5222
J . Org. Chem. 1981,46,5222-5224 Scheme I
Bu3SnCi
+
LIC
f
CH
THF __f
Scheme I CH
Bu3SnC
-I
+
I75'Al
?
Bu3SnC z C S n B u 3
5
125%)
I
RC-OCH3
t X-
-
RCO;
1.'C
Registry No. I , 994-89-8; 2, 994-71-8; 3, 14275-61-7; BuaSnC1, 1461-22-9. (11) Zavgorodnii, V. S.; Sharanina, L. G.; Petrov, A. A. J. Gen. Chem. USSR. (Engl. Trawl.) 1967, 37, 1469. (12) Wollenberg, R. H. Ph. D. Thesis, Harvard University, 1976.
Nonprotic Procedure for Transesterification of Methyl Esters' Dante Bencivengo and Joseph San Filippo, Jr.* Department of Chemistry, Rutgers University, New Brunswick, New Jersey 08903 Received June 16, 1981
In the course of an unrelated investigation in our laboratory if became necessary to carry out a series of transesterifications under rigorously neutral conditions. A survey of the literature produced only an isolated example which remotely met these needs.2 We report here our development of one procedure which does meet these qualifications and whose general synthetic applicbility appears to be moderate to good. Treatment of the methyl ester of various carboxylate acids with tetra-n-alkylammonium halide a t elevated (1) This work was supported by the National Science Foundation, Grant 80-17045. (2) Brasen, W. R.; Hauser, C. H.Org. Synth. 1954, 34, 58.
0022-3263/81/1946-5222$01.25/0
(2)
temperature leads to the production of the corresponding n-alkyl ester (eq 1). Thus, reaction of methyl cinnamate RC02CH3 + n-R'4N+XRC02R' CH3X (1) 1 2 with tetra-n-butylammonium bromide at 140 O C for 72 h under anhydrous conditions results in its quantitative conversion to n-butyl cinnamate (3). The same reaction repeated with the corresponding tetra-n-alkylmmonium chloride or iodide under optimized conditions produces, respectively, 78% and 69% yields of 3. The applicability of this procedure to a representative series of methyl ethers is summarized in Table I. Several aspects about this data deserve brief comment. For example, the rate of reaction, in HMPA solution, exhibits the following dependence on halogen: C1- > Br- > I-. No reaction is observed with n-Bu4N+BF4-. Second, higher concentrations of tetra-n-butylammonium halide promote higher product yields, a fact which may be related to the further observation that, in general, most solvents have a deleterious effect on the course of this r e a c t i ~ n . Thus, ~ in general, optimum results are obtained in the absence of any solvent. Although the use of hexamethylphoshoramide (HMPA) seems to cause little or no diminution in product yield, the workup, isolation, and purification of products obtained from reactions performed in HMPA are generally more difficult. Third, entries 31 and 32 summarize the influence which the structure of the alkyl group in the tetra-n-alkylammonium halide has on the transesterification. Lower yields appear to obtain with tetra-n-propyl- than with tetra-n-butylammonium halide. In general, the influences of time, solvent, and reactant ratios parallel those presented in Table I. Mechanistically, we believe the transformation defined by eq 1 is occurring by the pathway outlined in Scheme I. Thus, eq 2 defines the well-known 0-alkyl cleavage of a methyl ester by nucleophilic displacement while eq 3 presents the alkylation of a carboxylate anion by an onium ion. This scheme is consistent with (i) the previously observed influence which the nature of the group R in the OR functionality has on the 0-alkyl cleavages of esters,4s5 (ii) an increasing recognition of the moderately nucleophilic nature of carboxylate ions under aprotic conditions,6 and (iii) the influence exerted by the nature of the alkyl group
-
for 1h at which time 60.0 g (180 mmol) of tri-n-butyltin chloride were added dropwise over a period of 30 min. The reaction mixture was warmed to room temperature and stored under a closed acetylene atmosphere for 12 h. Water (10 mL) was added, and the reaction mixture was concentrated in vacuo. Heptane (250 mL) was added, and the reaction mixture was washed with one 1WmL portion of water, dried over MgS04,and concentrated in vacuo to furnish an oil. Distillation afforded 43 g (75%)of tri-n-butylethynylstannane(1): bp 200 "C (2 mm) [lit." bp 76 "C (0.2 mm)]. The NMR and IR spectra of 1 were identical with those of material previously reported.I2 Conversion of Bis(tri-n -butylstannyl)acetylene (2) to Tri-n-butylethynylstannane(1). A solution of 80.0 mL (124 mmol) of n-butyllithium in hexane in 250 mL of dry tetrahydrofuran was stirred under an acetylene atmosphere at 0 "C for 1h at which time 30.0 g of the accumulated residues, consisting largely of 2, was added dropwise over a period of 30 min. The reaction mixture was warmed to room temperature and stored under a closed acetylene atmosphere for 15 h. The isolation and purification procedure described above gave 22 g (65%)of 1. The remaining residue may be recycled for the preparation of more
t CH,X
+
(3) Experiments performed in dimethylformamide (DMF) and Nmethylpyrrolidone produced no significant yield of transesterification product. Transesterification can be effected by using ether tri- or tetraglyme solvent; however, the yields are 20-30% less than those observed for the equivalent reactions performed neat or in HMPA. (4) Consistent with earlier obaervationss that 0-alkyl cleavage of esters by nucleophilic displacement is effective only when the alkyl group is methyl, we have observed that treatment of ethyl octanoate with 4 equiv of (n-C4Hg),N+C1-at 140 O C for 36 h produced a 9 % yield of n-butyl octanoate. Similar treatment of n-propyl octanoate failed to yield any (
