Allenyl dimer

Heated in methanol for an extended period of time, propargyl azide 1147 experiences a [3,3] sigmatropic shift to allenyl azide 1148 that undergoes rapid cyclization to triazafulvene 1149. Addition of a molecule of methanol converts reactive intermediate 1149 to triazole 1150 that is isolated in 68% yield. In concentrated solutions, two molecules of intermediate 1149 may undergo cycloaddition to form dimer 1151 as a side product (Scheme 189) <2005EJ03704>. 

Disubstituted furans 62 are obtained from a gold-catalyzed cycloisomerization/dimerization pathway involving terminal allenyl ketones and a,(3-unsaturated ketones <00AG(E)2285>. 

Hashmi et al. investigated a number of different transition metals for their ability to catalyze reactions of terminal allenyl ketones of type 96. Whereas with Cu(I) [57, 58] the cycloisomerization known from Rh(I) and Ag(I) was observed (in fact the first observation that copper is also active for cycloisomerizations of allenes), with different sources of Pd(II) the dimer 97 was observed (Scheme 15.25). Under optimized conditions, 97 was the major product. Numerous substituents are tolerated, among them even groups that are known to react also in palladium-catalyzed reactions. Examples of these groups are aryl halides (including iodides ), terminal alkynes, 1,6-diynes, 1,6-enynes and other allenes such as allenylcarbinols. This che-moselectivity might be explained by the mild reaction conditions. 

With certain substitution patterns such as in the p-methoxybenzyl allenyl ketone 103, the products from reactions with Ag(I), Au(III), Pd(II) and Hg(II) are entirely different 104, the dimers 105 or 106 or even 107 were formed (Scheme 15.28) [63, 64]. 

Scheme 17.12 Cyclization-dimerization of a-allenyl acids and ketones.

Scheme 17.13 Alternative mechanism for the cyclization—dimerization reaction of an a-allenyl acid and a ketone.

The need for a base additive in this reaction implies the intermediacy of acetylide complexes (Scheme 9.10). As in the Rh(III)-catalyzed reaction, vinylidene acetylide S4 undergoes a-insertion to give the vinyl-iridium intermediate 55. A [l,3]-propargyl/ allenyl metallatropic shift can give rise to the cumulene intermediate 56. The individual steps of Miyaura s proposed mechanism have been established in stoichiometric experiments. In the case of ( )-selective head-to-head dimerization, vinylidene intermediates are not invoked. The authors argue that electron-rich phosphine ligands affect stereoselectivity by favoring alkyne C—H oxidative addition, a step often involved in vinylidene formation. 

Hydroalkoxylation of Allenes In the year 2000, during their investigation of transition metal catalyzed reactions of allenyl ketones [29], Hashmi et al. discovered that gold(III) salts were able to lead the cydoisomerization and dimerization of these substrates (Equation 8.2) with a considerable improvement related to other assays with Ag (I) or Pd (II) catalysts [18]. 

Rhodium(i)-catalyzed ene-allene carbocyclization strategy is suggested for the formation of seven-membered heterocycles, azepines and oxepines. In particular, treatment of an allenyl allyl ether with a catalytic quantity of chlorodi(carbonyl)rhodium dimer affords 4-alkylidene-5-alkyl-2,3,4,5-tetrahydrooxepines (Equation 28) in 40-55% yields <20040L2161>. 

As mentioned in the introduction, the reactivity of the allenes is high. Therefore the oxidative and cyclizative cross dimerization of two different allenic substrates just recently reported by Ma et al. can be considered to be a milestone in this field They discovered that one equivalent of 2,3-dienoic acids 69 and five equivalents of allenyl ketones 70 with the simple PdCl2(MeCN)2 catalyst can deliver up to 92% of the cross dimerization product 71... 

The cycloisomerization of allenyl ketones was initially described as being catalyzed by rhodium(I) or silver(I) by Marshall et al.21 The activity of copper, silver, and gold for this reaction was first compared in two papers published later (Scheme 12.7).22 In the case of copper and silver, only a cycloisomerization was observed (Table 12.4, entries 1 and 2) with gold, a dimer is obtained as well (entry 3). 

