Ravikumar et al.
JOCArticle
SCHEME 2. Typical Allenic Halide Synthesis
SCHEME 3. VinylAddition-Metalla-halo-[3,3] Rearrangement
metal bearing an appropriate halide was envisaged to access
the allylic alkoxide 9 and trigger a concerted rearrange-
ment to the corresponding allylic halide 10. As sporadi-
cally happens in chemical research,19 the same concept was
being simultaneously pursued with allylic chlorotita-
nium alkoxides (9, MXn = TiCl3).20 Mechanistic experi-
ments with these titanium alkoxides implicated a stepwise
ionization-halogenation sequence rather than a concerted
rearrangement, although in principle tuning the metal
oxophilicity and halogen nucleophilicity should favor a
concerted halogen transfer.21
Publication of the pioneering titanium-based allylic chlor-
ide synthesis20 was closely followed by communication22 of a
complementary niobium pentahalide procedure. Although
preliminary, the use of niobium broadenedthe substrate scope
and offered the promise of a general approach to both allylic
and allenic halides through a concerted rearrangement. Com-
plete details of these niobium pentachloride and pentabro-
mide rearrangements are provided with an emphasis on
mechanistic insight; extended substrate scope to include all-
ylic alcohols, aldehydes, enals, ketones, and enones; cascade
reduction-halogenation and addition-halogenation strate-
gies; and the synthesis of allenic bromides.
The inherent utility of allylic and allenic halides14 stimu-
lated a direct15 synthesis from carbonyl and alcoholic pre-
cursors. Conceptually the transformation centers on a
metalla-halo-[3,3] rearrangement16 predicated on metal
oxide eliminations17 and the privileged nature of six-mem-
bered transition structures (Scheme 3).18 Addition of a vinyl
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´
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Results and Discussion
The metalla-halo-[3,3] rearrangement strategy requires a
metal halide capable of simultaneously activating the allylic
alcohol, delivering a halogen in an SN20 displacement, and
forming a stable metal oxide. Addition of vinylmagnesium
bromide to 1-naphthaldehyde (1a),23 as with aldehydes in
general,24 forms an allylic magnesium alkoxide but does not
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