Allenyl lead

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Triphenylsilanol is one of several products obtained in the thermally induced decarbonylation of Ph3SiC02H (129). Acylsilanes react readily with aqueous base for example, Ph3SiC(0)Ph affords Ph3SiOH and PhC(0)H when treated with aqueous sodium hydroxide solution (130). Similarly, hydrolysis of the allenyl silane 22 leads to the formation of 23 via the acylsilane 24 [Eq. (24)] (131). (For further examples of acylsilane chemistry, see Refs. 132-134.)... 

Allenyl cations 1 are a stabilized form of vinyl cations1-3 in which the /1-carbon atom of the vinylic structure is part of the substituent which effects the stabilization of the ion via its electron-donating ability. This leads to a resonance hybrid having formally the alkynyl cation structure 2. Allenyl cations should be distinguished from the allenyl substituted carbenium ions 3 formulated as the mesomeric structures of the vinyl cations 4 (dienyl cations) stabilized by an w-vinyl group (equation 1). 

The cycloaddition of allenyl cations with monoolefins lead to [2 + 2]- or [3 + 2]-cycloadducts based on the substituents in the allenyl cations as exemplified in equation 1332. 

The Pd-catalysed carbonylation of alkynyl epoxides 60 and alkynyldioxolanones 61 leads to the allenes 62 which can then be converted to the same pyranones through a tandem conjugate addition-cyclisation (Scheme 40) <00JCS(P1)3188>. Carbonylation of allenyl alcohols is catalysed by Ru3(CO)i2 and yields 5,6-dihydropyran-2-ones . 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

The combination of a silyl-migration from carbon to oxygen and a prototropic isomerization leads to allenyl silyl ethers [246]. 

An analogous mechanism was proposed for the conversion of the triflate 416 to the vinyl-, allyl- and allenyl-A2-cephems 448 in yields of 47-71% by the respective tributyltin compounds in the presence of cuprous chloride (Scheme 6.91) [176]. Accordingly, the cyclic allene 417 should be liberated from 416 in the first step. Then, the organocopper species would transfer a hydrocarbon group to the central allene carbon atom of 417, leading to an allyl anion derivative, which is protonated during the workup. These reactions of 416 and 443 indicate that the cyclic allenes 417 and 444 behave toward nucleophiles as 1,2-cyclohexadiene (6) (Schemes 6.11— 13) and its non-polar derivatives such as 215 (Scheme 6.51), 221 (Scheme 6.52), 311 (Scheme 6.67) and 333 (Schemes 6.71 and 6.73), that is, they interact with nucleophiles at the central carbon atom of the allene system exclusively. 

Only a few examples exist describing the products from the allenylic/propargylic carbanion resulting from the deprotonation of 18 and reaction with other electrophiles instead of protons which lead to products analogous to 19 [48]. Thus, treating the propargyl compound 21 with tetrabutylammonium fluoride (TBAF) in the presence of benzaldehyde furnishes the C,C-connected compound 22 [41]. 

The nucleophilic addition of alcohols [130, 204-207], phenols [130], carboxylates [208], ammonia [130, 209], primary and secondary amines [41, 130, 205, 210, 211] and thiols [211-213] was used very early to convert several acceptor-substituted allenes 155 to products of type 158 and 159 (Scheme 7.25, Nu = OR, OAr, 02CR, NH2, NHR, NRR and SR). While the addition of alcohols, phenols and thiols is generally carried out in the presence of an auxiliary base, the reaction of allenyl ketones to give vinyl ethers of type 159 (Nu = OMe) is successful also by irradiation in pure methanol [214], Using widely varying reaction conditions, the addition of hydrogen halides (Nu= Cl, Br, I) to the allenes 155 leads to reaction products of type 158 [130, 215-220], Therefore, this transformation was also classified as a nucleophilic addition. Finally, the nucleophiles hydride (such as lithium aluminum hydride-aluminum trichloride) [211] and azide [221] could also be added to allenic esters to yield products of type 159. 

Based on nucleophilic addition, racemic allenyl sulfones were partially resolved by reaction with a deficiency of optically active primary or secondary amines [243]. The reversible nucleophilic addition of tertiary amines or phosphanes to acceptor-substituted allenes can lead to the inversion of the configuration of chiral allenes. For example, an optically active diester 177 with achiral groups R can undergo a racemization (Scheme 7.29). A 4 5 mixture of (M)- and (P)-177 with R = (-)-l-menthyl, obtained through synthesis of the allene from dimenthyl 1,3-acetonedicar-boxylate (cf. Scheme 7.18) [159], furnishes (M)-177 in high diastereomeric purity in 90% yield after repeated crystallization from pentane in the presence of catalytic amounts of triethylamine [158], Another example of a highly elegant epimerization of an optically active allene based on reversible nucleophilic addition was published by Marshall and Liao, who were successful in the transformation 179 — 180 [35], Recently, Lu et al. published a very informative review on the reactions of electron-deficient allenes under phosphane catalysis [244]. 

