corresponding homopropargyl alcohols 4–10 in 43–74% iso-
lated yields. Substitution at the 3-position retards the activity, as
in the case of 3-phenylprop-2-ynyl bromide (entry 8). The
reaction is also chemoselective towards aldehyde over ketone.
Activation of propargyl halides and esters by Pd(0) has been
the subject of intense investigation.9 With reference to carbonyl
propargylation, Tamaru, Marshall and others have recently
shown that facile redox transmetallation can occur between
allenylpalladium to zinc or indium.4 We anticipated that a
similar pathway involving redox transmetallation from alle-
1
nylpalladium to b-SnO could be responsible in our case. H
NMR monitoring of a solution of Pd2(dba)3 (9.14 mg, 0.01 mM)
and 1c (2.38 mg, 0.02 mM) in CDCl3 at regular time intervals
show the slow disappearance of peaks due to 1c (d 2.5, t, 1H and
d 3.9, t, 2H). Concomitant appearance of a peak due to
allenylpalladium is observed (d 4.1–4.3, m, 3H). The integrated
ratio of peak area in each case provides a measure of the relative
concentration of individual species. Treatment of the data yields
a good first order fit for the disappearance of the halide 1c with
a half-life value of 2.2 h21 (Fig. 2). Interestingly the data for the
appearance of allenylpalladium do not respond to first-order;
instead they are representative of slow conversion of one allenyl
isomer to another. Taking hints from the recent work of Ogoshi
et al.,10 we propose a tentative mechanism involving isomerisa-
tion of a mononuclear allenylpalladium I to dinuclear species II.
The latter can undergo redox transmetallation reaction with b-
SnO to afford allenyl-Sn intermediate III (Scheme 2). Ex-situ IR
studies indeed support such a process. Untreated b-SnO shows
absorptions at 315, 500 and 802 cm21. A sample of b-SnO, 1c
and Pd2(dba)3 (2 mol%) in THF is refluxed for 12 h, filtered,
washed several times with THF to remove the catalyst, and
dried under vacuum. The IR spectrum of b-SnO thus treated
show new peaks due to allenylmetal at (cm21): 820 (NCH2
wagging), 1030 (NCH out-of-plane bending), 1619 (–CNCNC–
stretching), and 2920 (NCH stretching). Further work is
underway to characterize the intermediate and extend the scope
of the present reaction.
Scheme 2
Notes and references
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Fig. 2 1H NMR monitoring for the reaction of 1a with Pd2(dba)3. Plot
showing peak area ratio (At/Aref) against time (A), and corresponding
logarithmic plot (B). Acetylenic proton (“). Allenic proton (1).
CHEM. COMMUN., 2003, 534–535
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