Page 668 - Advanced Organic Chemistry Part B - Reactions & Synthesis

2-butenylmagnesium bromide and 1-methyl-2-propenylmagnesium bromide are in 643
equilibrium in solution.
SECTION 7.2
Reactions of
CH CH CHCH MgBr CH CHCH CH
3
3
2
2
Organomagnesium and
Organolithium
MgBr
Compounds
Addition products are often derived from the latter compound, although it is the minor
component at equilibrium. 102 Addition is believed to occur through a cyclic process
that leads to an allylic shift.
Br
Mg O MgBr
O CH 2
CCHCH CH
R 2 2
R C
H CH
R H 3
CH 3
3-Butenylmagnesium bromide is in equilibrium with a small amount of cyclo-
propylmethylmagnesium bromide. The existence of the mobile equilibrium has
been established by deuterium-labeling techniques. 103 Cyclopropylmethylmagnesium
bromide 104 (and cyclopropylmethyllithium 105 ) can be prepared by working at low
temperature. At room temperature, the ring-opened 3-butenyl reagents are formed.

CH 2
CH 2 CHCH CD MgBr CHCH MgBr BrMgCH CD CH CH 2
2
2
2
2
2
CD 2
When the double bond is further removed, as in 5-hexenylmagnesium bromide, there
is no evidence of a similar equilibrium. 106
CH 2 CHCH CH CH CH MgBr × BrMgCH 2
2
2
2
2
The corresponding lithium reagent remains uncyclized at −78 C, but cyclizes

on warming. 107 -, -, and -Alkynyl lithium reagents undergo exo cyclization
to
-cycloalkylidene isomers. 108 Anion-stabilizing substituents are required for the
strained three- and four-membered rings, but not for the 5-exo cyclization. The driving

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103
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105
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106
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108
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