Eliminations via cyclic transition states

Substrate Transition state Product Temp, range Ref. 

The cyclic nature of the transition states in these reactions dictate that elimination will proceed with syn stereochemistry. A cyclic transition state involving only five or six atoms cannot accommodate an anti stereochemical relationship at the site of elimination. For this reason, this family of reactions is often referred to as thermal syn eliminations. 

Under the mild conditions of the reaction, there is no equilibration of the alkenes, so the product composition is governed by the relative stabilities of the various transition states. Usually, more of the trans olefin than of its cis isomer is formed, presumably because steric repulsion raises the energy of the transition state leading to cis olefin. The selectivity is not high, however, since the ratio of trans cis from some simple cases is in the range 3 1 to 2 1. In cyclic systems, conformational 

The pyrolysis of esters has usually been done with acetate esters. The thermal requirement for the reaction is very high, with temperatures above 500° usually required. The pyrolysis is thus a vapor-phase reaction. In the laboratory, this is usually accomplished in a packed glass tube heated with a small furnace. The reacting vapors and product are swept through the hot chamber at an appropriate rate by an inert gas such as nitrogen, and into a cold trap or other system for condensation. 

Similar reactions occur with esters derived from long-chain acids if the boiling point of the ester is high enough, the reaction can be carried out in the liquid phase. Vapor-phase acetate pyrolysis is, however, the most generally used procedure. 

That the elimination is syn has been established by use of deuterium labels. Deuterium was introduced stereospedfically by LiAlD4 reduction of cis- and frans-stilbene oxide. The product of the subsequent ester pyrolysis is frans-stilbene because of eclipsing effects in the transition state. Tlie syn elimination is demonstrated by retention of deuterium in the olefin from frans-stilbene oxide and its absence in the olefin from c -stilbene oxide.  

As with the amine oxides, mixtures of olefins are obtained when more than one type of jS -hydrogen is present. In noncyclic compounds, the olefin composition often approaches that expected on a statistical basis from the number of each type of hydrogen. The tram olefin usually predominates over the cis for a given isomeric pair. In cyclic systems, conformational features, ring strain, and related factors usually distort the mixture from that expected on a statistical basis. Elimination in a direction in which the syn mechanism can operate is strongly preferred over elimination in a direction where this is impossible. 

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Scheme 6.17. Thermal Eliminations via Cyclic Transition States... 

Alkenes are formed by the thermal decomposition of esters, xanthates, amine oxides, sulfoxides, and selenoxides that contain at least one (3-hydrogen atom. These elimination reactions require a cw-configuration of the eliminated group and hydrogen and proceed by a concerted process. If more than one (3-hydrogen is present, mixtures of alkenes are generally formed. Since these reactions proceed via cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

The thermolysis of xanthates derived from primary alcohols yields one olefin only. With xanthates from secondary alcohols (acyclic or alicyclic) regioisomeric products as well as fi/Z-isomers may be obtained see below. While acyclic substrates may give rise to a mixture of olefins, the formation of products from alicyclic substrates often is determined by the stereochemical requirements the /3-hydrogen and the xanthate moiety must be syn to each other in order to eliminate via a cyclic transition state. 

Upon addition of a base—triethylamine is often used—the sulfonium salt 7 is deprotonated to give a sulfonium ylide 8. The latter decomposes into the carbonyl compound 2 and dimethyl sulfide 9 through /3-elimination via a cyclic transition state. 

The other SiH4 decomposition pathway involves Hj elimination via a three-center cyclic transition state (Roenigk et ai, 1987) ... 

Oxophilic phosphomum compound 47 forms from tributylphosphine (46) and the phenylselenide 45, and this then suffers nucleophilic attack by alcohol 18. The resulting free selenium nucleophile 48 displaces phosphine oxide 50 with the formation of phenylsele-nide 51. Phenylselenide 51 is oxidi/ed by hydrogen peroxide to phenylselenoxide 52, which at room temperature undergoes an elimination reaction to (-)-A9tl2)-capnellene (1). The mechanism is similar to that of the Cope elimination, proceeding via a cyclic transition state. 

The gas-phase unimolecular pyrolysis of 2,4-dimethylpentane-2,4-diol has been found to occur by eliminative formation of acetone, isobutene, and H20 via a concerted six-membered cyclic transition state (Scheme 10).69 Single-pulse shock tube studies of the eliminative decomposition of ethoxy compounds have also been reported.70... 

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