The Chemistry of Organolithium Compounds, Volume 2: R-Li / Edition 1

by Zvi Rappoport, Ilan Marek

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Product Details

ISBN-13:	9780470023211
Publisher:	Wiley
Publication date:	02/03/2006
Series:	Patai's Chemistry of Functional Groups , #126
Edition description:	Volume 2 ed.
Pages:	768
Product dimensions:	6.18(w) x 9.11(h) x 1.80(d)

About the Author

Professor Zvi Rappoport, Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel.

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The chemistry of organolithium compounds

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John Wiley & Sons

Copyright © 2004 John Wiley & Sons, Ltd
All right reserved.
ISBN: 0-470-84339-X

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Chapter One

Theoretical studies in organolithium chemistry

ELUVATHINGAL D. JEMMIS and G. GOPAKUMAR

School of Chemistry, University of Hyderabad, Gachibowli, Hyderabad 500 046, Andhra Pradesh, India e-mail: [e-mail]

I. INTRODUCTION 1 II. THE NATURE OF THE C-Li BOND 2 III. STRUCTURE AND ENERGY 6 A. Effect of Solvation 6 B. Stability due to Sulfur 10 C. Lithium Amides 14 D. Oligomerization and Aggregation 18 E. Examples of Other Organolithium Compounds 18 IV. THEORETICAL STUDIES INVOLVING REACTIONS OF ORGANOLITHIUM COMPOUNDS 22 A. Regioselectivity in Addition 22 B. Self-condensation Reaction 25 C. Lithium Organocuprate Clusters 32 D. Organolithium Compounds Involving Aldehydes and Ketones 35 E. Other Reactions 41 V. APPLICATIONS IN SPECTROSCOPY 42 VI. CONCLUSIONS 44 VII. REFERENCES 44

I. INTRODUCTION

With their versatile structure, bonding and reactions, organolithium compounds continue to fascinate chemists. Tremendous progress has been made in each of these areas during the last few years. Theoretical studies have played an important role in these developments. Several reviews had appeared on the contribution of theoretical methods in organolithium compounds. Wave-function-based quantum mechanical methods at various levels continue to be used in these studies;theoretical studies based on Density Functional Theory (DFT) have also become popular in recent years. A major review on theoretical studies in organolithium compounds was published in 1995 by Streitwieser, Bachrach and Schleyer. We concentrate here on publications that have appeared since then. During these years considerable progress has been made in the application of theoretical methods to the chemistry of organolithium compounds at various levels of sophistication depending on the problem. Attempts have been made to further delineate the nature of C-Li bonding. Semiempirical calculations with the inclusion of solvent effects through various approximations have been used to study larger systems. Reactions have been modeled in the gas phase. Mechanistic details of several reactions have been studied theoretically. We discuss the developments in the nature of the C-Li bonding first. Theoretical studies on the structure and energetics, reactions and some applications involving NMR parameters are discussed in subsequent sections.

II. THE NATURE OF THE C-Li BOND

The nature of C-Li bond is still a dilemma for chemists due to the unusual behavior of the bond in different compounds. Although the electronegativity difference suggests the carbon-lithium bond to be essentially ionic, the solubility of some organolithium compounds in nonpolar solvents such as benzene makes the problem more complex. The nature of the C-Li bond is different from those of the heavier analogs of alkali-metal organic complexes; C-Na to C-Cs bonds are acknowledged to be even more ionic than the C-Li bond. It was therefore felt that a certain percentage of covalent character may be associated with the C-Li bond. But recent studies and developments of methodologies for the analysis of wave functions and charge distributions suggest a much higher polarity to the bond. In 1995, Streitwieser, Bachrach and Schleyer suggested: 'The carbon lithium bond in theory and in chemical properties can be modeled as an essentially ionic bond'. They described a number of examples, which support the ionic behavior of the carbon-lithium bond.

Later, Koizumi and Kikuchi used ab initio calculations of NMR spin-spin coupling constants in monomeric methyllithium, tert-butyllithium and methyllithium oligomers using self-consistent perturbation theory to probe the nature of C-Li bonding. Their studies suggested that solvation affects the nature of the C-Li bond and reduces the [sup.1][J.sub.CLi] value significantly. The calculations were also carried out using a truncated basis set (the MIDI-4 basis set for lithium which includes only the 1s function and corresponds to lithium cation), which models a purely ionic C-Li bond. The calculated coupling constants were in excellent agreement with experimental data, suggesting the importance of the ionic character of the C-Li bond in alkyllithiums. The calculated [sup.1][J.sub.CLi] value of methyllithium, 44.0 Hz, is found to be very close to that calculated for methyllithium with three solvating ligands. This result, which strongly suggests the ionic nature of the C-Li bond in methyllithium, does not change with the addition of ligands. The difference between the [sup.1][J.sub.CLi] values calculated by two different types of basis set for methyllithium tetramer is much smaller than that in monomeric methyllithium. This trend is in accordance with the observation that the coupling constants in methyllithium tetramer are independent of solvent. Comparing the coupling constants of the ring structures 1a, 1b and 1c (Figure 1) with the tetrahedral structure 1d (staggered and eclipsed form) implies that [sup.1][J.sub.CLi] depends on the state of aggregation rather than on the degree of aggregation. More clearly, [sup.1][J.sub.CLi] in methyllithium varies nearly inversely with the number of lithium atoms, which are bonded directly to the carbon atom. The implications are that the ionic nature of the monomeric MeLi increases on solvation and the tetrameric MeLi has more ionic C-Li bonding. In addition, further solvation is not desirable as the bridging nature of tetramer provides the effect of solvation.

