Regioselective Catalytic Transformations Involving β-Silyl-Substituted (η³-Allyl)palladium Complexes:  An Efficient Route to Functionalized Allylsilanes

Abstract

Various alkyl derivatives of 1-(trimethylsilanyl)but-3-en-2-ol acetate (1a−e) undergo regioselective palladium-catalyzed nucleophilic substitution via β-silyl-substituted (η3-allyl)palladium intermediates. With external nucleophiles, such as malonates and enolates, the nucleophilic substitution occurs with complete allylic rearrangement, providing functionalized allylsilanes as building blocks of high synthetic potential. Internal nucleophiles, such as disilanes and NaBPh4, afford bisallylic disilanes and (allylsilyl)benzene derivatives with good regioselectivity. For both types of nucleophiles, the double bond geometry of the resulting allylsilane is selectively trans. The β-silyl-substituted (η3-allyl)palladium intermediates of the reaction were also isolated. The 1H NMR studies indicate selective formation of the syn-isomer of the key (η3-allyl)palladium intermediates, which explains the high trans-selectivity of the double bond formation in the allylsilane products. According to the 13C NMR studies, the β-silyl functionality exerts deshielding effects on the nearest allylic terminal carbon (C3), which can be ascribed to hyperconjugative interactions between the silyl functionality and the allylpalladium moiety. It was concluded that, together with the steric effects of the silyl group, these electronic interactions are responsible for the high regioselectivity of the nucleophilic attack in the catalytic process.

Understanding the mechanism of Pd-catalyzed allylic substitution of the cyclic difluorinated carbonates

“…Unexpected and specific regioselectivity of Pd-catalytic asymmetric reactions of the gem -difluorinated allylic intermediates 1 and 4 prompted us to investigate further the mechanism of these reactions. Currently, one of the most direct tactics for mechanistic investigation of Pd-AA reaction was built on the analysis of crystal structure and 13 C NMR spectroscopy of Pd-π-allyl complex [ 19 – 20 ]. The orientation of attack of nucleophiles on the Pd-π-allyl complex could be illustrated via examining the 13 C NMR chemical shifts of three carbon atoms attached to the palladium.…”

Nature of the interaction between β-substituents and the allyl moiety in (η3-allyl)palladium complexes

“…2). 36 Comparison of the 13 C NMR shifts of the allylic terminal carbons in 22 and 21a-b reveals the perturbation effects of the SiMe 2 R group on the electronic structure of the allylic moiety. The chemical shift difference between the terminal carbons (Dd t ) increases in the presence of the silyl substituent.…”

“…38 The regiochemistry of the nucleophilic attack was also studied for b-silyl substituted (h 3 -allyl)palladium complexes. 23,36 Complex 19 was reacted with sodium diethyl malonate (Scheme 12), providing the 1,4-substituted product 20 with very high regio-and stereoselectivity. Since the external attack by the malonate proceeds by a trans mechanism, 37 it also involves steric influence of the 4-SiMe 2 Ph group.…”

“…Transformation of 16 afforded trans-1,4-disilyl compound 17b indicating that the nucleophilic attack is selective even in the absence of the steric influence of the silyl functionality. Similar results were found for acyclic silanes 21a-c and 23a-c. 36 The external attack with malonates proceeds with very high selectivity providing functionalized allylsilanes. The internal nucleophiles also predominantly attack at the less substituted allylic position, however the regioselectivity is somewhat lower than for the external attack.…”

Allylation of Aldehyde and Imine Substrates with In Situ Generated Allylboronates – A Simple Route to Enatioenriched Homoallyl Alcohols

“…[26,27] The next step is addition of the diboron reagent to the allyl moiety of 9b to form the transient allylboronate 11. Considering these type of reactions with analog dimetal reagents, [10,[17][18][19][28][29][30] such as hexaalkyl/aryldisilanes (R 3 Si-SiR 3 ) and distannanes (R 3 Sn-SnR 3 ) the boron-carbon bond formation probably takes place by a inner-sphere nucleophilic attack. Thus the nucleophilic attack is preceded by formation of complex 13, in which the boronate group is coordinated to palladium.…”