THE EFFECT OF CONJUGATION ON HYDROGEN IODIDE AND HYDROGEN-FLUORIDE ELIMINATION FROM 1-(F-ALKYL)-2-IODOALKENES - FROM 5-(F-BUTYL)-4-IODO-1-PENTENE TO 5-(F-BUTYL)PENTADIENES AND FURTHER TO 1-FLUORO-1-(F-PROPYL)-1,3,5-HEXATRIENES

Dehydrohalogenation of homologous l-CF-butyl)-2-iodoalkenes, [RFCH2CHI(CH2)rtCH:=CH2 (n = 2-4)] under defined conditions gave (F-butyl)-substituted alkadienes [RFCH=CH(CH2)riCH=CH2] in high yield. By contrast, RFCH2CHICH2CH=CH2(1) gave E,Z isomers of l-fluoro-l-(F-propyl)-l,3,5-hexatriene (3) in 95% yield by way of isomeric 5-(F-butyl)pentadienes (2 and 4). Trapping and identification of organic products by GC/MS and FT/IR gave rate of conversion of 1 to intermediate dienes 2 and 4 and of dienes to triene isomers 3. Samples were separately titrated for HI and HF. Kinetic order of reactions and rates by these independent methods were concordant. IR, UV, and mass supectra were useful in identifying separated GC peaks, and NMR supectra of isolated triene 3 confirmed its structure. Conjugated diene 3E-2 was the major intermediate on the path from 1 to 3; rate of disappearance of 2 and formation of 3 were almost identical. Surprisingly, elimination of HF from 2, and its unconjugated isomer 4E-4, occurred at similar rates. Elimination from 4E-4 may have involved a concerted loss of proton and fluoride ion, with a shift of the double bond to give the all-conjugated triene 3. Novel stereochemistry in these elimination reactions, mass supectrum fragmentation patterns, and other supectroscopic results are reported. © 1990, American Chemical Society. All rights reserved.