Hypercarbon compounds

Hypercarbon compounds contain one or more hypercoordinated carbon atoms bound not only by 2e-2c but also 2e-3c (or >3c) bonds. 

To show how hypercarbon compounds are closely related to many classically bonded systems and aromatic systems, and are not exotic species remote from mainstream organic chemistry. 

To show how the study of hypercarbon compounds helps us to understand the mechanisms of many organic reactions, reactions in which carbon atoms become temporarily hypercoordinated in intermediates or transition states even though the reagents and products contain only normally coordinated carbon atoms. 

In introducing the subject in Section 1.2, we dehne some of the terms we shall be using. In Section 1.3, we illustrate the various types of hypercarbon compounds now known. Since we shall rely heavily on the 3c-2e bond concept in their bonding, and since its usefulness is perhaps less widely appreciated in organic chemistry than in inorganic or organometallic chemistry, we devote Section 1.4 of this introductory chapter to discussion of that concept and illustrate its value for selected systems. We also demonstrate the relevance and value of some simple MO arguments applied to hypercarbon systems (Sections... 

For many years, a lively controversy centered over the actual existence of nonclassical carbocalions. " The focus of argument was whether nonclassical cations, such as the norbornyl cation, are bona fide delocalized bridged intermediates or merely transition states of rapidly equilibrating carbenium ions. Considerable experimental and theoretical effort has been directed toward resolving this problem. Finally, unequivocal experimental evidence, notably from solution and solid-state C NMR spectroscopy and electron spectroscopy for chemical analysis (ESCA), and even X-ray crystallography, has been obtained supporting the nonclassical carbocation structures that are now recognized as hypercoordinate ions. In the context of hypercarbon compounds, these ions will be reviewed. 

This book is concerned with an important area of organic (i.e., carbon) chemistry that has developed enormously over the past half-century, yet is still neglected in many organic textbooks. This is the chemistry of compounds in which carbon atoms are covalently bonded to more neighboring atoms than can be explained in terms of classical two-center, electron-pair bonds. Such carbon atoms are referred to as hypercarbon atoms (short for hypercoordi-nated carbon atoms ) because when first discovered, their coordination numbers seemed unexpectedly high. 

To illustrate the wide and developing scope of hypercarbon chemistry by illustrating the variety of compounds now known to contain hypercarbon atoms (carbocations,organometallics, carboranes, metal-carbon cluster compounds," and metal carbides ).They include bridged metal alkyls such as alkyl-lithium reagents (LiR) in which the hypercoordinated nature of the metal-attached carbon atoms, and the roles that the metal atoms play in their chemistry, are often overlooked. 

Figure 1.4. Mixed metal-carbon cluster compounds (metal-hydrocarbon n complexes) ( denotes hypercarbons).

The hypercarbon atom environment in this compound, with one silicon atom, two hydrogen atoms, and two copper atoms in the carbon coordination sphere, with a Cu-C-Cu bond angle of 74°, is consistent with the formation of three 2c-2e bonds to the silicon and hydrogen atoms, and a 3c-2e bond to the metal atoms. Hiis open cyclic structure, which may be contrasted with the more compact tetrahedral structures of typical tetrameric lithium alkyls, suggests that the metal atoms are sp hybridized, unable to make use of as many AOs as lithium atoms can. 

Compound 48 and the ferrocenyl-gold""" and ruthenocenyl-gold " compounds 49 provide interesting examples of a hypercarbon atom that is not only part of an aromatic cyclopentadienyl ring system, in which it is bonded to two other carbon atoms, but also bonds simultaneously to the sandwiched iron atom and (by a 3c-2e bond) to the two coinage atoms. The related dication 50 has also been isolated. " ... 

X-ray diffraction studies on yttrium and ytterbium" compounds (C5H5)2MR2A1R2 have established their structures as of type 58 with the characteristically acute M-C-Al bond angle at the hypercarbon atom that shows involvement in a 3c-2e bond to the metal atoms while bonding normally, by three 2c-2e bonds, to the methyl hydrogen atoms. 

In the second type of mixed hydride, hypercarbon atoms feature alongside boron atoms in the polyhedral molecular skeleton. Most of these mixed hydrides contain more boron atoms than carbon atoms, and their formulae and structures can be understood simply by isoelectronic replacement of B by C, B by C+, or BH units by C atoms in the parent borane. They are therefore known as carbaboranes, though the shorter name carboranes, coined soon after their discovery, "- is that most often used for this important category of compound and will be used here. Their now well-documented chemistry, " particularly structural and bonding aspects, is the concern of the present chapter. 

Hypercarbon chemistry is not restricted only for carbocations and their borane analogs but a few of examples of silicon- and germanium-containing bridged and homoaromatic compounds are also known. These will be discussed briefly in this section. 

In this chapter, we successively review reactions of electrophiles, coordinatively unsaturated metal compounds, carbenes, nitrenes, and heavy analogs of carbenes (silylenes, germylenes, stannylenes) with C-H and C-C bonds. Discussion of some electrophilic reactions of ti-donor systems is also included, along with Sn2 reactions. Tlie emphasis in all these discussions is centered on the involvement of hypercarbon intermediates (or transition states) of the reactions. 

A number of heteroatom-substituted dilakylaluminum compounds (R2AICH2-X) can undergo apparent a-, aP-, or ay-eliminations. The apparent a-elimination, when halomethylaluminum compounds cyclopropanate alkenes, is actually a combination of carboalumination and elimination [Eq. (6.87)]. Such eliminations involve hypercarbon intermediates or transition states. 

First, we survey the major types of compounds that contain hypercarbon. The relationships that link these apparently disparate species are demonstrated by showing how the bonding problems they pose can be solved by the use of three- or multicenter electron-pair bond descriptions or simple MO treatments. We also show the role played by hypercoordinated carbon intermediates in many familiar reactions (carbocationic or otherwise). Our aim here is to demonstrate that carbon atoms in general can increase their coordination numbers in a whole range in reactions. 

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