Cyclization of alkynals

TABLE III. Alkynylpyrazoles Prepared by Cyclization of Alkynes with Diazo Compounds [68CB3700 68LA113 88JOM247 91ZOB2286]. 

Chiu et al. developed a catalytic reductive aldol cyclization of alkyne-diones such as 115 and 117 using [Ph3PCuH]6 (10mol%) as catalyst and polymethylhydrosiloxane PMHS (200 mol %) as terminal reductant. The... 

Scheme 54 Cyclization of alkynes with metal-carbyne complexes containing Cr or W.

Scheme 55 Cyclization of alkynes and alkenes via three-, five-, and seven-membered metallacycles containing Mo or W.

Arylative cyclization of alkynals with arylboronic acids is catalyzed by rhodium-diene complexes and even proceeds enantioselectively in the presence of a chiral diene (Equation (48)).399... 

Alkoxides that arise from simple carbonyl additions have also functioned as excellent in situ nucleophiles for intramolecular hydroalkoxylation reactions. Garbinols derived from the addition of allyltin reagents have proved to be potent nucleophiles in reactions of this type (Equation (99)),349 and this approach has also been used for the combined addition-cyclization of alkynals under Pd(n)350 or Cu(i)351 catalysis, and alkynones under Pd(n) catalysis.352... 

The reaction of nBu2ZrCp2 with 2 equivalents of PhC CPh provides the novel bicyclic gem-dizirconium complex 140 [236] (Scheme 7.42). Protonolysis of complex 140 with 3 n HC1 gives bibenzyl in 88% yield, while its deuterolysis with D20 provides tetradeuterio-bibenzyl 141 with 92 % deuterium incorporation. The dual path nature (142 versus 140) of the reaction of Cp2Zr with alkynes is an important factor in designing Zr-promoted cyclizations of alkynes, enynes, and diynes. 

Thermal cyclization of alkynes with Fe(CO)5 proceeds predominantly with CO incorporation to afford (cyclopentadienone)Fe(CO)3 complexes, however small amounts of cyclobutadiene complexes can be isolated (see Section VI.B.)15. 1,6-FIeptadiyne and 1,7-octadiyne substrates 107 have been utilized to prepare bicyclo[3.3.0] and bicyclo[4.3.0] complexes 108 in excellent yield (equation 12)115, while 1,8-nonadiynes gave bicyclo [5.3.0] complexes in low yield. 

Low-temperature photochemical cyclization of alkynes bearing a bulky substituent, mediated by CpCo(CO)2, proceeds with CO insertion to give cyclopentadienone complexes. Higher reaction temperatures lead to cyclotrimerization. The intramolecular variant of this reaction gives the bicyclic cyclopentadienones 139 and 139 (equation 19)142. Cyclization of unsymmetrically substituted diynes with the chiral R CpCo(CO)2 (R = 8-phenylmenthyl) leads to the formation of a mixture of diastereomers modest diastere-oselectivity was found. 

While one of the first preparations of a cyclobutadiene-metal complex involved the cyclodimerization of diphenylacetylene in the presence of Fe(CO)5 at high temperature212, the thermal reaction of alkynes with Fe(CO)s gives predominantly cyclopentadienone complexes (Section IV.E.l.b). The cyclization of alkynes by a wide variety of metal complexes has been reported (Scheme 59)l 5-21 A—222... 

Figure 3.46. Scope of Rh/48-catalyzed asymmetric arylative cyclization of alkynals with arylboronic acids.

In a recent report, Toste and Shen developed a gold(I)-catalyzed cyclization of alkynes using silyl ketene amides that, by means of prior hydrolysis, provided 1,6-enyne (285) or 1,5-enyne systems (287) activated for the intramolecular cycloisomerization [160]. 

The term oxidative cyclization is based on the fact that two-electron oxidation of the central metal occurs by the cyclization. The same reaction is sometimes called reductive cyclization . This term is based on alkene or alkyne bonds, because the alkene double bond in 13 is reduced to the alkane bond 14, and the alkyne 15 bond is reduced to the alkene bond 16 by the cyclization. Cyclizations of alkynes and alkenes catalyzed by transition metal complexes proceed by oxidative cyclization. In particular, low-valent complexes of early transition metals have a high tendency to obtain the highest possible oxidation state, and hence they react with alkynes and alkenes forming rather stable metallacycles by oxidative addition or oxidative cyclization. 

