C O M M U N I C A T I O N S
5b, 5e, and 5f. Furthermore, the double bond geometry in the
products was exclusively trans.
the finding that electron-rich pincer complexes (such as 1a and
1c) are reluctant to undergo transmetallatation with allyl metal
species.6 Thus, the carbon-boron bond of the allylboronic acid
products 6 is not cleaved by 1a, allowing the subsequent isolation
of the products (7 and 8). The above-described reactivity of 1a has
at least three advantageous features compared to that of palladium(0)
catalysts, such as Pd2(dba)3 and Pd(PPh3)4: (i) the 1a-catalyzed
reactions proceed without formation of potentially unstable (η3-
allyl)palladium complexes (11), (ii) the electron-rich SeCSe
complex does not react with the allylboronate products to give bis-
allylpalladium (such as 12) or related complexes, and (iii) the pincer
complex catalyst is not reduced to palladium(0) under the applied
conditions, and therefore deactivation of the catalyst by precipitation
of palladium-black can be avoided. Furthermore, the above
presented transformations also widen the synthetic scope of
application of 2a in palladium-catalyzed coupling reactions. As far
as we know, the only previous use of 2a in these types of processes
was presented in a patent.5e
We have also employed other pincer complex catalysts in the
above process (eq 1), such as 1c and 1d. It was found that 1c
displayed a very low catalytic activity, while 1d proved to be
inactive as catalyst. Miyaura and co-workers5a,b have shown that
allyl acetates can be converted to allyl pinacolboronates (9) in the
presence of bis(pinacolato)diboron (2b) using Pd2(dba)3 catalyst
(eq 2). The allyl-allyl coupling process is a well-known side
reaction of this catalytic transformation. Formation of side product
10 can be explained by the reaction of allylboronate product 9 with
allylpalladium intermediate 11 to give bis-allylpalladium complex
12, which undergoes allyl-allyl coupling (eq 2).5c On the other
hand, Kabalka and co-workers4l obtained densely functionalized
allylboronates from palladium-catalyzed cross-coupling of Baylis-
Hillman acetate adducts with 2b without formation of allyl-allyl
coupling products (such as 10). These findings indicate that
formation of bis-allylpalladium complexes (such as 12) can be
avoided for certain types of allyl acetate substrates.
In summary, we have devised an efficient pincer complex-
catalyzed reaction for synthesis of functionalized allylboronates.
The primary products (6) can be converted to either trifluoro(allyl)-
borates (7) or other allylboronates (8), which are useful, highly
selective reagents in advanced organic synthesis and natural product
chemistry.3,4
Acknowledgment. This work was supported by the Swedish
Research Council (VR).
Supporting Information Available: Experimental procedures and
characterization and NMR spectra of the products. This material is
We have also attempted the present substitution reactions (eq 1)
using Pd2(dba)3 and Pd(PPh3)4 catalysts in place of 1a. These
transformations resulted in complex mixtures of several unsaturated
products. For example, the reaction of 5c and 2a with Pd(PPh3)4
led to full conversion of the starting material; however, formation
of 6h could not be detected (cf. entry 9). On the other hand, the
reaction of Pd2(dba)3 with allyl acetates 5c-f gave traces (5-30%
conversion) of the corresponding allyl boronic acid products (6c-
f). However, these transformations did not proceed with full
conversion of the starting materials, because of deactivation of the
catalyst accompanied by precipitation of palladium-black, which
was also observed for the reaction with 2b.5c The best result with
Pd2(dba)3 was achieved with 5e giving about 20% isolated yield
of 7j. Interestingly, the reaction of 5c with Pd2(dba)3 resulted in a
very low conversion (5%) to 6h; however, we observed formation
of a considerable amount of butadiene in the reaction mixture. This
finding can be explained by formation of a â-OAc-substituted
allylpalladium intermediate (11, R ) CH2OAc), which is known5d
to easily dissociate an acetate ion providing butadiene.
The above results clearly indicate that the catalytic activity and
selectivity of pincer complexes 1c,d and palladium(0) catalysts
Pd2(dba)3 and Pd(PPh3)4 are inferior to that of SeCSe complex 1a
in the presented boronate transfer reactions (eq 1). Although the
exact mechanism of the 1a-catalyzed reaction is not known, several
mechanistic features are probably similar to the trimethyltin transfer
reactions from hexamethylditin to allylic/propargylic substrates.2
In these transformations, the catalytic cycle is initiated by trans-
metalation of the dimetallic reagent to the pincer complex catalyst
followed by transfer of the organometallic group to the allylic
(propargylic) substrate in an SN2/SN2′-type reaction.2b,c Accordingly,
we assume that the first step of the present boronate transfer process
is formation of a boronate coordinated pincer complex intermediate
1b, and subsequently, the high energy Pd-B σ-bond initiates the
transfer of the B(OH)2 group from palladium to the allylic substrate.
The high selectivity observed in this reaction can be explained by
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