Carbon inverted

From an extensive set of data concerning annelation of cyclopropane rings to cyclopentanes, Christl and coworkers were able to employ y-syn shielding and y-anti shielding correlations to assign structures. Four compounds which contain all four valencies within one hemisphere, inverted carbons , were considered these were 41, 24, 42, 43 and their respective precursors without the cyclopropyl methylene group viz 44, 45, 11, 46. 

Duddeck and Klein also examined the pair of compounds 41 and 49 of which the former has an inverted carbon characterized by a long central bond formed by overlap of what are close to p orbitals. Cy is shifted downfield by ca. 23 ppm with respect to 49 a through-bonds transmission of the substituent effect is envisaged. 

More recently the chemical shifts of 43 have been considered with respect to 2,4-methanoadamantane In 43 the inverted carbons C(2), C(4) are shielded by 14.3 ppm with respect to the corresponding carbons of 53. This finding which is apparently inconsistent with the slightly earlier reports of Duddeck and Klein and Pincock and Fung is reconciled by the consideration of three factors which are responsible for the shift differences. These are ... 

Another husband-wife team (cf. Ref. 72a) has prepared 2,4-methano-2,4-de-hydroadamantane which contains a [3.1.1]propellane moiety (88) by intramolecular cycloaddition, of 87 " . (One may muse about what it is that attracts such teams to smallring propellanes. Imagine what great progress there might have been had Mrs Wiberg entered this field ) C-chemical shifts of the inverted carbons was discussed " (cf Ref 74c). 

Methanol, acetic acid, and HCl cleave the bicyclobutane system not necessarily by addition to the conjoining bond. Carbon tetrachloride, TsCl, and PhSH, however, behave in the catholic manner in this respect. The authors make the point that these results indicate high electron density at the back side of the inverted carbon atoms and a decrease of electron density between them. 

An alternative method of preparing L-ascorbic acid was reported by Bakke and Theander (Scheme 14) (43), In this synthesis D-glucose was first oxidized at C6, then at C5, and then reduced at Cl. This contrasts with the Reichstein-Griissner synthesis in which glucose was first reduced at Cl, then oxidized at C5, and then at C6 to achieve the requisite inverted carbon chain. The key intermediate in the Bakke-Theander synthesis, ketolactone (25) was prepared earlier (44,45) but was not converted to 1. Hydrolysis of 25 afforded 6-aldehydo-L-ascorbic acid (26, aldehydo-L-threo-hex-4-enurono-6,3-lactone) as an unisolated intermediate. Compound 26 was not previously synthesized. The reduction of 26 afforded 1. This synthesis of 1 is used effectively in the preparation of labeled derivatives of 1 (46). It is not useful for the preparation of analogues. 

From the time of van t Hoff (1) and Le Bel (2) the tetrahedral arrangement of substituents around a tetravalent carbon atom has been part of the foundation of organic stereochemistry. However, in the last 30 years, thanks to the rapid development of synthetic methods of organic chemistry as well as of experimental and theoretical methods of structure elucidation, many exciting saturated hydrocarbons with very unusual spatial structure have been synthesized. Cubane (1) was obtained as early as 1964 (3), triprismane (2) (4), a tetra-f-butyl derivative of tetrahedrane (3) (5), small-ring propellanes with structures involving inverted carbon atoms, 4-6 ([3.2.1], [4.1.1] and... 

As stated earlier, in spite of considerable interest in hydrocarbons possessing inverted carbon atoms, only eight systems (16, 8, 17a, 17b, 4-6, and 18) of this type are known. With the exception of bicyclobutane (16) they are all small-ring propellanes possessing symmetrical pairs of inverted atoms. In the course of the MM study of [k./.m]propellanes (17), with k, /, m equal to 2-4 (55), four questions have arisen ... 

Are there propellanes possessing medium or large rings exhibiting inverted carbons ... 

Is there a possibility of existence of an asymmetric pair of inverted carbon atoms ... 

These questions have been discussed elsewhere (34). Simple stereometric consideration of the spatial relationships in [n.l.ljpropellane (n>4) using a few simplifying assumptions was used to show that such a molecule should possess inverted atoms independently on the value of n, and the MM calculation for [5.1.1]propellane (19) revealed that this molecule should not be excessively strained. Therefore, this molecule and higher [n.l.l]propellanes appear to be prospective synthetic targets yielding an infinite family of molecules with inverted carbons. 