Unsaturated metallacycles have also had a fascinating history. Often these are readily formed by the coupling of alkyne units and may be involved in the catalytic cyclization of alkynes. There are numerous examples of structurally characterized metallacyclopentadienes, and even some examples of metal-lacycloheptatrienes and 4-benzene complexes (77). More recently metal-lacyclobutadienes have been implicated as the active intermediates in alkyne... 

The transition metal catalyzed or mediated cyclizations of alkynes are also classified into this category. The Ni complex catalyzed reaction of alkynes with... 

Among the classic examples is the coupling-cyclization of alkynes with Z-f)-bromo acrylic acid derivatives invented by Negishi and Kotora [110] as a sequential access to the y-alkylidcncbulcnolide 140 in excellent yield (Scheme 53). The constitutional isomeric a-pyranone 141 is only formed in 4% yield. 

Additionally, Arcadi and co-workers developed an electrochemical approach for the synthesis of 3,4-disubstituted-2(l//)-quinolones in moderate yields through an intramolecular cyclization of alkynes and malonyl moieties <07EJ02430>. 

Utimoto et al. [145] reported a procedure for the synthesis of macrocyclic ynones by intramolecular acylation of )-(trimethylsilyl)ethynylalkanoyl chlorides in the presence of Lewis acid. For example, in the synthesis of f — )-muscone 216), cyclization of alkyne acid chloride 238 gave the macrocycle 239 in 52% yield, which was hydrogenated to f — )-muscone (Scheme 80). 

At the same time, Reppe (158) discovered the catalytic properties of tetracarbonyinickel and its derivatives in the carbonylation reactions and in the cyclization of alkynes, and this gave a tremendous thrust forward to the experimental research on the chemistry of carbonylnickel derivatives. In the beginning the development of the chemistry of low oxidation states proceeded concomitantly with the chemistry of zerovalent... 

Polymerization and Cyclization of Alkynes (163). In 1940 Reppe and Schweckendieck 163) discovered that the substitution derivatives of tetracarbonylnickel with phosphines, and particularly those of the type Ni(CO)2(PR3)2, catalyze the linear polymerization and the cyclization to benzene derivatives of alkynes. Schrauzer 167) more recently showed that stoichiometric amounts of bisacrylonitrilenickel give the cyclization reaction. The reaction becomes, on the contrary, catalytic in the presence of a tertiary phosphine. Meriwether and co-workers (57, 136, 137, I40) studied the possible mechanism of Reppe s reaction. They showed that both the polymerization and the cyclization reactions are particularly effective with monosubstituted acetylenes, and that the cyclization reaction is infiuenced by steric and electronic effects. They then proposed a mechanism by which the induction time is ascribed to this rather slow reaction ... 

The cobalt-catalyzed co-cyclization of alkynes with heterofunctional substrates is not limited to nitriles. / -Cp-cobalt half-sandwich complexes are capable of co-oligomerizing alkynes with a number of C=C, C=N, C=0, or C=S bonds in a Diels-Alder-type reaction. Chen has observed that these cycloadditions are best performed in the presence of a small amount of ketones or esters [48]. This modified cycloaddition may be used for the formation of dihydroindole systems at the 7 -Cp-cobalt catalyst (eq. (17)). 

Cacchi, S. Heterocycles via cyclization of alkynes promoted by organopalladium complexes. J. Organomet. Chem. 1999, 576, 42-64. 

CoCp(CO)2 also is a versatile synthetic precursor, which reacts with a wide variety of organic compounds. It serves as a source of the reactive CoCp unit, and this species catalyzes cyclization of alkynes as well. This reaction has been applied to the one step synthesis of steroid analogs [22]. 

Thermal cyclization of alkynes with Fesmall amounts of cyclobutadiene complexes can be isolated (see Section 1,6-Heptadiyne and 1,7-... 

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