Allenylzinc bromide

X. COUPLING OF ALLENYLZINC BROMIDE REAGENTS WITH ARYL... 

TABLE 1. Additions of allenylzinc bromides to aldehydes and ketones... 

When the alcohol adduct from the allenylzinc reagent and diisopropyl ketone was treated with 80 mol% of allenylzinc bromide in HMPA, a mixture containing 12% of diisopropyl ketone and 88% of recovered alcohol was obtained after 7 days at ambient temperatures (equation 1). Thus, it may be deduced that the allenylzinc additions are reversible. Presumably, the propargyl adducts are intrinsically favored, but steric interactions between the R1 and R2 substituents in the propargyl product favors an increased proportion of allenyl adducts in a reversible process (see Table 1). HMPA would expectedly facilitate reversal of the addition by decreasing the ion pairing between the alkoxide anion and ZnBr cation of the adducts. This expectation was subsequently confirmed by a study of solvent effects. 

TABLE 3. Effect of carbonyl structure on allenylzinc bromide additions... 

Reactions of allenylzinc bromide reagents with aldehydes afford increased amounts of propargyl adducts compared to analogous additions to ketones (Table 3). Presumably, diminished steric interactions render the aldehyde propargylic adducts more stable than their ketone counterparts. Alternatively, equilibration of the kinetic propargylic adducts with the allenyl isomers is slower for the adducts of aldehydes. 

TABLE 12. Coupling of allenylzinc bromide reagents with haloarenes... 

Normant and Poisson prepared allenylzinc bromide reagents from TMS acetylenes along the lines of Epsztein and coworkers5, by sequential lithiation with s-BuLi to yield a lithiated species, and subsequent transmetallation with ZnBr2 (equation 35)27,28. Additions to racemic /J-silyloxy aldehydes proceed with low diastereoselectivity to afford mixtures of the anti,anti and anti,syn adducts (Table 17). The latter adducts are formed via an anti Felkin-Anh transition state. Additions to the racemic IV-benzylimine analogs, on the other hand, proceed with nearly complete Felkin-Anh diastereoselectivity to yield the anti,anti amino alcohol adducts (Table 18). 

The value of the silicon substituent in promoting the carbolithiation is illustrated by comparing the reactions of hex-l-en-3-yne 42 and l-(trimethylsilyl)but-3-en-l-yne 43 with u-BuLi in ether, as shown in Scheme 7-33. However, to be synthetically useful, the metallotropic equilibrium between the lithioallene and the lithioalkyne should be displaced by reaction with an electrophile in favor of only one derivative. Thus, the addition of zinc salts to a mixture of propargyl-allenyllithium derivatives quantitatively leads to the allenylzinc bromide intermediate (Scheme 7-34) and reaction of the latter with electrophiles, via an 8 2 process [34], only gives the propargyl derivative (Scheme 7-34). 

The high diastereoselectivity obtained from the reaction of the allenylzinc bromide with isobutyraldehyde to produce the y/i-homopropargyl alcohol is rationalized by the well-known six-membered ring transition state [34]. The scope of this reaction is quite broad a-mono-a,/S-disubstituted silylated enynes as well as -mono- and ,j5-disubstituted silylated enynols are easily carbometallated using this procedure [33]. 

The addition of zinc salts to a propargyl/allenyl organolithium reagent leads quantitatively to the allenylzinc bromide [60] and, considering that this latter reacts with the vinylmetal moiety, via an S 2 process [62, 107], the stereochemical result is thus explained... 

The same diastereomer is obtained from the ( )-vinyl Grignard reagent [107], and the scope of the reaction is quite broad the use of functionalized vinylmetals as well as the introduction of an alkyl group (different from the methyl group) on the allenylzinc bromide were shown to be possible without altering the diastereoselectivity [107],... 

Organometallics of allenic structure are usually prepared by the reactions of metals with propargylic or allenic halides, or by metallation with alkyllithiums of the corresponding hydrocarbons. The first preparation of allenylmagnesium bromide was accomplished by Prevost in I950. Allenylzinc bromide is easily obtained by the direct reaction of the metal with propargyl bromide at -100 °C in anhydrous tetra-hydrofuran. The yield is greater than 80%. ... 

---