Hypervalent carbon

J. C. Martin Structural factors influencing stability in compounds of hypervalent carbon, silicon, phosphorus and iodine [5]... 

As a part of a study of stable hypervalent carbon compounds, the selenonium salt 117 has was prepared by treatment of the carbinol 118 with perchloric acid. A smiliar outcome was observed for several sulfur analogues <05JA5893>. 

Proposed mechanisms for the initial C-C bond formation may be broadly classified as carbenic [lb,10], carbocationic, involving hypervalent carbon intermediates [4,5], oxoniumylide [6,7], and free radical [8,9], while recognizing that the classification is somewhat arbitrary since many mechanisms embody several of these categories. 

It is apparent that much resourceful, imaginative experimentation has been done to resolve the question of C-C bond formation from methanol. Although the answer remains elusive, these experiments tell us at least what is probably not involved in the bond formation, particularly in the presence of zeolite catalysts. The Stevens rearrangement of oxonium ylide can be ruled out, as well as the carbocationic route invoking hypervalent carbon transition states. Not excluded are surface-bound species such as carbenoids and ylides. Again there seems to be a consensus that surface methoxyls are precursors to these reactive C- intermediates, which seems somehow to be "coming full circle", since surface methoxyls were early shown to be intermediates in the formation of DME, which is itself an intermediate in hydrocarbon formation. Finally, if the free radical character of the initiation step proves correct, the implications to zeolite catalysis will be far-reaching. 

In superacid systems, methylation of DME gives trimethyloxonium (TMO) ion [19]. Mechanisms invoking hypervalent carbon intermediates therefore seem unlikely. 

Figure 1.6 Hypervalent carbon. In the complex [Co8C(CO)is) the isolated carbon atom has eight coordinating neighbors.

The 2-norbornyl cation is part of the group of hypervalent carbon compounds, which are discussed in the books of Olah et al. (1987) and Minkin et al. (1987). Hanack (1990) edited a volume of Houben-Weyl on carbocations. Berndt (1993) described interesting correlations between nonclassical carbocations of the type discussed above with related compounds among methylidene-boranes (-CH2 = B-). 

A similar associative intermediate having a hypervalent carbon center was also proposed to account for the reaction of the Pt complex. The intramolecular transmetalation of Pd complex with a stannyl group containing organic ligand was also observed (Eq. 5.27) [108]. 

This arrow is incorrect because it leads to a hypervalent carbon atom. 

We might anticipate, however, that the three center-two electron bonding of carbonium ions might lead to atypical values of Av. In particular, we might expect the chemical shifts of hypervalent carbons to be unusual, and indeed they are shifted far upheld from what is seen for carbenium centers. This leads to greatly reduced values of A. ... 

FIGURE 3.7 Gold stabilization of hypervalent carbon and nitrogen. 

Because carbon is a first-row element unable to extend its valence shell, hypervalence cannot exist in carbon compounds, only hypercoordination. 

Phosphorous ylides such as triphenylphosphine-metJhylidene may either be represented as hypervalent species incorporating a phosphorous-carbon double bond, or in terms of a zwitterion, that is, a molecule with separated positive and negative charges. 

However, on account of the particularly short C-S distances, multiple bonds (probably of the d-si-p-si type) between hypervalent sulfur and carbon occur in a number of species. 

In this chapter, we will consider examples of RIs characterized by a hypervalent or valency-deficient carbon, such as carbocations, carbenes, carbanions, and carbon radicals. In the first part, we will consider examples that take advantage of stabilization and persistence to determine their structures by single crystal X-ray diffraction. In the second part we will describe several examples of transient reactive intermediates in crystals. ... 

The best Lewis-type representation of the bonding in OCF3 would therefore appear to be as in 4, even though the carbon atom does not obey the octet rule. This molecule can be considered to be a hypervalent molecule of carbon just like the hypervalent molecules of the period 3 elements, such as SFfi. We introduced the atom hypervalent in Chapter 2 and we discuss it in more detail in Chapter 9. But it is important to emphasize that the bonds are very polar. In short, OCF3 has one very polar CO double bond and three very polar CF single bonds. A serious limitation of Lewis structures is that they do not give any indication of the polarity of the bonds, and much of the discussion about the nature of the bonding in this molecule has resulted from a lack of appreciation of this limitation. 

The use of resonance structures such as 7 and 8 to describe bond polarity led to a subtle change in the meaning of the octet rule, namely, that an atom obeys the octet rule if it does not have more than eight electrons in its valence shell. As a result, resonance structures such as 7 and 8 are considered to be consistent with the octet rule. However, this is not the sense in which Lewis used the octet rule. According to Lewis, a structure such as 7 would not obey the octet rule because there are only three pairs of electrons in the valence shell of carbon, just as BF3 does not obey the octet rule for the same reason. Clearly the octet rule as defined by Lewis is not valid for hypervalent molecules, which do, indeed, have more than four pairs of shared electrons in the valence shell of the central atom. 

Triheterapentalenes have numerous structural variations of the generic structure 6. They can possess either S, Se, or Te as its central hypervalent atom Y. The number of permutations increases dramatically since each of the atoms X and Z can be replaced by S, Se, O, or NR. Also, one or more of the remaining ring carbons can also undergo aza replacements. 

It was found that treatment of a mixture of 120 and 121 with tris(diethylamino-sulfonium) trimethyldifluorosilicate [TASF(Et)] resulted in smooth addition-elimination to the naphthoquinone to form the y-alkylation product 125 (85 %). TASF(Et) is a convenient source of soluble, anhydrous fluoride ion [47]. It is believed that exposure of 121 to TASF(Et) results in fluoride transfer to generate a hypervalent silicate anion, as depicted in structure 124. The transfer of fluoride between TASF(Et) and 121 may be driven by stabilization of the anionic species 124 by delocalization of the carbon-silicon bond into the LUMO of the unsaturated ketone. 1,4-Addition-elimination of this species to the naphthoquinone 120 would then form the observed product. 

Transformation of the thiadiazolopyrimidine compound 138 to the fused dithiazole 140 also follows a fairly complicated pathway <2004JHC99>. When the derivative 138 is treated with carbon disulfide, a cyclization reminiscent of 1,3-dipolar cyclization takes place with the reagent acting as a dipolarophile to give a />OT-fused tricyclic intermediate containing a hypervalent sulfur atom 139. This intermediate can undergo isothiocyanate elimination to furnish 140. It is interesting to note that the sulfur atom of the thione moiety in the product is derived from carbon disulfide. 

Denmark utilized chiral base promoted hypervalent silicon Lewis acids for several highly enantioselective carbon-carbon bond forming reactions [92-98]. In these reactions, a stoichiometric quantity of silicon tetrachloride as achiral weak Lewis acid component and only catalytic amount of chiral Lewis base were used. The chiral Lewis acid species desired for the transformations was generated in situ. The phosphoramide 35 catalyzed the cross aldolization of aromatic aldehydes as well as aliphatic aldehydes with a silyl ketene acetal (Scheme 26) [93] with good yield and high enantioselectivity and diastereoselectivity. 

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