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Chapter 1 Planarizing Distortions at Carbon Chapter 1 Planarizing Distortions at Carbon 1.1 Introduction 1 1.2 Inversion of Methane 4 1.3 Designing Planar Carbon 6 1.4 Electronic Stabilization 9 1.4.1 Lithium and Boron Substituents 9 1.4.2 Dimetallic Complexes 13 1.4.2.1 Complexed Aromatics 13 1.4.2.2 Olefinic Complexes 14 1.4.2.3 Allene Adducts and Analogues 15 1.4.2.4 Other Transition-Metal Complexes 17 1.4.3 Annulenes 18 1.5 Structural Approach 19 1.5.1 The Effect of Small Rings 23 1.5.2 Fenestranes 24 1.5.3 Vesiprenes and Fenestrindan 30 1.5.4 Paddlanes and Bowlane 31 1.5.5 Bridged Spiropentanes 35 1.6 Concluding Remarks 38 References 40 1.1 Introduction In 1874 it was suggested independently both by van’t Hoff1 and le Bel2 that the arrangement in space of the substituents at a tetracoordinate carbon is such that they form a tetrahedral geometry. Van’t Hoff wrote that,† “… theory is brought into accord with the facts if we consider the affinities of the carbon atom directed toward the corners of a tet- rahedron of which the carbon atom itself occupies the center.” Similarly, le Bel, who was considering the same problem of isomerism in the fatty acids, wrote that, † Van’t Hoff’s proposal was general but was born of the problem of uniting the structural theory and exper- imental evidence for the isomers (both structural and “physical”) of the fatty acids. 2 • Chapter 1: Planarizing Distortions at Carbon “… we are obliged to admit that the four atoms A occupy the angles of a regular tetrahedron, whose planes of symmetry are identical with those of the whole molecule MA4; in this case no bisubstitution product can have rotatory power.” These concepts quickly propelled structural chemistry into the three-dimensional realm which today seems only too natural. Remarkably, van’t Hoff was attacked vehemently for his theory by Hermann Kolbe.† Kolbe3 called it a “play of imagination” which “for- sakes the solid ground of fact and is quite incomprehensible to the sober chemist.” It is likely that Kolbe’s tirade was the inspiration for van’t Hoff’s inaugural address as pro- fessor of chemistry in Amsterdam, which dealt with the role of the “imaginative faculty” in science. It was almost 100 years later before the very real prospect of exceptions to what has become the van’t Hoff/le Bel precept was seriously considered. In 1970, Hoffmann, Alder and Wilcox4 proposed in their seminal work on this subject, that it might be possi- ble to stabilize a structure in which the four substituents attached to a tetracoordinate carbon atom are arranged in a plane. There ensued a great interest in the possibility of planar-tetracoordinate carbon. At first examination it was clear to Hoffmann et al. that because molecules with a simple chiral center at carbon do not racemize we should expect that an achiral geome- try, such as a planar one, will lie at least 250 kJ mol–1 above the tetrahedral form. Their initial calculations suggested that the barrier to inversion was in fact considerably greater than this proposed lower limit and that it was unlikely that a simple carbon com- pound could be found that actually prefers a planar arrangement. It was suggested, instead, that the possibility of a planar geometry at tetracoordinate carbon might arise, in a more indirect fashion, as the transition structure involved in non-dissociative racemiza- tion of a suitably substituted methane derivative. However, the challenge had been laid and a search to find systems which might contain planar-tetracoordinate carbon began. Some degree of success came very quickly with the work of Pople, Schleyer and coworkers, published in 1976.5 They identified a number of small lithium- and boron-containing molecules (which included 1-1 – 1-3) that were predicted from ab initio calculations to prefer a planar-tetracoordinate geome- † O. T. Benfey gives some details of this affair in his translation of van’t Hoff’s article.1 Introduction • 3 try. These simple structures have, however, proven to be difficult to synthesize (see Section 1.4.1 on page 9). At about the same time experimental work began which was H H H Li Li B B B C C C BB B Li H H H Li 1-1 1-2 1-3 directed towards the synthesis of the fenestranes (1-I), and related hydrocarbons6,7 that were expected to induce planarizing distortions at the central tetracoordinate carbon atom. Experimental success was then achieved in Erker’s group8 where a facile route was found to organometallic compounds with a planar carbon center which is clearly † associated with four substituents, two of which are metals (1-II). (CH2)n R′ (CH2)n (CH2)n C R′′ Cp2M L M′R2 (CH2)n 1-I 1-II Lack of success in achieving significant planarizing distortions