Allylzinc bromides

Table 1. Addition of in situ Generated Allylzinc Bromides to Carbonyl Compounds...

Enantiomerically enriched formyl irimclhyleiiemethane -irontricarbonyl complexes add to allylzinc bromides with high stereoselectivity. This was used, after photochemical deprotection, in an enantioselcctive synthesis of the insect pheromone (-)-ipsdienol4s. 

The combination of metal tuning and double stereodifferentiation helps to prepare chelation and nonchelation products in the imine series7. In the case of an alkoxy substituent adjacent to the aldimino, the chelation product 10 is predominantly obtained with allylmagnesium chloride, chloromagnesium allyltriethylaluminate or allylzinc bromide, while the use of allyl-boronates or allyltitanium triisopropoxide, which lack the requisite Lewis acidity for chelation, gives 11 with good Cram selectivity. 

The results show that modest chelation control can be obtained using organomctallics like allylmagnesium chloride, chloromagnesium allyltriethylaluminate and allylzinc bromide, which are capable of /1-chelation. In contrast to this, 9-allyl-9-borabicyclo[3.3.1]nonane (9-allyl-BBN)... 

A solution of 1 mmol of butyl (-)-(S)-2-(1-phenylethylimino)acetate in THF is cooled under N2 to —78 CC. 1.1 mmol of 9-allyl-9-borabicyclo[3 3 1 Jnonane or 1.1 mmol of allylzinc bromide, respectively, is added and the resulting mixture is warmed to r.t. over a period of 12 h. A few drops of coned HC1 are added at 0°C... 

The stereoselective allylation of carbon-nitrogen multiple bonds have also been studied. The addition of allylzinc bromide to aromatic imines derived from (. S j-valine esters was affected by reversibility, which caused the lowering of the diastereoisomeric ratio with increasing reaction time. The retroallylation reaction could be avoided by performing the reaction in the presence of trace amounts of water or by using CeC - 7H2O as the catalyst with a decreased reaction rate.71... 

Mistryukov, E. A. et al., Mendeleev Comm., 1993, (6) 242 Adding formamide to a reaction mixture of allylzinc bromide and a ketone as a proton source for a Barbier reaction in ether or tetrahydrofuran gave uncontrollable explosion-like reaction. The reaction proceeded smoothly when starting from zinc dust and allyl bromide, with both formamide and the ketone in situ. 

Allylzinc bromide (Bromo-2-propenylzinc) [18925-10-5] CH2=CHCH2ZnBr Formamide, Ketones See Formamide Organozinc compounds See also GRIGNARD REAGENTS C3H5BrZn... 

One of the important new directions in the study of addition reactions of organozinc compounds to aldehydes is the use of ionic liquids. Usually, application of these compounds in reactions with common organometallic reagents has a serious problem ionic solvents are usually reactive toward them, particularly Grignard and organolithium derivatives. It has been recently reported that carbonyl compounds react with allylzinc bromide formed in situ from allyl bromide and zinc in the ionic liquid 3-butyl-l-methylimidazolium tetrafluoroborate, [bmim][BF4].285 Another important finding is that the more reactive ZnEt2 alkylates aldehydes in a number of ionic liquids at room temperature.286 The best yields (up to 96%) were obtained in A-butylpyridinium tetrafluoroborate, [bpy][BF4] (Scheme 107). 

Reactions of allylzinc halides with carbon nucleophiles are rare examples of uncatalyzed transformations of organo-zinc compounds. Allylzinc bromide, formed in situ, reacted with a number of structurally diverse acyl chlorides furnishing the corresponding ketones 222 in high yields (Scheme 129).330 No isomerization to the corresponding a,(3-unsaturated ketones was observed under the conditions used (RT, 5min-6h). 

Aniinophenyl)-a,(J>-ynones react with nitrile oxides by domino [3 + 2] cycloaddition/annulation reactions, giving rise to isoxazolo[4,5-c]quinolines in satisfactory yields (434). Nitrile oxides undergo addition to allylzinc bromide to generate 5-butenylisoxazolines in good yields. The domino reaction combines 1,3-cycloaddition with Wurtz coupling (435). 

However, with allylzinc bromide the reaction afforded the vinylic metal intermediate 111 with the allyl group connected to the carbon linked to a methoxy group, which may afford a new allenic product via /3-elimination. 

With allylzinc bromide, 3,4-allenylamines delivered a mixture of trans- and ds-ally-lation products [153],... 

Synthesis of copolysiloxanes 14, a candidate for dental and medical devices and sensors, required the preparation of the substituted allylbenzene 13 (Scheme 20) °. However, attempted cross-coupling reaction of 12 with allylzinc bromide resulted in failure. Alternatively, the magnesiation of 12 with lithium tributylmagnesate followed by the addition of allyl bromide provided 13 in excellent yield. 

Jones and Knochel found that the adduct of allylzinc bromide and di-i-butyl ketone, upon conversion to the bromozinc alkoxide, serves as a source of allylzinc bromide,... 

The initial reaction between allylzinc bromide (69) and vinyllithium (101) gives a stable complex 102. In the next step this intermediate forms, via transition state 103, the lithio zinc adduct 104, which then oligomerizes to give very stable 105. 

