Page 5 - From -Arylation of Olefins to Acylation with Aldehydes: A Journey in Regiocontrol of the Heck Reaction

R-Arylation of Olefins to Acylation with Aldehydes Ruan and Xiao

TABLE 5. Arylation of Unsaturated Alcohols in DMSO[bmim][BF 4 ] TABLE 7. Ligand Effect on the Arylation in [bmim][BF 4 ]

ligand bite angle (deg) conversion (%) R/β E/Z of β product
none 4
PPh 3 66 76/24 75/25
P(o-toly) 3 <1
dppm 72 <1
dppe 85 17 >99/1
dppp 91 100 >99/1
BINAP 92 100 93/7 89/11
dppf 96 100 50/50 75/25
dppb 98 100 45/55 79/21
xantphos 112 100 31/69 72/28

TABLE 8. Effect of Ligand Electronic Properties on the Arylation in
[bmim][PF 6 ]

TABLE 6. Arylation of 5-Hexen-2-one in [bmim][BF 4 ]

(pathway B, Scheme 3). 8,25 While ligands with smaller bite
angles would be less inhibitive toward olefin rotation, dis-
sociation of the halide ion or the insertion itself could
be slow.
We also examined the electronic effect of the ligand.
Contrary to the Heck reaction proceeding via the neutral
appears to be a correlation between the catalyst activity and pathway A, where electron-rich monophosphine ligands are
3
selectivity with the bite angles of diphosphines. 23 Full con- generally favored, the results in Table 8 show that the
versions were only observed with ligands with relatively R-arylation of electron-rich olefins benefits from electron-
large bite angles (ca. >90°), but high regiocontrol was only deficient diphosphine ligands. 26 The higher rates associated
possible with ligands of medium bite size. Large bite angles with these ligands may be attributed to a reduced olefin
could facilitate the dissociation of halide ions from palla- insertion barrier, and this could in turn stem from an easier
dium due to steric repulsions, 24 increasing the concentration rotation of the olefin preceding insertion due to reduced π
of Pdolefin cations and R-selectivity as a result. However, back-donation from Pd(II), and from an enhanced positive
when the bite angle becomes too large, olefin rotation from charge on the coordinated olefin. 10,27 DFT calculations
the initial out-of-plane coordination to an in-plane position revealed indeed a lower insertion barrier for the electron-
necessary for insertion could be inhibited. Previous studies deficient dppp analogues. 26
suggest that the ionic pathway is rate-limited by the olefin An inevitable question is why the ionic liquid is so
insertion step and there is a pre-equilibrium between the effective in promoting the R-arylation. We believe that the
þ
neutral [L 2 Pd(Ar)X] and the ionic [L 2 Pd(Ar)(olefin)] species formation of the key cationic Pd(II)olefin species is favored

618 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 614–626 ’ 2011 ’ Vol. 44, No. 8