Allene derivatives reactivity

To prepare the other cross-conjugated allene, 4-methylene-l,2,5-hexatriene ( 2-allenyl-1,3-butadiene ) (23), the allene alcohol 215 was first converted into the phosphate 216, that readily underwent an SN2 -type substitution with allenylmagnesium bromide to yield the target hydrocarbon as a highly reactive allene derivative (Scheme 5.32) [76],... 

Donor- and acceptor-substituted allenes with general structures 1 or 2 (Scheme 8.1) have the most obvious synthetic potential among functionalized allene derivatives and therefore they serve as versatile building blocks in many synthetic endeavors [1], As expected, the reactivity of the double bonds of 1 or 2, which are directly connected to the activating substituents, are strongly influenced by these groups. Hence there is enol ether or enamine reactivity of 1 and Michael acceptor type chemistry of 2. In addition, the terminal double bonds are also influenced by these functional groups. 

The Pd(0)-catalyzed reactions of propargylic compounds so far discovered can be classified into four types, I, II, III, and TV, from a mechanistic viewpoint. The allenyl intermediate complex 8 undergoes three types of transformation, depending on reactants. The reactions of Type I proceed by insertion of unsaturated bonds into the a-bond between palladium and sp carbon in 8. This a-bond has a reactivity similar to the a-bond formed by the oxidative addition of alkenyl halides to Pd(0) in the Heck reaction [3]. Therefore, reactions similar to those observed in the Heck reaction are expected to occur witli the intermediate 8. Alkenes and carbon monoxide are known to insert into the palladium-carbon a-bond. The allene derivatives 9 are formed by these reactions (Scheme 11.3). 

The reactivity of e/z dizincioalkenes (prepared by metal allylation of alkynyl metals (see Section 5.1)) with aldehydes leads to the corresponding allene derivatives in moderate yields (Scheme 38). As already seen for e/z dizincioalkanes, an internal chelation of the 1,1-dimetalloalkene with a Lewis functional group allows differentiation of the reactivity of the two metal centers towards electrophile see ElectrophilS reagents. Scheme 39 summarizes the reactivity... 

Allene derivatives are reactive toward Pd-catalyzed allylic alkylation reactions. The addition of bis(benzenesulfonyl)-methane to a functionalized allene in the presence of dimer (1) and dmppp [(l,3-bis(di(2-methoxyphenyl)phosphino)propane)] ligand gave the substituted alkene as the exclusive -isomer in 64% yield (eq 34). ... 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

All the synthetic protocols described above have limitations to some extent and the yields of the products were modest. In some cases the formation of 1,2-alkadi-enephosphonate derivatives is essential for obtaining the final cyclic products. This is the reason why many authors have used the higher reactivity of 1,2-alkadi-enephosphonates, discovered by Mark [42] in 1962 for the preparation of 2,5-dihydro-l,2-oxaphosphole-2-oxide derivatives. Since then, the oxaphospholic cyclization of 1,2-alkadienephosphonate system of double bonds has become the easiest method for the synthesis of these compounds. The special structure of phosphorylated allenes is responsible for their special properties, which has attracted the attention of chemists for a long time [43 16],... 

Pyrrolo[l,2- ][l,2]oxazines are a class of compounds with very few references regarding synthesis and reactivity. An interesting preparation has been described by intramolecular cyclization of IV-hydroxy pyrrolidines carrying a methoxyallene substituent at C-2 (242, Scheme 32). These compounds were obtained by addition of a lithiated allene to chiral cyclic nitrones 241. Cyclization occurred spontaneously after some days at relatively high dilution (0.05 M). Compounds 243 (obtained with excellent diastereoselectivity) can be submitted to further elaboration of the double bond or to hydrogenolysis of the N-O bond to form chiral pyrrolidine derivatives (Section 11.11.6.1) <2003EJ01153>. 

Mikami and Yoshida extended the scope of this method considerably by using propargyl phosphates and chiral proton sources [94], The propargylic phosphates thereby have been found to be advantageous owing to their high reactivity towards palladium and the extremely low nudeophilicity of the phosphate group [95]. In some cases, it was even possible to obtain allenes from primary substrates, e.g. ester 194 (Scheme 2.60) [96]. A notable application of this transformation is the synthesis of the allenic isocarbacydin derivative 197 from its precursor 196 [97]. 

Two types of derivatives of 1,2-cyclohexadiene with two heteroatoms were proposed as reactive intermediates more than 20 years ago. Lloyd and McNab [168] observed the reaction of the 5-bromo-l,2-dihydropyrimidinium ions 411 with thiourea in refluxing ethanol to give the bromine-free cations 413. Suspected as intermediates, the 5d2-dihydropyrimidines 412 were initially considered as zwitterions of the type 414-Zj. However, quantum-chemical calculations on the parent systems suggested an unambiguous preference of the allene structure 414 over the zwitterion 414-Za [169]. 

Saalfrank, Hoffmann and co-workers performed a number of reactions with tetra-alkoxyallenes such as 196 (Scheme 8.47) [1, 41, 105, 114—116] and demonstrated that this class of donor-substituted allenes can serve as a 1,3-dianion equivalent of malonic acid. Treatment of 196 with cyclopropyldicarboxylic acid dichloride 197 produces 2,4-dioxo-3,4-dihydro-2H-pyran 198 through release of two molecules of ethyl chloride [115]. Similarily, the reaction of this allene 196 with oxalyl chloride gives 3-chloromalonic acid anhydride derivative 199. This intermediate is a reactive dieno-phile which accepts 2,3-dimethyl-l,3-butadiene in a subsequent [4+2] cycloaddition to afford cycloadduct 200 in good yield [116]. 

Although allene itself is reluctant to react with ordinary 1,3-dienes, it underwent successful [4+2]-cycloadditions with relatively reactive cyclopentadienes to afford 5-methylenebicyclo[2.2.1]hept-2-ene derivatives [145]. 

The ability of (Z)-l,2,4-heptatrien-6-ynes (enyne-allenes) and the benzannulated derivatives to undergo cyclization reactions under mild thermal conditions to produce biradicals has been the main focus of their chemical reactivities [1-5]. With the development of many synthetic methods for these highly conjugated allenes, a variety of biradicals are readily accessible for subsequent chemical transformations. Cyclization of the enyne-allene 1 could occur either via the C2-C7 pathway (Myers-Saito cyclization) leading to the a,3-didehydrotoluene/naphthalene biradical 2 [6-10] or via the C2-C6 pathway (Schmittel cyclization) producing the fulvene/benzofulvene biradical 3 [11] (Scheme 20.1). 

Allenic amino acid derivatives 50, which are of special interest as selective vitamin Bg decarboxylase inhibitors [35], are accessible through 1,6-cuprate addition to 2-amino-substituted enynes 49 (Eq. 4.22) [36]. Because of the low reactivity of these Michael acceptors, however, the reaction succeeds only with the most reactive cuprate the t-butyl cyano-Gilman reagent tBu2CuLi-LiCN. Nevertheless, the addition products are obtained with good chemical yields, and selective deprotection of either the ester or the amino functionality under acidic conditions provides the desired target molecules. 

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