Aldehydes alkenyl halides

Aryl or alkenyl halides attack the central carbon of the allene system in the 2,3-butadien-l-ol 120 to form the 7r-allyl intermediate 121, which undergoes elimination reaction to afford the o,/3-unsaturated ketone 122 or aldehyde. The reaction proceeds smoothly in DMSO using dppe as a ligandflOl]. 

Aldehydes can also be prepared by the carbonylation of aryl and alkenyl halides and triflate, and benzyl and allyl chlorides using tin hydride as a hydride source and Pd(PhjP)4 as a catalyst[377]. Hydrosilancs arc used as another hydride source[378]. The arenediazonium tetralluoroborate 515 is converted into a benzaldehyde derivative rapidly in a good yield by using Et ,SiH or PH MS as the hydride source[379]. 

E-Alkenyl halides.1 Iodoform in combination with CrCl2 reacts with an aldehyde at 0° to form an (E)-alkenyl iodide in 75-90% yield. Replacement of CHI, by CHC13 and use of higher temperatures results in (E)-alkenyl chlorides. (E)-Alkenyl bromides can be obtained by use of CHBr3 and CrBr2. The E/Z ratio increases in the order I < Br < Cl, but the rate is in the order I > Br > Cl. Ketones undergo this reaction very slowly. 

GRIGNARD-TYPE COUPLING REACTIONS BETWEEN ALKENYL HALIDES (OR TRIFLATES) AND ALDEHYDES, MEDIATED BY THE CrCI2-NiCI2 SYSTEM3... 

Aromatic carboxylic acids, a,/f-unsaturated carboxylic acids, their esters, amides, aldehydes and ketones, are prepared by the carbonylation of aryl halides and alkenyl halides. Pd, Rh, Fe, Ni and Co catalysts are used under different conditions. Among them, the Pd-catalysed carbonylations proceed conveniently under mild conditions in the presence of bases such as K2CO3 and Et3N. The extremely high toxicity of Ni(CO)4 almost prohibits the use of Ni catalysts in laboratories. The Pd-catalysed carbonylations are summarized in Scheme 3.9 [215], The reaction is explained by the oxidative addition of halides, and insertion of CO to form acylpalladium halides 440. Acids, esters, and amides are formed by the nucleophilic attack of water, alcohols and amines to 440. Transmetallation with hydrides and reductive elimination afford aldehydes 441. Ketones 442 are produced by transmetallation with alkylmetal reagents and reductive elimination. 

The 4-alkenyl /3-sultams 50 and 51 can be prepared by stereoselective alkylation of the 4-monosubstituted /3-sultams 68 with alkenyl halides. Reaction of product 50 with ethylaluminium dichloride in toluene gives the aldehyde 69. When /3-sultam 51 is used, a tandem cyclization is observed yielding the bicyclo[3.2.1] 7-sultam 70 (Scheme 12). In the... 

The insight that zinc ester enolates can be prepared prior to the addition of the electrophile has largely expanded the scope of the Reformatsky reaction.1-3 Substrates such as azomethines that quaternize in the presence of a-halo-esters do react without incident under these two-step conditions.23 The same holds true for acyl halides which readily decompose on exposure to zinc dust, but react properly with preformed zinc ester enolates in the presence of catalytic amounts of Pd(0) complexes.24 Alkylations of Reformatsky reagents are usually difficult to achieve and proceed only with the most reactive agents such as methyl iodide or benzyl halides.25 However, zinc ester enolates can be cross-coupled with aryl- and alkenyl halides or -triflates, respectively, in the presence of transition metal catalysts in a Negishi-type reaction.26 Table 14.2 compiles a few selected examples of Reformatsky reactions with electrophiles other than aldehydes or ketones.27... 

Terminal attack occurs with water, methyl iodide, and trimethylchlorosilane, whereas central attack was preferred with alkenyl halides, aldehydes, and ketones at low temperatures under kinetic control [Eq. (5)]. The Et3SiO group is readily removed from 6 by potassium fluoride in isopropanol to give the vinyl ether RCH2CH2COCH=CH2 (61). Some of these reactions have also been used in elegant syntheses of terpenes (99-102). 

Takai et al. [101] found that alkenyl halide is readily reduced with CrCl2 to give the corresponding organochromium species which adds selectively to an aldehyde... 

Table 9 CiCl2-mediated Addition of Alkenyl Halides to Aldehydes and Ketones (equation 62) Entry Alkenyl halide Aldehyde/ketone Yield (%) Time (h) Product...

Entry Alkenyl halide Aldehyde/ketone Yield (%) Time (h) Product... 

Table 15 Selective Synthesis of ( )-Alkenyl Halides (265) from Aldehydes (264) and CHX3-C1CI2/THF...

Aldehydes may be converted to ( )-alkenyl halides by the reaction of CrCh with a haloform in THF. The highest overall yields for the conversion were with iodoform, but somewhat higher (E) (Z) ratios were observed with bromoform or chloroform. Other low-valent metals, such as tin, zinc, manganese and vanadium, were ineffective. As the examples in Table 19 indicate, the reaction is selective for the ( )-isomer, except in the case of an a,3-unsaturated aldehyde. In addition, the reaction with ketones is sufficiently slow for chemoselectivity to be observed for mixed substrates. 

Takai-Utimoto olefination The chromium(ll)-mediated one-carbon homologation of aldehydes to the corresponding ( )-alkenyl halides. 452... 

The scope of vinyl metals as sources of nucleophilic vinyl groups is very great. As well as the expected electrophiles such as halogens, alkyl and acyl halides, aldehydes and ketones, unsaturated carbonyl compounds and epoxides, they also combine with aryl and alkenyl halides with palladium catalysis. The usual stereochemical course is retention at the vinyl group. It is necessary to decide whether the vinyl metal is reactive enough or whether it must first be transformed into an ate complex. Since most of these vinyl metals can be converted into each other with retention, this is an unusually versatile group of reagents. 

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