In 1996, Bickelhaupt and coworkers investigated C[H.sub.3]Li, [(C[H.sub.3]Li).sub.2] and [(C[H.sub.3]Li).sub.4] using Density Functional Theory (DFT) and conventional ab initio Molecular Orbital Theory (MOT). This study highlighted the important role of a small covalent component in the polar C-Li bond, especially in the methyllithium tetramer. It was suggested that the lithium outer 2p orbital serves only as 'superposition functions', helping to describe the carbanion, and does not play any part in covalent interaction. However, there appears to be a small contribution from the inner parts of the Li 2p orbital. Streitwieser and coworkers showed that calculations using a truncated basis set on lithium with only s-type basis functions yield essentially the same result (including the energetic ordering of isomers) as calculated using the full basis sets. They concluded that the bonding is governed by electrostatic interactions. The extended 6-31+[G.sup.*] basis set used in the evaluation of aggregation energies was expected to minimize the basis set superposition error as suggested by Bickelhaupt and coworkers. The result showed that the oligomerization energies ([DELTA][E.sub.oligo] + [DELTA]ZPE) calculated with truncated basis set are up to 20% lower than those obtained using the full 6-31+[G.sup.*] basis. This indicated that the bonding mechanism is more complicated than suggested by the purely electrostatic model.

Charges on lithium calculated using the Voronoi Deformation Density (VDD) decrease from 0.38 via 0.26 to 0.13e along C[H.sub.3]Li, [(C[H.sub.3]Li).sub.2], [(C[H.sub.3]Li).sub.4] showing that the shift of electron density from lithium to methyl decreases upon oligomerization. Similarly, Hirshfeld lithium charges decrease from +0.49 via 0.42 down to +0.30e along the same series of methyl lithium oligomers (Table 1). The fragment molecular orbital analysis shows [(C[H.sub.3]).sub.n] and [(Li).sub.n] fragments to have triplet and quintet electronic structures in [(C[H.sub.3]Li).sub.2] and [(C[H.sub.3]Li).sub.4], respectively. Thus the interacting fragments are two singly occupied molecular orbitals ([SOMO.sub.low] and [SOMO.sub.high]) in each [(C[H.sub.3]).sub.n] and [(Li).sub.n]. The above trend of decrease in electron density transfer from lithium is in accordance with the increasing population of the [(Li).sub.n] fragment orbitals [SOMO.sub.low] and [SOMO.sub.high] from [(C[H.sub.3]Li).sub.2] [[SOMO.sub.low] = 0.57 and [SOMO.sub.high] = 0.63] to [(C[H.sub.3]Li).sub.4] [[SOMO.sub.low] = 0.91 and [SOMO.sub.high] = 0.85]. This is indicative of the increasing importance of a covalent component in the carbon-lithium bond. Also, the carbon-lithium bond is much less ionic according to Hirshfeld (50-30%) than according to NPA charges (90%). These factors suggest that the degree of ionicity of a bond obtained on the basis of atomic charges should not be regarded as an absolute quantity, rather it will be more meaningful to consider trends in atomic charges across a series of molecules using the same method. Even though Bickelhaupt emphasized the importance of covalent contributions to the C-Li bonding, the results imply the 'dual nature' of the C-Li bond. It can be concluded that the appearance of a covalent or ionic aspect depends strongly on the physical and chemical context.

From their analysis of the conformational energies of pentadienyl anion and the pentadienyl metal compounds, Pratt and Streitwieser in 2000 pointed out that the stabilization of the planar forms of the organometallic structures results from both conjugation and electrostatic attraction between the negative carbons and the alkali metal cations. To determine the relative magnitude of these effects, the reaction energies were determined for hypothetical reaction, shown in Scheme 1 where M represents any alkali metal.

