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Palladium-catalyzed enyne cyclizations

In analogy to the mechanism of the palladium-catalyzed enyne cyclization, it is postulated that exposure of palladium(O) to acetic acid promotes in situ generation of hydridopalladium acetate LnPd"(H)(OAc). Alkyne hydrometallation affords the vinylpalladium complex A-10, which upon r-carbopalladation of the appendant alkyne provides intermediate B-7. Silane-mediated cleavage of carbon-palladium bond liberates the cyclized product along palladium(O), which reacts with acetic acid to regenerate hydridopalladium acetate to close the cycle (Scheme 33). [Pg.512]

Trost [10] discovered a palladium-catalyzed enyne metathesis during the course of his study on palladium-catalyzed enyne cyclization. Treatment of the 1,6-enyne 25 with palladacyclopentadiene (TCPT, 26a) in the presence of tri-o-tolyl-phophite and dimethyl acetylene dicarboxylate (DMAD) in dichloroethane at 60°C led to cycloadduct 27 and vinylcyclopentene 28 in 97% yield in a ratio of 1 to 1 (Eq. 10). The latter compound 28 is clearly the metathesis product. [Pg.148]

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. [Pg.149]

Scheme 6-17 Possible mechanisms for palladium-catalyzed enyne cyclizations. Scheme 6-17 Possible mechanisms for palladium-catalyzed enyne cyclizations.
Few other examples of such reaction sequences have been described to date. Oh has reported the palladium-catalyzed reductive cyclizations of 1,6-enynes in the presence of formic acid or triethylsilane via an alkylpalladium intermediate and its application to organic synthesis. Palladium complexes also catalyze the conversion of a range of enynes to cyclic 6,7-unsaturated carboxylic acids in the presence of CO.260... [Pg.333]

The palladium-catalyzed hydrostannylative cyclization of enynes is dealt with first, since mechanistically it is closely related to hydrometallation. Lautens262 reported the formation of homoallyl stannanes through the reaction of 1,6-enynes with tributyltin hydride in the presence of a catalytic amount of Pd(OAc)2.263 The active catalytic species is... [Pg.333]

Table 9 Palladium-catalyzed silastannative cyclization of enyne derived from diethyl malonate... Table 9 Palladium-catalyzed silastannative cyclization of enyne derived from diethyl malonate...
A similar reaction is catalyzed by palladium or copper complexes, using fluoroalkyl iodides as additives. In an intramolecular version, a 1,2-disubstituted heterocycle was obtained with low stereoselectivity34-33. Palladium-catalyzed carbohalogenative cyclization of enynes in the presence of copper(II) and lithium halides has been used in the stereoselective synthesis of halo-genated methylene butyrolactones52-33,... [Pg.520]

The concept is applied to hydroxy-directed cyclization of 1,6-enynes (Scheme 5.16) [16]. Palladium-catalyzed borylative cyclization of 10 affords homoallylic alcohol 11 with high stereoselectivity under the direction of the hydroxy group. [Pg.172]

After investigation of the palladium-catalyzed cascade cyclization/Suzuki coupling reactions of 1,6-enynes, Zhu and Zhang turned their attention to the analog 1,2,7-triene 333 for the synthesis of various functionalized five-membered cycles 334/335 [115] (Scheme 6.88). Oxidative addition of allyl hahides or carbonates to Pd(0) generates 7i-allylpalladium species 336, followed by intramolecular cycUzation to form vinylpalladium complex 337. Subsequent transmetallation with aryl boronic acids... [Pg.270]

Corkey BK, Toste FD (2007) Palladium-catalyzed enantioselective cyclization of silyloxy-1, 6-enynes. J Am Chem Soc 129 2764-2765... [Pg.339]

The stereocontrol and functional group tolerance exhibited by the palladium-catalyzed silane-mediated reductive enyne cyclization has led to its use as a key bond formation en route to structurally complex natural products. These include /3-necrodol,59 (—)-4a,5-dihydrostreptazolin,S9b ( )-laurene,S9c and, as illustrated by the conversion of 1,6-enyne 35a to furan 35b, ( )-phyllanthocin (Scheme 25).S9a... [Pg.506]

The first rhodium-catalyzed reductive cyclization of enynes was reported in I992.61,61a As demonstrated by the cyclization of 1,6-enyne 37a to vinylsilane 37b, the rhodium-catalyzed reaction is a hydrosilylative transformation and, hence, complements its palladium-catalyzed counterpart, which is a formal hydrogenative process mediated by silane. Following this seminal report, improved catalyst systems were developed enabling cyclization at progressively lower temperatures and shorter reaction times. For example, it was found that A-heterocyclic carbene complexes of rhodium catalyze the reaction at 40°C,62 and through the use of immobilized cobalt-rhodium bimetallic nanoparticle catalysts, the hydrosilylative cyclization proceeds at ambient temperature.6... [Pg.506]

The Alder-ene cyclization of allylic silyl ethers represents a clever use of cycloisomerization chemistry, as the enol ether products can be easily unmasked to yield aldehydes. Palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes containing an allylic oxygen most often gives rise to 1,3-dienes (see Section 10.12.4.1). However, enynes of type 63 underwent facile Alder-ene cyclization to the corresponding five- or six-membered rings (Equation (40)) using both [CpRu(MeCN)3]PF6 41 and the Cp analog ([Cp Ru(MeCN)3]PF6, 64).53... [Pg.573]

