Reductive cyclizations, triethylsilane

The triethylsilane/Pd2(dba)3 combination is also used for these reductive cycli-zations, although lower yields are reported.247 1,6-Diynes are reductively cyclized to 1,2-dialkylidenecyclopentanes in good yields with Et3SiH/Pd2(dba)3 CHCl3 (Eq. 104).248... 

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... 

For example, a dienyl aldehyde reductively cyclizes in the presence of an Ni(0)/PPh3 complex and triethylsilane to give homoallylic cyclopropentanol with high regio- and stereoselectivities, while bishomoallylic cyclopropentanol is obtained as major product under the conditions using stoichiometric Ni(0)-diene complexes (Scheme 85). 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Another reaction is reductive cyclization. 1,6-Diynes and 1.6-enynes undergo reductive cyclization using hydrosilanes as a hydrogen source in AcOH. The 1,6-diynes 91 and 95 are converted into the 1,2-dialkylidenecyclopentane derivatives (1,3-dienes) 94 and 96. Triethylsilane is used as a hydrogen donor for the reaction[48]. The reaction involves the formation of a vinylpalladium bond in 92 via the insertion of an alkyne into the Pd—H bond, followed by the alkyne insertion to give 93, which is hydrogenolyzed with Si—H to give the 1,3-dienes 94 and 96. 

In their prominent synthesis of brevetoxin B, Nicolaou et al. have investigated, among some other methods for construction of oxepine ring system, the reductive cyclization of keto alcohol 37 with triethylsilane and trimethylsilyl triflate giving oxepane 38 (Equation 12) <1995JA1173>. 

Another reaction is reductive cyclization of 1,6-diynes and 1,6-enynes using hydrosilanes as a hydrogen source in AcOH. Triethylsilane is used as the hydrogen donor for this reaction [142], The reaction can be understood by the formation of vinylpalladium bond 350 via the insertion of alkyne 349 to the Pd—H bond in 328. Then intramolecular alkyne insertion gives vinylpalladium 351, which is hydro-genolysed with Si—H to give the 1,3-diene 352 by transmetallation and reductive... 

Reductive cyclization of 1,6-diynes.2 Cyclization of these substrates to 1,2-dialkylidenecyclopentanes can be effected with 1 and a phosphine as catalyst and triethylsilane (10-fold excess) in place of polymethylhydrosiloxane. 

Reductive cyclization of 1,6-diynes in the presence of triethylsilane and a catalyst prepared in situ from palladium(O) and acetic acid leads to dialkylidenecyclopentanes. Thus the diester 505 gave compound 506. ... 

Studies by Mori demonstrate that triethylsilane and dienals undergo reductive cyclization in the presence of bis(T] -cycloocta-l,5-diene)nickel(0) (2) and triphenylphosphine (1 2) to produce the silyl ether of cycloalkanols P l in this instance, y.b-misaturated products are obtained. However, if the reaction is carried out in the presence of cyclohexa-l,3-di-ene, an analogous reaction proceeds to give 8,e-unsaturated products. This effect is reported to be derived from selective diene hydrometalation followed by addition of the or-ganonickel intermediate to the tethered aldehyde. The reaction proceeds with five-, six-, and seven-membered ring formation and with heterocyclic substrates. Several synthetic applications of this cyclization methodology are reported (Scheme 8). Intermolecular processes with simple dienes and aldehydes to afford y.S-unsaturated silyl ethers are also possible. 

Similar to the results discussed for the silylcarbocyclizations of carbon-carbon multiple bonds, reductive cyclizations in the presence of carbonyl compounds are readily achieved. Crowe has developed a titanium-catalyzed procedure for the intramolecular reductive coupling of i5, -unsaturated carbonyl compounds in the presence of triethoxysilane (eq 18).The electronic advantage of triethoxysilane is demonstrated by the lack of reductive coupling in the presence of less reactive silanes, such as triethylsilane and diphenylsilane. With this method, Mori has utilized nickel(O) catalysts to generate five- and six-membered carbocycles and pyrrolidine derivatives. Furthermore, coordination of a chiral phosphine ligand to the nickel catalyst renders the reaction moderately enantioselective. ... 

The reductive cyclization of N-(w-iodoalkyl)succinimides induced by samarium(II) iodide was disclosed by Ha et al. as a novel method for making pyrrolizidines and indoUzidines (Scheme 46). " In the apphcation of the method to the synthesis of (+)-lentiginosine (127), reaction of N-(4-iodo-butyl)tartarimide (+)-335 with samarium(II) iodide in the presence of the iron(III)—tris(dibenzoylmethane) complex as catalyst produced the unsaturated indolizidin-3-one (+)-336 in 82% yield. Reduction of the bridgehead alkene was accomplished with triethylsilane and trifluoroacetic acid via an intermediate acyliminium ion, giving (+)-337 as the sole product in 93% yield. Routine hydrolysis of the silyl ethers produced the known diol (+)-177, after which reduction of the lactam with Hthium aluminum hydride then completed this short synthesis of (+)-127. 

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