Alkenylzinc reagent preparation

Heterosubstituted 1,3-dienes. Some alkenylmetals substituted by OR, SR, or SiR, can be coupled with alkenyl halides (or aryl halides) in the presence of this Pd(0) catalyst. Alkenylzinc reagents, prepared by reaction of alkenyllithiums with ZnCh, and alkenyl-alanes are the most useful. Yields from reactions with organoboronates are low. 

Exchange with boranes can also be used to prepare alkenylzinc reagents.138... 

A portion of the side chain of calyculin was prepared by a tandem reaction sequence that combined an alkenylzinc reagent with 2-bromoethenylboronate, followed by Suzuki coupling with a vinyl iodide in the same pot.230... 

An alternative preparation of stereodeflned cw-a,/3-substituted alkenylzinc reagents involves transmetallation of the corresponding trialkenylboranes or alkenyl tellurides with diethylzincJ f The alkenylzincs thus obtained undergo Negishi coupling with aryl iodides in good yields (Scheme 31). 

Scheme 4.22 Preparation of alkenylzinc reagents by nickel-catalyzed carbozincation of alkynes [100].

Preparation of alkenylzinc reagents by boron-zinc transmetalation from stable organoboronic esters and dialkylzinc is hampered by the instability of the evolved alkenylzinc mixtures of the two reagents turned black and the reaction of the evolved alkenylzinc on remaining pinacol alkenylboronic ester was speculated on the basis of the structure of by-products. Conversely, this boron to zinc exchange can lead to good results if the evolved alkenylzinc is trapped in situ by an electrophile. The reaction of an alkenylboronic ester... 

Preparation of the alkenylzinc reagent 323 via hydrozirconation of a terminal alkyne ... 

Scheme 7.20 Preparation of an alkenylzinc reagent bearing an azide function...

The mixed 1,2-bimetallic Zn/Si reagent 108 is a versatile species, which reacts with aldehydes in high diastereo-selectivity (Scheme 40).314 It is prepared by a bromine/lithium exchange reaction, followed by a transmetallation with ZnCl2- The reaction with acetaldehyde leads initially to the alkenylzinc species 109, which reacts with MesSiCl providing the alkenylsilane 110 in 41% yield and with a diastereoselectivity >9 1. 

Zinc and zirconium 1,1-bimetallic reagents 4.104, prepared by the hydrozirconation of alkenylzinc halides 4.103, react with carbonyl compounds to produce alkenes with high R-stereoselectivity (Scheme 4.51). Ketones give an E/Z mixture of stereoisomers . 

Boro zincioalkane and 1,1-boro zincioalkene can be prepared by a different route, that is, the reaction of Q -haloboronic esters with zinc dust iu N,N-dimethylacetamide (DMA). The ziuc insertiou (see Insertion) into Q -haloalkenylboronic esters is not stereospecific, so a pure Z reagent will be converted to an E/Z mixture of the corresponding alkenylzinc iodide (equation 53). 

Hajjem and Baccar used cyanomethylimidates as substrates for addition of organozinc reagents to prepare (3-ketoimidates that cyclized to 2,5-disubstituted-oxazoles as shown in Scheme 1.95. For example, addition of an alkenylzinc... 

At present, (Z)-/3-substituted alkenylmetals containing other metals are less readily accessible than those mentioned above, even though the Zr-catalyzed carboalumination of ethyne has been shown to produce (Z)-/S-substituted alkenylalanesJ They are generally prepared via metallation-transmetallation of (Z)-/S-substimted hahdes. Despite this drawback, (Z)-/3-substituted alkenylzincs generated by this procedure have been shown to be superior reagents in the subsequent Pd-catalyzed cross-coupling reactiont (Table 11). 

Synthetic applications of the alkenyl-alkenyl Negishi cross-coupling are intimately related to the preparation method of the alkenylzinc nucleophile. Reagents obtained by Zn(0) insertion from the corresponding halide, or by a halogen-hthium or a tin-lithium exchange/transmetallation sequence, are weU suited, as illustrated with the preparation of compounds 202 [170] and 205 (Scheme 4.46) [46]. Further evidence for the applicability of this method is its use in recent total syntheses of natural products, such as xerulinic acid [171], 6,7-dehydrostipiamide [172], epolactaene [173], and the side chains of mycolactones A and B [174]. 

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