Allene derivatives complexes

As typical examples of crystal-to-crystal thermal reactions, the cyclization of allene derivatives to four-membered ring compounds and the transformation of a racemic complex into a conglomerate complex are described. 

If doubly-bonded compounds >M = X (M = Si, Ge, Sn) are now well known, this is not yet the case for the heavy allenic derivatives >M = C = X. Transient silaallenes >Si = C = C 160-167 or silaazaallene >Si = C = N—168 were postulated some years ago, but the first stable derivative of this type, a formal 3-stanna-l-azaallene >Sn = C = N —, was isolated only in 1992 by Griitzmacher et al169 The compound Tbt(Mes)Si = C = NR170 has also been obtained, but according to the authors it is closer to a silylene Lewis base complex than to a silacumulene. The first stable silaallene was only reported in 1993 by West et al.111 and the first metastable >Si = C = P— was characterized still more recently.172... 

The reactions of type II proceed by transmetallation of the complex 5. The transmetallation of 5 with hard carbon nucleophiles M R (M = main group metals) such as Grignard reagents and metal hydrides MH generates 8. Subsequent reductive elimination gives rise to an allene derivative as the final product. Coupling reactions of terminal alkynes in the presence of Cul belong to Type II. 

The reaction of ate complexes (24), formed from trialkylboranes and trimethyl-silylpropargyl phenyl ether, with a mixture of acetic acid and hexamethylphosphoric triamide (HMPT) gives trimethylsilylacetylenes (25) selectively (Eq. 54) whereas the corresponding trimethylsilylallenes (26) are selectively prepared by the reaction with sodium methoxide instead of acetic acid and HMPT (Eq. 54) In the latter, when primary alkylboranes are used, the corresponding allene derivatives are obtain in high purity, but secondary alkylboranes give reverse ratios of the isomer distribution. 

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). 

Since the 7r-allylnickel complexes are tolerant to polar amide and ester groups, the catalysts mentioned in the previous sections could polymerize allene derivatives with functional groups. The polymerization of N-allenyl-... 

Novak found the polymerization of allene catalyzed by [( -allyl)Ni (OCOCF3)]2 (Mw/Mn=1.23) and its copolymerization with propyne [138]. Although [(7r-allyl)Ni(OCOCF3)]2 does not initiate the homopolymerization of propyne, the copolymerization of allene with propyne is catalyzed by the complex to give the copolymer which has alignment of the allene-derived unit, -CH2-C(=CH2)-, and the propyne-derived unit, -CH=C(-CH3)-(Eq. 37) [139]. [(7r-Allyl)Ni(OCOCF3)]2 also initiates smooth polymerization of 1,3-butadiene [65] and isocyanides [140]. 

In contrast to the behavior of free alkynes, 1,3-isomerization of alkyne complexes to produce complexed allenes [140,141) is rather rare. A synthetically useful example [140] is provided by the CpMn(CO)2 complexes of electrophilic alkynes 89 which undergo alumina-promoted isomerization to the corresponding allene derivatives 90 these, in turn, can be oxidatively demetalated to the free allenes 91 (Scheme 4-45). 

In the vertical mode [88], constructing a trisubstituted furan (102) was considered a suitable starting point. Subsequent cyclo-addition reaction with the allene derivative 103 gave the Diels-Alder adduct which with lithiumaluminiumhydride-titanium trichloride complex and triethyl amine opened up to form the penta substituted benzene derivative 104. This was converted into 86a in five steps as depicted in Scheme 21. 

The second type of reactions proceed by transmetallation of the complexes 1. MR (M = main group metals) and metal hydrides MH undergo the transmetallation with 1 to generate 6. Subsequent reductive elimination gives the allene derivative 7. Also reactions of 1 with 1-alkynes in the presence of Cul to afford allenylalkynes belong to this type. 

The Larock method for annulation between vicinal iodo-arylamines and 1,2-dienes in the preparation of indoles can be adapted for preparation of azaindoles using corresponding azine substrates. Thus, substituted-3 f-pyrrolo[2,3-fc]pyridin-3-ones can be prepared from 2-amino-3-iodopyridine derivatives by a palladium carboannulation process with al-lenic compounds (Scheme 104). The bicyclic products, the methylene derivatives 308, and the alkylidenes 309 can be oxidatively cleaved with ketone formation. The reaction may proceed by formation of a pyridinylpalladium complex followed by the 7r-allyl complexa-tion of allenic derivatives 310. Since the polar substituents on terminal carbons of the rr-allyl system influence the regiochemistry of the reactions, nucleophilic attack of the nitrogen atom on the most electron-deficient carbon atom of the rr-allyl system affords either of the... 

