Negative Hyperconjugative Interactions Research Articles

Structures and reactivities of 2-Trityl- and 2-(Triphenylsilyl)pyrrolidine-derived enamines: evidence for negative hyperconjugation with the trityl group.

X-ray structures of enamines and iminium ions derived from 2-tritylpyrrolidine (Maruoka catalyst) and 2-(triphenylsilyl)pyrrolidine (Bolm-Christmann-Strohmann catalyst) have been determined. Kinetic investigations showed that enamines derived from phenylacetaldehyde and pyrrolidine (R = H) or 2-(triphenylsilyl)pyrrolidine (R = SiPh3) have similar reactivities toward benzhydryl cations Ar2CH(+) (reference electrophiles), while the corresponding enamine derived from 2-tritylpyrrolidine (R = CPh3) is 26 times less reactive. The rationalization of this phenomenon by negative hyperconjugative interaction of the trityl group with the lone pair of the enamine nitrogen is supported by the finding that the trityl group in the 2-position of the pyrrolidine increases the electrophilic reactivity of iminium ions derived from cinnamaldehyde by a factor of 14. The consequences of these observations for the rationalization of the reactivity of the Jørgensen-Hayashi catalyst (diphenylprolinol trimethylsilyl ether) are discussed.

Probing the structural and electronic effects to stabilize nonplanar forms of thioamide derivatives: A computational study

Density functional calculations have been performed to examine the stability of nonplanar conformations of thioamide derivatives. Electrostatic, orbital, and ring strain effects were invoked to stabilize the nonplanar conformations of thioamide systems 2-7. Electrostatic interactions helped to achieve the twisted forms of thioamide derivatives; however, pyramidal forms predicted to be the global minimum. Negative hyperconjugative type interactions enhanced the stability of the twisted form 4b when compared with the planar form 4a. The influence of ring strain effect to achieve the twisted form of thioamide was observed with azirine ring. The predictions made with B3LYP/cc-pVDZ+ level of theory was found to be in good agreement with more accurate CBS-QB3 method. The solvent calculations performed with polarized continuum solvation model suggest that the relative stabilities of the nonplanar forms of thioamide derivatives are in general similar to the gas phase results. The importance of hydrogen bonding interactions between the solvent molecules and thioamide derivatives was observed toward the enhanced stability of twisted forms using a combination of explicit solvent molecules and continuum model. The natural bond orbital analysis confirmed the participation of n(N) → π*(C=S) delocalizations in the planar forms and corroborated the earlier reports on larger delocalizations in thioamide systems. Furthermore, the influence of electrostatic and ring strain effects on the amide, natural amides, and selenoamide has also been studied.

What governs nitrogen configuration in substituted aminophosphines?

AbstractThe trigonal planar geometry of the nitrogen atom in commonly used phosphoramidite ligands is not in line with the traditional valence shell electron pair repulsion (VSEPR) model. In this work, the effects governing nitrogen configuration in several substituted aminophosphines, A2PNB2 (A or B = H, F, Cl, Br, Me, OMe, BINOP), are examined using modern computational analytic tools. The electron delocalization descriptions provided by both electron localization function (ELF) and block localized wavefunction analysis support the proposed relationships between conformation and negative hyperconjugative interactions. In the parent H2PNH2, the pyramidal nitrogen configuration results from nitrogen lone pair electron donation into the σ* PH orbital. While enhanced effects are seen for F2PNMe2, placing highly electronegative fluorine substituents on nitrogen (i.e., Me2PNF2) eliminates delocalization of the nitrogen lone pair. Understanding and quantifying these effects can lead to greater flexibility in designing new catalysts. Copyright © 2008 John Wiley & Sons, Ltd.

NMR spin‐spin couplings involving nuclei in the neighborhood of a carbonyl group. 3JCH couplings in α‐substituted acetamides

In this work 3JCH spin-spin coupling constants (SSCCs) for the cis- and trans-conformers for alpha-X-acetamides (X = F, Cl, Br and CN) (1-4) were studied in detail since they were found to be notably different for both conformers. These differences are rationalized as originating in the changes of the strong negative hyperconjugative interactions that take place within the carbonyl group. Such changes are found to depend not only on conformation, but also on solvent. For the cis-conformers there is a close proximity between the X-substituent and the in-plane oxygen lone pair of pure p character, which affects notably their respective negative hyperconjugative interactions. Both the efficiency for transmitting the Fermi contact (FC) term through the coupling pathway of 3JCH SSCCs and its potential as a probe to study the stereochemical properties of the XH2C group are discussed.

