Haloalkene preparation

C8H17CHO, CHl3 82% ( /Z= 83 17) /7-C8H17CHO, CHBra 37% E/Z = 89 11) Scheme 5.29. (E)-Haloalkene preparation from aldehydes and CHX3-CrCl2. 

More than fifteen years have elapsed since this method was developed, but it has not lost any utility in natural product synthesis. It remains one of the most utilized haloalkene preparation methods. Even in cases where a substrate possesses a lot of sensitive functional groups and chiral centers, selective transformation to the ( )-haloalkene can be accomplished with this reagent. 

Scheme 8.48 E-Haloalkene preparation from aldehyde-ChD -CrClj.

Scheme 8.50 Examples of E-haloalkene preparation in natural product synthesis with aldehyde-CHXj-CrClj.

The F / Cl exchange in chloroalkanes is a route to HFCs. For example, different routes can be possible for the synthesis of CF3CH2F [1,2 ]. Our focus is on its preparation from CF3CH2CI and HF with chromium (HI) oxide as a catalyst. This fluorination is accompanied by a dehydrofluorination which produces chloroalkenes (mainly CF2=CHC1) resulting in a deactivation of the catalyst Indeed this haloalkene could polymerise and thus lead to coke formation. The reactions involved are ... 

Alkynes have been prepared from 1,1- and 1,2-dihaloalkanes and from haloalkenes under the influence of a quaternary ammonium salt geminal- and vic-dibromoalkanes are converted into alkynes under liquid liquid [10, 11] and soliddiquid [12-14] conditions with Aliquat or tetra-n-octylammonium bromide... 

Trihalocyclopropanes, prepared from haloalkenes with dihalocarbenes under phase-transfer conditions, rapidly undergo a base-mediated ring-opening under liquiddiquid two-phase conditions in the presence of ethanol and benzyltri-ethylammonium chloride to yield isomeric 3,3-diethoxyprop-l-ynes in moderate yields [12] (Scheme 9.5). 

In addition to their use as precursors for alkynylboron, tin or zinc compounds, alkynylmagnesium reagents show a moderate reactivity toward the coupling with haloarenes and haloalkenes. They are often commercially available, or easy to prepare. Their main drawback is their low chemoselectivity and high nucleophilicity, which implies incompatibilities with functional groups such as nitro and carbonyl. 

For a lisl of references to the preparation of haloalkenes by Wittig reactions, with references, see Ref. 64, pp... 

In some cases, especially when alkynes are prepared by dehalogenations of haloalkenes, the intermediate organometallic compounds, depending on the nature of the metal, are sufficiently stable to be identified or even isolated at ambient temperatures. This property can be exploited in transmetalation reactions with suitable organometallic compounds, and thus new polylluorinated organometallic species may be prepared. 

A Prelude Preparation of Haloalkenes and Alkenylboronic Acid Derivatives, Important Building Blocks for Palladium-Mediated C,C Couplings Carbocupration of Alkynes... 

Vinyl halides represent yet another important class of intermediates in the conversion of ketones to alkenes. The most widely applied conditions for the conversion of ketones into vinyl halides are those developed by Barton et a/. ° for the conversion of 3p-acetoxyandrost-5-ene-17-one into 3P-hydroxyandrosta-5,16-diene (Scheme 44). These conditions of vinyl halide formation and subsequent reduction have been useful in a number of steroid systems for the introduction of a A -carbon-carbon double bond and have been shown to be compatible with such functional groups as alcohols, isolated double bonds and acetals. The scope of vinyl iodide formation from hydrazones has been studied by Pross and Stemhell, and recently the original reaction conditions were improved by using sterically hindered guanidine bases rather than triethylamine. Haloalkenes have also been prepared from the corresponding ketones by treatment with iodoform and chromium chloride or with phosphorous penta-halides. ... 

Bombykol is a well-known pheromone, first isolated from Bombyx mori L. Bombykol the three related isomers were synthesized by cross-coupling reaction. Three alkenylboronates or boronic acids (35 and 36) required for the coupling are prepared by starting from two alkynes. The stereoselective syntheses of ( )- and (Z)-l-alkenylboronic acids or esters have been discussed in Section 2.2.1. Halogenation of alkenylboronic acid 34 with either iodine or bromine provides ( )- and (Z)-haloalkenes [71]. The palladium-and base-assisted coupling of the Cj-boronate 32 or 33 with the C,-halide 35 or 36 stereoselectively provides bombykol and its three geometrical isomers (Scheme 2-29) [72]. 

Alkenyl perfluorosulfonates have gained importance as substitutes for haloalkenes - at least in laboratory scale preparations - as they are easily obtained from the corresponding carbonyl compounds [137]. Many successful reactions with alkenyl trifluoromethane-... 

Good yields are achieved for five- and six-membered ring cyclizations by the intramolecular cross-coupling reaction of haloalkenes [139, 152], but the scope of reaction is still limited by the availability of the haloalkenes, particularly because of the absence of a simple method for preparing cyclic haloalkenes from ketone precursors. The ready availability of triflates from carbonyl compounds now offers a valuable tool for the annulation of ketones (Scheme 2-56) [160]. 

In most cases, bromo or chloro derivatives are used as starting materials. The 1,2-dihaloalkanes are usually prepared from the alkenes by halogen addition, the 1,1-dihaloalkanes from the corresponding carbonyl compounds with PCI5 or other acid chlorides (Scheme 31). Some 1,1-dichloroalkanes can be prepared from vinyl chloride by addition of a secondary or tertiary alkyl chloride (Scheme 32). The 1-haloalkenes can be prepared from either the 1,2- or the 1,1-dihaloalkanes by elimination of 1 mol of hydrogen halide or by a Wittig reaction (Scheme 33). 

Haloalkenes can be prepared by dehydrohalogenating saturated hydrogen-containing polyhalocarbons using liquid alkali metal acid fluoride and/or alkali metal fluoride compositions [75], HCFC-133a can be converted to CF2=CHC1 using these catalyst systems as shown in eq 21. 

The coupling of ArCl with benzophenone imine is a fast reaction, therefore it represents an option for preparation of arylamines, as the products are easily hydrolyzed/ A simple protocol also avails for the preparation of the A-Boc derivatives of A-alkenylhydrazines from A-haloalkenes/ ... 

Haloalkenes, CH2=CH-(CH2)nX (n = 3-7 X = Cl, Br), are interesting synthons for use in the preparation of long chain alkenols and alkenoic acids 2 in cyclization reactions,3 9 as intermediates in the synthesis of pheromones 10 11 in the preparation of silica gel having a functional surface 12 and for the synthesis of oo-iodochloroalkanes,13 and organogermanium dendrimers.14 They are usually prepared from the corresponding alkenols. 

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