In the Laboratory
Advanced Chemistry Classroom and Laboratory
edited by
Joseph J. BelBruno Dartmouth College Hanover, NH 03755
A Simple and Convenient Method for Generation and NMR Observation of Stable Carbanions An Advanced Undergraduate Laboratory Experiment† Hamid S. Kasmai* Department of Chemistry, East Tennessee State University, Johnson City, TN 37614; *
[email protected] The study of carbanions and their role as reactive intermediates is an integral part of an undergraduate organic chemistry curriculum. The electronic structures of cyclic conjugated carbanions such as cyclopentadienide ion (CPD{) are usually discussed under the Hückel’s 4n + 2 pi electron rule of aromaticity and Hückel molecular orbital (HMO) theory. With the increased inclusion of computational chemistry in the undergraduate curriculum, the use of semiempirical and more advanced calculations in the study of the structure of these reactive intermediates is becoming part of organic chemistry instruction at an advanced undergraduate level (1). To date the only tangible and direct method to demonstrate the existence of carbanions for an undergraduate audience has been the formation of the characteristic colors associated with stable carbanions such as cyclopentadienide (2). Although NMR spectroscopy has been employed to observe carbanions, it remains primarily a research tool. A recent online JCE index search of the field “Title” for “NMR Spectroscopy” yielded 32 records, none of which dealt with reactive intermediates. The primary reason for the lack of experiments dealing with the NMR of carbanions in the undergraduate curriculum may be that the published procedures for the preparation and study of these species are too elaborate and inconvenient for an undergraduate laboratory experiment. For example, indenide ion has been prepared by three different methods (3) for the purpose of NMR spectroscopy, but none is suitable for adoption in an undergraduate laboratory.1 We now report a simple and convenient method for the generation and NMR (1H and 13C) observation of stable carbanions. The 1H and 13C NMR data obtained by this method for a number of cyclic and heterocyclic carbanions are presented, and some significant and instructive features of spectra associated with the method are discussed. Experimental Procedure
Safety Considerations The experiments suggested in this paper are designed for advanced laboratory courses. Caution must be exercised in their execution. The direct supervision of the experiments by the laboratory instructor is recommended. Pure KH is a moisture-sensitive flammable solid and reacts violently with water. The ether washings of KH contain † Presented at the 16th International Congress of Heterocyclic Chemistry Conference, August 10–15, 1997, Montana State University, Bozeman, MT.
830
some solid KH and should be decomposed carefully by reacting with alcohol reagent before its disposal. DMSO-d6 is a harmful liquid and is readily absorbed through skin.
Materials Cyclopentadiene was prepared from dicyclopentadiene (4 ). Thioxanthene was prepared according to the literature (5). All other conjugate acid progenitors were obtained commercially and were either recrystallized from appropriate solvents or distilled before use.2 Methyl sulfoxide-d6 (99.9 atom% D, containing 1% v/v TMS) and potassium hydride (35% by weight in mineral oil) were obtained from Aldrich.