TABLE 12.4. Cycloisomerization or Cycloisomerization/Dimerization of Allenyl Ketones (R1 = Me)... 

Mechanistically, this unusual multicomponent trimerization can be rationalized as a sequence of a Pd-catalyzed alkyne dimerization [154,155] giving rise to the regioselective formation of the enyne 218, which undergoes subsequent Pd-catalyzed [4 + 2]-benzannulation [158,159] with a butadiyne as an enynophile to furnish the benzene 216 via an allenyl-Pd species 219 (Scheme 90). Upon submitting an electron-deficient alkyne together with a terminal alkyne in equimolar amounts to the sequence, the unsymmetri-cal alkyne dimerization gives a trisubstituted enyne to set the stage for the formation of pentasubstituted benzene derivatives 217. 

Hashmi et al. have found that terminal allenyl ketones give dimerization products when Pd(ll)-catalysts are employed furnishing 2,4-disubstituted furans (Equation 15) <1995AGE1581, 1997JOC7295>. Tetrakis(2,2,2-trifluoroethoxycarbonyOpalladacyclopentadiene proved to be the most effective catalyst to obtain predominantly dimerization instead of cycloisomerization. 

In the presence of 1 mol% AuCls, allenyl ketones undergo a rapid cycloisomerization and can be reacted with a Michael acceptor. In a cross-dimerization of terminal allenyl ketones and a,/3-unsaturated ketones, 2,5-disubstituted furans can be obtained (Equation 16) <2000AGE2285>. 

The conversion of a dimeric rr-bound methyl propargyl ether complex [Mo2(CO)4Cp2(At-Tj, i7--CH=CCH20Me)] (35) to the cationic allenyl complex [Mo2(CO)4Cp2(m-i7% i -CH=C=CH2] (36) has been described by Curtis et al. (25). Protonation of 35 with HBF4 induced the loss of methanol and formation of the required complex. Alternatively the same complex was reported accessible via the acid-promoted elimination of methanol from [Mo2(CO)4Cp2 M-T7%Tj -MeO(H)C=C=CH2 ], a rare example of an intact T7%Tj -bound allene (Scheme 9). 

Dickson er al. have generated an allene from the metal-induced decarboxylation of a diketene (52) by [Rh2(CO)Cp2(M-V i7"-CF3C=CCF3)]. Subsequent C-H bond activation led to the vinyl-allenyl-bridged dimer [Rh2(CO)Cp2(M-T), T)--CF3C=CHCF3)(M-r). 7 2-HC=C=CH2)] (97) (52). The reactivity of this dimer is discussed further in Section VII,B,2. 

Two forms of the /i-T>, T> -bridging allenyl ligand have been identified. The most common form for both homo- and heterometallic dimers is that in which the internal double bond (C -Cjb) forms the interaction (type H). However, the previously unknown bonding type I in which the external double bond (C -Cy) is coordinated in an manner to the adjacent metal center has been established (38b). The bimetallic compounds [Ru2(CO)6(/A-PPh2)(M-i . i -PhC=C=CPh2)] and [Ru2(CO)6(/i-PPh2)(/i-T, Tj -PhC=C=CH2)] are of structural types H and I respectively, and provide an informative comparison. These types can be distinguished by the nomenclature fi-r), r) a,p and which specifies the internal... 

Allenyl ketones undergo cycloisomerization on exposure to (PhCN)2PdCl2. Dimeric products are produced if an a-substituent (at the aUenyl group) is absent from the substrates. ... 

We have demonstrated the first example of oxidative dimeric cyclization between two different classes of allenes, i.e., 2,3-allenoic acids and 1,2-allenyl ketones. The product is a dumb-bell-type bicyclic product 4-(3 -furanyl)-butenolide71 (Scheme 33) [24]. 

Intramolecular domino reactions proceed smoothly to afford cyclic compounds [2]. Domino 5-exo cyclization of the carbonate 22 generated the allenyl intermediate 23, and 3-exo cyclization of 23 gave the cyclopropane 24, which was converted to a dimeric product 25 [3]. 

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