Transmetallating reagent

At this point, the other ligands were re-examined with t-butylethyne, but no improvement could be made. Clearly, the greater steric bulk of the zinc species resulting from t-butylethyne substantially diminished the stereoselection of the catalyst. In a final attempt to decrease the steric demand of the zinc species, dimethylzinc (3 equiv) was employed as the transmetalation reagent, which should have resulted in formation of the slightly less demanding zinc species (Table 2.1.3.1). 

Another alternate to the Sonogashira coupling was reported by Blum and Molander, where sodium tetraalkynylaluminatcs were coupled with bromoazines and bromoazoles in the presence of a palladium-triphenylphosphine catalyst system. 5-Bromopyrimidine coupled with the TMS-acetylide, for example, to give the cthynylpyrimidinc in excellent yield (7.41.),59 The transmetalating reagents were prepared in situ by the reaction of the appropriate acetylene derivative with sodium aluminiumhydride. 

The group-IIB metal dihalide is added to the solution of the transmetallating reagent either in small portions or in solution. The solubility of Hg(II) halides is not considerable, even in ether, and if solid HgXj is added, vigorous stirring is advisable. 

Optimization of the titanium homoenolate formation consisted of selecting the most appropriate titaninm ligands (i.e., the ratio of chloride to aUcoxide, as well as aUcoxide structure). In our original commnnication, we described the nse of ClTi(OiPr)3 as the transmetallation reagent. This selection was made partly based upon prior precedent, but also upon an observation made when employing... 

The anti-addition products were observed, with the absolute and double bond configuration dependent on the transmetallating reagent. The stereochemical course was rationalized by transmetallation to the titanium or alumi-... 

Ali, M. McWhinnie, W.R. Hamor. T.A. The crystal structure of phenylazophenyl-C,N9mercury(II) chloride A dimeric transmetallation reagent. J. Organomet. Chem. 1989. 371. C37. 

The lower reactivity of the Si-C bond requires the use of activating reagents to enhance the reactivity of silanes and to promote the Si-Pd transmetallations. Fluoride is the common additive, although other nucleophiles such as hydroxide, metal oxides, and aUcoxides are also effective [30, 293, 294]. Fluoride converts the starting silanes into pentacoordinate fluorosilicates, which are the actual transmetallation reagents. 

Reported lithium salts of NHCs remain relatively few despite the fact that they offer potential to be effective and affordable transmetallation reagents. It should be recognized that these salts would compete with Ag(I) complexes as transmetallation reagents and that some have already shown their potential (vide infra). In 1999, Arduengo et al. described the synthesis of the first complexes in which a lithium atom is coordinated to only carbon centers in the NHC-stabilized lithium cyclopentadienide salts shown in Figure 15.3 [22]. 

Transmeta nation reactions have been used for the preparation of relatively simple metallacycles, but the method is limited by the availability of suitable transmetallation reagents. Some examples involving the preparation of ligand-free, anionic nickelabicycles are presented in Section 8.02.2. Transmetallation reactions shown in Equations (81)-(83) provide examples involving Li, and Mg reagents. 

Addition of aldehydes to the lithium enolate derived from propanoyl complex 6 requires prior transmetallation for optimum results. In particular, the use of diethylaluminium chloride has proved to be most valuable when the anti-aldol adduct 14 is required, whilst copper cyanide is the transmetallation reagent of choice when the syn-aldol adduct 15 is the target (Scheme 4.8). Oxidative cleavage and formation of the threo and erythro -hydroxy acids, respectively, is easily achieved by treatment with aqueous bromine solution or CAN. The aluminium(III)-mediated sequence has been employed in the synthesis of an enantiomerically pure degradation product of a marine cyclic peroxide, thereby proving its absolute configuration. ... 

Zinc COMPLEXES excellent transmetallation reagents Negishi coupling... 

Recently, the redox transmetallation/ligand exchange (RTLE) route in the presence of Ph3Bi as the transmetallation reagent has provided the heavy alkaline-earth... 

Given the large numbers of simple Lewis adducts that can be prepared by coordination of a carbene to a Group 1 or 2 metal, this section will be limited to the discussion of carbene complexes of the Group 1 and 2 elements that serve as (i) useful transmetallation reagents, (ii) precursors to more elaborated carbene ligands, or (iii) have unique structures or bonding situations that merit discussion. 

In 1998, Siebert reported the synthesis of 3-borane-l,4,5-trimethylimidaz-ol-2 1idene, whieh was isolated as the lithium adduct (4-Li), by deprotonation of the eonjugate aeid using n-BuLi (Seheme S.l). " The dimeric structure of 4-Li was eonfirmed by X-ray diffraetion and, notably, the C NMR resonanee for the earbene carbon was shifted upheld to 191.3 ppm compared to the free ear-bene (213.7 ppm). Complex 4-Li was shown to be an excellent transmetallation reagent, which allowed for the preparation of [(4)Mn(CO)s], [Li][4.CO-Fe(CO)4], and 4-B(NMe2)2 (5,6, and 7, respeetively). This further demonstrated the exeel-lent ligand ability of NHCs and their potential utility in catalysis. ... 

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