Butterfly conformation

Anthracenes are planar by virtue of the necessity of maintaining aromaticity. When the central ring is reduced, an overall "butterfly" conformation is achieved. For reasons that are not yet understood at the molecular level, this conformation is often associated with central antidepressant activity. 

The neutral decanuclear complex [Mn°(tptz)(H20)]2[Mn (tptz) (MeOH)2l4[W (CN)g]4 (152) exhibits a structure in which three cyanide Mu2W2 squares are connected by shared W(V) comers [Fig. 54(a)]. Another view of this cluster [Fig. 54(Z>)] reveals that the peripheral squares are quite distorted and adopt a butterfly conformation. The overall shape of the cluster core resembles two boats fused at the Mn(II) centers. 

The seven-membered ring in the 2-phenyldioxaarsepine 54 has a chair conformation with a mean As—O separation of 1.79 There are significant differences in the structures of the related 10, lO -bis(phenoxarsine) oxide and selenide 55, X = O or Se, particularly in the deviations from planarity of the phenoxarsine rings . Two structurally similar, independent molecules are found in the asymmetric unit for the epoxy-dihydroarsanthrene 56 both show the butterfly conformation with As—O distances of 1.81 A and angles at arsenic varying between 88.8 and 93.2° . 

Each Ti bonds to 3C via a bonds and each C bonds to 2Ti and one C. The all-carbon analogue, C20, is not expected to be stable because of severe internal strain (it would be the smallest possible fullerene, p. 280). Note, however, that dodecahedrane, C20H20, is known.An alternative description of the structure (Fig. 8.18b) would be as a weakly bonded cube. Tig, each face of which is capped by a C2 unit. The calculated distances are Ti Ti 302 pm, Ti-C 199 pm and C-C 140 pm (implying some multiple bonding cf. 140 pm in benzene). An alternative Th structure for TigC, which is calculated to have a lower energy, has also been proposed.In this, the Tig array is a tetracapped tetrahedron containing six Ti4 faces in butterfly conformation each of these Ti4 faces can then accommodate a C2 unit as shown in Fig. 8.18c. 

The two-electron reduced, or hydroquinone, isoalloxazine is pale yellow. It is an electron-rich heterocycle and, when planar, is antiaromatic according to Hiickel s rule. The ring system of the hydroquinone in some small-molecule structures and some protein structures is bent by as much as 30° along the N5—NIO axis, presumably to relieve the antiaromaticity. However, the majority of hydroquinones bound to proteins do not deviate from planarity much more than oxidized isoalloxazines. This is likely to be influenced by protonation of Nl, whose pA a is 6.7 in aqueous solution. Quantum calculations show that neutral hydroquinone adopts butterfly conformation but the anion is planar. A survey of crystal structures agrees with this correlation — anionic hydroquinones are generally nearly planar, while the few instances of the butterfly conformation belonged to neutral hydroquinones. Hydroquinones react as single-electron donors, as hydride donors, or as nucleophiles at N5 or C4a. 

The reaction of benzcne-1,2-dithiol (H2bdt) with [Ph4As][TcOCl4] in aqueous ethanol resulted in the red-brown complex salt [Ph4As][TcO(bdt)2] with m.p. 208-21 () C. [Ph4As [TcO(bdt)2] crystallizes in the monoclinic space group Ce with r/=12.652(1), h= 15.783(1). c=16.662(l) A, /1=93.03(1)°, and Z=4. The Tc atom lies 0.732(1) A above the plane of the four sulphur atoms, lire sulphur and carbon atoms are located in two different planes. The dihedral angle of 164.91(1) between the normals of the planes renders a butterfly conformation to the complex anion [111]. 

The structure of the 9,10-distannaanthracene 155 has been determined by X-ray crystallography and shows that the six-membered stannacycles have a boat conformation and that each half of the molecule has a butterfly conformation. The Sn-Sn bond length of 2.7836(5) A is in the normal range <2005CL1018>. 

A different conformational picture is encountered when the eight-membered ring is fused with two aromatic rings as illustrated by the conformational analysis on the dibenzo[d ][l,3>6]dioxazocine (7) skeleton carried out by Farkas et al. <85JST131>. The possible structures can be divided into the quasi-rigid Q symmetric butterfly conformation (7a), which has the hetero ring in the crown conformation, and two flexible conformational families with an enantiomeric relationship... 

To the small extent to which trimethoprim binds to chicken liver DHFR, X-ray crystallography reveals it to be squeezed into a butterfly conformation, less energetically favourable than the position it occupies in the E. coli enzyme where the two rings are nearly perpendicular to one another (Matthews, 1981). 

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