The reaction energies for the formation of pentadienyllithium are found to be much greater than those for pentadienyl sodium, which indicate a greater electrostatic attraction for the shorter Li-C bond. The calculated regional charges for the pentadienyllithiums (HF/6-311+[G.sup.*]) indicate that the most positive charge is concentrated on lithium and the most negative charge is concentrated on the carbon atom coordinated to the lithium. These results imply an ionic nature of the C-Li bond in pentadienyllithium. However, the larger magnitude of electrostatic interaction may be due to the shorter distance of the C-Li bond, and not necessarily to a larger charge separation. In other words, it is possible that the charge on lithium may be less than that on sodium in the corresponding sodium derivative and yet the electrostatic interaction may be larger in the former due to the shorter distance.

Density functional theory calculations on methyllithium, tert-butyllithium and phenyllithium oligomers by Kwon, Sevin and McKee support the ionic character of the C-Li bond. Their calculations of carbon lithium Natural Population Analysis (NPA) charges and dipole moments for C[H.sub.3]Li, t-BuLi and Ph-Li oligomers (Table 2) indicate the ionic behavior of the C-Li bond. Comparison of the charges of various oligomers suggests that charges of lithium and carbon atoms are almost independent of the size of oligomers. There are minor variations in the charge of the Li on going from C[H.sub.3]Li via t-BuLi and PhLi, implying that there are changes in the nature of C-Li bonding as a function of the organic group. Thus it is not correct to say that all C-Li bonds are 100% ionic. There are minor variations.

Ponec and coworkers reconsidered the conventional concept of C-Li bond in C[H.sub.3]Li and C[Li.sub.6]. Their calculations were based on two recently proposed methodologies: the Atoms in Molecule (AIM) generalized population analysis and Fermi hole analysis. These results support the ionic nature of C-Li bonding in C[H.sub.3]Li, but in C[Li.sub.6] a different description than the one published earlier is suggested. The bonding description of C[Li.sub.6] proposed by Schleyer and coworkers in 1995 involves a [C.sup.4-] ion surrounded by [Li.sub.6.sup.4+] in an octahedral fashion (Figure 2). The two electrons in the lithium cluster are placed in an orbital, which is completely symmetric, being a Li-Li bonding orbital among all lithium atoms, with a small contribution from the carbon 2s orbital. This extra electron pair was considered as a part of the Li ... Li bonding interactions. According to Ponec and coworkers the AIM analysis suggests that this electron pair is also shared between the carbon and lithiums and the contributions of C and Li are roughly equal to 1.2 and 0.8e, respectively. Although the oxidation state of the central carbon is indeed close to the NPA estimate (-IV), the interactions between the central atom and the surrounding cage need not be purely ionic as expected so far. This is supported by the result of generalized population analysis, which detects the presence of 3-center bonding interactions in Li-C-Li fragments as seen from the values of the corresponding indices, such as the C ... Li cage interactions. The Li ... Li bond indices drop from 0.167 (Mulliken-like analysis) to 0.020 (AIM generalized value), correlating the conclusions above.

Thus it is evident from all these studies that the nature of the C-Li bond varies from compound to compound; hence any generalization of the nature of bonding is to be taken cautiously. As Schleyer and Streitwieser have discussed in the past, the C-Li bond is essentially ionic; however, the covalent components cannot be neglected. The unusual behavior of the C-Li bond has been a subject of discussion from the initial years of applying theoretical methods, and the debate continues in an interesting manner due to the developments of new theoretical methodologies. In fact, we support the implications of Bickelhaupt that there is a covalent contribution to the C-Li bonding, however small this turns out to be in specific examples.

III. STRUCTURE AND ENERGY Theoretical studies of the structure of organolithium compounds continue to attract much attention for several reasons. Often, it is not possible to obtain detailed structural information from experiments. Experimental realization of a single crystal, which is good enough for X-ray diffraction studies, is not always easy.

Continues...

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Table of Contents

1 Dynamics of the reorganization behavior of organolithium compounds (Gideon Fraenkel).

2 Organolithiums as useful synthetic intermediates for tandem reactions (Alvaro J. Vàzquez, Raquel G. de Waisbaum and Norma Sbarbati Nudelman).

3 The chemistry of acyllithium derivatives (Carmen Najera and Miguel Yus).

4 Intramolecular carbolithiation reactions (Francisco J. Fañanás and Roberto Sanz).

5 Structure and dynamics of chiral lithium amides (Göran Hilmersson and Johan Granander).

6 Chiral lithium amides in asymmetric synthesis (Daniel Pettersen, Mohamed Amedjkouh and Per Ahlberg).

7 The lithium metal reduction of π-conjugated hydrocarbons and fullerenes (Ivan Aprahamian and Mordecai Rabinovitz).

8 Aspects of the synthesis, structure and reactivity of lithium enolates (Jean-Yves Valnot and Jacques Maddaluno).

9 Chemistry of ketone dilithio dianions (Ilhyong Ryu and Hiroyuki Nakahira).

Author index.

Subject index.

Table of Contents of Volume 1.