Lone-Pair Orientation Effect of an α-Oxygen Atom on (1)JCC NMR Spin-Spin Coupling Constants in o-Substituted Phenols. Experimental and DFT Study.

The well-known N lone-pair orientation effect on (1)JCC spin-spin coupling constants (SSCCs) in oximes and their derivatives was used to study how negative hyperconjugative interactions of type LP1(O) → σ*CC depend on ortho interactions involving the OH group. This study demanded the following analyses: (i) a qualitative estimation of how (1)JCC SSCCs are affected by hyperconjugative interactions, (ii) a study of similar stereochemical effects to those in oximes, but in (1)JC1C2 and (1)JC1C6in a series of 2-substituted phenols, and (iii) a quantitative estimation, with the natural bond order approach, of some key electron delocalization interactions. A few unexpected results are quoted. LP1(O) → σ*CC interactions are affected by proximity interactions as follows: (a) they are enhanced by hydrogen bonds transferring charge into the (O-H)* antibonding orbital; (b) they are enhanced by proximity interactions of type LP1(O)···H-C; (c) they are inhibited by interactions of type LP(O1)···H-O. Consequences of these observations are discussed.

α-Carbonyl Substituent Effect on the Lifetimes of Triplet 1,4-Biradicals from Norrish-Type-II Reactions

Triplet 1,4-biradicals were generated by Norrish-Type-II hydrogen transfer from alpha-heteroatom-substituted beta-branched butyrophenones 1-6 and detected by laser flash absorption measurements. For three oxy-substituted compounds 2-4 (R(alpha)=OH, OCOMe, OCOOEt) comparable lifetimes were determined in acetonitrile (roughly 1.5 micros). In benzene, divergent trends were observed: for the hydroxy compound 2 a lower lifetime of 790 ns was determined, whereas for 3 and 4 the lifetimes increased to 4.9 micros. Photolyses of the alpha-amino-substituted compounds 1 and 6 resulted in transient species with significant lower lifetimes (for 1 160 ns in benzene and 450 ns in acetonitrile; for 6 <100 ns in both solvents). The mesyloxy substrate 5 undergoes rapid C-O bond cleavage upon photolysis and no transient triplet species were detected. Computational (UB3 LYP/6-31G* and natural don orbital (NBO) analyses) results supported the assumption of a negative hyperconjugative interaction strongly stabilizing alpha-oxy-substituted over alpha-amino-substituted radicals.

Transmission mechanisms of NMR long-range J(13C,19F) scalar couplings in 1-F,4-X-cubanes: a DFT and experimental study

In order to get insight into transmission mechanisms of the title spin–spin coupling constants, nJ(C, F1) couplings (n = 4, 5) were calculated at the DFT-B3LYP-6-311-G**/EPR-III level, and couplings were measured in 9 members of the series 1-F,4-X-cubanes, where the α atom of the X group is a carbon atom. Hyperconjugative interactions were evaluated within the NBO approach, using the same level of theory as that employed for calculating spin–spin coupling constants. The unusual 4J(C4,F1) spin–spin coupling constants known in 1-F,4-X-cubanes are rationalized in this work as originating mainly due to two factors, namely, (i) the strong σ-hyperconjugative interactions involving as donors the cage C–C bonds; (ii) the particular arrangement of cage bonds not involving either the C1 or the C4 carbon atoms, C i and C j . For linear X substituents, the direction F1–C1··· C4–X is a three-fold symmetry axis and there are six C i –C j equivalent cage bonds, which are involved in (C i –C j )→(C1–F1)* and (C i –C j )→(C4–X)* hyperconjugative interactions. This means that coupling pathways involving such interactions are amplified six times, rendering the substrate very efficient for transmitting ‘trans-cage’ coupling constants. The large halogen substituent effects observed for 4J(C4,F1) spin–spin coupling constants in 1-F,4-X-cubanes (X = halogen atom) are rationalized in terms of interactions between σ-hyperconjugative (involving C–C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs.

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

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.

Electronic Structure of Sulfimides: A Theoretical Study

Abstract The electronic structure of sulfimides has been studied using the model system HN=SH2, 1, by performing ab initio calculations. There are two minima on the path of rotation across the S–N bond in 1. G2 calculations showed that the S–N rotational barrier in 1 is 8.50 kcal/mol. The inversion around S in 1 goes through a high-energy barrier of 26.85 kcal/mol at the same level. Substituent effects on the electronic structure were studied using Me, Cl, and F as substituents. Charge analysis using the NPA method has been performed to understand the electronic factors responsible for the observed trends in the S–N interactions. The NBO method was employed to quantitatively estimate the second-order interactions, which indicate that the S–N partial double-bond character in sulfimides is mainly attributable to negative hyperconjugative interactions, rather than pπ–pπ interactions.

