Substituent-Dependent Anomeric Effects as a Source of Conformational Preference in Pyridinium Methylides

Abstract

A systematic density functional theory level investigation of differently substituted pyridinium methylides was carried out to determine the role of C(ylidic) lone-pair-associated hyperconjugative and negative hyperconjugative interactions in deciding conformational preferences. Deviation from the coplanar orientation of the carbanionic center with the pyridine ring and its substituent dependence has been found to correlate well with the relative opportunities for conjugative and negative hyperconjugative interactions of a ylidic moiety with different substituent groups present at the ylidic carbon. The contribution of individual n-->pi* conjugative, n-->sigma* negative hyperconjugative, and sigma-->pi* hyperconjugative interactions in a particular conformation of pyridinium dichlorophosphinomethylides was assessed from donor-acceptor stabilization energies, as obtained from natural bond orbital (NBO) analysis. The relative extent of conjugative and negative hyperconjugative interactions with the substituents present at the ylidic carbon plays an important role in permitting the delocalization of ylidic charge into the pyridine ring, thereby controlling the relative orientation of the latter with the carbanionic plane.

Carbanions and Electrophilic Aliphatic Substitution

“…The lithium enolate formed from methyl S-trityl mercaptoacetate (8) has been Calkylated in high yield at or below −40 • C (Scheme 4). 37 At higher temperatures, the [1,2]-thio-Wittig rearrangement of the enolate was the predominant process.…”

“…A systematic density functional theory level investigation of differently substituted pyridinium methylides has been carried out to determine the role of C ylidic lone-pairassociated hyperconjugative and negative hyperconjugative interactions in deciding conformational preferences. 8 Deviation from the coplanar orientation of the carbanionic centre with the pyridine ring and its substituent dependence have been found to correlate well with the relative opportunities for conjugative and negative hyperconjugative interactions of an ylidic moiety with different substituent groups present at the ylidic carbon.…”

Conformations of Dimethoxydimethylsilane: Matrix Isolation Infrared and ab Initio Studies

“…To understand the nature of the orbital interactions in determining conformational preferences, Natural bond orbital (NBO) (Version 3.1) analysis, invoked through Gaussian G98W, were performed at HF and B3LYP levels of theory, using the 6-31++G** basis set. The use of NBO has been shown to provide a detailed understanding of conformational preferences in several such systems. − To ascertain the role of orbital interactions, we also performed calculations, where specific orbitals were deleted from the function space, using the NBODEL option.…”

What governs nitrogen configuration in substituted aminophosphines?

“…d p bonding) and other stereochemical factors influencing the P-N rotational barrier in aminophosphines. [24][25][26][27] Negative hyperconjugation has also been used to explain conformational preferences of fluoroalkyl groups, fluoroamines, [28] aminoborane, [29,30] alkyl-lithium complexes, [31] phosphazenes, [32,33] and pyridinium methylides, [34,35] to name only a few. In this paper, recent computational methodologies are used to examine the role of lone-pair delocalization and other stereoelectronic effects that establish nitrogen planarity in aminophosphines and phosphoramidite ligands.…”