at a quaternary car- bon in purely organic systems (such as 1-I) and the difficulty in synthesizing candidates of a purely organic nature led to a lull in this approach to planar-tetracoordinate car- bon. However, it had always been recognized that the search for planar-tetracoordinate carbon was the sort of problem for which computational chemistry was well suited.4 Computational techniques allow for the examination of a wide range of molecules, each of which might take years to synthesize in a laboratory. Exploitation of this computa- tional approach led to the identification by McGrath, Radom and Schaefer of bowlane † However, as is explained later (see Section 1.4.2.2 on page 14), there is some ambiguity as to the nature of the planar-tetracoordinate carbon in these compounds, which might be better considered as a tricoordi- nate, planar, sp2-hybridized carbon (typical of olefins), with a fourth, partially-bonded, in-plane ligand, than as a planarized, tetracoordinate carbon. 4 • Chapter 1: Planarizing Distortions at Carbon (1-4)9 as a purely organic molecule with considerable flattening at a tetracoordinate car- bon atom. Since then we have proposed other target molecules, including octaplane (1- 5),10 and spiro[2.2]octaplane (1-6).11 Finally, we have identified dimethanospiro[2.2]- octaplane (1-7)12 as the first saturated hydrocarbon predicted to contain a planar-tetraco- ordinate carbon atom. 1-41-5 1-6 1-7 This endeavour has proven to be well served by the use of both van’t Hoff’s “imagi- native faculty” and detailed analysis. Previous work in the search to locate structures containing a planar-tetracoordinate carbon, their relevance to the present work, and fur- ther analysis by us that relates directly to these compounds, are described in detail in the remainder of this Chapter. 1.2 Inversion of Methane Because methane is the simplest possible tetracoordinate carbon molecule, it serves as a good first model for examining the issues involved in designing a molecule contain- ing a planar-tetracoordinate carbon atom. Many researchers have examined methane from the slightly different perspective of a model for the inversion (or, in the case of methane itself, the automerization) process at tetracoordinate carbon, which was thought originally to proceed via a square-planar transition structure (Figure 1-1). Monkhurst13 was the first to examine inversion of tetracoordinate carbon theoreti- cally, although he was preempted by a brief paper on the deformation of a general AH4 species by Saturno.14 Monkhurst proposed that inversion may occur either through a planar or a pyramidal structure. Using methane as a model he then determined, via approximate electronic structure calculations, that the planar structure was lower in energy than the pyramidal structure and that the barrier to inversion was of the order of Inversion of Methane • 5 Cs/C4v H H H H H H H H H T HH T d H H d H H H D4h H H H H Cs/C4v Figure 1-1. The automerization of methane, initially thought to proceed via a D4h structure is expected to proceed via a pyramidal-like structure. 1000 kJ mol–1. With such a large barrier, dissociation and recombination would clearly be the preferred pathway. Many workers4,15–17 (including Hoffmann, Alder and Wilcox) made estimations of the difference in energy between the planar and tetrahedral geome- ∆ –1 tries ( EPT = EPlanar – ETetrahedral) in methane which ranged from 600 to 1000 kJ mol . In 1977, Minkin et al.18 used a number of methods including HF calculations employing a double-zeta basis to show that in fact the pyramidal C4v symmetry struc- ture is expected to be lower in energy than the planar D4h structure. This preference was noted by others19 and led to the elucidation in 1993 by Gordon and Schmidt20 of the pre- ferred route for the hypothetical classical inversion (automerization) of methane, which is seen to proceed via a pyramidal-type structure with Cs or possibly C4v symmetry (Figure 1-1). The most accurate calculations published to date for the relative energies of the var- ious methane geometries come from the work of Pepper and coworkers.21 Like Gordon and Schmidt before them, they find that the lowest-energy, planar structure for methane does not have four equivalent hydrogen atoms and D4h symmetry, as had been sup- posed, but has C2v symmetry and a structure that suggests a complex between CH2 and 22 2+ H2 (reminiscent of the structure for CH4 ). Further, they find that the energy of the 6 • Chapter 1: Planarizing Distortions at Carbon C2v (D4h) symmetry, planar structure relative to the tetrahedral geometry is about 510 (546) kJ mol–1 (after correction for the zero-point vibrational energy (ZPVE)).
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