90-110 OC to form a 0.003 M solution. The organic substrate was then added such that the resulting solution was ca. 0.1 M in substrate. Excess inorganic substrate in water was added to give a two-phase mixture which was stirred magnetically. The use of a Vibromixer for agitating the reaction mixture was briefly examined but was not continued since no significant increase in reaction rate was noted.16 Kinetic data given in Table I were obtained by periodically removing aliquots from the organic phase and analyzing them by GC using hydrocarbon internal standards. The observed rates are such that half-lives of these reactions are in the range 1-8 h at convenient catalyst concentrations. These half-lives are comparable to those seen with other insoluble polymerbound phase-transfer catalysts and are short enough to be synthetically practical. In order to evaluate the activity of functionalized ethylene oligomers as phase-transfer catalysts and to compare them to known low molecular weight and insoluble polymer-bound phase-transfer catalysts, we have examined a series of reactions. In the case of phasetransfer catalysis under liquid-liquid conditions, we studied the nucleophilic substitution of iodide for bromide (eq 6). Numerous literature studies have measured the NaI
(12) We have now further improved the procedure for
'H NMR
analysis of linear ethylene oligomers by using tert-butyllithium as the initiator in the oligomerization reaction 1. The methyl groups of this tert-butyl group are better resolved from the methylene envelope of the ethylene oligomer and more readily analyzed by 'H or 13C NMR spectroscopy: Bergbreiter, D. E.; Treadwell, D. T., unpublished results. (13) Ford et al. (Ford, W. T.; Periyasamy, M.; Spivey, H. 0. Macromolecules 1984, 17, 2881-2886 and references therein) describe some aspects of 13C NMR of functionalized DVB cross-linked polystyrenes. Examples of 31PNMR spectroscopy of functionalized DVB cross-linked polystyrene can be found in: Naaktgeboren, A. J.; N o h , J. M. R.; Drenth, W. J. Am. Chem. SOC.1980, 102, 3350-3354.
110 o c
(14) Oxidized polyethylene containing 0.6 mequiv of CO,H/g of polymer is available from Aldrich Chemical Co. (15) Nuzzo, R. G.;Smolinsky, G.Macromolecules 1984,17,1013-1019. (16) Balakrishnan and Ford (Balakrishnan, T.; Ford, W. T. J . Org. Chem. 1983,48,1029-1035) reported no advantage for use of a Vibromixer instead of use of magnetic or overhead stirrers.
5830 J. Org. Chem., Vol. 50, No. 26, 1985
Bergbreiter and Blanton
Table I. Phase-Transfer Catalysis under Liquid-Liquid Conditions Using Functionalized Ethylene Oligomers as Recoverable Catalystsn substrate catalystb organic PE-P(C~H~)Z-~-C~HB+ CH3(CHz)6CHzBr PE-P(CSH5)2-n-C4Hg+ CH3(CHz)6CHzBr PE-P(C,jH5)2-n-C,Hg+ CH3(CHZ)&H2Br P E - P ( C G H ~ ) ~ - ~ - C ~ H ~ + CH3(CH2)&HZCl PE-P(CGHb)z-n-CdHg+ CH,(CH,)&HO PE-P(C&)2-n-C4Hg+ c-C&,,Br C ~ B H ~ ~ P ( C G H ~ ) Z - ~ - C ~ CH3(CH2),CHZBr H' CH3(CHZ),CH,Br PS-(CHZ)$'(~-C~H~)~ benzo-15-crown-5 CH3(CH2),CH2Br PE-benzo-15-crown-5 CH3(CHz),CHZBr PE-benzo-15-crown-5 CHdCH2),CH2Br PE-CO-aza-15-crown-5 PS-CO-aza-15-crown-5 PE-benzo-15-crown-5 benzo-15-crown-5
CH3(CHZ)&H2Br CH3(CHZ)&HzBr CH3(CH2)6CHO CH3(CH2)&HO
inorganic NaI NaI KI NaBr NaBH, NaI NaI NaI NaI KI NaI NaI NaI NaBH, NaBH,
product CHdCHz)&HJ CH~(CHZ)&HZI CHdCHz)&HzI CH3(CH2),CH2Br CH3(CH&CHzOH C-C6H111 CHB(CHZ)&HZI CHdCHz)&HzI CHB(CHZ)&HZI CHB(CHZ)&HZI CH~(CH~)~CHZI CHdCHz)&HJ CHdCHz)5CHzI CH3(CHz)&HzOH CH3(CHz)&HzOH
105kobsd, s-l (mequiv of cat.)-' a3 8< 83 42 33 d
100 48' 72 0 2a
271 358 5 43h 27 67
Reactions were run at 110 "C using 2 mM xylene solutions of the catalyst ( M , = 1300) and a 10-fold excess of the inorganic substrate dissolved in 5 mL of water. Agitation of the reaction mixtures was achieved with magnetic stirring. The reaction progress and the product identities were measured with GC or GC/MS. bPE stands for an oligomeric ethylene chain. PS stands for 2% DVB-crosslinked polystyrene. CThecatalyst was heated at 135 "C for 3 h before being used. d
