Allenes, with alcohols

The products from this reaction are not invariably homogeneous since the first formed alkyne may isomerise, probably by way of an intermediate allene. With alcoholic potassium hydroxide at 170-180 °C, for example, terminal alkynes tend to rearrange to internal alkynes. The rearrangement process may be minimised by using sodamide in liquid ammonia as the reagent. 

The hydroesterification of allenes with alcohol and CO, when catalyzed by ruthenium complexes, gives acrylates [48, 49]. In the presence of amines, acrylamides are formed in high yields (Eq. 11.17). 

Nishina and Yamamoto have also reported the gold(I)propenyl ether in 98% yield (Eq. (12.39)). The protocol was most effective for monosubstituted and 1,3-disubstituted allenes and gave no transfer of chirality for the hydroalkoxylation of 1-phenyl-l,2-butadiene. Horino has reported the gold(I)-catalyzed intermolecular addition of alcohols to the allenyl moiety of 4-vinylidene-2-oxazolidinones [110]. 

Zhang Z, Widenhoefa RA (2008) Regio- and stereoselective synthesis of alkyl ally lie ethers via gold(I)-catalyzed intermolecular hydroalkoxylation of allenes with alcohols. Org Lett 10 2079-2081... 

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]. 

In situ transmetallation of allenylpalladium intermediates with Sml2 provides a route to allenylsamarium reagents [123]. These undergo protonolysis with alcohols to afford mainly allenes (Table 9.59). Additions to ketones yield either allenylcarbi-nols (A) or homopropargylic alcohols (B) depending on the allene substituents (Table 9.60). 

The reaction pathway depends on the steric hindrance and/or the electron density of the allene moiety. With alcohols bearing a less or unsubstituted allene moiety, the addition of CaC03 may help the cyclization [166,167]. 

The reaction of l-cyano-l,2-allenes with o-aminobenzyl alcohol afforded a mixture of conjugated 2-enenitriles 594 and dihydro-4H-3,l-benzoxazines 598. On heating to 300 °C, 598 eliminates MeCN to give the 4H-3,l-benzoxazines 599. However, under similar conditions, 594 affords 2-alkyl-3-cyanoquinolines 597 as the major product [269],... 

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. 

The use of acetylenic alcohols in place of the vinylacetylene affords allenic amino alcohols on condensation with amines [88e]. 

Yamamoto described a cascade cyclization reaction to prepare allene-substituted isochromenes 39 (Scheme 5.18).68 Diynones 38, when submitted to the action of AgSbI V, (5mol%), formed a benzopyrylium intermediate Z (identified by NMR spectroscopy), which could undergo a 1,4-Michael-type addition with alcohol nucleophiles to produce allenylisochromenes 39 (Scheme 5.18). 

Reactions of complexes of 1,2-cycloheptadienes have received only cursory attention. 1,2-cycloheptadiene is readily displaced from bisftriphenyl-phosphine)platinum(O)118 [Eq. (54)], but no reagent has been found that will displace the allene from iron.119 Reaction of the iron complex with alcohol in the presence of base (e.g., 312 — 322) is typical of Fp+ complexes of acyclic allenes.131132 The thermal chemistry of 312 is unusual in its decomposition to 324 (Scheme 41). This is probably attributable to the presence of the triflate counterion, since the corresponding fluoroborate salt is stable when warmed to 40°C for 16 h.119 A mechanism to 324 via carbene complex 321 appears likely. 

Reductive deoxygenation-rearrangement of 2-yne-l,4-diols to 1,3-dienes is a useful synthetic procedure since a large variety of ynediols are available in a few steps by sequential reaction at both ends of acetylene with aldehydes. Acetylenic 1,4-diols can be deoxygenated reductively by lithium aluminium hydride to form conjugated dienes of high stereoisomeric purity (equation 9). A modification to this procedure is the use of acetylenic 1,4-diol mono-THP derivative. Allenic tertiary alcohols which are intermediates in the reaction can be separated and subjected to reductive elimination rearrangement... 

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). 

The alkyllithium dehalogenation of dihalocyclopropanes can be used for derivatives having heteroatomic substituents (O, N, S) and is even compatible with alcohol functions and the preparation of allene alcohols. In the latter case two or more equivalents of the alkyllithium reagent must be used (see Table 5). Improved yields of allene alcohols can be obtained by protecting the alcohol function as the trimethylsilyl derivatives. Competing processes which can occur include C-H and O-H insertion, reductive dehalogenation and intramolecular insertion of cyclopropylidene at unsaturated sites of the substituent. 

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. ... 

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