Controlled haptotropic rearrangements – towards a stereospecific molecular switch based on chiral arene chromium complexes

Abstract

A stereospecific molecular switch has been designed based on a reversible thermo- or photo-induced haptotropic shift of a Cr(CO)3 fragment along a naphthohydroquinone skeleton.

Dötz Benzannulation Reactions: Heteroatom and Substituent Effects in Chromium Fischer Carbene Complexes

“…[68,69] We had not yet considered the haptotropic migrations between a functionalized ring (6) and a nonsubstituted fused one (7) until now. As found experimentally in similar cases, [70] coordination to the nonsubstituted ring is more favorable than coordination to the substituted ring. In terms of relative energies with respect to complex 6, the stability of 7 increases marginally on going from OH to OCH 3 substituents and decreases on going from NH 2 to NA C H T U N G T R E N N U N G (CH 3 ) 2 ligands.…”

“…The barriers of about 20 kcal mol À1 are in perfect agreement with those found experimentally for molecules having the same naphthohydroquinone skeleton but different substituents on the substituted ring. [70] Although haptotropic migration is only symmetric for 4-naphthohydroquinone, other substitutions at the para position do not seem to affect energetically the preference of Cr(CO) 3 to move from one side or the other onto the less substituted ring. A very small difference is noted for the amino side-chain in 4-dimethylamino-1-naphthol in which the organic skeleton adopts the 6 x spatial orientation shown in Scheme 5, but no amino interaction was observed as a possible stabilizing factor.…”

Theoretical investigation of thermally and photochemically induced haptotropic rearrangements of chromium ligands on naphthalene systems

“…The inspiration to the present piece of work was given by the work of Jahr et al in 2005 [3], who designed a potential stereospecific molecular switch. They observed in experiments that a tricarbonyl chromium ligand (Cr(CO) 3 ) which is h 6 -coordinated to one ring of a naphthohydroquinone-like skeleton, can undergo a "haptotropic rearrangement" on this aromatic system (shown in Fig. 1).…”

“…In the literature the reaction is also known as the Dötz or WulffeDötz reaction. In our case, a (OC) 5 Cr[C(OMe)(C 6 H 6 )] carbene metal complex reacts with 3-hexyne and in a subsequent step the phenolic functional group of the product is protected [3] in order to yield (R)-1.…”

“…By applying heat to compound (R)-1, the thermally more stable product (S)-2 is formed. In addition, one is able to regenerate complex (R)-1, albeit with a yield of only about 60% [3], through a photoinduced reaction including a coligand interchange of one carbonyl fragment by cyclooctene (cOctene). This type of a photoinduced rearrangement is just one of a variety of rearrangement reactions where transition metals are involved and the reader is advised to find more general information in an overview by Vos and Pryce [6].…”