Zinc reagents uncatalyzed reactions

The si facial preference displayed by the reagent is enhanced in reactions proceeding through Lewis acid-catalyzed open transition states.Thus, when reacted with the ketene silyl acetal shown (eq 2) under zinc iodide catalysis, a 96 4 ratio of products was obtained. The corresponding uncatalyzed reaction led to an 85 15 mixture of the same products in similar yield. ... 

Most of the described procedures of forming tetrazoles (catalyzed and uncatalyzed) suffer from some disadvantages hke the use of both toxic metals and expensive reagents, drastic reaction conditions, water sensitivity, and the presence of dangerous hydrazoic acids. The zinc(II)-catalyzed reactions to form tetrazoles by Sharpless and coworkers [212,224,225] are safe, but in the case of stericaUy hindered aromatic or alkyl inactivated nitriles, high temperatures (140-170 °C) are required. Amantini et aL reported TBAF to be an efficient catalyst in the synthesis of 5-substituted IH-tetrazoles by using TMSN3 without solvents (Scheme 65) [226]. 

The uncatalyzed reaction of acid chlorides with organozincs is sluggish and inefS-cient. It is often complicated by side reactions and leads usually to low yields of the desired acylation products. In contrast, the CuCN 2LiCl-mediated acylation of various zinc reagents affords ketones in excellent yields. The reaction of the zinc-copper reagent 328 obtained by the direct zinc insertion to the iodide 329 followed by a transmetallation with CuCN 2LiCl reacts with benzoyl chloride at 25 °C leading to the ketone 330 in 85% yield [52b]. The functionalized benzylic bromide 331... 

Whereas uncatalyzed substitution reactions of organozinc compounds are limited to very reactive electrophiles, metal-transmetallated organozinc compounds are able to perform substitution reactions on various electrophiles. In the case of conjugated electrophiles, these zinc copper reagents can follow a Sn2 or Sn2 mechanism. 

Recently, many efforts have been focused on the development of the enantioselective (iodomethyl)zinc-mediated cyclopropanation of allylic alcohols. Kobayashi and co-workers reported that moderate to good enantioselectivities were observed if a Ci-symmetric chiral disulfonamide was added. " To reduce the rate of uncatalyzed process responsible for decrease of enantioselectivity, Charette and Brochu studied the effect of addition of Lewis acid, and proved that TiCU accelerates the reaction. The addition of the chiral titanium catalyst 548 allowed the conversion of 3-aryl and 3-heteroaryl-substituted allylic alcohols to the corresponding cyclopropane derivatives in enantiomeric ratios up to 97 3 (Scheme 2-154, eq. (a)). The dioxaborolane ligand 549 is an efficient chiral reagent for the enantioselective cyclopropanation not only of allylic alcohols but also of unconjugated and conjugated... 

---