Organozinc reagents mechanism

TADDOL 104 and 126 afford 95-99% ee in the asymmetric addition of organozinc reagents to a variety of aldehydes. The best enantioselectivities are observed when a mixture of the chiral titanium TADDOL compound 127 and excess [Ti(OPr1)4] are employed (Scheme 2-49). The mechanism of the alkylzinc addition involves acceleration of the asymmetric catalytic process by the... 

The mechanism has not been fully clarified, but pure carbenes can be excluded, and a metal carbenoid is likely to be involved. The following results may be interpreted to indicate a possible complexation of the active species by hydroxy groups leading to reaction on the same face as this substituent. This would only be possible if an organozinc reagent is present. 

The electrosynthesis of /Miydroxy esters (220), 2,3-epoxy esters (221) and /Miydroxy nitriles (222) was achieved under nickel-catalyzed conditions, obviating Reformatsky reaction 70(equation 112-114). The reaction proceeds in excellent yield when a sacrificial zinc rod is used as the anode. A mechanism has been proposed which involves reduction of a Ni(II) complex to a Ni(0) complex, oxidative addition of the a-chloroester to the Ni(0) complex, and a zinc (II)/Ni(II) exchange, leading to an organozinc reagent, in analogy to the Reformatsky reactions. 

Nucleophilic addition of organozinc reagents is crucial step in Reformatsky reaction which was employed for fullerene derivatization. Ethylbromoacetate reagent (5 equiv.) in conjunction with zinc dust (20 equiv.) reacts with fullerene to obtain product 48 in high yield. In this process, known cyclopropane molecule 49 was formed as a side product, as well as two products unknown in the literature, 50 and 51 (Scheme 7.14) [26]. Formation of these products is rationalized by mechanism which starts with formation of organozinc reagent, which is followed by nucleophilic addition to fullerene. 

This reaction, described by the authors as a Diels-Alder cycloaddition via the o-xylylene (p. 99), was employed in carbohydrate chemistry to produce anthra-cyclinone analoguesNq reaction occurs without sonication. The mechanism was not established with certainty, and it can be postulated that a mono-organozinc reagent adds to the activated olefin and the enolate anion undergoes an alkylation from the second benzylic bromide. 

Active zinc is an effective mediator in intramolecular conjugate additions. For example, a spirodecanone was formed from the 1,4-addition of the organozinc reagent, which was readily available from the primary iodide (Equation 3.10) [50]. Other types of ring closures also occur (Equations 3.11 and 3.12) and are thought to proceed by a mechanism that does not involve a free radical pathway. 

As in the transmetalation leading to the trans product 3 (Fig. 4.5), the first step in this mechanism corresponds to the dissociation of a THF molecule from the organozinc reagent ZnMeCl(THF)2 and the concomitant bridging chloride coordination to Zn (Fig.4.7). The intermediate that results from this first step is CC-1 (analogous... 

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