7
542
P. C. Andrews et al. / Tetrahedron Letters 43 (2002) 7541–7543
water (ca. 0.5 ml) and extracted with Et O (3×15 ml).
attention due to an influx of these compounds into the
2
1
The products were analysed by H NMR and GC–MS.
environment, resulting from their wide and varied use
9
,10
in industry and as marine anti-fouling agents.
Toxi-
Good yields were obtained for both aromatic and
aliphatic aldehydes (80–100%) (Table 1) comparable to
those produced in equivalent reactions performed in
cologically, the tetra-substituted and tri-substituted tin
organyls are more harmful than the mono- and di-sub-
stituted analogues. For example there is a significant
6
2
–4
H O and ionic liquids. The reaction with trans-cin-
increase in acute oral toxicity values observed in rats,
when moving from butyltin tetrachloride at LD 2140
2
namaldehyde produced consistently only the 1,2-addi-
tion product while the reactions involving ketones, such
as acetophenone and benzophenone, failed to react,
most likely as a result of the reduced reactivity of the
carbonyl group.
5
0
6
mg/kg to tributyltin chloride LD50 129 mg/kg.
It would clearly be of concern and contrary to our aim
of developing benign synthetic protocols if these com-
pounds persisted to any degree in the reaction products.
Unlike the analogous reactions with elemental In, Zn
and Bi, the Sn mediated reactions required ultrasonic
activation of the metal powder and longer reaction
times (ca. 12 h). Also, to achieve high yielding reactions
the use of 4 equiv. of allyl bromide was necessary. In
reactions where only 2 equiv. was added, significantly
lower yields were obtained. However, in contrast with
more traditional synthetic methods for the production
of allylstannanes, normally involving a combination of
From GC–MS studies the presence of both allyltin, and
allyltin bromide species were detected in the organic
extracts of the reactions. Such species have also been
observed in reactions performed in ionic liquids using
2
(CH ꢁCHCH ) Sn. The presence of highly toxic
2
2 4
allyltin compounds contrasts with the results of our
study on In, Bi and Zn, where no intermediate allyl
metal species were detected in the products extracted
into ether, in any of the reactions.
a catalyst (e.g. HgCl or HBr) and heat, the solvent-free
2
7
approach is significantly less cumbersome.
From an analysis of the final reaction mixtures the
The need for induction can be understood by consider-
ing the relative kinetic stability of the metallic Sn
surface, which results from the stable octet configura-
tion of the outer electrons. While the longer reaction
times can also be traced to the relatively high kinetic
stability of the CꢀSn bond in comparison with those of
most reactive Sn species appears to be (CH ꢁ
2
CHCH ) SnBr . The reactions which led only to a
2
2
2
complete recovery of starting compound (i.e ketones)
showed a large amount of (CH ꢁCHCH ) SnBr in the
2
2 2
2
ether extract with relatively small amounts of
(CH ꢁCHCH ) Sn, (CH ꢁCHCH ) SnBr and (CH ꢁ
2
2 4
2
2 3
2
8
other main group metals. To investigate whether ultra-
CHCH ) Sn. Corresponding reactions which produced
2 2
sound was required for both the formation of the
allyltin bromide intermediate and the subsequent allyla-
tion reaction allyl bromide and tin powder were
allowed to react under ultrasonic irradiation for 12 h.
Benzaldehyde was then added to the preformed allyltin
bromide species and the reaction mixture simply stirred
for 4 h. On quenching, the allylic alcohol was obtained
in relatively high yield (ca. 68%). The comparative
reaction with no ultrasound gave no allylic alcohol.
Thus, ultrasonic irradiation is necessary to accelerate
the reaction of the metal, presumably initially activat-
ing the metal surface, but also contributes slightly to
the formation of the Sn alkoxide intermediate.
homoallylic alcohols in high yields showed no trace of
(CH ꢁCHCH ) SnBr , only (CH ꢁCHCH )SnBr ,
(CH ꢁCHCH ) SnBr and (CH ꢁCHCH ) Sn, suggest-
2
2 2
2 3
2
2
2
3
2
2
2 2
ing that (CH ꢁCHCH ) SnBr is the reactive Sn species.
2
2 2
2
To prove this hypothesis we followed the established
reaction protocol without the addition of any carbonyl
compound. On analysis of the ether extract by GC/MS
it was observed that the predominant allyltin species
was in fact (CH ꢁCHCH ) SnBr (m/z 359), with no
2
2 2
2
observations of allyltin derivatives of type R SnBr,
3
RSnBr3 or R Sn (R=CH ꢁCHCH ). Thus, (CH ꢁ
2
3
2
2
CHCH ) SnBr is both the most stable Sn intermediate
2
2
2
formed and the one most responsible for allylation of
Recently the toxicological effects of low molecular
weight alkyl derivatives of Sn have received a lot of
the aldehyde. Surprisingly an additional peak at m/z
439 was also observed which was attributed to
−
[
(CH ꢁCHCH ) SnBr ] (Fig. 1). Formation of such
2
2 2
3
negatively charged species is a known phenomena for
tin complexes of the type R SnX (R=Ph and X=Cl or
3
Table 1. % Yields for allylation reactions of various car-
Br), in negative ion electrospray mass spectrometry and
1
119
bonyl compounds in the presence of Sn
also in H and
Sn NMR, which are conducted in
11
halogen rich environments.
Carbonyl compound Allyl bromide:aldehyde/ketone:Sn Yielda
From this, theoretically only two equivalents of allyl
PhCHO
-HOC H CHO
PhCHꢁCHCHO
4:1:1
4:1:1
4:1:1
4:1:1
4:1:1
4:1:1
98
96
87
82
0
bromide would be required to produce (CH ꢁ
2
2
6
4
CHCH ) SnBr and efficient synthesis of the homoal-
2
2
2
lylic alcohols. However, in reactions where only 2
equiv. were added significantly reduced reaction yields
were obtained and 4 equiv. of allyl bromide were
necessary to obtain high yields. The reason is physical
rather than chemical and stems from the volatility of
3
,4-(MeO)C H CHO
6 3
Ph CꢁO
2
Ph(Me)CꢁO
0
a
Estimated by GC–MS.
Andrews, Philip C.
