Negishi-type

Intramolecular allylations can also provide flve-membered rings with ease, and the entropic benefits facilitate additions to ketone substrates. For example, the allylic boronate 121, formed by a Negishi-type coupling between alkenyl... 

Negishi-type reactions to construct a fluoroolefin depsipeptide precursor (29)... 

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... 

Negishi-type cross-coupling reactions of primary and secondary alkyl iodides 1 and alkylzinc bromides 2 proceeded with 10 mol% of Ni(py)4Cl2/(sBu)-PyBOX 5a (entry 6) [48]. Based on calculations, an alkylNi(I)(PyBOX) complex is formed by initial SET reduction, which carries much of the spin density in the ligand, similar to Vicic s catalysts 9. Based on this result a Ni(I)-Ni(II)-Ni(III) catalytic cycle was proposed to operate. 

Fig. 5.8 Hexa(ferrocenyl)benzene, prepared in Peter Vollhardt s group by sixfold Negishi-type ferrocenylation of hexaiodobenzene and characterized by X-ray crystallography...

An easier route to the same unsymmetric zirconacyclopentadiene 20 is the use of Cp2ZrEt2 as reagent (easily prepared from Cp2ZrCl2 with two equivalents of EtMgBr). This reagent (Scheme 9), equivalent to a zirconocene-ethene complex 24, reacts similarly to the Negishi-type reagent, which was prepared in the... 

Some 2-carbonyl-l-propargyl-l//-indoles 159 are prepared by means of Suzuki- and Negishi-type reactions <2006OL4839>. 2-Carbonyl-1/f-indoles 158 were converted into the corresponding A -propargyl derivatives in good to excellent yield by means of PTC nucleophilic substitution in toluene/aqueous NaOH and BuaNBr as the catalyst (Equation 28). l-[l-(2-Propyn-l-yl)-12/-pyrrol-2-yl]ethanone was obtained in 86% yield by similar route. 

Utilization of stereodefined alkenylalanes or alkenylzirconium reagents in palladium-catalyzed cross-coupling reactions greatly enhances the versatility of Negishi-type coupling reactions. These organometallics are readily available by hydroalumi-nation, carboalumination, and hydrozirconation of alkynes, respectively. 

The Suzuki-Miyaura reaction is certainly the most widely used, but each of the others has its own particular advantages (and in some cases disadvantages ). Other metals, such as aluminium, zirconium and indium are occasionally used in variants of Kumada/Negishi-type reactions. 

Zinc Derivatives Oxazoles and benzoxazoles can be lithiated in the 2-position by alkyllithium. In transmetallation of lithiated oxazole excess zinc dichloride gave the highest yield. The zincated substrates 106 and 107 were used in Negishi-type coupling (Scheme 45). Reactions of zincated oxazoles and benzoxazole with aryl halides all proceed well. A similar reaction sequence was used in the preparation of a 2-oxazolyl-substituted tricyclic structure 108. 

Zinc Derivatives. 2-(Benzopyran-4-yl)pyridine Al-oxide has been prepared from 2-chlorozinciopyridine 7V-oxide and benzopyran-4-yl triflate in a Negishi-type coupling from 2-chlorozinciopyridine N-oxide (Scheme 66). A five-fold excess of the the zincated pyridine A-oxide had to be used for optimal formation of the cross-coupled product 161. 

Zinc Derivatives. Direct zincation of 6-bromoxazolo[4,5-b]pyridmes can be effected using activated zinc (Scheme 69). The zincated species 164 and 165 from oxazolo[4,5- ]pyridin-2(3 0-ones or 2-phenyl- or 2-benzyloxyoxazolo[4,5-b]pyridmes were used in Negishi-type coupling in a one-pot reaction with benzyl bromide for the preparation of 6-benzyl derivatives. ... 

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