Bromo-3-methyl-l,2--butadiene

Both l-bromo-3-methyl-l,2-butadiene (182) and 3-bromo-3-methyl-l-butyne (183) give in aqueous ethanol the same product mixture, containing mainly propargyl alcohol and ether, but no allenic derivative (Shiner and Humphrey, 1967). In the presence of both PhS and OH ions, an identical product mixture (32% allenic and 52% propargylic thioether) is again formed from both isomers. This and the rate acceleration by base were ascribed to the formation of a common allene-carbene intermediate which was written as (184a 184b) (Scheme 8). Since the... 

Bromo-l,2-butadiene, 180 - (33) l-Bromo-3-methyl-l,2--butadiene, 181 - (32) l-Brorao-l,2-nonadiene, 182 - (33) l-Bromo-3--phenyipropadiene, 183 - (34) 1-Iodo-l,2-butadiene, 183 - (35) 1-Iodo--3-phenylpropadiene, 184 - (36) l-Methylthio-3-iodo-l-propyne, 184 -(37) 3-Cyclopentyl-l-propyne, 185 - (38) Benzyl acetylene, 186 - (39)... 

Methyl propargyl ketone, 193 - (48) 5-Chloro-3-hexen-l-yne and 5-bromo-3-hexen-l-yne, 193 - (49) 4,5-Hexadien-2-yn-l-ol, 194 -(so) l-Phenylthio-3-methyl-l,2-butadiene, 195... 

A number of unsaturated carbenes have been generated under phase transfer conditions. The most extensive work has been reported for dime thy Ivinylidene carbene. Dimethylvinylidene carbene has been generated by base catalyzed 7-elimination of HCl from 3-chloro-3-methyl-l-butyne and by a-elimination of HBr from l-bromo-3-methyl-1,2-butadiene (see Eq. 4.13). Both quaternary alkylammonium salts and various crown ethers have proved to be effective catalysts for this reaction. Yields of dimethylvinylidenecyclopropane products obtained under phase transfer catalytic... 

If the conjugated diene is not symmetrical, the major products of the reaction are those obtained by adding the electrophile to whichever terminal sp carbon results in formation of the more stable carbocation. For example, in the reaction of 2-methyl-l,3-butadiene with HBr, the proton adds preferentially to C-1 because the positive charge on the resulting carbocation is shared by a tertiary allylic and a primary allylic carbon. Adding the proton to C-4 would form a carbocation with the positive charge shared by a secondary allylic and a primary allylic carbon. Because addition to C-1 forms the more stable carbocation, 3-bromo-3-methyl-l-butene and l-bromo-3-methyl-2-butene are the major products of the reaction. 

To a large bottle are added 470.0 gm (5.6 moles) of 2-methyl-3-butyne-2-ol, 1000 ml of 48 % technical grade hydrobromic acid, 200.0 gm (2.04 moles) of ammonium bromide, and 70.0 gm (0.71 mole) of cuprous chloride. The bottle is sealed, shaken at room temperature for 4J hr, opened, and the organic layer is separated. The organic layer is washed twice with sodium bicarbonate solution, once with a saturated sodium bisulfite solution, dried over calcium chloride, and fractionally distilled through a glass-helix-packed column to afford 500 gm (61 %) of almost pure product (ir 1956 cm-1 allene), b.p. 34°C (18 mm), d5 1.5163. The ir showed that the product contained a trace of l-bromo-3-methyl-1,3-butadiene (1580 and 1620 cm-1). 

The addition of HBr to 2-methyl-l,3-pentadiene, 1-bromo-1,3-butadiene, 1-phenyl-1,3-butadiene and 2,4-hexadiene produces 2,4-dibromo-2-methylpentane, 1,3-dibromo-l-butene, 3-bromo-l-phenyl-1-... 

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