Allylzinc reagent

Previous work on the reactions of allylzinc reagents, which usually does not include stereochemical aspects, is collected in a review4. 

In particular 1- or 3-hetero-substituted allylzinc reagents are often prepared in situ by adding anhydrous zinc dichloride or dibromide to the solution of the allyllithium compound. 

Scheme 40 Diastereoselective addition of y-substituted allyllithium and allylzinc reagents to a chiral diimine...

Nitrostyrene reacts with allylzinc reagents in dry DMF at room temperature to give the addition products in excellent yield (Eq. 4.80).102 The reaction of allyl tin compounds or allyl silanes with nitroalkenes requires the assistance of Lewis acids to give the addition products in good yield (Eq. 4.81).103... 

Nakamura et al.141 reported a closely related reaction, that is, the enantioselective addition of allylzinc reagent to alkynyl ketones catalyzed by a bisox-azoline catalyst 137. High ee values were obtained in most cases (Scheme 2-55). 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

TABLE 9.33. ENANTIOSELECTIVE ADDITION OF ALLYLZINC REAGENTS TO CYCLIC IMINES ... 

Addition of an allylic zinc reagent to cyclic aldimines has been reported129. Lithiated bisoxazoline 61 and allylzinc reagent form the reactive species, which adds to a cyclic aldimine enantioselectively with up to 97.5% (equation 29)129. 

Allenyl metal compounds also act as an acceptor of allylzinc reagent. As shown in Scheme 33, the obtained allylic bismetal species reacted sequentially. A terminal allene... 

An alkoxy group at the -position induces a cyclopropanation reaction of the dimetallic species as shown in equation 46. The formation of the gem-dimetal species proceeded diastereoselectively as described above. The ring closure proceeds with inversion of configuration to form stereospecifically the cyclopropane ring. An alkoxy substituent at the -position of the allylzinc reagent also induces the cyclopropanation reaction (equation 47)61a 72. 

With A-benzyl or A-allyl amides, the a-lithiated products are more stable. N,N-dibenzylbenzamide 48, for example, is a-lithiated with LDA to give 49 and hence 50,38 while two equivalents of BuLi or LDA are sufficient toa-lithiate the secondary amides 51 and 54 to give 52 or 55 which react with electrophiles to produce 53 and 56.39 The lithiated A-Boc allyl amides 57 react with selectivity which is highly electrophile dependent, but transmetallation of 58 to the allylzinc reagent leads to good a-selectivity.40... 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Primary allylmagnesium halides and allylzinc reagents react similarly, presumably by an ene-type reaction, to give selectively the cyclopropyl derivative in which the more substituted allylic carbon is attached to the ring (equation 24). Allylzinc compounds have been employed in the same manner in carbozincation of cyclopropenone acetals ... 

Reaction of allylzinc bromide with various reactants affords, in addition to the normal products, compounds containing the group CH2== C(CH2CH=CH2)CH2- in yields of 10-15% (96). These products may be formed via a dimeric allylzinc reagent (XXXII). Reaction of (XXXII)... 

Enantioselective allylmetallation of chiral cyclopropene ketal is also possible [135] as well as allylmetallation of the achiral ketal but with the acid of chiral bisoxazoline (BOX) ligands [136] (Scheme 7-117). The chiral allylzinc reagents were much more reactive than the parent allylzinc bromide. 

Scheme 7-94 Addition of a heterosubstituted allylzinc reagent to a (Z) or ( ) vinylmetal.

Rnochel has developed an effective [3 + 2] cycloaddition strategy which involves a nucleophilic addition of a ferf-butylsulfonyl-containing allylzinc reagent with... 

Diastereoselective reactions of oxime (198) and phenylsulfenimines such as (201) with allyl metal reagents have been described (Scheme 33). The reaction of (198) and allylboronate (144) provides (199) with modest selectivity, while excellent diastereoselectivity was realized in the reactions of (201) and its C-2 epimer with the allylzinc reagent." The corresponding ketone derivatives, however, gave 70 30 mixtures of (204) and (205) upon reaction with diallylzinc, while with allyl Grignard, (205) is almost the exclusive product." The latter result is suggestive of a chelated transition state. 

Addition reactions. Allylzinc reagents add to t-butanesulfinylimines in a straightforward fashion. Interestingly, the products can be different from those using the corresponding Grignard reagents. 

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