Grignards, 1-alkenyl

We have synthesized a series of organogermane dienes following a similar procedure to that described in the synthesis of stannadienes, namely the Grignard alkenylation of dialkylgermanium dichlorides. 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Especially in the early steps of the synthesis of a complex molecule, there are plenty of examples in which epoxides are allowed to react with organometallic reagents. In particular, treatment of enantiomerically pure terminal epoxides with alkyl-, alkenyl-, or aryl-Grignard reagents in the presence of catalytic amounts of a copper salt, corresponding cuprates, or metal acetylides via alanate chemistry, provides a general route to optically active substituted alcohols useful as valuable building blocks in complex syntheses. 

Further utility of the Andersen sulphoxides synthesis is demonstrated by the preparation of optically active unsaturated sulphoxides which were first prepared by Stirling and coworkers359 from sulphinate 276 and the appropriate vinylic Grignard reagents. Later on, Posner and Tang360 prepared in a similar way a series of ( )-l-alkenyl p-tolyl sulphoxides. Posner s group accomplished also the synthesis of (+)-(S)-2-(p-tolylsulphinyl)-2-cyclopentenone 287, which is a key compound in the chiral synthesis of various natural products361 (equation 159). 

Organotins. The organotin reagents have much lower nucleophilicity than that of the Grignard reagents, thus allowing the use of a variety of functionalized monomers for the polymerization. Aryl-alkenyl iodides, bromides and tosylates have been used as substrates. Palladium complexes are commonly employed as catalysts for the reaction. Because the catalysts can be destroyed... 

Addition of an organolithium and Grignard reagent across the peroxy bridge of endoperoxides gives c/s-cyclohex-2-en-l,4-diols. Alkyl, cycloalkyl, alkenyl and aryl moieties can be transferred to oxygen <96TL6635>. 

Alkenyl sulfoxides (42 and 43) were first prepared in optically active form by Mulvaney and Ottaviani , described in an article overlooked by most workers in the field, and a year later by Stirling and coworkers through the reaction of the appropriate vinyl Grignard reagent with sulfinate ester 19. Both groups studied the addition of nucleophiles to the carbon-carbon double bond . More recently, Posner and coworkers reported a similar synthesis of ( )-l-alkenyl sulfoxides, e.g. 44 and In the synthesis of 45,... 

Nickel(II) salts are able to catalyze the coupling of Grignard reagents with alkenyl and aryl halides. A soluble 6 -phosphine complex, Ni(dppe)2Cl2, is a particularly effective catalyst.266 The main distinction between this reaction and Pd-catalyzed cross... 

Stereo- and regioselective synthesis of trienes and tetraenes has been reported by palladium-catalysed coupling of (E)- or (Z)-l-alkenyl boronates with (E)- or (Z)-2-bromo-1-phenylthio-l-alkenes followed by treatment with a Grignard reagent in the presence of a nickel catalyst (equation 146)259. 

Likewise, the sulfoxide-metal exchange reaction of /3-acetoxy sulfoxides 164 (/3-mesyloxy sulfoxides can also be used), which are prepared from alkenyl aryl sulfoxides 163 and aromatic aldehydes, with a Grignard or alkyllithium reagent at low temperatures gives the allenes 162 in good to excellent yield (Scheme 5.24) [65],... 

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