Stereoselective Cyclopropane Syntheses by “Oxy‐Homodienyl Hydrogen Shifts”

Abstract

A suspension of tetraphenylphosphonium chloride (1 2.438, ~3 3 mmol) in THF (100 ml) is added dropwise with stirring to the yellow suspension at 220 K. After warming to 298 K the suspension is stirred for ca. 20 h at 298 K.Red, rod-shaped crystals (length = 0.4 mm) crystallize out from the filtrate within a week. Later, yellow amorphous "higher phosphanes" also separate out. However, the red crystals of (Ph4P)2P16 can easily be separated mechanically from the by-products. Compared to salts like Na3P7, Na3Pll and their derivatives, the hexadecaphosphide is remarkably stable. The crystals, which are practically insoluble in anhydrous solvents, hydrolyze and turn brown within two days on exposure to air. They decolorize within minutes in H20/ethanol mixtures, but their morphology is preserved. The phosphane odor typical for such reactions does not occur. Up to 570 K the mass spectrum (QMS) shows only the fragments of the cation and of P4. In the presence of Me3SiCl the fragment P16R: (m/z=642) is observed.The X-ray structure analysis[41 gave a surprising result: The compound does not contain the anions p;-, as would be expected in the synthesis, but the novel polycyclic hexadecaphosphorus dianion P: , (Fig. 1). Its skeletal structure, with mm2 symmetry, is made up of two P7 units which are coupled via a common P2 dumbbell; it can also be described in terms of the combination of a P,-unit with a P9-unit. The bond lengths show some peculiarities: The P-P distance a of 232 pm is very long. Here the strain caused by the 3m symmetry of the nortricyclic unit and the yr-tetrahedral configuration of the bridgehead atoms linked directly to the P2-dumbbell is effective. The shortest bond lengths (hand i) occur at the two divalent P'-atoms. The height 312 pm of the nortricyclene moieties lies directly between those in p;and those in the neutral P,R3I5'. D 2-Fig. I . The dianion in the (Ph4P),P16 structure. The bonds of the P,-fragment are shown in bold lines. Bond lengths P-P in pm ( 0 5 0 . 6 ) : a=232.0, b=219.7, c=220.6, d=219.6, e=222.3, f=223.6, p 2 2 1 . 4 , h=217.0, i=214.1, k=220.2, 1=220.4, m=219. 7.The slow crystallization of the sparingly soluble salt would indicate slow formation of the PI,-system. A number of disproportionation reactions are conceivable, which, e.g., start with P:-+Pi-+P4 and then lead from 2P:-and P4 directly or via Pi-as intermediate to P: , and P:-. The Pg-fragment of the dianion is isomeric with the framework of the prismane derivative P9R3 discussed earlierI2I, into which it can be converted via a simple route by a double 1,2-bond shift. pm, B=137.05(3)0, 2=4, pcAic= 1.435 g ~m -~; diffractometer data, 2443 reflections hkl, Mo,, radiation 2 0 5 4 0 " , largedropin intensity, 0=0.06 A: R=0.085.The thermally induced 1,5-homodienyl hydrogen shift (1)+(2) ( n = 1) belongs to the intramolecular ene reactions, which are of importance in the stereoselective synthesis of cyclic compounds [']. With n> 2, the equilibrium (l)*(2) lies on the side of the carbocycle, whereas with n = 1 ring strain effect...

Transition structures for the oxy–ene reaction

“…In fact, such a reaction had been described in 1981 by Klärner and co-workers who named it “oxy-homodienyl hydrogen shift” by analogy with the oxy-Cope rearrangement. 9 The stereoselective thermolysis of ( Z )-hexa-2,5-dienols like compound 3 into cis -cyclopropane 4 was achieved at ca 260 °C, overcoming an experimentally determined Gibbs free energy of activation of 43.5 kcal mol −1 (Scheme 3). Borschberg designed his own substrates such as 5 and investigated their thermally-induced reaction, coined “oxy–ene” in 2001 (Scheme 1C), 10 pointing out that stereoselective transformations could be achieved at high temperature.…”

“…The Gibbs free energy of activation to reach TS AB is 42.1 kcal mol −1 , which is consistent with the experimental value of 43.5 kcal mol −1 reported by Klärner et al for the intramolecular cyclization of ( Z )-hexa-2,5-dienol. 9 Of note, A was taken in its s- cis conformation, leading to a cis enol. With the s- trans conformation, a slightly higher barrier of 42.3 kcal mol −1 was obtained.…”