Rieke s zinc

Interestingly, Rieke s zinc cannot afford this synthesis from 3-bromothiophene into one step. Indeed, the substrate has to be first transformed in the corresponding 3-iodothiophene, which is more reactive than the bromide analogue (equation 33)42. 

Lithium halides (bromide or Iodide) may well modify the Lewis character of the zinc atom, probably via a zincate species [53], and prevent the efficient coordination of the zinc atom to the double bond, coordination which is required for the carbocyclization. Thus, in the Rieke method, it is essential to wash the active zinc thoroughly since the lithium naphthalenide reduction of zinc bromide also generates lithium bromide, which is detrimental to the success of the reaction. Indeed, the insertion of Rieke s zinc in the presence of LiBr leads to the linear organozinc iodide but not to the cyclic product [52]. 

Treatment of 6-iodo-l-heptene 52 with Rieke s zinc, in ether, leads quantitatively to the cyclized organozinc iodide in less than 20 min [55, 56], as shown by iodinolysis of the reaction mixture [57] (Scheme 7-46). 

The zinc-mediated Reformatsky reaction is one of the classical methods for carbon-carbon bond formation. To date, various main group metals and transition metals have been used for this reaction. Rieke s activated indium powder mediates readily the coupling of ethyl a-bromoacetate and a variety of carbonyl compounds yielding /3-hydroxy esters in good yields (Scheme 87).3 Later, commercially available indium powder has been found to be equally effective for the indium-based Reformatsky reaction in THF.28 This indium Reformatsky reaction is accelerated by ultrasound irradiation (Scheme 88).322,323 Indium(i) iodide also mediates the Reformatsky reaction of aldehydes and ketones to give /3-hydroxy esters, presumably via organoindium(m) diiodide (Scheme 89).27... 

General procedure for the eyelization involving Rieke s activated zinc (Scheme 7-39)... 

Rieke s method quantitatively reacts dibromothiophene 8 with Rieke zinc (Zn ) to generate a ca 9 1 or higher ratio of isomers 9 and 10 (Scheme 2.2) [33], The ratio varies with the reaction conditions, temperature and, in some cases, substituent bulk. Despite this, polymerization affords exclusively HT polythiophene. Chen and co-workers demonstrated that steric congestion on the catalyst determines selectivity in the coupling reaction [33]. 

Rieke, R. D., Uhm, S. J. Activated metais. Xi. improved procedure forthe preparation of 3-hydroxy esters using activated zinc. Synthesis 1975,452453. 

A useful technique for the preparation of n and Cu powders) is by electrolysis with puls< efficiency in allylation achieved by using s involves reduction of ZnCl with Na in liquid 1 Ketones. Rieke zinc produced by the r reacts with alkyl halides, and the organozin treatment with CuCN and acid chlorides. S readily prepared in this direct manner. a-Chkj method using chloroacetyl chloride in the couf 3-Thienylzinc halides. These reagenii Rieke zinc. They undergo Ni-catalyzed coupln... 

Recently, a systematically regiocontrolled synthesis of poIy(3-alkylthiophene)s mediated by Rieke zinc has been proposed [187] and a series of poly(3-alkylthiophene)s with different regioregularities has been obtained starting from the same compounds. Thus it was possible to prepare a completely regioregular HT polymer and a totally regiorandom one. 

Rieke zinc is prepared by placing lithium metal (10 mmol), a catalytic amount of naphthalene (1 mmol), and 12-15 ml of THF in one flask placed in an ice bath. Once this mixture has stirred for about 30-60 s, it will turn dark green, indicating the formation of lithium naphthalenide. Zinc chloride dissolved previously in 12-15 ml of THF is then cannulated dropwise (ca. 3 s per drop) into the lithium naphthalenide, and stirring is continued for 30 min after the transfer is complete. This method is not only safer due to the use of lithium... 

Finally, the most reactive Rieke zinc yet prepared is generated by the reduction of Zn(CN)2 with lithium using an electron carrier such as naphthalene. Some of this zinc s chemistry will be described later in the text. The reaction details are the same as Method 2. 

The lithium reduction of zinc (II) cyanide using naphthalene or biphenyl as a catalytic electron carrier yields a more reactive form of Rieke zinc. This new form of Rieke zinc is able to undergo direct oxidative addition to alkyl chlorides under mild conditions and tolerates the presence of nitriles and bulky tertiary amides [17]. Table 3.5 shows representative reactions of alkyl zinc chloride reagents with benzoyl chloride. The activation of the zinc surfeice could originate by the adsorption of the Lewis base cyanide ion on the metal surface. The adsorbed cyanide ion can affect the metal s reactivity in two possible ways. One possible mode of activation would be the reduction of the metal s work function in the vicinity of the adsorbed cyanide ion, and the second could be that the cyanide ion is acting as a conduction path for the transfer of the metal s electrons to the alkyl chloride. One or both processes could account for the observed enhanced chemical reactivity. 

Chen, T.-A., Wu, X., Rieke, R.D. Regiocontrolled s)mthesis of poly(3-alkylthiophenes) mediated by rieke zinc their characterization and solid-state properties. J. Am. Chem. Soc. 117, 233-244 (1995)... 

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