Allenes, with alcohols formation

The intermolecular reaction of allenes with alcohols in the presence of catalytic amounts of PtCl2 was recently reported by Sierra and coworkers [155]. The reaction leads to an unexpected aliphatic acetal formation by attack of two molecules of methanol to the terminal carbon of monosubstituted allene systems with complete reduction of the allene (Scheme 92). 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

By using the same catalytic system, alkylations of 1,3-dimethylbarbituric acid with alcohols were also accomplished (Scheme 5.31) [68]. The Cp lr-catalyzed alkylation using 2-iodobenzyl alcohol, followed by palladium-catalyzed carbon-carbon bond formation with allene, gave spirocyclic barbituric acid derivatives in a one-pot process. 

A new perspective was opened up recently when Denmark demonstrated diat with chirally modified phosphoryl-activated allenes an asymmetric induction could be effected. From easily generated allenyl phosphoramidates containing an optically active amino alcohol, the diastereomeric adducts (37) and (39) could be obtained by addition of dlyl alcohol. When the separated adducts were employed in the carb-anionic Claisen rearrangement, a remarkable asymmetric induction (90 10) could be achieved with preferential formation of the diastereomers (38) or (40) respectively, whereas in a thermal reaction no stereoselection was observed (Scheme 63). Another example of an asymmetric induction in Claisen rearrangements is reported by Welch. ... 

The diazotization route is frequently accompanied by products derived from solvolysis of the initially formed cyclopropylidene or the rearranged cyclopentenylidene (Skattebol rearrangement) in the case of vinylcyclopropylidene with alcohol solvent, although allenes still account for the major products in the case of vrc-disubstituted cyclopropylidenes. It is noteworthy that the stereochemistry of the ring substituents (Table 2, entries 2 and 3) is an important factor in affecting yields of allenes. gcw-Disubstitution of vinylcyclopropanes diminishes formation of allenes in favor of products from the Skattebol rearrangement (entry 6). 

A TT-allylpalladium complex derived from coupling of an allene with ArB(OH)2 is reactive toward an aldehyde, therefore formation of cyclic alcohols from 4,5-hexadienal,... 

Zhang and Widenhoefer also employed gold(I)-NHC complex IPrAuCl together with silver triflate for the regioselective and stereoselective addition of various alcohols to substituted allenes (Scheme 4-109). The efficiency of chirality transfer was remarkably influenced by the concentration of the alcohol A 0.44-M solution of benzylic alcohol led to an enantiomeric excess of 64% after 30 min, whereas the use of a 1.76 M solution delivered the allyl ether with 79% ee after 20 min. Treatment of the allene with the catalyst in the absence of an alcohol led to complete racemization within 30 min. This is probably caused by formation of a 7t-allylgold intermediate. ... 

The submitters and the checkers prepared the sodium ethoxide in the conventional manner. However, sodium eth-oxide and sodium methoxide are very conveniently prepared by the inverse procedure, as described by Tishler in Fieser, Experiments in Organic Chemistry, 2nd Ed., 1941, D. C. Heath and Company, New York, p. 385 (bottom) The metal is placed in the flask, and the alcohol is added through the condenser at such a rate that rapid refluxing is maintained. It is necessary, as a precautionary measure, to clamp the flask and not to trust to the friction between a rubber stopper and the flask to hold the flask in place. When this precaution is taken, a cooling bath may be used with safety. It is necessary to cool the flask the metal must not be allowed to melt, as this will result in the formation of one large mass with a greatly decreased metallic surface. (Private communication, C. F. H. Allen.)... 

In 1974, Vermeer et al. described formation of allenic alcohols 61 by the reaction of alkynyl epoxides 60 with Grignard reagents in the presence of 10mol% of Cul (Scheme 3.33) [71]. In the absence of Cul, a complicated mixture of products was obtained. Furthermore, the Cu-catalyzed reactions exhibited higher yields and higher selectivity than analogous reactions of alkynyl epoxides with lithium dialkylcup-rates [72], This method was applied to a reaction of allylmagnesium bromide with an alkynyl epoxide [73]. 

A single example of allene formation was briefly described for a reaction of 2-methylbut-l-en-3-yne (148) with catecholborane (149) [116]. The allenylborane 150 was not isolated but converted into the homopropargyl alcohol 151 in 57% yield by quenching with benzaldehyde (Scheme 3.76). 

One of the rare applications of selenium-substituted allenes was recently reported by Ma et al. [182]. The allenyl selenide 352 undergoes an iodohydroxylation or iodo-amination, depending on the amount of water used, leading to the formation of allyl alcohol 353 and allylacetamide 354 (Scheme 8.97). When the reaction is performed with 12-16 equiv. of water, allyl alcohol 353 is exclusively formed, whereas the use of 1 equiv. of water exclusively provides the amide 354 in 64% yield. 

These reactions are thought to proceed by initial formation of the lithio propargylic alcohol adduct, which undergoes a reversible Brook rearrangement (Eq. 9.14). The resulting propargyllithium species can equilibrate with the allenyl isomer and subsequent reaction with the alkyl iodide electrophile takes place at the allenic site. An intramolecular version of this alkylation reaction leads to cyclic allenylidene products (Eq. 9.15). 

Reaction of the transient zinc intermediates with various electrophiles yielded the acetylenic substitution products and only minor amounts of allenes (Table 9.49). Reactions with aldehydes were non-selective, affording mixtures of stereo- and regioisomeric adducts. However, prior addition of ZnCl2 resulted in the formation of the homopropargylic alcohol adducts with high preference for the anti adduct, as would be expected for an allenylzinc chloride intermediate (Table 9.50). 

The enyne-allene 12 having a methyl substituent at the allenic terminus was likewise prepared from the corresponding enediynyl propargylic alcohol 11 (Scheme 20.4). The presence of a methyl group accelerates the rate of cyclization by approximately sixfold and 12 cyclizes with a half-life of -3.6 min at 78 °C. The formation of a more stable secondary benzylic radical is apparently responsible for the rate enhancement. 

The propargylic alcohol 102, prepared by condensation between 100 and the lithium acetylide 101, was efficiently reduced to the hydrocarbon 103, which on treatment with potassium tert-butoxide was isomerized to the benzannulated enyne-allene 104 (Scheme 20.22) [62], At room temperature, the formation of 104 was detected. In refluxing toluene, the Schmittel cyclization occurs readily to generate the biradical 105, which then undergoes intramolecular radical-radical coupling to give 106 and, after a prototropic rearrangement, the llJ-f-benzo[fo]fluorene 107. Several other HJ-f-benzo[fo]fluorenes were likewise synthesized from cyclic aromatic ketones. 

Treatment of the propargylic alcohol 144, readily prepared from condensation between benzophenone (143) and the lithium acetylide 101, with thionyl chloride promoted a sequence of reactions with an initial formation of the chlorosulfite 145 followed by an SNi reaction to produce in situ the chlorinated and the benzannulated enyne-allene 146 (Scheme 20.30) [62], A spontaneous Schmittel cyclization then generated the biradical 147, which in turn underwent a radical-radical coupling to form the formal [4+ 2]-cycloaddition product 148 and subsequently, after a prototropic rearrangement, 149. The chloride 149 is prone to hydrolysis to give the corresponding 11 H-bcnzo h fluoren-ll-ol 150 in 85% overall yield from 144. Several other llff-benzo[fc]fluoren-ll-ols were likewise synthesized from benzophenone derivatives. 

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