Diorganozinc mixed

Since the early disclosure by Negishi that zinc halide salts accelerate Pd(0) -catalyzed crosscouplings between vinyl zirconocenes and various halides [78], several methods have been developed that extend the utility of this metathesis process from a zirconium chloride to a zinc chloride (79 Scheme 4.47). Alternatively, routes to more reactive diorganozinc intermediates, e. g., using Me2Zn, convert readily available zinc derivatives to mixed species 80, which selectively couple with various electrophiles [14]. 

Mixing a diorganozinc compound with an organometallic reagent in an appropriate solvent is another approach to the synthesis of triorganozincates (Scheme 3). 

Thus, the hydroboration of 1-phenylcyclopentene with (—)-IpcBH, (99% produces, after crystallization, the chiral organoborane 126 with 94% ee. The reaction of 126 with Et2BH replaces the isopinocamphenyl group with an ethyl substituent (50 C, 16 h) and provides, after the addition of i-Pr2Zn (25 °C, 5 h), the mixed diorganozinc 127. Its stereoselective allylation leads to the fraw5-disubstituted cyclopentane 128 in 44% yield (94% ee trans cis = 98 2) see Scheme 43 ° . This sequence can be extended to open-chain alkenes and Z-styrene derivative 129 is converted to the anf/ -zinc reagent 130, which provides after allylation the alkene 131 in 40% yield and 74% ee (dr = 8 92). 

SCHEME 70. Mixed diorganozincs for conjugate addition reactions... 

Mixed zincates R1 (R2)2ZnLi are prepared either from (R2)2Zn and R1 Li or by successive addition of R2Li (2 equiv) and R Li (1 equiv) to ZnX2. Sodium and potassium zincates are prepared from diorganozinc according to Wanklyn s procedure (equation 5)1. 

Reaction of dicrotylzinc with 1,3-butadiene readily took place at 20 °C and addition competed between C2 and Cl, leading respectively to a primary dialkylzinc 127 or a secondary ally lie diorganozinc 128 (or any of the mixed metallotropic forms)93. Consequently, hydrolysis led to a mixture of three major hydrocarbons 129a-c but the actual regioselectivity of the addition could not be accurately evaluated. Indeed, 129c could also in principle arise from the addition of dicrotylzinc, without allylic transposition, to the C(2) atom of butadiene (equation 61). 

Preparation of 3-pivaloxypropyl-(trimethylsilylmethyl)zinc by iodine-zinc exchange, formation of a mixed diorganozinc and its enantioselective addition to an aldehyde preparation of (SM-)-4-hydroxy-4-phenylbutyl pi va late23... 

The same experimental procedure can be used with alkylidenemalonate and other related Michael-acceptors, as well as nitroolefins.26 However, under these reaction conditions only one of the two R groups of R2Zn is transferred to the enone. This problem can be solved by using mixed diorganozincs of the... 

In these conditions, excess of R2Zn and the catalyst (transition metal) are not required. In addition, when a solution of an alkyl iodide is mixed with 1 equiv of R2Zn the mixed diorganozinc compounds (R-Zn-R ) are obtained (equation 5). This reaction takes place only between diaUcylzincs and alkyl iodides however, in comparison with the few other methods described for the preparation of mixed diorganozincs, this procedure represents a fantastic enhancement for alkyl group exchange (R-Zn-R ). ... 

The more difficult cross-coupling reaction between Csp -Csp can be accomplished by mixed zincate compounds, prepared by the reaction of diorganozinc derivatives with Me2CuMgCl-MgCl(CN) (equation 59). Substitution reaction at Csp centers are also efficiently catalyzed by Ni salts when the alkyl halide contains a double bond in remote position (Scheme 22). Changing the double bond for a triple bond leads to an intramolecular carbozincation reaction (equation 60). 

Cyclopropylcarbinols are prepared from dicyclopropylzinc, and t)ie use of mixed diorganozincs such as MejSiCHjZnEt for the addition has also been explored. Chiral tertiary alcohols are obtained from organozinc addition to ketones in the presence of functionalized isobomeols. (15,2/f)-2-(A -piperidinyl)-l-phenylpropane-l-thiol acetate is a ligand prepared from (+)-norephedrine ° and it catalyzes asymmetric addition of diethylzinc to aldehydes very effectively. 

Addition reactions. Mixed diorganozincs of the type RZnCH SiMej donate the R... 

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