Stannaboration of diynes and enyne provide cycloalkanes incorporating both stannyl and boryl groups, which are otherwise difficult to synthesize. Palladium-catalyzed reactions of 1,4-, 1,5-, and 1,6-diynes with 9 give the corresponding cyclization products in good yields (Equation (99)).247 The reaction is so efficient that even a strained four-membered ring is readily formed. [Pg.768]

Wang s approach for the synthesis of enyne-allenes focused on ene-allenyl iodide 45 (Scheme 14.12) [24]. Palladium-catalyzed Sonogashira reaction of 45 with terminal alkynes 46 (R= Ph or CH2OH) proceeded smoothly under mild reaction conditions in the presence of the cocatalyst cuprous iodide and n-butylamine. The initially formed enyne-allene 47b with substituent R= CH2OH cyclized spontaneously to the corresponding a-methylstyrene derivative 48. [Pg.854]

Enamines (cf. 63JCE194, 82T1975,88MI1,08H(75)1849) play an important role in the syntheses under review, both as target substances (see Schemes 16 and 17) and as precursors (see Scheme 9 and following Schemes 19-21). Thus, noble-metal-catalyzed enyne and diene cyclizations have been described (Scheme 19) palladium-catalyzed cycloisomerization of... [Pg.77]

Yamamoto has proposed a mechanism for the palladium-catalyzed cyclization/hydrosilylation of enynes that accounts for the selective delivery of the silane to the more substituted C=C bond. Initial conversion of [(77 -C3H5)Pd(GOD)] [PF6] to a cationic palladium hydride species followed by complexation of the diyne could form the cationic diynylpalladium hydride intermediate Ib (Scheme 2). Hydrometallation of the less-substituted alkyne would form the palladium alkenyl alkyne complex Ilb that could undergo intramolecular carbometallation to form the palladium dienyl complex Illb. Silylative cleavage of the Pd-G bond, perhaps via cr-bond metathesis, would then release the silylated diene with regeneration of a palladium hydride species (Scheme 2). [Pg.370]

Lautens has developed an effective palladium-catalyzed protocol for the cyclization/stannylsilylation of 1,6-enynes to form bis(functionalized) alkylidenecyclopentanes. Eor example, reaction of dimethyl allylpropargylmalonate and trimethylsilyltributylstannane (1.2 equiv.) catalyzed by a 1 1 mixture of PdBr2(dmmdiy)... [Pg.402]

Table 9 Palladium-catalyzed cyclization/stannylsilylation of 1,6-enynes... Table 9 Palladium-catalyzed cyclization/stannylsilylation of 1,6-enynes...
The key step in this synthesis is the palladium-catalyzed domino- 1.6-enyne cyclization. which creates the bicyclic skeleton of the natural product in a single diastereoselective step. [Pg.121]

Enynals 104 can undergo a palladium-catalyzed cyclization to form the pyrylium intermediate 105, which upon deprotonation affords the triene 106. In the presence of DM AD, a [2+2]-cycloaddition then occurs to form the tricyclic 4//-pyran intermediate 107, which undergoes expansion of the cyclobutene ring to afford the bicyclic 4//-pyrans 108 (Scheme 34) <2004S1409>. [Pg.450]

Trost has discovered that Pd(OAc)2 and other Pd" compounds catalyze the cyclization of terminal 1,6-enynes to give ene-type adducts. Treatment of (117) with 5 mol % Pd(OAc)2 at 60-66 C gives metal-locyclopentene (118), which reacts further to give a 6 94 mixture of (119) and (120) in 80% yield. Flash vacuum pyrolysis of (117) at 625 C gives a 1(X) 0 mixture of (119) and (120) in 83% yield. The palladium reaction not only occurs under much milder conditions but gives different isomer ratios. In many other cases the palladium-catalyzed cyclization gives products which are not available from a thermal ene reaction. [Pg.17]

Trost and co-workers have made great strides in developing the palladium-catalyzed cycloisomerization of enynes into a powerful ring-forming method [39]. In most cases, the intimate details of these reactions are unknown. They are considered here, since a Heck cyclization is a potential step of one possible mechanistic sequence [40]. Two plausible mechanisms for palladium-catalyzed cycloisomerization of enynes are depicted in Scheme 6-17. In the Heck pathway (101 102 103104), hydropalladation of the alkyne is... [Pg.135]

Knierzinger, A., Grieder, A., and Schdnholzer, P, Palladium-catalyzed ene-type cyclizations of terpenoid 1,6-enynes, Helv. Chim. Acta, 74, 517, 1991. [Pg.41]


See other pages where Palladium-catalyzed enyne cyclizations is mentioned: [Pg.405]    [Pg.405]    [Pg.534]    [Pg.143]    [Pg.406]    [Pg.340]    [Pg.402]    [Pg.117]    [Pg.437]    [Pg.260]    [Pg.330]    [Pg.506]    [Pg.803]    [Pg.402]    [Pg.407]    [Pg.953]    [Pg.456]    [Pg.171]    [Pg.120]    [Pg.1290]    [Pg.156]    [Pg.71]   
See also in sourсe #XX -- [ Pg.246 ]

See also in sourсe #XX -- [ Pg.246 ]




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Catalyzed Cyclizations

Cyclization Palladium

Enyne cyclization

Enynes

Enynes cyclization

Palladium cyclizations

Palladium enynes

Palladium-Catalyzed Cyclizations

Palladium-catalyzed 1,6-enyne

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