625-1
T. L. [CONTRIBUTION FROM
THE
JACOBS,
E. G. TEACH AND D. WEISS
DEPARTMENT OF CHEMISTRY OF
THE
Vol. 77
UNIVERSITY OF CALIFORNIA, Los ANGELES~
The Dehalogenation of Allenic Halides1 BY
THOMAS
L.
JtZCORS,
EUGENE G. TEACH^
.&ND DANIEL
WEISS
RECEIVED JUNE 10, 1955 The allenic halides l-chloro-1,2-hexadiene, l-chloro-3-methyl-l,2-butadiene and bromopropadiene reacted with a zinccopper couple to give the same mixtures of allenic and acetylenic hydrocarbons as the corresponding propargyl halides and 3-bromopropyne. Rearrangement of the propargyl halides to the allenic 3-chloro-l-hexyne, 3-chloro-3-methyl-1-butyne halides did not occur under these conditions. These same compounds were dehalogenated with lithium aluminum hydride. The propargyl halides gave results qualitatively like those obtained with zinc-copper, but the ratio of allenic hydrocarbon to acetylenic hydrocarbon was different except with 3-chloro-3-methyl-1-butyne which gave only 3-methyl-1,2-butadiene with either reagent. Bromopropadiene gave results but little different from 3-bromopropyne, but the two chloroallenes gave mainly saturated hydrocarbons and oldns, along with minor amounts of the corresponding acetylenic hydrocarbons.
The dehalogenation of propargyl halides I (R = and the corresponding propargyl compounds were alkyl, R’ = alkyl or H) to yield allenic hydrocar- dehalogenated both with lithium aluminum hydride bons I11 mixed in most instances with some of the and with a zinc-copper couple in ethanol, The corresponding acetylenic hydrocarbons I V has results are summarized in Table I. been effected in several ways; the process has pracTABLEI tical value as a method for the synthesis of allenes 111 when the dehalogenating agent is lithium alu- DEHALOGEY.4TIO\ O F PROPARGYLIC AhD ALLENICHALIDES Yield of Ratio of minum hydride314or a zinc-copper c ~ m b i n a t i o n . ~ - ~ hydroallene t o RR’CXC=CH I R R ’C=C=CHX 11
IR R f c = c = c H 2 7
I11
+ RR’CHC=CH IV
Halogen compound
Solventa
BrCHgC=CH BrCH=C=CH* Z-C~H~CHCICSCH n-C3H,CH=C=CHCl (CHs)?CClCECH
Zn-Cu EtOH EtOH EtOH EtOH EtOH
carbon,
82
yo acetylene
2 2 36 36
88
63
-It was of interest to compare allenic halides I1 93 with the propargyl halides from which they are ob51 b tained by rearrangement to discover whether the L141H1 former are intermediates in the dehalogenation of DEC 8%5 ( 0 02 the latter and to investigate the course of the reac- RrCH2C=CH c DEC 90 tion in which halogen is replaced by hydrogen in BrCH=C=CHI n-C3H7CHClCZCH Et20 56 c both types of compound. DEC 31 d The reduction of l-chloro-3-methyl-l,2-butadi-n-CXH?CH=C=CHC 1 THF 54 h ene (11,R = R’ = CH3; X = C1) by zinc and cop- (CH,)zCClC=CH d THF 61 per powder in ethanol or butanol was reported to (CHi)zC=C=CHCl d DEC 74 give pure 3-methyl-lJ2-butadienein excellent yield and free from 3-methyl-l-b~tyne.~3-Chloro-3Solvents: EtOH = anhyd. ethanol, DEC = DiethylZ)~~ =, diethyl ether, THF methyl-1-butyne (I, R = R’ = CH3; X = Cl), carbitol ( C ~ H ~ O C H ~ C HEtzO = tetrahydrofuran. No 3-methyl-1-butyne could be degave the same product in slightly lower yield (63% tected with ammoniacal silver nitrate solution. No allene compared with SFI%).~ The dehalogenation of 1- could be detected by infrared. The product appeared to be chloro-3-ethyl-1,2-pentadiene (11, R = R’ = CzHs; pure propyne. -4 small amount of acetylenic material X = CI), required a higher temperature and was could be removed by distillation before hydrolysis. This was shown to be 1-alkyne by reaction with ammoniacal carried out in boiling 2-etho~yethanol.~No other silver nitrate. Hydrolysis of the reaction mixture was an reports of dehalogenations of allenic halides were exothermic reaction which gave a mixture of hexane and encountered. hexenes from 1-chloro-1,a-hexadiene; isopentane and 3The allenic halides used in the present investiga- methyl-1-butene from l-chloro3-methyl-l,2-butadienein tion were 1-chloro- 1,2-hexadiene, 1-chloro-3- tetrahydrofuran; and mainly isopentane in Diethylcarbitol. methyl-1,2-butadiene and bromopropadiene. These TVhen zinc-copper dehalogenation was employed, (1) This work was carried o u t largely under a contract with the the allenic halides were found to give the same reOffice of h-aval Research, b u t a few of t h e experiments were done under sults as the propargyl compounds. l-Chloro-1,2a contract with t h e Office of Ordnance Research. Reproduction in hexadiene and 3-chloro-1-hexyne were found to rewhole or in part is permitted for a n y purpose of t h e United States act a t about the same rate under the conditions Government. P a r t of t h e material is taken from t h e Ph.D. thesis of Daniel Weiss. A preliminary report of this work was given a t t h e described by Hennion and Sheehans and the same New York meeting of t h e American Chemical Society, September, product was obtained from both as judged by re1954 (see Abstracts of Papers Presented, page 2 4 - 0 ) . fractive indexes. -An experiment starting from .?(2) Western Regional Laboratory, U. S . Department of Agriculture, chloro-1-hexyne was interrupted after about half Albany, California. ( 3 ) n‘.J . Bailey and C. R. Pfeifer, J . Oug. Chem., 2 0 , 55 (19.55). the chloride had reacted and the unused chloride (4) J. H TVotiz, THIS JOURNAL, 73, 693 (1951). was recovered ; this recovered material was un( 5 ) 17a. I. Ginzburg. Z h u r . Obshchei K h i m . , 10, 513 (1940); C. A . , 34, changed S-chloro-1-hexyne and no l-chloro-1,Z-hex7843 (1940). adiene could be detected. It was found that 1( 0 ) G. F. Iiennion and J. J. Sheehan, THIS J O U R N A L , 71, 1964 (1949). hexyne was unchanged by refluxing in ethanol with (71 T. L . Jacobs, R . Akawie and R . G . Cooper, ibid., 73, 1373 a zinc-copper couple or with the zinc-copper residue (1951). which had been used to dehalopenate 3-chloro-I(8) Ya. I. Ginzburg. Z h w . Obshchci K h i m . , 1 5 , 442 (1945); C. A . , hexyne. 40, 4665 (1546). I
+
Dec. 5 , 1965
THEDEHALOGENATION O F ALLENIC HALIDES
6255
e @ Dehalogenation of l-chloro-3-methyl-l,2-butadiene by zinc and copper6 was not repeated, but the addition of the reagent as H --- A1
