In (alkyl)di(aryl)(8-dim ethylamino-naphth-l-yl)phosphonium salts and in the parent phos phines, there is no experimental evidence of dative N -^P interactions. Conclusions to the contrary are based on the choice of improper standards of comparison.In tro d u c tio n /?e/7 -D isubstituted naphthalenes continue to be cherished objects o f research for the investigation of donor/acceptor (D ->A) interactions. This includes com pounds w ith donor substituents D o f only m od erate nucleophilicity and "acceptor" substituents A w hose electrophilicity is, at best, rather w eak or even doubtful. E .g ., a /?m -dim ethylam ino group or even a m ethoxy group has frequently been cho sen as the donor substituent [2 ], though 1-dim ethylam ino-naphthalene and 1 -m ethoxy-naphthalene are o f the dim ethylaniline and anisole type, respec tively.A m ong tertiary am ines, dim ethylaniline is a fairly w eak nucleophile. In 8-substituted 1-dim ethylam ino-naphthalenes, sterical hindrance in the peri space forces the (C H^^N group out of the Cio plane (see form ula 1) [3] so that efficient resonance o f the lone pair at N and the arom atic system is precluded; however, the N atom, bound to tw o sp3-carbons and one sp2 -carbon atom , neverthe less is a m uch poorer nucleophile than in aliphatic tertiary am ines (for com parisonmutatis mutan dis -note that 1-dim ethylam ino-naphthalene is less basic and N ,N ,2,6-tetram ethyl-aniline (2) only very slightly more basic than dim ethylaniline w hereas all three o f them are less basic than trim ethylam ine by more than five pow ers o f ten in the K B scale [4]). In 1 -m ethoxy-naphthalenes, sterical inhibition of res onance is m uch sm aller even in the presence of 8-substituents, and the cr-donicity of the anisole-type oxygen atom is, indeed, very poor.Am ong the "acceptor" substituents, am ide [2], silyl [5] and phosphino groups [6 -9] have been used w hich range, at best, at the bottom end of any electrophilicity scale. Such choice is surprising be cause the geom etry o f the fairly rigid naphthalene skeleton puts the peri substituents at a "natural" dis tance o f ca. 250 pm [10,11], much too long for a conventional bonding interaction [12]. The form a tion o f a dative bond D^A [13] of norm al length would require a considerable distortion of the Cio skeleton w hose energetic bill could be paid only by a strong p eri bond (such as a C-C or an am ide N-CO bond [14]). For the bonding interaction between a weak nucleophile and a p o o r electrophile, the naph thalene skeleton w ould act as a spacer which pre vents the substituents from com ing into bonding distance.Though still very m oderately so, a phosphonium phosphorus atom is a better electrophile than P in phosphines [15]. Earlier, we had investi gated the possibility o f a dative (C H 3)2N M P + in teraction in (alkyl)(8-dim ethylam ino-naphth-l-yl)-[=D A N ]di(phenyl)phosphonium salts [16]. Such in teraction could be described either as an isomerism (3A ^ 3B) or as a bond/no bond resonance (3A <->• 3B) [16]. In the form er case...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.