Allenyl enolate intermediate

DFT study of interception of the allenyl enolate intermediate of Meyer-Schuster rearrangement using aldehydes and imines has shown that the active form of the vanadium catalyst bears two triphenyl siloxy ligands and that vanadium enolate is directly involved in the C-C bond formation." DFT calculations have elucidated the mechanisms and diastereoselectivities of phosphine-catalysed [4-1-2] annulations between allenoates and ketones or aldimines (Scheme 44). 

The transient allenyl enolate intermediate of the vanadium-catalysed Meyer-Schuster rearrangement with aldehydes and imines has been studied computationally and the active form of the catalyst is considered to contain two triphenylsiloxy ligands. The transesterification of vanadate occurs via tr-bond metathesis and the vanadium enolate is directly involved in the key C-C bond formation (Scheme 175). ... 

In the [4 + 2] cycloadditions discussed so far, the enol ether double bond of alkoxyallenes is exclusively attacked by the heterodienes, resulting in products bearing the alkoxy group at C-6of the heterocycles. This regioselective behavior is expected for [4+2] cycloadditions with inverse electron demand considering the HOMO coefficients of methoxyallene 145 [100]. In contrast, all known intramolecular Diels-Alder reactions of allenyl ether intermediates occur at the terminal C=C bond [101], most probably because of geometric restrictions. 

The isomeric propargylic stannylated aldehyde intermediate, on the other hand, could be prepared from the alcohol precursor without competing cyclization to an seven-membered enol ether product (Eq. 9.105). Treatment of this stannane with SnCl4 afforded the cis-disubstituted tetrahydrofuran stereoselectively. Presumably, this reaction proceeds through an allenyl trichlorostannane intermediate. 

As previously mentioned, allenes can only be obtained by 1,6-addition to acceptor-substituted enynes when the intermediate allenyl enolate reacts regioselectively with an electrophile at C-2 (or at the enolate oxygen atom to give an allenyl ketene acetal see Scheme 4.2). The regioselectivity of the simplest trapping reaction, the protonation, depends on the steric and electronic properties of the substrate, as well as the proton source. Whereas the allenyl enolates obtained from alkynyl enones 22 always provide conjugated dienones 23 by protonation at G-4 (possibly... 

Addition to an enynone (1). Cuprates, particularly higher-order ones, undergo 1,6-addition to 1 to provide the less stable (Z)-dienone 2 as the major product. The (Z)-stereoselectivity increases with the bulk of the transferred R group. The addition may involve an allenyl enol. Addition of QHjSLi to 1, however, provides (E)-dienones, possibly via an intermediate 1,2-adduct. 

Regioselective Protonation of Enolates. In a study of the regioselectivity of protonation of allenyl enolates, pivalic acid demonstrated superior selectivity in the example shown (eq 2), allene 1, resulting from protonation at C-2, was formed exclusively when the intermediate enolate was quenched with pivalic acid at-80°C. ... 

The elimination reactions of /l-acetoxy sulfones 114 to give the donor-acceptor-substituted allenes 115 by a Julia-Lythgoe process are less conventional (Scheme 7.18) [157]. A new one-step synthesis of allene-l,3-dicarboxylates 118 from acetone derivatives 116 was developed by the use of 2-chloro-l,3-dimethylimidazolinium chloride 117 [158, 159]. This elimination of water follows also the general Scheme 7.17 if a derivative of the enol, resulting from 116, is assumed as an intermediate for an elimination step. More complex processes of starting materials 119 furnished allenyl ketones 120 in high yields [160-162]. 

By analogy with the mechanistic pathway described for the enol ether 119 (Scheme 44), we believe that the transformation of 123a-c also occurs via the formation of the /J-metalated allenyl intermediate, generated from the -elimination of the corresponding zirconacyclopropane and subsequent rearrange-... 

The dehydrogenation of bis(a-bromobenzyl) sulfide (41) with Fe2(CO)9 in the presence of furan affords (43). The iron-stabilized sulfur ylide (42) has been proposed as intermediate. Reaction of C -di-phenylnitrone (44) with l,2-bis(methylene)-3,3.4,4,5,5-hexamethylcyclopentane (45) in benzene at 80 C gives a [4 -I- 3] cycloadduct (46 18%), along with some other products. Landor et al. have shown that the dehydrobromination of allenyl bromide (47) in the presence of furan affords bicyclic ketone (49) in 9% yield. Apparently, a strain alkyne intermediate reacts with r-butyl alcohol to afford an enol ether (48), which is then converted to (49) (Scheme 11). 

A number of intermediates related to the cyclopropylmethyl radical undergo rapid ring opening. 1-Cyclopropylvinyl radicals 35 rearrange to 2-(allenyl)ethyl radicals 36, while radical anions 37, formed by one-electron reduction of cyclopropyl ketones, ring open to give enolate radicals... 

The addition of a chiral allenyl metal to an aldehyde generating a 2-substituted butynyl structure is named Marshall-Tamaru MT) reaction (Scheme 5-13). An allenyl palladium species is generated via a formal Sn2 substitution of the mesylate, which in turn undergoes a transmetalation with diethylzinc yielding a nucleophilic species (Scheme 5-13). The reaction of the electrophile proceeds via a similar -ester enolate transition state as depicted in Scheme 5-13. Corresponding allenylindium reagents can also be used instead of allenylzinc intermediates. ... 

The reaction of lithium diisopropylamide with propargyl acetate 3.559 followed by treatment of the enolate formed with trimethylchlorosilane at —78°C affords ketene acetal 3.561. After heating the reaction mixture to 40°C, the intermediate vinylpropargyl ether 3.560 undergoes the Claisen rearrangement to produce silyl ester 3.561, which after hydrolysis forms allenyl-acetic acid 3.562 with a 50% overall yield (Scheme 3.42) [281]. This reaction is of interest as a synthetic strategy for obtaining the benzoannelated enyne-allene that by radical cyclization forms polycyclic aromatic compounds with an embedded fluorene section [281]. (Scheme 3.43)... 

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