Vasu Dev
California State Polytechnic College Pomona, 91766
I
I
Synthesis of Cyclopentanecarboxaldehyde An undergraduate organic chemistry laboratory experiment
Interest in the synthesis of alicyclic compounds is quite evident from the chemical literature published during the last several years. This area has recently been outlined (1) to demonstrate both the synthetic and theoretical challenges encountered in the field of alicyclic compounds. The information obtainable from these studies can be applied in understanding the behavior of compounds in other areas of organic chemistry. It is, therefore, important to bring some of this information into contact with students at an early stage of their undergraduate work and thus delineate both the theoretical and experimental aspects of this field. Cyclopentanecarboxaldehyde can be used advantageously for such instructional purposes. Formation of cyclopentanecarboxaldehyde IV, in a reaction involving cyclohexcne oxide I, with magnesium bromide etherate has been explained (8) to proceed
I
Br
OM^&
%+
1
H
111 !-M~B=*
I
XIII through a transition state such as 11. The mechanism of this transformation is quite similar to the pinacol rearrangement of 1,2-diols. However, in pinacol rearrangements the stereochemistry of the l,&diol is of importance in predicting the products of reaction. It has been pointed out that in pinacol rearrangements the alkyl group which undergoes migration is the one which is located in a trans-position with respect to the leaving group (3). Such a stereochemical restriction on the starting material makes trans-1,2-cyclohexanediolVI, an appropriate precursor for the preparation of cyclopentanecarboxyaldehyde VI + IV. Cyclohexene oxide I, needed for the preparation of VI has been prepared from trans-2-bromocyclohexanol, XII, by treatment with aqueous sodium hydroxide solution. The mechanism of this reaction presumably involves a neighboring group participation by the alkoxide ion XIII, from the backside of the carbon-bromine bond (4). Trans-2bromocyclohexanol, in turn, could be conveniently prepared from a reaction involving cyclohexene, X, and N-bromosuccinimide in water. Formation of XI1 under these conditions is believed to involve an electrophilic attack on the olefin by protonated N-bromosuccinimide I X to form a bromonium ion X I (5). The bromonium ion then undergoes a nucleophilic attack from the back side of carbon-bromine bond leading to the desired trans-2-bromocyclohexanol, XII. Contribution from the California Association of Chemistry Teachers.
476
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Journal of Chemical Education
The stereospecific conversion of cyclohexene oxide to trans-1,2-cyclohexanediol is accomplished by reacting the epoxide with water and the reaction is catalyzed by a trace of strong acid. The mechanism of this reaction, which is well established, involves a protonation of the oxygen atom in the epoxide ring. The resulting oxonium ion provides the driving force for an SN2type of back side attack by the nucleophile (in this case water), yielding the trans-diaxial dial VI. Interestingly, no cis-dial is formed in this reaction. It is, therefore, proposed that an SN1type of mechanism involving a planar carbonium ion does not contribute towards the formation of trans-dial to any extent (6). Last, the pinrtcol rearrangement of trans-1,2-cyclohexanediol to cyclopentanecarboxaldehyde can be carried out in 4 M sulfuric acid solution at elevated temperatures. Preparations Tya-2-Bromocyclohezanol, XZZ. The preparation of 1tansQhromocyclohexanol involved modification of the procedure described elsewhere (7). A 500-ml Erlenmeyer flask containing a mixture of 24.6 g (0.30 mole) cyclohexene. 75 ml water. and 100 ml tetrahydrofuran was A thermometer was attached
