Photoirradiation would require an antarafacial shift of hydrogen. Although such reactions are rare, there are examples where antarafacial shifts are favored:
In contrast to hydrogen [1,5] shifts, there have never been any observed [1,5] alky shifts in an open-chain system. Chemists have determined rate preferences for [1,5] alkyl shifts in cyclic systems: carbonyl and carboxyl> hydride> phenyl and vinyl» alkyl.
Alkyl groups undergo [1,5] shifts very poorly and usually require high temperatures. However, for cyclohexadienes, the temperature for alkyl shifts is not much higher than that for carbonyls, the best migratory group. A study showed that this is because alkyl shifts on cyclohexadienes proceed through a different mechanism. First the ring opens, followed by a [1,7] shift, and then the ring reforms electrocyclically:
This same mechanistic process is seen below, without the final electrocyclic ring-closing reaction, in the interconversion of lumisterol to vitamin D.
[1,7] sigmatropic shifts are predicted by the Woodward-Hoffmann rules to proceed in an antarafacial fashion, by a Mobius topology transition state. An antarafacial [1,7] shift is observed in the conversion of lumisterol to vitamin D, where following an electrocyclic ring opening to previtamin D, a methyl hydrogen shifts.
Bicyclic nonatrienes also undergo [1,7] shifts in a so called walk rearrangement, which is the shift of divalent group, as part of a three-membered ring, in a bicyclic molecule.
[3,3] sigmatropic shifts are well studied sigmatropic rearrangements. The Woodward-Hoffman rules predict that these six electron reactions would proceed suprafacially, using a Huckel topology transition state.
Discovered in 1912 by Rainer Ludwig Claisen, the Claisen rearrangement is the first recorded example of a [3,3]-sigmatropic rearrangement. This rearrangement is a useful carbon-carbon bond-forming reaction. An example of Claisen rearrangement is the [3,3] rearrangement of an allyl vinyl ether, which upon heating yields a γ,δ-unsaturated carbonyl. The formation of a carbonyl group makes this reaction, unlike other sigmatropic rearrangements, inherently irreversible.
The ortho-Claisen rearrangement involves the [3,3] shift of an allyl phenyl ether to an intermediate which quickly tautomerizes to an ortho-substituted phenol.
When both the ortho positions on the benzene ring are blocked, a second ortho-Claisen rearrangement will occur. This para-Claisen rearrangement ends with the tautomerization to a tri-substituted phenol.
The Cope rearrangement is an extensively studied organic reaction involving the [3,3] sigmatropic rearrangement of 1,5-dienes. It was developed by Arthur C. Cope. For example 3,4-dimethyl-1,5-hexadiene heated to 300 °C yields 2,6-octadiene.
The Oxy-Cope rearrangement a hydroxyl group is added at C3 forming an enal or enone after Keto-enol tautomerism of the intermediate enol:
The Carroll rearrangement is a rearrangement reaction in organic chemistry and involves the transformation of a β-keto allyl ester into a α-allyl-β-ketocarboxylic acid. This organic reaction can be followed by decarboxylation and the final product is a γ,δ-allylketone. The Carroll rearrangement is an adaptation of the Claisen rearrangement and effectively a decarboxylative allylation.
The Fischer indole synthesis is a chemical reaction that produces the aromatic heterocycle indole from a (substituted) phenylhydrazine and an aldehyde or ketone under acidic conditions. The reaction was discovered in 1883 by Hermann Emil Fischer.
The choice of acid catalyst is very important. Successful acid catalysts include: Bronsted acids such as HCl, HSO, polyphosphoric acid and p-toluenesulfonic acid. Lewis acids such as boron trifluoride, zinc chloride, iron chloride, and aluminium chloride are also useful catalysts.
Several reviews have been published.
Similar to [3,3] shifts, the Woodward-Hoffman rules predict that [5,5] sigmatropic shifts would proceed suprafacially, Huckel topology transition state. These reactions are rarer than [3,3] sigmatropic shifts, but this is mainly a function of the fact that molecules that can undergo [5,5] shifts are rarer than molecules that can undergo [3,3] shifts.
The migration of a divalent group, such as O, S, NR or CR, which is part of a three-membered ring in a bicyclic molecule, is commonly referred to as a walk rearrangement. This can be formally characterized according to the Woodward-Hofmann rules as being a (1, n) sigmatropic shift. An example of such a rearrangement is the shift of substituents on tropilidenes (1,3,5-cycloheptatrienes). When heated, the pi-system goes through an electrocyclic ring closing to form bicycle[4,1,0]heptadiene (norcaradiene). Thereafter follows a [1,5] alkyl shift and an electrocyclic ring opening.
Proceeding through a [1,5] shift, the walk rearrangement of norcaradienes is expected to proceed suprafacially with a retention of stereochemistry. Experimental observations, however, show that the 1,5-shifts of norcaradienes proceed antarafacially. Theoretical calculations found the [1,5] shift to be a diradical process, but without involving any diradical minima on the potential energy surface.
Group transfer reaction
A Group transfer reaction is pericyclic reaction where one pi bond is converted to one sigma bond, at the same time that a sigma bond migrates.
The best known group transfer reaction is the ene reaction.
Mercedes-Benz W201
The Mercedes-Benz W201 is a car produced by Mercedes-Benz. It was introduced in 1982 and came before the C-Class, that replaced the W201 in 1993.
Cadel Evans
Cadel Evans (born 14 February 1977 in Katherine, Northern Territory) is an Australian professional bicycle race driver. In 2011, he is the first Australian to win the Tour de France.
Cadel Evans was born on 14 February 1977 at the Katherine Hospital, Katherine, Northern Territory, Australia. He is the son of Helen Cocks, a bank manager and Paul Evans, a council foreman. He spent his early childhood in the small aboriginal community of Barunga, 80 km east of Katherine. At the age of seven, he was hit in the head by a horse and spent seven days in an induced coma. In 1986, his parents got divorced and he moved with his mother to Armidale, New South Wales. Later they moved to the state of Victoria. Skateboarding was one of his interests when he was a teenager. His father describes him as a good student, but otherwise just an ordinary kid who would leave his toys around. "Not in [my] wildest dreams" would he imagine that his son would become a top world athlete.
Evans attended Newling Public School in Armidale, and Eltham High School in Melbourne.
Grand Army Plaza (Manhattan)
Grand Army Plaza is at 59th Street and Fifth Avenue in Manhattan, New York. It is in front of the Plaza Hotel. It is opposite the southeast corner of Central Park. The sculptor Karl Bitter first had the idea for the Plaza in 1898. It is named after the Grand Army of the Potomac. The Carrère and Hastings architecture firm designed it. Construction was finished in 1916.
The Plaza is oval. 59th street divides it into northern and southern parts. The northern part has a golden statue of William Tecumseh Sherman designed by Augustus Saint-Gaudens. The statue features Sherman on a horse with the goddess "Victory" in front of him. There are often temporary sculpture exhibits at the north side of the plaza.
The southern part is the Plaza of the Pulitzer Fountain of Abundance. This fountain was designed by Bitter and paid for by Joseph Pulitzer. A bronze statue of the Roman goddess Pomona is at the top of the fountain. The statue of Pomona was the last one that Bitter designed.
The plaza was renovated in 1990 at a cost of $3.7 million. The Plaza Hotel is named for the Plaza, and is at its southwest corner.
USS Enterprise (CV-6)