One of the first reports dealing with the carbozincation of alkenes by allylic organozinc reagents was the addition of allylzinc bromide to the homoallylic amine 118 which occurred in refluxing THF. A secondary organozinc 119 was regioselectively produced and provided the amine 120 after hydrolysis. A small amount of aminoalcohol 121, resulting from oxidation of 119, was also isolated when the reaction mixture was exposed to air (equation 52)80,81. 

Failure for Grignard reagents to undergo similar additions to homoallylic amines may be explained by the greater stability of the complexes between an amine and zinc rather than magnesium. By contrast, addition of allylzinc bromide to allyl or homoallyl alcohols proceeded considerably less efficiently compared to amines and the regioselectivity was reversed (equation 55)81. 

Although the presence of an appropriately located heteroatom seemed to be required for the success of the allylzincation, it is in fact not necessary. Actually, one of the first reports of allylzincation of an unactivated olefin was related to an observed dimerization reaction when a solution of allylzinc bromide was heated at reflux (following its preparation from allyl bromide and metallic zinc in THF) or if reaction with an electrophile required... 

The first example of enantioselective allylzincation of an alkene was also reported for the cyclopropenone ketal 78 as substrate. The chiral allylzinc complex 135 was prepared from the corresponding bis-oxazoline derived from (,V)-valine by deprotonation with n-BuLi and transmetallation with allylzinc bromide. This reagent reacted with 78 and afforded the allylated product 136 with high optical purity ( = 99%) (equation 66)101. 

As a general rule, allylic organozinc reagents do not react with unactivated a, ji-disubstituted alkynes. A few exceptions to this trend have been observed in the case of substrates bearing an appropriately located heteroatom such as the homopropargylic tertiary amine 147. The latter reacted regioselectively with allylzinc bromide but led to a 67/33 mixture of two geometric isomers, apparently derived from syn and anti additions (equation 71)82,105. 

Allylzinc bromide is able to add to a variety of alkenyl Grignard reagents in THF at 35 °C123. The structure of the hydrocarbons 184, obtained in moderate yields after hydrolysis (due to their volatility), suggested that 1,1-dimetallic species tentatively formulated as 185 were regioselectively formed (equation 89). Additional examples were subsequently reported but the scope of this intriguing reaction remained rather unexplored and the reactivity of 185 towards various electrophiles was not investigated124,125. 

Preliminary studies demonstrated that, under the above conditions, the more substituted the alkenylmetal, the less reactive it was towards addition of allylzinc bromide. Disubstitution at the /J position significantly retarded the addition127. The reaction still occurred efficiently in THF at 35 °C if silyl or aryl groups were present at the a-position... 

Another option for the preparation of allenyllithium reagents involves the carbolithia-tion of activated conjugated enynes15. Thus, addition of BuLi to the enyne 281 led to an allenyllithium compound 282 (in metallotropic equilibrium with its propargylic counterpart). Subsequent addition of allylzinc bromide generated the allylic 1,1-dimetallic species... 

The presence of heteroatoms, not only for the regio- and stereoselective functionalization of the allylic 1,1-diorganometallic species, and/or silyl substituents was found to exert a dramatic influence on the course of the allylzincation of allenyllithium reagents. Indeed, when 1,2-decadiene (285) was metaflated and treated with allylzinc bromide, the reaction followed a different course and led after hydrolysis to the bistoo-methylenejcyclohexane... 

The ability of allylzinc bromide to add to terminal alkynes was first observed with acetylenic amines as substrates. Thus, the propargylic amine 290 underwent addition of allylzinc bromide in refluxing THF and afforded after hydrolysis the allylic amine 291. This reaction occurred much more easily compared to the case of ethylenic amines (see Section III.A) and the regioselectivity was also noteworthy as a branched product was obtained predominantly82,105. It was suggested that metallation of the terminal alkyne was responsible for the observed regioselectivity. Indeed, the formation of an alkenyl... 

It was demonstrated that a variety of terminal alkynes, including propargylic alcohols or ethers, could also react with allylzinc bromide in THF. These reactions afforded mixtures of mono- and bis-addition products 295 and 296 respectively, presumably arising from... 

Addition of excess allylzinc bromide to the conjugated enynol 297 chemoselectively affected the triple bond and afforded a mixture of monoaddition and bis-addition products. The formation of di- and trimetallic species was unambiguously ascertained by deuteriolysis (equation 137)171. 

It was observed that the metallation of terminal alkynes by organomagnesium (or organolithium) reagents considerably accelerated the rate of the subsequent addition of allylzinc bromide. The products resulting from a bis-addition predominate even in the case of sterically more demanding alkynes (equation 140). 

The addition of 3-(phenylthio)allylzinc bromide to 1-octenylmagnesium bromide leads to the formation of 2-hexyl-3-vinylcyclopropylzinc bromide (equation 32)83, presumably by intramolecular cydization of the initially formed gem-dimetallic homoallylic intermediate. Hydrolysis gave a mixture of two isomeric 2-hexyl vinylcyclopropanes in a 7 3 ratio. 

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