Theoretical investigations on R(O) nS–NO ( n=0,1,2) systems

In the current article we report the ab initio study on the stability of S-Nitrosothiols (MeSNO, 1 ) and their oxidised derivatives (MeS(O)NO, 2 ) and (MeS(O) 2NO, 3 ). The bond length, bond order, rotational barrier and bond dissociation energy have been calculated and compared with that of sulfenamide (HS–NH 2) and its oxidised derivatives sulfinamide (H(O)S–NH 2) and sulfonamide (H(O) 2S–NH 2). The S–N bond dissociation energy in the oxidised state is very small compared to parent RSNO indicating the weakness of sigma bond. NBO analysis suggests that the negative hyperconjugative interactions are very strong in S-nitrosothiols and their oxidised derivatives, which weaken the sigma bond and facilitate the release of nitric oxide.

Synthesis, spectral characterization and crystal structures of organophosphonic diamides: pyramidal nitrogen centers and hydrogen bonding in [PhP(O)(NHtBu)2], [PhP(O)(NHDipp)2] (Dipp = 2,6-iPr2C6H3) and [tBuP(O)(NHiPr)2

Organophosphonic diamides of general formula [R1P(O)(NHR2)2] (1–8; R1 = Ph, Cy or tBu and R2 = Cy, iPr, tBu or Dipp) have been prepared by the addition of P,P'-dichloro(alkyl/aryl)phosphine oxide, R1P(O)Cl2, to a solution of primary amine in toluene. The synthesis of PhP(O)(NHtBu)2 (1) in high yield was achieved by slow oxidation of PhP(NHtBu)2 in air over several days. All new compounds have been characterized by elemental analysis and by IR, EI mass and NMR (1H and 31P) spectroscopy. The molecular structures of [PhP(O)(NHtBu)2] (1), [PhPO(NHDipp)2] (2) and [tBuP(O)(NHiPr)2] (7) have been determined by single crystal X-ray diffraction studies. The amido nitrogen atoms in 1, 2 and 7 show considerable deviation from the expected trigonal-planar geometry. The observed dihedral angles and the orientation of the nitrogen lone pair (l.p.) in these compounds point to the role of l.p.(N) → σ*(P–X) type negative hyperconjugative interactions in P–N multiple bonding. Compounds 1 and 7 form interesting polymeric structures in the solid state with the aid of N–H⋯OP hydrogen bonding interactions while the presence of the very bulky Dipp substituent on nitrogen in 2 prevents the participation of the N–H protons in hydrogen bonding with phosphoryl oxygen atoms.

Negative hyperconjugative interactions in S-nitrosothiols: a theoretical study

The S–N interactions are very weak in S-nitrosothiols because the n O→σ* S–N negative hyperconjugative interaction weakens the σ bond, though the pπ–pπ interactions are reasonably strong.

Natural J coupling (NJC) analysis of the electron lone pair effect on NMR couplings: 2. The anomeric effects on 1 J (C, H) couplings and its dependence on solvent

The electronic origin of the influence of the anomeric effect (negative hyperconjugative interaction, NHI) on the Fermi contact (FC) term of 1 J(C, H) couplings has been studied from a theoretical point of view at the DFT-B3LYP level. The HN=CH2, molecule was chosen as the primary model compound, in which both FC 1 J(C, H) couplings were decomposed into bond contributions with the natural J coupling dissection approach (NJC). Differences between the 1 J (C, H)FC couplings for C——H bonds in synperiplanar and antiperiplanar orientations with respect to the nitrogen non-bonding electron pair closely follow the experimental trend. They are made up chiefly of three NJC contributions: ‘bond’, ‘direct lone pair’ and the ‘carbon-core orbitals’. The NHI influence on these terms was studied by applying the natural bond orbital (NBO) deletion procedure to the charge transfer interaction into the antiperiplanar (C——H) antibond (n(N)→(C——H)*) prior to the NJC dissection calculation. The dielectric solvation effect on both the total FC terms and the respective NJC contributions was estimated by carrying out the calculations using the polarization continuum model. Inhibition of the anomeric effect is evident when the solvent polarity is increased. NHI saturates rapidly with increasing solvent dielectric. Specific solute-solvent interaction effects on 1 J(C, H) couplings were estimated by evaluating molecular complex models of the form CH2=HN…S (S = H2O and DMSO).

