Hoffmann-type elimination

When LDA was used in place of butyllithium, l,l-bis(trimethylsilyl)ethylene (Hoffmann-type elimination product) was obtained as the main product (71 %).214... 

Treatment of the ot/ o-quinones 687 with 2equiv of carbethoxymethylenetriphenylphosphorane 688 affords coumarins in good yield. The reaction proceeds via Wittig olefination of the C-6 carbonyl to form the intermediate ort o-quinone methanides 689. Michael addition of a second equivalent of the phosphorane 688 followed by a Hoffmann type elimination of triphenylphosphine affords the intermediate 690. Intramolecular lactonization then forms the desired coumarins (Scheme 167) <2002J(P1)1455>. 

Now let us go one step further and investigate the homologue mesylate which containes one methylene group more in the side chain. When the alcohol 21 ) was attempted to mesylate, the desired derivative could not be isolated. The formation of the internal quaternary derivative is so fast, that this is the only product isolated in 71 % yield. This compound was very sluggish to react with sodium cyanide. The reaction has been performed in ethylene glycol at 200 Now no attack at an a-carbon with respect to the charged nitrogene occured, only an epimer-pair i2 ) was isolated in 68 % combined yield. Presumably a Hoffmann-type elimination is... 

One would think that the electrophilicity of the indicated centers in Fig.l does not change much while varying n from 0 to 2. Nevertheless the "main attack", the most conclusive attack is shifted from the carbon in a-position with respect of the charged nitrogene to the hydrogene of the C-H bond being in 3-position, resulting in Hoffmann-type elimination only because of a small... 

Because of the harsh conditions and high basicity of fluoride anions, these reactions are often accompanied by base-induced y3-elimination and decomposition of the TAA catalyst via the Hoffmann-type degradation. Particularly, secondary alkyl halides, cyclohexyl chloride, and 2-octyl bromide, in the reaction with KF under these conditions undergo -elimination. 

RX - [Ni(I), RX ], is superseded by more examples of the four types of processes concerted, radical, radical-chain, and two-step ionic. Under the mantle of reductive elimination, perhaps not quite correctly, /3 elimination of hydrogen is included since this process seems able to take place under very similar conditions to those under which a concerted reaction occurs. It is interesting to see the use of Hoffmann-type calculations in rationalizing elimination reaction pathways. Mention should be made also of the mechanism of processes involving two metal centers. 

Reaction with a-silyltertiary aliphatic amines leads to results of the same type, including silyl- and non-sily 1-products, proceeding either from a Stevens rearrangement or a Hoffmann elimination.364... 

The same MO methods can be applied to four- and five-center additions and eliminations, subject to certain restraints. Thus, the synchronous addition of X—X to a double bond in any unsaturate must be syn. The termolecular addition of X and Y to an unsaturate can be syn or anti. Provided that X and Y are nonbonded, YB theory, the Walsh-type correlations (Fig. 4), and extended HMO theory (Hoffmann, 1963) suggest an anti preference. In the next section, the powerful application of symmetry restrictions will confirm these predelictions. 

Evidence for the transient formation of adamantene in the dehalogenation of the dihalides (198 X = Br or I) and of homoadamantene in the Cope elimination of the iV-oxide [199 X = N(0)Me2] and the Hoffmann elimination of the quaternary ammonium hydroxide (199 X = NMe3 0H") has been adduced from the isolation of cyclobutane-type dimers of these highly strained olefins. The present state of knowledge of Bredt s rule and bridgehead olefins has been reviewed by Buchanan. ... 

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