Dialkylzinc compound

The direct reaction of zinc metal with organic iodides dates back to the work of Frankland(67). Several modifications have been suggested since that time to increase the reactivity of the metal. The majority of these modifications have employed zinc-copper couples(68-72), sodium-zinc alloys(73), or zinc-silver couples(77). Some recent work has indicated that certain zinc-copper couples will react with alkyl bromides to give modest yields of dialkylzinc compounds(74,73). However, all attempts to react zinc with aryl iodides or bromides have met with failure. The primary use of zinc couples has been in the Simmons-Smith reaction. This reaction has been primarily used with diiodomethane as 1,1-dibromides or longer chain diiodides have proven to be too unneactive even with the most reactive zinc couples. 

Table 2 Structural data of selected dialkylzinc compounds from gas electron diffraction (GED) and computational (DFT) studies...

Experimental and estimated thermodynamic data of homoleptic dialkylzinc compounds are listed in Table 3. Like many organometallic compounds, the lower dialkylzincs have a positive enthalpy of formation, and only the incorporation of silicon atoms in the /3-position imparts significant thermodynamic stability. The mean Zn-C bond rupture enthalpies, all of which are quite low, follow a similar trend as the bond lengths in these compounds. Thus, the presence of methyl substituents in the a-position weakens the zinc-carbon bonds, while silyl substituents strengthen them. 

Dialkylzinc compounds have a tendency to form colored, and sometimes paramagnetic, adducts with 7r-acceptor ligands. The potentially bridging nitrogen donor pyrazine (and 4,4 -bipyridine) formed dinuclear complexes, Scheme 31, while the chelating 2,2 - bipyridine formed mononuclear adducts with diisopropylzinc.78 In the bis(diisopropylzinc)-pyrazine adduct 33, the presence of an unpaired electron was detected, but the resolution of the ESR spectra was insufficient for an unambiguous structural assignment. 

Many 1,4-diazabutadiene adducts of dialkylzinc compounds show unusual reactivities (see also Section 2.06.10.7), such as the intramolecular electron transfer from zinc to the chelating ligand and the subsequent dimerization of these radicals.127 In solution, the carbon-carbon coupled dimer [MeZn(ButN=CHCH=NBut)2ZnMe] 71 is in equilibrium with its radical monomers (Scheme 56). Addition of potassium to a THF solution of the dimer produced cleanly 72, the first heteroleptic alkyldiamidozincate. 

In terms of their chemical behavior, dialkylzinc compounds generally react to transfer alkyl groups to other metals. Dialkyls of cadmium are usually prepared by the reaction of a Grignard reagent with a cadmium halide. 

Nucleophilic addition of metal alkyls to carbonyl compounds in the presence of a chiral catalyst has been one of the most extensively explored reactions in asymmetric synthesis. Various chiral amino alcohols as well as diamines with C2 symmetry have been developed as excellent chiral ligands in the enantiose-lective catalytic alkylation of aldehydes with organozincs. Although dialkylzinc compounds are inert to ordinary carbonyl substrates, certain additives can be used to enhance their reactivity. Particularly noteworthy is the finding by Oguni and Omi103 that a small amount of (S)-leucinol catalyzes the reaction of diethylzinc to form (R)-l-phenyl-1 -propanol in 49% ee. This is a case where the... 

Scheme 2.32. Preparation of polyfunctional primary dialkylzinc compounds by boron-zinc exchange.

Dialkylzinc compound 46 contains two 2,4-di-tert-butyl-2,4-dienyl anions (Figure 23). In the solid state the two organic groups are / -bonded to zinc via C(l) with a Zn—C... 

The complex of Me2Zn with (5, 5 )-ebpe, 107, has been applied successfully as catalyst in the enantioselective reduction of ketones by polymethylhydrosiloxane and combines excellent product yields with high ee values . Its structure comprises the iV,iV-chelate coordination of the ebpe ligand to the MeiZn unit (Figure 51). It is remarkable that in this case the two secondary amine functionalities are coordinated to zinc and leave the Zn—C bonds unaffected. Indeed, usually secondary amines undergo a fast deprotonation reaction with dialkylzinc compounds. 

TABLE 1. Enthalpies of vaporization of dialkylzinc compounds (kJmol )... 

A novel approach to allenylzinc reagents, reported by Oku and coworkers, employs propargylic mesylates as starting materials12. Treatment with triorganozincates, prepared in situ by the reaction of dialkylzinc compounds with organolithium reagents, affords transient allenyl dialkylzinc intermediates. These intermediates were found to react with various... 

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