Synthesis and Comparison of the Reactivity of Allyl Fluorides and ...

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Chapter Three
In the reaction of (115) in solvents such as benzene, DCM and THF, syn addition occurred
almost exclusively (Table 3.1). In contrast, when MeCN and DMSO were used as the
solvents anti addition was observed. This was the case for both Pd2(dba)3 and
Pd(COD)(MA) starting materials. This may be due to the fact these solvents can prevent
Pd-Cl bond formation inherent in syn addition through coordination to Pd, or by stabilising
the anti addition transition state in which charge separation takes place to a greater extent
than in syn addition. Whereas syn addition which leads to the trans isomer may be
proceeding via an SN2’ mechanism.
With (114) both DCM and THF afforded equimolar amounts of the syn and anti addition
products. Whilst MeCN exclusively yielded the product of anti addition and benzene
afforded predominantly the syn addition product.
Interestingly, when (115) was reacted with Pd(PPh3)4 (Scheme 3.3), anti addition occurred
when both benzene and DCM were used affording the cis isomer exclusively.
Scheme 3.3 Reaction of (115) with Pd(PPh3)4
Further work by Kurosawa et al. examined the influence of the ligand bound to Pd(0), that
was also found to affect the stereochemistry of the oxidative addition. [20] The results in
Table 3.2 reveal that the reactions of Pd(COD)(MA) and Pd(NBE)2(MA), in the absence of
additives proceed with the preferential formation of the syn addition product. Upon the
addition of additives, the percentage of anti product obtained increases. This may be due to
the replacement of the maleic anhydride ligand by the additive, forming anti addition
directive complexes such as Pd(COD)2 or Pd(NBE)3. However, little if any, evidence of
free maleic anhydride was observed when 1 H NMR spectroscopy was conducted on the
mixtures of Pd(COD)(MA) with COD or that of Pd(NBE)2(MA) with NBE [each in a (1:10
ratio)]. This suggests that the Pd(0) complexes possessing the more electron-donating olefin
ligands, Pd(COD)2 or Pd(NBE)3, undergo anti oxidative addition much faster than
Pd(COD)(MA) and Pd(NBE)2(MA), undergo syn addition. In contrast, Pd(NBE)3 formed
almost exclusively the anti product. However, addition of electron-withdrawing additives
caused the selectivity to decrease forming more syn product.