Can Polarization of Triple Bond in Tolanes Be Deduced from ¹³C NMR Shifts? Re-evaluation of Factors Affecting Regiochemistry of the Palladium-Catalyzed Hydrostannation of Alkynes

Abstract

Polarization of the triple bond in a series of differently substituted ortho- and para-tolanes has been studied by NMR and computational methods in order to examine if (13)C NMR data can be effectively used for the assessment of electronic polarization of a triple bond in diarylacetylenes. DFT calculations of both natural charges and NMR properties revealed that chemical shifts concur with effective charges on sp-carbon atoms in para-tolanes, whereas in ortho-analogues magnetic anisotropy complicates the analysis making (13)C NMR data inapplicable for ascribing triple bond polarization. The obtained information was used to reevaluate factors affecting the regiochemistry of the Pd-catalyzed hydrostannation of the triple bond in tolanes. Computational study on the polarization of triple bonds taken together with the experimental data on hydrostannation of various mono- and disubstituted tolanes bearing para- and ortho-substituents demonstrated that the regioselectivity of hydrostannation is governed by a combination of electronic and steric factors. In para-tolanes, the electronic effect prevails and alpha- and beta-vinylstannanes are obtained predominantly for substrates with electron-withdrawing and electron-donating groups, respectively. In the ortho-series, steric factors dominate over electronics and alpha-isomers are produced with high selectivity regardless of the substituents' nature. However, it was found that in disubstituted "push-pull" tolanes steric control of an ortho-group can be overruled by the very strong electronic effect of an electron-withdrawing substituent in para-position.

Rhodium‐Catalyzed Hydrosilylation of Internal Alkynes with Silane Reagents bearing Heteroatom Substituents. Studies on the Regio‐/Stereochemistry and Transformation of the Produced Alkenylsilanes by Rhodium‐Catalyzed Conjugate Addition

“…Gevorgyan recently reported regiochemical studies on the palladium-catalyzed hydrostannylation of unsymmetrical internal alkynes. [6] The course of the regiochemistry was shown to be similar to that of the present rhodium-catalyzed hydrosilylaton.…”

“…The stereochemical outcome of hydrosilylation contrasts to that with a ruthenium catalyst reported by Trost to induce the trans-addition. [5,6] ð2Þ COMMUNICATIONS …”

Microwave‐Promoted Synthesis of N‐Heterocycles by Tandem Imination/Annulation of γ‐ and δ‐Ketoalkynes in the Presence of Ammonia

“…It is well‐known that one of the most important parameters determining the NMR chemical shift is the shielding effect determined by the electron density around the nucleus of interest. Moreover, the chemical shift may depend also upon the presence of more or less proximate anisotropic groups,27 and for this reason, we did not evaluate the chemical shift of alkynyl‐benzaldehydes bearing an o ‐substituted aryl group on Cβ. Thus, taking into account that both the shielding cone of the triple bond and the substituent on Cα are the same for all substrates 2 , the differences in Cα and Cβ chemical shifts for 2b , 2c , 2e and 2i are only related to the nature of the substituent bonded to Cβ.…”

Regular Acyclic and Macrocyclic [AB] Oligomers by Formation of Push–Pull Chromophores in the Chain‐Growth Step

“…Compound 3 was commercially available. Compounds 1 a, [8a] 1 b, [13] 1 c, [14] 1 d, [15] 2 a, [13] 2 b, [13] 4, [16] 5 a, [8a] 5 c,…”