When compared with C2 symmetric ligands, Ci symmetric and bidentate NHCs induced the highest ee values (<99% ee) with unsaturated C-C bonds and [B(pin)]2 in MeOH. For instance, in the p-boration of a- and p-unsatu-rated aldehydes and esters, which are more reactive than other C-C unsaturated substrates, a 213-derived system is more enantioselective than the 214-derived complex (74% ee vs. 59% ee, respectively) (Scheme 13.8). Improved results were obtained by Hoveyda and co-workers, using bidentate 217-220 with acyclic and cyclic internal alkenes, alkynes, 1,6-dienes and various allenes ee s of <98% were achieved (Scheme 13.8). For the regio-and enantioselective hydroboration of a,p-unsaturated carbonyl compounds... 

A derivative of cyclopentyne has been trapped in a matrix. Although cycloheptyne and cyclohexyne have not been isolated at room temperatures, Pt(0) complexes of these compounds have been prepared and are stable." The smallest cyclic allene" so far isolated is l-/err-butyl-l,2-cyclooctadiene 107." The parent 1,2-cyclooctadiene has not been isolated. It has been shown to exist transiently, but rapidly dimerizes." " The presence of the rert-butyl group apparently prevents this. The transient existence of 1,2-cycloheptadiene has also been shown," and both 1,2-cyclooctadiene and 1,2-cycloheptadiene have been isolated in platinum complexes." 1,2-Cyclohexadiene has been trapped at low temperatures, and its structure has been proved by spectral smdies." Cyclic allenes in general are less strained than their acetylenic isomers." The cyclic cumulene 1,2,3-cyclononatriene has also been synthesized and is reasonably stable in solution at room temperature in the absence of air." ... 

AUylzirconium complexes are conveniently obtained by the regio- and stereoselective hydrozirconation of allene [127-133] and can be, for example, used subsequently for the MAO-catalyzed allylzirconation of alkynes to prepare enyne derivatives [132]. Alternatively, the preparation of (E)-l,3-dienes from aldehydes and the appropriate allylstannane zirconocene derivative (R = SnBu,) is accomplished (Scheme 8-17) [131], Note that addition of [Cp2Zr(H)Cl[n (1) on the aUenyl reagent with the... 

The insertion of allenes in the Pd-C bond of cyclopalladated 3-arylisoquinoline derivatives 327 afforded compounds 328, derived from the berberinium cation (Scheme 71). This reaction takes place via the formation of an intermediate (r/ -allyOpalladium complex <2003JOM313>. This chemistry has been extended to the preparation of other cationic N-heterocycles, including naphtho[r+/ ]( uinolizinium derivatives <2004EJ01724>. 

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

A proposed mechanism of the bis(allene) cyclization involves the formation of the allyl(stannyl)palladium species 6, which undergoes carbocyclization to give vinyl(stannyl)palladium intermediate 7 (Scheme 36). Reductive elimination and cr-bond metathesis may lead to the formation of the m-pentane derivative and the bicyclic product, respectively. The cyclization of allenic aldehydes catalyzed by a palladium complex was also reported.163... 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

C-NMR spectrum, only the Ru(3) carbonyl groups are not affected by proton decoupling 102). An intermediate isomeric form of the complex HRu3(CO)9(C6H9) has also been isolated, and the X-ray structure (Fig. 13) establishes the coordination of a ir-allene group 103). This isomer converts to the 7r-allyl derivative on warming. 7r-Allyl derivatives may... 

The alternative building scheme C2 + Q was used by Petasis and Hu [89], who reacted various aldehydes and ketones with alkenyltitanocene derivatives 172 to obtain the corresponding allenes 173 in high chemical yields (Scheme 2.54). The reaction probably proceeds via titanocene vinylidene complexes, which can also be trapped with alkynes and isocyanides to afford allenylketene imines [90],... 

By chance, the existence of the borane complex 330 of 329 was discovered. The liberation of 330 occurred with the best efficiency with sodium bis(trimethylsilyl)-amide from the borane complex 327 of 326. When styrene or furan was used as the solvent, three diastereomeric [2 + 2]-cycloadducts 328 and [4 + 2]-cycloadducts 331, respectively, were obtained in 30and 20% yield (Scheme 6.70) [156]. With no lone pair on the nitrogen atom, 330 cannot be polarized towards a zwitterionic structure, which is why its allene subunit, apart from the inductive effect of the nitrogen atom, resembles that of 1,2-cydohexadiene (6) and hence undergoes cycloaddition with activated alkenes. It is noted that the carbacephalosporin derivative 323 (Scheme 6.69) also does not have a lone pair on the nitrogen atom next to the allene system because of the amide resonance. 

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]. 

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