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

Allenes containing an allyl unit at a suitable position within the molecule can undergo an intramolecular ene reaction. Thus, allenyl sulfones 423, prepared by oxidation of the corresponding sulfoxides, lead to ring closure products 424 on heating [371]. 

Among the many useful electrophiles, carbonyl compounds and their derivatives lead to products of the highest value for synthetic endeavors. First experiments with aldehydes and ketones were performed by Hoff, Brandsma and Arens (Scheme 8.15) [12b]. The primary allenyl adducts 60, which are isolable in moderate to excellent yields [12b, 47], serve as starting materials for subsequent cyclizations (see Section 8.2.2.2). 

Tius and co-workers investigated a number of cationic cyclopentannelations of allenyl ethers [113] and found that 1-lithio-l-alkoxyallenes 180 react with a,/3-unsatu-rated carbonyl compounds 181 leading to highly functionalized cyclopentenones 182 (Scheme 8.44). The primary products are a-allenyl ketones 183, which form pentadienyl cations 184 by protonation. The latter undergo a thermally allowed 4jt-conrotatory ring closure to intermediates 185, which with elimination of R1 finally lead to the expected products 182 (Scheme 8.45). 

In contrast to the rich chemistry of alkoxy- and aryloxyallenes, synthetic applications of nitrogen-substituted allenes are much less developed. Lithiation at the C-l position followed by addition of electrophiles can also be applied to nitrogen-containing allenes [10]. Some representative examples with dimethyl sulfide and carbonyl compounds are depicted in Scheme 8.73 [147, 157]. a-Hydroxy-substituted (benzotriazo-le) allenes 272 are accessible in a one-pot procedure described by Katritzky and Verin, who generated allenyl anion 271 and trapped it with carbonyl compounds to furnish products 272 [147]. The subsequent cyclization of 272 leading to dihydro-furan derivative 273 was achieved under similar conditions to those already mentioned for oxygen-substituted allenes. 

A stereoselective route to 2-(phenylthio)-l,3-butadienes such as 327 or 328 was developed by Pearson et al. [167] with allylboranes as crucial intermediates. Addition of 9-BBN to allenyl sulfide 324 gives the allylborane intermediate 325, which subsequently adds to aldehydes (Scheme 8.89). Typical of Peterson olefinations, this reaction can also be terminated by two different work-up procedures, either acidic conditions leading to anti-elimination, which affords Z-configuration of dienes 327, or basic work-up resulting in a syn-elimination to form (E)-dienes 328. 

Tricyclic sulfur heterocycles 341 were prepared utilizing an intramolecular [4 + 2] cycloaddition. Heating of allenyl sulfides 340 to 110 °C leads to Diels-Alder products 341 in reasonable yields (Scheme 8.93) [163], Unfortunately, this method does not allow general access to these heterocycles, since a particular substitution pattern of the substrate is required. No reaction occurred with substrates lacking the thioacetal moiety. 

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). 

Allenyl iodides can be prepared from propargylic mesylates by Sn2 displacement with LiCuI2 (Eq. 9.143) [118]. The reaction proceeds primarily by an anti pathway with slight racemization. Metallation of these iodides with powdered indium in various donating solvents leads to transient allenylindium intermediates which react in situ with aldehydes to afford anti homopropargylic alcohols (Table 9.52). Additions... 

Allenyl bromide accepts the nucleophilic attack of amines or potassium thio-acetate leading to propargylic amines or thioesters via the alkenylidenecarbene 132 or zwitterion intermediate 133 [66],... 

The iodohydroxylation of 1,2-allenyl sulfoxide 171 with I2 in the presence of H20 exhibits E-selectivity leading to (E)-2-iodo-3-hydroxy-l-alkenyl sulfoxide [88]. By using Br2, CuBr2 or NBS, (E)-2-bromo-3-hydroxy-l-alkenyl sulfoxide is produced. For the chlorohydroxylation of a sulfoxide, CuCl2 and silica gel were used to mix with the sulfoxide to deliver the (E)-chlorohydroxylation products highly stereoselec-tively [88]. The chirality in the allene moiety can be efficiently transferred to the final allylic alcohols. Under the catalysis of a Pd or Ni complex, the C-X and C-S bonds can be coupled to introduce different substituent(s) into the different locations of the C=C bond. 

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