Dehydration of 3,4-dimethyl-3,4-hexanediol to the six possible C8H14 dienes and proof of structure of the substituted butadienes

Abstract

Dimethyl-3,4-hexanediol (1) has been dehydrated, using a variety of reagents, to mixtures of the six isomeric CgHu dienes. The use of acidic reagents, or iodine in propionic anhydride, favors the formation of cis,cis-3,4-dimethyl-2,4-hexadiene (2) and cis,<rares-3,4-dimethyl-2,4-hexadiene (3), the dehydration following the Saytzeff pathway. An initial cis double bond is favored over an initial trans configuration because the conformations of the diol leading to a cis double bond exhibit less steric crowding. Phenyl isocyanate as a dehydrating agent is unusual in that double bond formation predominately follows the Hofmann pathway; even 2,3-diethylbutadiene can be prepared in this manner.

Cyclobutene photochemistry. Involvement of carbene intermediates in the photochemistry of alkylcyclobutenes

“…ldentification of E-and 2-4 was not so straightforward. The 'H nmr spectra of the two isomers exhibit substantially different chemical shifts for the four methyl groups; these allow a tentative assignment to be made on the basis of comparisons with the nmr spectra of the isomeric 3,4-dimethyl-2,4-hexadienes, whose identities have been previously assigned (20). The isomers have been more rigorously identified by uv photoelectron (pe) spectro~copy,~ comparing the magnitude of the splitting between the two lowest energy bands in the pe spectra with calculated (MNDO) values.…”

“…The filtered extracts were distilled to remove the solvent and then bulb-to-bulb distilled under vacuum (ca. 0.2Torr) to yield a colourless liquid (5 g, 0.04mo1, 56%), identified as 4,5-dimethylhex-2-yn-4-01 (7) Alkenol8 (1.0 g, 0.0075 mmol), propionic anhydride (5 mL), and a small amount of iodine were placed in a 10 mL roundbottom flask equipped with a reflux condenser and magnetic stirrer (20). The mixture was stirred at ca.…”

Cyclobutene photochemistry. Substituent and wavelength effects on the photochemical ring opening of monocyclic alkylcyclobutenes

“…The solvent was removed on the rotary evaporator to yield a slightly yellow oil (2.3 g, 75%); the major component of this product was identified as E-4-hexen-3-01 on the basis of its 'H NMR spectrum ( 1H). ' The crude alkenol from above was combined with propionic anhydride (10 mL) and a crystal of iodine in a 25-mL round-bottom flask fitted with a reflux condenser and magnetic stirrer (29). After the initial frothing had subsided, the mixture was stirred at 130°C for 1 h. After cooling to room temperature, the mixture was poured into ether (30 mL) and the solution was washed with saturated aqueous bicarbonate (3 X 40 mL) and water (2 X 20 mL), and then dried over anhydrous sodium sulfate.…”

“…The second isomer is one of two major products from thermolysis of cis-3 (vide infra). The first two dienes that eluted exhibit the following spectral characteristics: 3,4-Dimethyl-2,4-hexadienes (E,E-, E,Z-, and Z,Z-6) Authentic samples of E, E-and E,Z-6 were prepared by dehydration of 3,4-dimethyl-3,4-hexanediol with iodine and propionic anhydride (29), and isolated from the reaction mixture by semi-preparative VPC (column (a)). The Z,Z isomer was prepared from E,E-6 by direct photoisomerization (20).…”

“…The solvent was removed on the rotary evaporator, the residue was bulb-to-bulb distilled under vacuum, and the dienes were isolated by semi-preparative VPC (column (b)). They were identified on the basis of comparisons of their 'H NMR spectra to those previously published (29). The three isomers elute from the SPB-1 column in the order (Z,Z) < (E,Z) < (E,E).…”