C O M M U N I C A T I O N S
a methanol molecule in the six-membered ring TS (6) of the process.
In 7, the hydroxy group is converted to a better leaving group, and
moreover, the cleavage of the B-B bond is also facilitated by
coordination of the water molecule produced in the esterification.
Application of small amounts of PTS (entries 10 and 11, Figure 1)
is supposed to catalyze the ester formation.
Figure 1. Formation of 4a from 3a (b, 4) and cinnamyl acetate (0) using
2 and 5 mol % of 1 in DMSO-d6/MeOH-d4 mixture at 55 °C. Effects of
addition of 5 mol % PTS (4).
In summary, we have shown that allyl alcohols can be converted
to allyl boronates using 2 and catalytic amounts of 1. Functionalized
allyl boronates are very useful precursors in natural product and
advanced organic synthesis,3a-d,7a,b which explains the great current
interest to find efficient synthetic methods for preparation of these
species.4a,7 The above presented robust catalytic transformation
employs the least expensive allylic precursors reported for synthesis
of functionalized allyl boronates, and therefore, this method is also
a good example for the synthesis of highly value-added chemicals.
withdrawing substituents usually give the other regioisomer, the
1,4-substituted product.1a,5
In the presence of COOR groups (3j,k), the rate of boronation
reaction drops considerably. However, we found that addition of
catalytic amounts (3-5 mol %) of strong acids, such as p-toluene
sulfonic acid (PTS), considerably accelerated the conversion of 3j,k,
affording the corresponding boronated products 5i and 5j. Again
5i was obtained as a single diastereomer, clearly indicating that
the boronation reactions proceed with high trans regioselectivity.
We found that using a DMSO/MeOH mixture is indispensable
to obtain high reaction rates and high yields. When the reactions
were performed in pure DMSO, a very slow reaction with low
conversion (about 5-20%) of the allyl alcohol substrate occurred.
On the other hand, catalyst 1 is poorly soluble in pure MeOH, and
therefore, only the most reactive allyl alcohols 3h,i react under mild
conditions using MeOH as solvent (entry 8). As mentioned above,
allyl hydroxy boronates tend to decompose to 1,3-dienes. This
decomposition is particularly fast for 4h in MeOH, decreasing the
yield of the boronation reaction. We found that in a DMSO/water
mixture the rate of consumption of 3i is slower than that in MeOH;
however, the corresponding boronated product 4h/5h could be
obtained in high yield (entry 9), indicating that 4h is stable in the
presence of water as cosolvent.
Acknowledgment. This work was supported by the Swedish
Research Council (VR).
Supporting Information Available: Experimental procedures as
well as characterization and NMR spectra of the products. This material
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To gain insight into the activation of the hydroxy group, we
carried out competitive boronation experiments (Figure 1) monitored
1
by H NMR. We have previously shown4a that cinnamyl acetate
and 2 in the presence of catalytic amounts of 1 afford cinnamyl
boronic acid 4a. Surprisingly, under the same reaction conditions,
cinnamyl alcohol 3a was converted significantly faster to boronic
acid 4a than was cinnamyl acetate. Thus, 3a was converted
quantitatively to 4a in 8 h, while its acetate derivative was still
present in the reaction mixture after 11 h.6 Addition of 5 mol % of
PTS had a dramatic effect on the rate of the reaction, as boronation
of 3a was complete in 2 h accounting for a 4-fold acceleration of
the reaction. Allylboronic acid 4a was surprisingly stable7b in the
presence of PTS, as in 9 h only 5% of 4a was decomposed (Figure
1).
The above studies clearly indicate that under the employed
reaction conditions the hydroxy group of the allyl alcohol is
converted to an excellent leaving group, which is easier to displace
than an acetate. A possible explanation is that 2 acts as a Lewis
acid catalyst by interacting with the free electron pairs of the oxygen
of 3a-k. A similar activation is suggested in the Tamaru reaction
employing BEt3 for activation of allyl alcohols.1c-e On the other
hand, boronic acids are far less efficient Lewis acids than are alkyl
(or halo) boranes. Therefore, we envision another type of activation
of the hydroxy group involving formation of allyl boronic acid ester
7 (eq 2). This esterification is probably facilitated by inclusion of
JA060468N
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