The extension of Paquette s idea (56) of fused bicyclics (called geminanes) to smaller molecules allowed Dodziuk (34) to propose a group of molecules 20-29 possessing inverted carbon atoms that differ from small-ring propellanes. Molecular mechanics calculations for the molecules revealed that molecules 20-23, cis- and trans-21, the two stable rrans-conformers of 28, denoted as I and II, and the single conformer of tram-29, (denoted analogously as I)... 

In particular, they seem to indicate that the strain in molecules 20-29 is not large enough to preclude their synthesis. The synthesis of [l.l.ljgeminane (21) and that of molecules 22, 23, and tru s-28(I) would allow one to study the properties of a bond formed by inverted carbon atoms different from the central bond in small-ring propellanes while that of tricycloheptane 20 and trans-28(II) and trans-29(I) would provide the opportunity to analyze the bond between the carbon atoms with inverted and tetrahedral configuration of substituents, respectively. 

The possibility of the existence of a body-diagonal bond in 1,4-dehydro-cubane (50), a prototype new molecule possessing inverted carbon atoms, has recently been discussed by Hassenriick etal. (59), Hrovat and Borden (60a), and Eaton and Tsanaktsidis (60b). 

The existence of an interesting molecule (30) (61) with structure analogous to that of [2.2.2]propellane (17c) was proposed by Wiberg etal. (61), but in an attempted synthesis the authors were not able to detect it. Some other interesting propellanes without inverted carbon atoms will be mentioned in the next sections. Normal coordinate treatment with calculation of the infrared intensities of bicyclobutane (16) and of [l.l.l]propellane (8) was carried out... 

The fact that bicyclobutane (16) also has an inverted carbon atom is sometimes overlooked, although its derivatives such as tricyclo[2.1.0.0 - ]pentane (38) reviewed in (31) exhibit unusual patterns of electron density distribution analogous to that found in small-ring propellanes (47). 

As already stated, Wiberg s definition of inverted carbon atoms is not limited to such atoms but includes also pyramidal ones. According to another concept by Minkin etal. (17) not only pyramidane (39) or pyramidal fenestrane (14b) but also cubane (1), triprismane (2), tetrahedrane (3), and the (CH)j cation... 

There are only a few studies of these molecules (30, 36,124). They have been summarized in a review (30) together with those of unsaturated members of the CgHg family. All but one of the semiempirical quantum chemical studies quoted in Table 1 of the review yielded cubane (which is [4] prismane) as the most strained member of the family. Except in ref 36, the existence of inverted carbon atoms (the bridgehead ones in the bicyclobutane units) in the octabisvalene molecule has been overlooked. There is no agreement in the literature concerning the relative stability of cuneane and octabisvalene. As mentioned earlier, the experimental determination and ab initio calculation have been carried out only for 1 thus, more experimental measurements and further ab initio calculations for molecules 76a and 83 are necessary. 

So far, considerable efforts have been made to examine thermal and acid catalysed rearrangements of propellanes. However, PET reactions and direct photolyses with high-energy photons (e.g. 185-nm excitation) have so far been neglected. It would be a challenging task for future research to fill this gap, because new information can be expected on the nature of the cleavage of the central propellane bond with its inverted carbon hybridization. 

By inverted carbon we will mean a fragment CWXYZ in which all four bonds lie in the same half of a sphere centered on the carbon atom C, for example, 3 Equivalent definitions are that the plane determined by W, X and Y passes between C and Z or that C lies outside the tetrahedron determined by W, X, Y and Z. Numerous compounds containing inverted carbon are known. 

MNDO appears to provide satisfactory geometry for derivatives of [3.1.1]propellane (30), and HAM/3 calculations based on MNDO geometry reproduce reasonably well the experimental ionization energies of 31, which is a [3.1.1]propellane. Molecular mechanics calculations are useful here only for less distorted systems, such as [3.2.2]pro-pellane (32) and larger systems that do not contain inverted carbon . 

Inverted carbon atoms can be incorporated into many other ring systems of lower symmetry than those discussed above. Molecular mechanics calculations have been reported for several such hydrocarbons, along with the opinion that the results indicate... 

Less progress has been made in preparation of strained paddlanes than in the other areas under discussion. A number of compounds formally represented by 7 or heteroatom derivatives of 7 are known, including some, such as 35 ", 36 and 37, that were prepared without specific interest in inverted carbon. Others result from exploration of routes that might be adapted to highly strained systems but have led so far to compounds with conformational restrictions but not great angle strain. One of these is 39, which is available on Diels-Alder addition of dicyanoacetylene to furanophane 38 . The proton NMR... 

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