Complexes allyl zinc

The use of other metal cations such as those derived from zinc, lithium, or aluminium proved less effective (136). Treatment of allyl alcohol with diethyl zinc in the presence of a catalytic amount of diisopropyl (/ ,/ )-(+ )-tartrate (DIPT) in 1,4-dioxane, however, afforded the corresponding (5/f)-2-isoxazolines with excellent selectivity er >92 8) (178). Addition of dioxane was necessary in order to avoid precipitation of the complex of zinc salts containing the DIPT moiety. Without this solvent, lower stereoselectivity was found, probably due to the precipitation mentioned above, which prevents the favorable catalytic cycle proposed (Scheme 6.32) (178). 

A formal 13+2 -cycloaddi lion can be accomplished by adding bis(2-carbethoxyethyl) zinc to acetylenic esters . This reaction allows the construction of complex cyclopen-tenones, such as 388, which is a precursor of ( )-bilobalide (Scheme 101) . The allylzincation of tiimethylsilylacetylenes can be performed intramolecularly providing a functionalized alkenylzinc which cyclizes in the presence of Pd(PPh3)4 at 25 "C within 3.5 h, leading to the bicyclic product 389 in 84% yield (Scheme 102). The addition of allylic zinc halides to various alkynes occurs in the absence of copper salts. The related... 

Complexes of various metal salts and chiral hw-oxazolines have also been examined as catalysts for allylations of aldehydes (Eq. 21) [38], The most effective salts were ZnCl2, ZnBr2, Znl2, Sn(OTf)2, and InCl3. Octanal and cyclohexanecarboxaldehyde afford adducts of 40 5% ee with complexes of zinc salts. 

Zinc metal reacts with aUyhc bromides to form an allylic zinc complex, which... 

Other allyl metal complexes can be used to generate carbon bonds, including allyl zinc complexes. Stille showed that 3-methyl-l-bromo-2-butene reacted with zinc chloride (ZnCl2) to generate the it-allyl zinc species. Coupling required the use of an organotin species such as a-trimethyltin isoprene (466). Reaction in refluxing THF led to a 94% yield of myrcene (467).303... 

Many carbamates have been used as protective groups. They are arranged in this chapter in order of increasing complexity of structure. The most useful compounds (not necessarily the simplest structures) are /-butyl (BOC), readily cleaved by acidic hydrolysis benzyl (Cbz or Z), cleaved by catalytic hydrogenol-ysis 2,4-dichlorobenzyl, stable to the acid-catalyzed hydrolysis of benzyl and /-butyl carbamates 2-(biphenylyl)isopropyl, cleaved more easily than /-butyl carbamate by dilute acetic acid 9-fluorenylmethyl, cleaved by /3-elimination with base isonicotinyl, cleaved by reduction with zinc in acetic acid 1-adamantyl, readily cleaved by trifluoroacetic acid and allyl, readily cleaved by Pd-catalyzed isomerization. 

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

Zinc-tartrate complexes were applied for reactions of both nitrones and nitrile oxides with allyl alcohol and for both reaction types selectivities of more than 90% ee were obtained. Whereas the reactions of nitrones required a stoichiometric amount of the catalyst the nitrile oxide reactions could be performed in the presence of 20 mol% of the catalyst. This is the only example on a metal-catalyzed asymmetric 1,3-dipolar cycloaddition of nitrile oxides. It should however be no-... 

Lewis acids such as SnCl4 also catalyze the reaction, in which case the species that adds to the alkenes is H2C —O— SnC. Montmorillonite KIO clay containing zinc(IV) has been used to promote the reaction. The reaction can also be catalyzed by peroxides, in which case the mechanism is probably a free-radical one. Other transition metal complexes can be used to form allylic alcohols. A typical example is. ... 

Isomerization of allylic amines is another example of the application of the BINAP complex. Rh BINAP complex catalyzes the isomerization of N,N-diethylnerylamine 40 generated from myrcene 39 with 76-96% optical yield. Compound (R)-citronellal (R)-42. prepared through hydrolysis of (R)-41, is then cyclized by zinc bromide treatment.49 Catalytic hydrogenation then completes the synthesis of (—)-menthol. This enantioselective catalysis allows the annual production of about 1500 tons of menthol and other terpenic substances by Takasago International Corporation.50... 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

The tartaric acid scaffold also led to the design of one of the most effective and general methods to generate enantiomerically enriched substituted cyclopropyhnethanol derivatives. Indeed, the chiral dioxaborolane ligand 19, prepared from tetramethyltartramide and butylboronic acid, is a superb chiral additive in allylic alcohol-directed cyclopropanation reactions (equation 83) . The best procedure requires the use of the soluble bis(iodomethyl)zinc DME complex . The reaction affords high yields and enantiomeric... 

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