Electrocyclic reactions torquoselectivity

The Rondan-Houk theory rationalizes many experimental results and leads to several predictions which have since been confirmed. Exceptions have been explained by Houk and co-workers, using numerical calculations. In a sense, the problem of electrocyclic reaction torquoselectivity may be considered solved. 

Actually, the first example of torquoselectivity dates back to the 1960s, though it was not identified using this terminology. The solvolysis of cycloproyl halides and tosylates proceeds via a two-electron electrocyclic reaction to yield allyl cation. disrotatory pathways are allowed, only one occurs ... 

A six-electron cychzation will proceed with disrotatory motion. Torquoselectivity in a six-electron electrocyclic reaction was first examined in the ring opening... 

Torquoselectivity is described as the preference of clockwise (38) or counterclockwise (39) rotation of substituents in an electrocyclization reaction, giving rise to... 

In contrast, the C-C triple bond shows excellent effect on torquoselectivity. Alkynyl aUcyl ketones 81 are olefinated by ynolates to afford the tetrasubstituted olefins 83 in good -selectivity (Fig. 28) [72]. In the electrocyclic reaction of 82, the alkynyl group prefers outward rather than inward rotation over the alkyl and alkenyl... 

The formation of cyclic nitrones (150) from pericyclic mechanism. Kinetic and computational studies have provided evidence for the involvement of a novel pseudo-pericyclic electrocyclization in the conversion of o-vinylphenyl isocyanates into quinolin-2-ones. " Such reactions have also provided evidence of torquoselectivity in a 6jt system. Hash vacuum thermolysis of triazoles (151) has been found to afford dihydroquinolines (155), presumably by generation of a-oxoketenimines (152) which can undergo a [1,5]-hydrogen shift to the o-quinoid imines (153)7(154) and subsequent electrocyclization (see Scheme 57). 

The Nazarov reaction, in which the key electrocyclic step is the conrotatory process 6.505, has one more atom in the ring but the same number of electrons. The question with respect to torquoselectivity now, since this reaction is taking place in the opposite direction, namely ring-closing, is which reacts faster, a dienone... 

The torquoselectivity of the 4 r electrocyclic ring-opening reaction of 2-azetines has been reported to be controlled by the Brpnsted acidity of the catalyst and the polarity of the solvent." A review of the origin of the torquoselectivity in the thermal ring opening of cyclobutenes based on the Cplex-isoelectronic theory and expanded Robinson electronic theory has been reported." Cplex-isoelectronic theory is said to provide a rational for the torquoselectivities in the ring opening of cyclobutenes." ... 

The formation of j8-lactams from aliphatic acid chlorides and imines has been reviewed. The reaction is assumed to proceed through in situ formation of a ketene, followed by interaction with the imine to form a zwitterionic intermediate, which undergoes an electrocyclic conrotatory ring closure to give the jS-lactam ring (Scheme 8). The stereoselectivity of the reaction is sensitive to both the electronic and steric effects of the substituents on the ( )- or (Z)-imine, and the former manifest themselves in the electronic torquoselectivity that occurs during the conrotating cyclization of a zwitterionic intermediate [shown for ( )-imine in Scheme 8]. ... 

Hulot, C., Amiri, S., Blond, G. L, Schreiner, P. R., Suffer , J. (2009). Understanding the torquoselectivity in 8x-electrocyclic cascade reactions synthesis of fenestradienes versus cyclooctatrienes. Journal of the American Chemical Society, 131, 13387-13398. 

Since the torquoselectivity concept was proposed in the 1980s, this concept has been used in various aspects of organic chemistry. Most of these investigatimis involved syntheses of 1,3-dienes via electrocyclic ring-opening reactions of... 

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