Allylic zinc compounds

Addition of allylic zinc compounds to vinylic Grignard and lithium... 

Allylic zinc bromides add to vinylic Grignard and lithium reagents to give the cm-dimetallo compounds 65. The two metallo groups can be separately reacted with various nucleophiles. ... 

The addition of a large excess of bis(cj-alkenyl)zinc compounds to the TiC -catalyzed polymerization of propene resulted in an increased polymer yield, but a reduction in the molecular weights of the polymers.64 This suggests that the diorganozinc compounds are both co-catalysts and chain-transfer agents in this polymerization. The catalyst activity decreased in the order bis(3-butenyl)zinc < bis(7-octenyl)zinc < chlorodiethylaluminum. Bis(7-octenyl)zinc was co-polymerized with propene to afford hexylzinc side chains, whose zinc-carbon moieties were converted to vinyl groups by the addition of allyl bromide. 

Both the palladium- and the nickel-catalysis enables the use of allylic acetate as starting reagent. In the two approaches a zinc compound is evoked as key intermediate, though its formation has been demonstrated only indirectly. In the two methods allylic transposition is observed. The authors have then concluded that these electrochemical allylation reactions closely parallel the chemical allylation reactions involving allylzinc intermediates. 

By analogy with allylic organometallic compounds (see Section . ), the possibility of achieving intramolecular related zinc-ene reactions involving allenylzinc species acting as ene-components has been investigated. Such reactions benefit from favorable thermodynamics and were thus expected to proceed more readily than the related intermolecular additions of allenylzincs to alkynes or alkenes. 

Asymmetric phosphines, polymer-supported, 12, 701 Asymmetric pinacol coupling reactions, characteristics, 11, 48 Asymmetric reactions, with allylic tins, 9, 355 Asymmetric substitution reactions, with zinc compounds, 9,99 Atmosphere studies... 

Two examples using azo compounds in allylic amination reactions will be presented in the following. Leblanc et al. have used the more reactive trichloro derivative 82 of DEAD, 81, and found that the ene reaction proceeds at various temperatures and without any Lewis acid catalyst present, for both cyclic and acyclic alkenes to give allyl amines in good yields. The reaction of the alkene 85 with 82 gave the allylic aminated compound 86 in 85 % yield (trans. cis = 85 15) (Eq. (21)) [53f. The allyl amine 87 was formed in good yield after treatment with a suspension of zinc powder in acetic acid solution. 

Transformations involving chiral catalysts most efficiently lead to optically active products. The degree of enantioselectivity rather than the efficiency of the catalytic cycle has up to now been in the center of interest. Compared to hydrogenations, catalytic oxidations or C-C bond formations are much more complex processes and still under development. In the case of catalytic additions of dialkyl zinc compounds[l], allylstan-nanes [2], allyl silanes [3], and silyl enolethers [4] to aldehydes, the degree of asymmetric induction is less of a problem than the turnover number and substrate tolerance. Chiral Lewis acids for the enantioselective Mukaiyama reaction have been known for some time [4a - 4c], and recently the binaphthol-titanium complexes 1 [2c - 2e, 2jl and 2 [2b, 2i] have been found to catalyze the addition of allyl stannanes to aldehydes quite efficiently. It has been reported recently that a more active catalyst results upon addition of Me SiSfi-Pr) [2k] or Et2BS( -Pr) [21, 2m] to bi-naphthol-Ti(IV) preparations. 

Althou little studied, asymmetric copper-catalyzed substitution reactions of dialkylzinc [236] and Grignard reagents [237] vrith allylic substrates have been achieved vrith ferrocenylamidocopper and arenethiolatocopper catalysts, respec-tivdy. Good enantiosdectivity can be achieved vrith the zinc compounds (87% ee), but the method is limited to stericaUy hindered dialkylzinc reagents v/hile Grignard methodology gives only modest selectivities (18-50% ee). 

The structure (1) proposed by Rilling and Epstein for presqualene alcohol, the elusive biological intermediate between farnesol and squalene, has been confirmed by three independent rational syntheses. Altman et al selected the allylic diazo-compound (2) as starting material. Addition of this to trans,trans-farnesol in the presence of zinc iodide gave a 25 % yield of presqualene alcohol (1) and its isomer (3). 

---