Crotyl bromide and α-methylallyl bromide each react with magnesium in ether to yield the same butenyl Grignard reagent, which conceivably could be crotylmagnesium bromide (I), α-methylallyl-magnesium bromide (II), a more or less mobile equilibrium mixture of I and II, or an intermediate, bridged structure such as III.
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Nordlander et al. (1961) studied this question.