Base-Free Allylation

C02Me 2. (COCI)2, DMSO, EtgN MeMgl, Ni(acac)2 S. C02Me 3. TICI4, Zn  

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So far, there is no conclusive evidence that a free allyl carbanion is generated from allylsilanes under fluoride ion catalysis. A hypervalent silyl anion, with the silicon still bonded to the allylic moiety, accounts equally well for the results obtained. Based on a variety of experimental results, it is in fact more likely that a nonbasic hypervalent silyl anion is involved rather than the basic free allyl carbanion first postulated14-23. When allylsilanes are treated with fluoride in the presence of enones. 1,4-addition takes place along with some 1,2-addition13. 

The effect of hydrogen bonding on allylic alkylation was studied in the base-free reaction of phenylallyl carbonate with dimethyl malonate.1 31,1321 While the reaction proceeds rapidly in THF in the presence of four equivalents of PPh3, it is very sluggish in [C4Ciim][BF4], The reaction in THF was significantly inhibited when small quantities of the ionic liquid were added. Only with excess external base did the reaction proceed in a comparable rate in the ionic liquid. It was shown that the oxidative addition of allylic acetate to Pd(0) is reversible and in THF the resulting acetate anion... 

Postsynthetically modified frameworks were also used as heterogeneous catalyst for the same reaction. Proch et al. [104] loaded MOF-177 with platinum nanoparticles via gas-phase infiltration of volatile [MejPtCp ] precursor under reduced pressure followed by reduction under hydrogen pressure. While this material efficiently catalyzes the oxidation of benzylic and allylic alcohols under solvent and base-free conditions at room temperature, it could not be recycled because of stability issues. 

Under a different manner of cinchona alkaloid activation, azodicarboxylates were utilized as substrates for enantioselective aUylic aminations. The electrophilic addition of nucleophiles to electron-withdrawing aUyUc C-H bonds (21) was feasible via activation by a chiral Bronsted base, DHQ(2PYR) (Scheme 13.6) [15]. This discovery, from Jorgensen s group, highUghts the first enantioselective, metal-free allylic amination using alkyUdene cyanoacetates with dialkyl azodicarboxylates. 

By analogy with the catalytic allylic alkylation performed with C-nucleo-philes described in Section 7.2.2.3, Plietker et al. used benzyl mercaptan as an S-nucleophile toward an isobutyl carbonate in the presence of an NHC-Fe complex 9. Allylic thioethers were obtained in 82% yield as a mixture of two regioisomers [eqn (7.7)]. The same group also investigated the sulfonylation of allylic carbonates with a-sulfonyl succinimides as S-nucleop-hiles. In this case, a base-free catalytic system generated in situ by thermal release of the SIMes carbene from its chloroform adduct in the presence of [Bu4N][Fe(CO)3(NO)] was adopted. Addition of DBU to the reaction mixtures allowed the one-pot synthesis of vinyl sulfones through a tandem iron-catalysed allylic sulfonylation/amine-catalysed isomerisation process. Remarkably, base-induced decomposition of the iron catalyst was not observed. 

Allyltrimethylsilanes can be activated by a nucleophilic Lewis base such as TBAF and metal alkoxides to undergo carbonyl allylation. The reaction of crotylsilane gives a regioisomeric mixture of carbonyl addition products, suggesting that free allylic anion is possibly involved (Scheme 3-116). On the other hand, the reaction of crotyl(trifluoro)silane shows perfect ) selectivity derived probably Irom a six-... 

A solution of 24.6 g of o-allyl-epoxypropoxybenzene dissolved in 250 ml of absolute ethanol saturated with ammonia was placed in an autoclave and heated on a steam-bath for 2 hours. The alcohol was then removed by distillation and the residue was redissolved in a mixture of methanol and ethylacetate. Hydrogen chloride gas was introduced into the solution. The hydrochloride salt was then precipitated by the addition of ether to yield 11.4 g of product. Five grams of the amine-hydrochloride thus formed were dissolved in 50 ml of methanol and 9 ml of acetone. The resulting solution was cooled to about 0°C. At this temperature 5 g of sodium borohydride were added over a period of 1 hour. Another 2.2 ml of acetone and O.B g of sodium borohydride were added and the solution was kept at room temperature for 1 hour, after which 150 ml of water were added to the solution. The solution was then extracted with three 100-ml portions of ether which were combined, dried over potassium carbonate, and evaporated. The free base was then recrystallized from petrol ether (boiling range 40°-60°C) to yield 2.7 g of material having a melting point of 57°C. 

Allyl anion is too strongly basic to be studied as the free anion in solution. Bordwell developed an acidity scale based on equation 1 in dimethyl sulfoxide (DMSO) at 25 °C3 and applied the method to a number of more acidic substituted allylic systems. A summary of some results is shown in Table 10. DMSO is sufficiently polar that there is little ion... 

Aluminum-free titanocene-methylidene can be generated by thermolysis of titana-cyclobutanes 6, which are prepared by reaction of the Tebbe reagent with appropriate olefins in the presence of pyridine bases [9]. Alternatively, the titanacyclobutanes are accessible from titanocene dichloride and bis-Grignard reagents [10] or from 71-allyl titanocene precursors [11]. The a-elimination of methane from dimethyltitanocene 7 provides a convenient means of preparing titanocene-methylidene under almost neutral conditions [12] (Scheme 14.5). 

Commercially available ethyl nitroacetate is an interesting pronucleophile, because it can serve as the synthetic equivalent of either nitromethane or glycine. The ethoxycarbonyl group can also be considered as a protecting group against dialkylation. The allylic alkylation with ethyl nitroacetate did not require an additional base (salt-free conditions). As a consequence of the high acidity of the chirality center a to N, 1 1 mixtures of epimers were formed. 

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