Halo-terminated intermediate

Example Procedure for Preparation of Halo-Terminated Intermediate Monomer/Oligomer Mixtures... 

Pandey and colleagues reported the intramolecular photocycliza-tion of 6-halo terminal alkenes using organosilicon compounds having Si-Se bond as shown in Scheme 13." The 1,5-dimethoxy-naphthalene-sensitized photoreaction generates RjSi and PhSe" by the dissociation of the radical anion of PhSe-SiRj. Key intermediates in this complex system are RjSi and PhSe. The former radical abstracts a halogen atom and the latter radical is trapped by a terminal radical giving a cyclopentane derivative. 

The reaction of allyl halides with terminal alkynes by use of PdClifFhCNji as a catalyst affords the l-halo-l,4-pentadienes 297. 7r-AlIylpalladium is not an intermediate in this reaction. The reaction proceeds by chloropalladation of the triple bond by PdCh, followed by the insertion of the double bond of the allyl halide to generate 296. The last step is the regeneration by elimination of PdCh, which recycles[148]. The cis addition of allyl chloride to alkynes is supported by formation of the cyclopentenone 299 from the addition product 298 by Ni(CO)4-catalyzed carbonylation[149]. 

Haloboration of terminal alkynes yielded stereodefined (Z)-2-halo-l-alkenylboron compounds e.g., 114, 115 that allowed further functionalization of the resulting C-Br or G-B bond. 2,2-Disubstituted-l-alkenylboronic esters were stereoselectively obtained when the haloboration was followed by cross-coupling with 1-alkynylzinc chlorides (Equation (18)).192 The iodoboration-conjugate addition sequence gave alkenyl iodides that were used as intermediate for the total synthesis of deoxyepothilone derivatives (Equation (19)).193-195... 

Silver also activates terminal alkynes to form halo acetylides (144). In 2006, Yamamoto et al. (145) employed this method to perform an intramolecular diiodination with excellent yields, which gives a key intermediate in the production of the synthetically intriguing bicyclic / -diiodobenzene (Fig. 35). 

Triazoles can also be prepared from in situ formation of azides from halo compormds. For example, 1,4-disubstituted-1,2,3-triazoles 138 were obtained in excellent yields by a convenient one-pot procedure from a variety of aryl and alkyl iodides 136 and terminal alkynes 137 without isolation of potentially unstable organic azide intermediates 05SL2941>. Efficient one-pot synthesis of 1,2,3-triazoles from in situ formation of azides from benzyl and alkyl halides with alkynes has also been reported <05SL943>. 

Asymmetric synthesis of 8-functionalized optically active secondary alcohols was realized hy TarB-N02-catalyzed enantios-elective reduction of a-halo ketones to an intermediate terminal epoxide and sequential ring opening with various nucleophiles. Optically active st)Tene oxide was prepared from a-bromoacetophenone with NaBH4 and TarB-N02 in high yield and with high enantioselectivity (98% 3ueld and 94% ee). fi-Functionalized secondary alcohols could be obtained from the epoxides by nucleophilic attack under appropriate conditions (eq41). 

The intramolecular carbopalladation starting from a 2-halo-1,6-enyne leads to a terminal aUcenylpalladium halide relay that can be trapped by an external alkene or alkyne. With the former, the cascade process leads to a 1,3,5-hexatriene that undergoes rapid 67T-electrocyclization to a flve-ring-annelated cyclohexadiene and this, in turn, is easily dehydrogenated to the corresponding aromatic compound (Scheme 15). With an external alkyne trapping the intermediate, a 1,3,5-hexatrienylpalladium intermediate will be formed and can either cyclize by intramolecular carbopalladation or 67r-electrocycliza-tion before termination by dehydropalladation will occur. ... 

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