Planarization of Tetracoordinate Carbon Atom. Synthesis of 13‐oxa‐14‐oxo‐pentacyclo [5.5.2.1.0^4,150^10,15]pentadecane, a bridged ‘tetraquinacane’ preliminary communication

Abstract

SummaryStructural aspects of planoid deformation of tetracoordinate carbon atom, particularly those in annulenes with central carbon atom are discussed. [ 12lAnnulene 11 with central carbon atom is proposed as compound with strong planoid deformation around the internal carbon centre. The synthesis of a saturated precursor 15 is described.The theory of Van't Hoffand Le Be1 requires 4 equal ligands of a tetracoordinate carbon atom to be arranged as a tetrahedron [ l ] [2]. Although the tenet is basic to organic chemistry, no test has been made of how far planoid deformation without bond breaking is possible. Experimental evidence has shown that racemization of chiral carbon compounds does not occur without breaking a bond 131.The bonding and energetic situation of tetracoordinate tetrahedral as well as of planar carbon atom has been discussed extensively [4] [5]. In tetrahedrally coordinated carbon compounds the bonds can be described as being formed from one 2s and three 2 p orbitals and an appropriate linear combination of ligand orbitals, giving rise to 4 bonding molecular orbitals, each occupied by 2 electrons, and 4 antibonding ones. For planar coordination the central carbon atom can only use one 2s and two 2 p atomic orbitals for a-bonding to the 4 ligands giving rise to 3 orbitals. occupied by 6 electrons. The orthogonal nonbonding 2p orbital is occupied by the remaining 2 electrons (Fig. I ) .According to this bonding model, the difference in stability between the tetrahedral and the planar configuration is mainly due to the increasing energy of one of the 3 degenerate (T,) MO's, which is correlated with the 2p-A0 of A,,-symmetry in the square planar form. This suggests in terms of average C-H bond energies that the planar arrangement should be roughly 80-100 kcal/mol less stable than tetrahedrally coordinated carbon atom. It is thus not too surprising that autoracemization without bond breaking has not been observed. I)

Planarisation of Tetracoordinate Carbon Atom. A Further Route ot (all‐cis)‐[5.5.5.5]Fenestrane

“…-(all-cis)-[5.5.5.5]Fenestrane (3) has been obtained from 4 by a reaction sequence which shows remarkable differences from that reported [3] [7]. In view of these earlier results, the lack of Dieckmann-type reactions of 9 as well as the missing transannular insertion of hydrazones derived from 12 under thermal or photolytic conditions are surprising.…”

“…However, the potassium salt of this hydrazone did not give the expected derivative of fenestrane under thermal or photolytical conditions. This result is particularly surprising because in a related case a substituted fenestrane could readily be prepared by this method [7].…”

“…'exo'-Configuration is also assigned to the CH,O-group of the acetal moiety in 9, because the acetal proton appears as a s in the 'H-NMR spectrum. Because of earlier experience in a related case [7], we had expected that Dieckmann condensation of the diester 9 would give a P-ketoester, which upon further transformations could have led to the desired ketolactone 12. Since the diester 9 gave, even under high-dilution conditions, less than 5 YO of the desired intramolecular condensation product, it was transformed into the dinitrile 10 by reaction of 9 with NH, and subsequent dehydration of the diamide [8].…”

Acylation of Esters, Ketones and Nitriles

“…85 In competition between an ester and the PhS02-stabilized anion the latter cyclizes even where it is secondary and the ester is primary, (62) to (63), presumably in part a reflection on the electrophilicity of the two groups and the stabilities of the products. Where cyanide replaces one of the esters, cyclization occurs from the cyano-stabilized anion.…”

Planar Hexacoordinate Carbons: Half Covalent, Half Ionic

“…[3] Although, for more than two years the presence of two ptCs in this complex remained unnoticed. [4] These findings inspired the design of more ptCs, and later extended to explore for penta-and hexacoordinate carbon species. [5][6][7][8][9][10][11] However, the more the coordination number increases, the more elusive these species become.…”