And dehydrohalogenation

As we saw in Chapter 5 dehydrations and dehydrohalogenations are typically regiose lective m the direction that leads to the most stable double bond Conjugated dienes are more stable than isolated dienes and are formed faster via a lower energy transition state... 

Alkenylbenzenes are prepared by the various methods described m Chapter 5 for the preparation of alkenes dehydrogenation dehydration and dehydrohalogenation... 

Hydrogenation of alkynes to alkenes using the Lindlai catalyst is attractive because it sidesteps the regioselectivity and stereoselectivity issues that accompany the dehydration of alcohols and dehydrohalogenation of alkyl halides. In tenns of regioselectivity, the position of the double bond is never in doubt—it appears in the carbon chain at exactly the sane place where the triple bond was. In tenns of stereoselectivity, only the cis alkene forms. Recall that dehydration and dehydrohalogenation normally give a cis-trans mixture in which the cis isomer is the minor product. 

Nitrile oxides are usually prepared via halogenation and dehydrohalogenation of aldoximes [11] or via dehydration of primary nitro alkanes (Scheme 1) [12]. However, it is important to note that nitrile oxides are relatively unstable and are prone to dimerization or polymerization, especially upon heating. 1,3-Dipolar cycioaddition of a nitrile oxide with a suitable olefin generates an isoxazoline ring which is a versatile synthetic intermediate in that it provides easy access to y-amino alcohols, )5-hydroxy ketones, -hydroxy nitriles, unsaturated oximes, and a host of other multifunctional molecules (Scheme 1) [5a]. Particularly for the formation of )5-hydroxy ketones, nitrile oxide-olefin cycioaddition serve as an alternative to the Aldol reaction. 

Vinylation or arylation of alkenes with the aid of a palladium catalysts is known as the Heck reaction. The reaction is thought to proceed through the oxidative addition of an organic halide, RX onto a zero-valent [PdL2] species followed by coordination of the olefin, migratory insertion of R, reductive elimination of the coupled product and dehydrohalogenation of the intermediate [HPdXL2] (Scheme 6.1). 

Halogenation and dehydrohalogenation have been performed as a one-step operation from aldoximes using NCS in the presence of base such as pyridine (67), NBS and triethylanfine (68), or a heterogeneous base such as basic alumina or Florisil (69). Solvent-free reaction conditions also proved useful for the preparation... 

Similarly, the 2-cyano-6-oxazolopiperidine 75 (Scheme 16) can be used to provide a variety of substituted piperidines <99TL3731, 99H(51)2065>. Conversion to the enamide 76 provides a means to introduce C-3 alkyl groups by Michael reaction <99TL3699>. Electrochemical bis-bromination and dehydrohalogenation affords the vinyl bromide 77, which can imdergo substitution at the 4-position by the addition of nucleophiles as simple as water <99T8931>. 

By far the most thoroughly investigated azocines are the 2-methoxy derivatives prepared and studied by Paquette and coworkers (7lAG(E)ll). The synthesis involves addition of chlorosulfonyl isocyanate to a cyclohexadiene, conversion to the imidate and introduction of another double bond by allylic bromination and dehydrohalogenation. Valence isomerization then ensues and 2-methoxyazocine (98) or alkyl homologs are isolated from this sequence in multigram quantities as stable yellow oils. NMR data (see 98) clearly indicate the monocyclic azocine structure the spectra are invariant from -70 to 180 °C and indicate less than 2% of the bicyclic valence isomers. 

In dehydration and dehydrohalogenation the preferential order for removal of an H is 3°>2°> 1 (Saytzeff rule). We can say the poor get poorer. This order obtains because the more R s on the C==C group, the more stable is the alkene. The stability of alkenes in decreasing order of substitution by R is... 

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