Ab initio study of conjugative and negative hyperconjugative interactions in disubstituted radicals

Electronic interactions in disubstituted radicals CHXY (X, Y is F, OH or NH 2) have been probed by examining the energetics of numerous conformations at the UMP3/6-31G(d)//ROHF/4-31G level. Negative hyperconjugative interactions in these radicals are indicated to be comparable in magnitude to π conjugation. Most of these disubstituted radicals are predicted to adopt unsymmetrical geometries in order to benefit from both types of stabilizing interactions. These results have interesting implications for reactions involving carbohydrate radicals, for which the simple systems serve as models. Thus, the structures and energetics of the dihydroxymethyl radical have been used to explain the observed stereoselectivity of a few reactions involving sugar-based radicals. The non-additivities of substituent effects in the disubstituted radicals have been compared with the magnitude of the anomeric effect in the corresponding disubstituted methanes. The inter-substituent interactions in the radicals should not be overlooked when using these systems as reference species in estimating other non-additive effects, such as the captodative effect.

Reactions of the hexachlorocyclodiphosphazane [MeNPCl3]2 with primary aromatic amines: formation of highly basic bisphosphinimines

Reactions of hexachlorocyclodiphosphazane [MeNPCl3]2 with primary aromatic amines afforded the bisphosphinimine hydrochlorides [(RNH)2(RN)PN(Me)P(NHMe)(NHR)2]+Cl- (R = Ph 1, C6H4Me-4 2 or C6H4OMe-4 3). Dehydrochlorination of 2 and 3 by methanolic KOH yielded highly basic bisphosphinimines [(RNH)2(RN)PN(Me)P(NMe)(NHR)2] (R = C6H4Me-4 4 or C6H4OMe-4 5). Compounds 1-5 have been characterised by elemental analysis and IR and NMR (H-1, C-13, P-31) spectroscopy. The structure of 2 has been confirmed by single-crystal X-ray diffraction. The short P-N bond lengths and the conformations of the PN, units can be explained on the basis of cumulative negative hyperconjugative interactions between nitrogen lone pairs and adjacent P-N sigma* orbitals. Ab initio calculations on the model phosphinimine (H2N)3P=NH and its protonated form suggest that (amino)phosphinimines would be stronger bases compared to many organic bases such as guanidine.

Synthesis, spectral characterisation and single-crystal structure of a bicyclic tetraphosphapentazane tetraoxide, N5P4ET5O4(OC6H3Me2-2,6)2

Reaction of the bicyclic phosphazane N5P4Et5Cl2 with 2,6-dimethylphenol and subsequent oxidation of the product by aqueous hydrogen peroxide yields N5P4Et5O4(OC6H3Me2-2,6)2 in 85% yield. Its structure has been established by NMR spectroscopy and single-crystal X-ray diffraction. The compound crystallises in the monoclinic space group C2/c with a= 21.245(5), b= 10.879(2), c= 16.450(6)Å, β= 123.94(2)°, Z= 4, R= 0.066. The structural features are compared with those of bicyclic λ5-phosphazenes of type N5P4R3(NR1R2)5(NHR3)(R1,R3= Me or Et, R2= H or Me). The observed conformation of the N3P3 rings in the present compound is mainly dictated by the maximisation of the stabilising influence of ‘negative hyperconjugative interactions’ between the nitrogen lone pairs and the adjacent P–X σ* orbitals.

The influence of lone-pair repulsions on C–C bond lengths: a critical evaluation of the experimental and theoretical evidence

X-Ray structural data, as well as semiempirical and ab initio molecular orbital calculations, reveal no systematic and substantial difference between the C–C bond lengths of cis and trans 1,2-diketones. Additional results on various conformations of 1,2-diimines and 1,2-dithiones follow the same pattern. Therefore, lone-pair repulsions cannot be implicated in the observed lengthening of C–C bonds in isatin and several related molecules. Conjugation in these systems occurs peripherally avoiding the participation of the central C–C bond. Negative hyperconjugative interaction between the oxygen lone pairs and the adjacent C–C σ* orbital is suggested to be the principal reason for the relatively long C–C bond in diketones. This effect is found in both the cis and trans conformations.

Geometry changes induced by negative hyperconjugative interactions involving carbonyl and thiocarbonyl groups

MNDO geometry optimizations have been carried out on a series of acyclic and cyclic unsymmetrically disubstituted carbonyl and thiocarbonyl compounds. The C=X unit shows a consistent and often sizeable tilt towards one of the substituents, following the order O > S∼N > C > B. Reference ab initio calculations and available experimental results support the MNDO results. The effect, which is particularly dramatic in small rings, is attributed primarily to favorable negative hyperconjugative interaction between the lone pair on X and a low lying adjacent σ* orbital. Such an interaction can lead to highly distorted structures, including perhaps to a planar molecule with an inverted sp 2 carbon center.