Cross-coupling alkyl-alkenyl

Stereoselective synthesis of 1,4-dienes This Pd complex markedly catalyzes the cross-coupling of alkenyl and aryl metals containing Al, Zr, or Zn with allylic halides, but not of alkyl metals. The coupling is markedly stereo- and regioseleclive. Examples ... 

Scheme 5.15 Cross-coupling of alkenyl-Grignard reagents with alkyl electrophiles.

The hydroboration-coupling approach for the construction of carbon skeletons affords several advantages [139]. The high stereoselectivity of the hydroboration reaction provides a stereodefined alkyl center on boron. For instance, in the reaction shown in Scheme 2-49, the hydroboration occurs chemoselectively at the less hindered C(19)-C(20) double bond. In addition, the alkylboron group thus constructed can be readily cross-coupled with alkenyl or aryl halides under mild conditions. 

The three principal palladium-mediated reactions of interest in the synthesis of vitamin h(l) and carotenoids are (a) allylic alkylation of 7C-allylpalladium complexes, (b) Heck-type couplings, and (c) cross-coupling of alkenyl and alkynyl metal derivatives. 

The thioboration of terminal alkynes with 9-(alkylthio)-9-borabicyclo[3.3.1]-nonanes (9-RS-9-BBN) proceeds regio- and stereoselectively by catalysis of Pd(Ph,P)4 to produce the 9-[(Z)-2-(alkylthio)-l-alkeny)]-9-BBN derivative 667 in high yields. The protonation of the product 667 with MeOH affords the Markownikov adduct 668 of thiol to 1-alkyne. One-pot synthesis of alkenyl sulfide derivatives 669 via the Pd-catalyzed thioboration-cross-coupling sequence is also possible. Another preparative method for alkenyl sulfides is the Pd-catalyzed cross-coupling of 9-alkyl-9-BBN with l-bromo-l-phe-nylthioethene or 2-bromo-l-phenylthio-l-alkene[534]. 

The hydrosi(ly)lations of alkenes and alkynes are very important catalytic processes for the synthesis of alkyl- and alkenyl-silanes, respectively, which can be further transformed into aldehydes, ketones or alcohols by estabhshed stoichiometric organic transformations, or used as nucleophiles in cross-coupling reactions. Hydrosilylation is also used for the derivatisation of Si containing polymers. The drawbacks of the most widespread hydrosilylation catalysts [the Speier s system, H PtCl/PrOH, and Karstedt s complex [Pt2(divinyl-disiloxane)3] include the formation of side-products, in addition to the desired anh-Markovnikov Si-H addition product. In the hydrosilylation of alkynes, formation of di-silanes (by competing further reaction of the product alkenyl-silane) and of geometrical isomers (a-isomer from the Markovnikov addition and Z-p and -P from the anh-Markovnikov addition. Scheme 2.6) are also possible. 

Aryl-alkenyl cross-coupling is straightforward. Simple alkylmagnesium reagents (Me, Et, CH2SiMe3, etc.) can be easily involved in Ni-catalyzed cross-coupling (27),139,140 while more complex alkyl halides—particularly branched ones prone to /3-hydride elimination—require Pd catalysts with bidentate phosphines, such as dppf, to achieve good selectivity (Section 9.6.3.4.7). 

The same ligand allowed the cross-coupling of various boronic acids (aryl, alkenyl, alkyl) with alkyl bromides in the system (Pd(OAc)2/PMe Bu2, BuOK. amyl alcohol, r.t.).411... 

Recently, with a view to overcome the difficulty on the preparation of aryl or alkenyl halides or sulfonates, thioamides and their S-alkyl derivatives have been proposed as a new class of electrophilic partners. This palladium cross-coupling methodology was developed by Liebeskind and mostly applied to heteroaromatic templates.118 121... 

Nickel(II) salts are able to catalyze the coupling of Grignard reagents with alkenyl and aryl hahdes. A soluble bis-phosphine comples, Ni(dppe)2Cl2, is a particularly effective catalyst.167 168 The main distinction between this reaction and Pd-catalyzed cross coupling is that the nickel reaction can be more readily applied to saturated alkyl groups because of a reduced tendency for /(-elimination. 

Aryl and alkyl organozincate reagents, generated in situ by reaction of Grignard reagents and sub-stoichiometric amounts of ZnCl2, cross-couple smoothly in refluxing THF with functionalized aryl and alkenyl as well as primary and secondary alkyl chlorides in the presence of Pd(dppf)Cl2. ... 

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