NMR Spectroscopy Proton NMR spectra at 90 MHz and 13C NMR spectra at 22.5 MHz were recorded on a JEOL FX-90Q spectrometer. Proton chemical shifts were measured from TMS signal and carbon chemical shifts were measured from the central signal of DMSO-d6 (δ TMS = 39.44 ppm) and are reported from the TMS signal. Preparation of the Anions The following procedure is typical. An oven-dried 25-mL three-necked round-bottom flask was fitted with a nitrogen inlet adaptor with a stopcock, a small magnetic stirring bar, and two rubber septum stoppers. The entire assembly was tared. A homogeneous dispersion of KH in mineral oil (35 wt. % KH) was added dropwise via a Pasteur pipet until a mass of about 150 mg for oil dispersion was achieved. Nitrogen gas was introduced via a bubbler, and anhydrous ethyl ether (2 mL) was added via a dry syringe. The suspension was stirred for 2 minutes and then allowed to stand until KH powder had settled to the bottom of the flask. The supernatant ether was withdrawn by a syringe. The washing was repeated two more times using a total of 3 mL of anhydrous ether. The grayish-white powder remaining at the bottom of the flask was then dried with magnetic stirring under an aspirator vacuum for 25 minutes. Nitrogen gas was gently introduced into the flask. The weight of dried KH could be determined at this time if desired. DMSO-d6 (99.9 atom% D containing 1% v/v TMS, 0.6 mL) was added via a syringe, initiating the evolution of a gas. The reaction mixture was stirred for 25 minutes. During this period, a vertically clamped NMR tube containing xanthene was prepared as follows: a slurry of xanthene (91 mg, 0.5 mmol) in 0.25 mL of DMSO-d6 was introduced into a medium-wall, 9-in., dry
Journal of Chemical Education • Vol. 76 No. 6 June 1999 • JChemEd.chem.wisc.edu
In the Laboratory Table 1. 1H and Carbon Acid
13
C NMR Data and pKa Values for Carbon Acid Progenitors δ (ppm) b pK aa 1 c 13 H NMR C NMR d
Cyclopentadiene (1)
18
3.02 (t, 2H, H-5), 6.57 (m, 4 H, H-1 & H-2)
41.17 (C-5), 131.97 (C-1), 133.00 (C-2)
Indene (2)
20.1
3.36 (br.s, 2H, H-5), 6.57 (m, 1H, H-3), 6.91 (m, 1H, H-2), 7.20 (m, 2H, Ar-H), 7.42 (m, 2H, Ar-H)
38.52 (C-1), 120.59 (C-6), 123.46 (C-4), 124.23 (C-7), 125.90 (C-4), 131.59 (C-3), 134.30 (C-2), 143.24 (C-8), 143.24 (C-9)
Fluorene (3)
22.6
3.89 (s, 2H, H-9), 7.36 (m, 4H, Ar-H), 7.53 (dd, 2H, Ar-H), 7.89 (d, 2H, Ar-H)
36.2 (C-9), 119.73 (C-1), 119.73 (C-4), 124.87 (C-2), 126.55 (C-3), 140.96 (C-10), 142.75 (C-11)
Xanthene (4)
30
4.04 (s, 2H, H-9), 7.17 (m, 8H, Ar-H)
26.76 (C-9), 115.72 (C-4), 120.33 (C-11), 122.87 (C-2), 127.42 (C-3), 128.83 (C-1), 151.09 (C-12)
Thioxanthene (5)e
28.6
3.84 (s, 2H, H-9), 7.15–7.48 (m, 8H, Ar-H)
37.92 (C-9), 126.23 (C-2), 126.23 (C-3), 127.74 (C-4), 127.74 (C-1), 132.78 (C-12), 135.60 (C-11)
3.87 (s, 2H, H-9) , 7.23 (m, 8H, Ar-H)
35.16 (C-9), 125.74 (C-2), 126.98 (C-1), 136.41 (C-11)
9,10-Dihydroanthracene (6) 30.1
a From ref 11. b NMR data were obtained in DMSO-d solvent. c Measured from TMS. d Measured 6 from the central signal of DMSO- d6. e The NMR solution contained ~0.15 mL of CDCl3.
NMR tube under a stream of nitrogen gas through a clamped disposable pipet containing a tight small plug of glass wool. The resulting pale yellow potassium dimsyl-d6 (K+ DMSYLd6{) solution in DMSO-d6 was then transferred via a dry syringe into the disposable pipet containing the glass wool plug. To do this, the rubber tubing delivering a stream of nitrogen into the NMR tube was disconnected momentarily. The solution was immediately filtered into the NMR tube under nitrogen gas by reconnecting the rubber tubing to the top of the pipet. A glass wool plug was then pushed about halfway down the NMR tube and the tube was capped tightly under a stream of nitrogen. The contents of the NMR tube were mixed gently by inverting the tube until a homogeneous in-
tensely colored solution had formed. The 1H and 13C NMR spectra were recorded at the probe temperatures of 32 and 39 oC, respectively. Results and Discussion The most common preparation of carbanions is based on the acid–base reactions of carbon acids (R–H) (eq 1). R–H + R′ { M+ → R{ M + + R′–H
(1)
A variety of strong bases (R′ { M +) including KNH2–NH3(,) (6 ), M+DMSYL{–DMSO (7) (M+ is an alkali metal cation), cesium cyclohexylamide (CsCHA)–cyclohexylamine (CHA) (8), and n-butyllithium–hexane (3a) have been used. We chose
Table 2. 1H and 13C NMR Data for Carbanions Carbanion
δ (ppm) a 1
H NMR
b
Ref
C NMR c
13
Cyclopetadienide (7)
5.50 (s, H-1)
103.42 (d, 155.03,d C-1)
14
Indenide (8)
6.00 (d,
