Allenolate

Allenols can be converted into terminal vinylepoxides by a Pd(0)-catalyzed insertion of aryl or vinyl halides [81]. The reactions take place with high trans-selectivity and in good yields (Scheme 9.17a). Chiral 2,3-allenols, which can be easily... 

To support the 2,3-allenol as an intermediate in this reaction, 2,3-allenols were employed in test experiments as starting materials (Scheme 10, route F). The crossover aldol product was obtained as the sole product, when the reaction was run in the presence of the suitable aldehyde. In the absence of an aldehyde the corresponding ( )-a,p-unsaturated ketone was obtained (Scheme 24) [37]. 

Scheme 24 Coupling of 2,3-allenols and aldehydes promoted by iron(III) halides...

Trost et alJ2 also explored the compatibility of di-, tri-, and tetrasubstituted allenes with their intermolecular Alder-ene protocol. Multiple substituents present the opportunity for a mixture of products to arise from differing regio- and chemoselectivity. 1,1-Disubstituted allenes were coupled to methyl vinyl ketone with excellent chemo-selectivity only when one set of /3-hydrogens was activated by an cy-ester or amide (Equation (69)). If the /3-hydrogens were of similar acidity, a mixture of products was obtained, as in the coupling of allenol 103 with methyl vinyl ketone dienes 104 and 105 are produced in a 1.3 1 mixture (Equation (70)). 

Regarding the first problem, the most elemental treatment consists of focusing on a few points on the gas-phase potential energy hypersurface, namely, the reactants, transition state structures and products. As an example, we will mention the work [35,36] that was done on the Meyer-Schuster reaction, an acid catalyzed rearrangement of a-acetylenic secondary and tertiary alcohols to a.p-unsaturatcd carbonyl compounds, in which the solvent plays an active role. This reaction comprises four steps. In the first, a rapid protonation takes place at the hydroxyl group. The second, which is the rate limiting step, is an apparent 1, 3-shift of the protonated hydroxyl group from carbon Ci to carbon C3. The third step is presumably a rapid allenol deprotonation, followed by a keto-enol equilibrium that leads to the final product. 

The electrophilic addition of I2 to 2,3-allenols 340 in Et20 was highly regioselec-tive with respect to the terminal C=C bond, leading to the diiodination products 341 with a preponderance of the Z-isomer. The diiodide 341 may be further converted to trans/cis vinylic epoxide 342upon the treatment with a base (Scheme 10.137) [159]. 

Friesen et al. reported that 2,3-allenols 340 can be converted to the corresponding carbamate derivatives 343, in which the terminal C=C bond of the allene moiety can be iodinated to afford diiodides 344. Under the catalysis by an Ag+ salt, compounds 344 react to give iminocarbonates 345 and oxazolindinones 346, leading to diols 347 and amino alcohols 348, respectively, after hydrolysis (Scheme 10.138) [160-162], A similar reaction was observed with trichloroacetimidates 350 [163],... 

Under the catalysis of 1 mol% of HgCl2, 4-methoxy-2-thiophenyl-2,3-allenols 365 and 367 can undergo cyclization to afford 3-phenylthiofurans 366 and 368 (Scheme 10.144) [169]. In this cycloelimination reaction, the methoxy or phenylthio substituent served as the leaving group. 

With hexylamine at room temperature, 4,4-difluorobuta-2,3-dienols also cyclize to 2,5-dihydrofuran 390 [176], A similar cydization was also observed with 2-benzotri-azole-substituted-2,3-allenol 391 [177]. 

The reaction of 2,3-allenols 398 with Ph2PCl afforded dienyl phosphine oxide 400 via the intramolecular nucleophilic attack of the phosphorus atom in 399 [179]. 

Recently, Trost et al. reported the vanadium-catalyzed addition reaction of 2,3-allenols [180], Here the oxygen in 401 served as an intramolecular nucleophile to attack the center carbon atom of allene to form a vanadium enolate 402. Aldol condensation of 402 with an aldehyde afforded (2-hydroxy)alkyl vinylic ketones 403. 

With NIS and aqueous NaHC03, the carbamates of tertiary 2,3-allenols 404 reacted to give 4,4-dialkyl-l-iodo-3-buten-2-ones 405 via the sequential intramolecular attack of the carbonyl oxygen or intermolecular attack of OH in 406 and iodina-tion [181]. 

Palladium-catalyzed reaction of a 3,4-allenol with iodobenzene proceeds through an oxypalladation-reductive elimination sequence to give a 2,3-dihydrofuran efficiently (Scheme 16.8) [13,14],... 

Ma and Zhao reported a highly regio- and diastereoselective synthetic method for 2-amino-3-alken-l-ols and 4-amino-2-( )-alken-l-ols by the palladium-catalyzed reaction of 2,3-allenols, aryl iodides and amines (Scheme 16.24) [29]. Carbopalladation of PhPdl to the allene probably generates a thermodynamically more stable anti-Jt-allylpalladium species for steric reasons. Regioselectivity of the amine attack depends largely on the stereoelectronic effect on the a-substituents. 

In the addition of Me2CuLi reagents to electron-deficient acetylenes [85-88], DCD-type complexes have been identified by NMR [84, 89]. As shown below, an ynoate affords a vinylcopper intermediate, while an ynone instead affords an allenolate (Eq. 10.9). The origin of this diversity remains unclear. A related carbocupration mechanism has also been proposed for the reaction with allenylphosphme oxide [53]. Olefin carbocupration of dienes [90] and cyclopropenes [34, 36] is known, but these mechanisms also remain unclear. 

As shown in Scheme 1, aliphatic phosphines such as P(n-Bu)3 catalyze the addition of alcohols (2) to methyl propiolate (3) [35]. The mechanism is believed to involve an initial addition of the phosphine to the C = C moiety to give a zwitterionic allenolate (I), which then deprotonates the alcohol, yielding a vinyl phosphonium salt (II). An alkoxide addition to give an enolate (III), followed by phosphine elimination gives the product 4 and regenerates the catalyst. Several experiments suggest that when alcohols are used in excess, the catalyst rests as the original phosphine [34]. 

If the addition involves an alkynyllithium such as 34, the first-formed alkoxide intermediate 35 isomerizes into the propargylic-allenic lithium reagent. Reactions with electrophiles lead to either 36a or the allenol silyl ethers 36b (equation 13). ... 

In 2003, FUrstner and Mdndez reported an elegant reaction between optically active propargyhc epoxides and organomagnesium compounds in the presence of iron(III) acetylacetonate (Scheme 50). Interestingly, the chirality of the starting product is transferred to the final 2,3-allenols which are obtained with a good enantiomeric purity. 

The iron-catalyzed addition of Grignard reagents to propargylic epoxides developed by Furstner and Mendez allows one to prepare a yw-allenol, which is an important intermediate for the synthesis of a precursor of the amphidinolide X (Scheme 67). 

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