Cobalt-alkynyl complexes

Treatment of epoxy-alkyne derivatives 314 with dicobalthexacarbonyl [Co2(CO)6] and a catalytic quantity of Lewis acid, followed by oxidative demetallation of the intermediate cobalt complex 315 affords 2-alkynyl-3-acetoxy tetrahydropyrans 316 in excellent yield. Syn- or tf /7-tetrahydropyrans are obtained by selection of either the anti- or syn- starting epoxides 314 respectively (Scheme 79, Table 10) <1998T823, 2000JOC6761>. 

Pauson-Khand cyclization3k 143 of tV-allyl (l-alkynyl)carbene complexes 134 (M = Cr, W R = Ph, Et R1 = H, Me) affords cyclopentenone derivatives 136144 via cobalt complexes 135145 (Scheme 53), as well as chromium complexes.146 Cyclopentenones also have been derived from 7V-diallyl(l-alkynyl)carbene complexes.39 Stable cobalt complexes of type 135 are obtained from O-allyl (l-alkynyl)carbene complexes. Interestingly, the last-named compounds do not form a cyclopentenone on heating instead, they form an enyne by elimination of M(CO)6 in a retro-Fischer reaction. 147... 

Ethoxy(l-alkynyl)carbene complex la forms a stable cobalt complex 182, whose structure has been elucidated by X-ray analysis.1453 11 Cobalt complexes of similar type have been derived from 0-allyl-147 and iV-allyl(l-alkynyl)carbene complexes 135 (Scheme 53).145 In contrast to thermolysis of compounds 135 (Scheme 53), thermolysis of compound 182 affords isomers 183 (structure based on X-ray analysis) and 184 (structure based on spectroscopic evidence only) in 48% total yield (Scheme 76). 

Ethoxy(l-alkynyl)carbene complex, cobalt complex, 227-228... 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

The use of cobalt species to catalyze cross-coupling reactions of activated or unactivated alkyl halides with alkynyl Grignard reagents in order to form C(sp)-G(sp ) bonds is also of interest. Cobalt compleres promote the crossbenzyl chlorides and bromides react with 2-trimethylsilylethynyl- and 1-octyn-l-ylmagnesium halides to provide l-aryl-2-alkynes in the presence of a catalytic amount of a cobalt complex [100,101]. 

The oxidative addition step in this reaction is postulated to involve singleelectron transfer from a low-valent cobalt complex to the alkyl hahde with the intermediate generation of alkyl radicals. This procedure enables sequential radical cychzation-alkynylation reactions of 6-halo-1-hexene derivatives [102]. 

French workers have extended this reaction to develop a stereoselective synthesis of -ene-ynes (Descoins and Samain, 1976). Whereas cyclopropyl alkynyl carbinols gave mixtures of ene-ynes of variable isomeric composition under acidic conditions, the corresponding cobalt complexes yielded the E isomers almost exclusively after oxidative decomplexation. Yields for the three step sequence were good. The enhanced selectivity presumably is a result of the steric demand of the cobalt carbonyl moiety. The conjugated ene-ynes are useful intermediates in the preparation of insect pheromones, several of which contain the ,Z-diene unit. 

The chemistry of alkynylated cyclobutadiene(cyclopentadienyl) cobalt complexes has been reviewed recently. ... 

We envisioned that an alkynyl substituent on a cyclopropane could be complexed as a cobalt hexacarbonyl derivative [8], which would enable activation of the cyclopropane toward ring opening in order to undergo subsequent cycloadditions. This is explained by the ring-opening of a cobalt complexed cyclopropanediester I under the influence of a Lewis acid to form the Nicholas carbocation 2 as illustrated in Scheme 10.3. 

Finally, reference is made to the remarkably facile cyclization which alkynyl Fischer carbene complexes can undergo under conditions normally leading only to alkyne-cobalt complex formation (eq 66) (M = Cr or W R=Ph or Et). 

A hydrosilylation/cyclization process forming a vinylsilane product need not begin with a diyne, and other unsaturation has been examined in a similar reaction. Alkynyl olefins and dienes have been employed,97 and since unlike diynes, enyne substrates generally produce a chiral center, these substrates have recently proved amenable to asymmetric synthesis (Scheme 27). The BINAP-based catalyst employed in the diyne work did not function in enyne systems, but the close relative 6,6 -dimethylbiphenyl-2,2 -diyl-bis(diphenylphosphine) (BIPHEMP) afforded modest yields of enantio-enriched methylene cyclopentane products.104 Other reported catalysts for silylative cyclization include cationic palladium complexes.105 10511 A report has also appeared employing cobalt-rhodium nanoparticles for a similar reaction to produce racemic product.46... 

All the reactions described so far are carried out using stoichiometric amounts of Co2(CO)8. However, a coordinatively unsaturated 02(00)5 species would be liberated when the rearrangement proceeds, and if this cobalt species could form an alkyne-Co2(CO)6 complex with another molecule of 1-(1-alkynyl) cyclopropanol, the reaction should proceed with only a catalytic amount of 02(00)8 [13, 18]. In practice, when 1-phenylethynylcyclopropanol (11b) is treated with a 10 or 20 mol% amount of 02(00)8, the rearranged product 13b is obtained in 43 and 59 % yield, respectively. Thus, the reaction in fact proceeds with a catalytic amount of 02(00)8 however, the efficiency is low and a complex mixture of by-products is also obtained. As this low efficiency could be ascribed to instability of the unsaturated cobalt species, the catalytic reaction could be made more efficient by the addition of a stabilizing additive. 

The Ojima group has extended their studies of silylformylation to include more complex substrates, such as alkenyne, dialkyne, alkynyl nitrile, and ethynyl pyrrolidinone. Use of rhodium or rhodium-cobalt metal complexes catalyzes the silylformylation of these substrates with high chemoselectivity, as the other functionalities present are inert to the reaction.122b,c d... 

In contrast to the thermal reactions of 1-alkynylboronic esters, the cycloaddition smoothly takes place under very mild conditions in the presence of metal catalysts (Scheme 28). A cobalt(i) complex catalyzed the [4+2]-cycloaddition of alkynyl boronates with 1,3-dienes to give cycloalkenyl boronates 266431 and with ct,cu-diynes giving arylboronates... 

Nuclear magnetic resonance (NMR) amino(l-alkynyl) carbenes, 169 2-Amino-l-metalla-l-en-3-ynes, 195-197 cobalt-alkyne complexes, 82, 84, 95-96... 

X-ray crystallography amino(l-alkynyl) carbenes, 170 chalcogen-bridged metal-carbonyl complexes, 244-248, 250-253, 255-256, 258-264, 266-272, 274-279, 281, 283-284, 287, 289, 292-293, 295-310 cobalt-alkyne complexes, 76-77, 82-83, 89-90, 94-96... 

Reaction with an amine, AIBN, CO and a tetraalkyltin catalyst also leads to an amide.Benzylic and allylic halides were converted to carboxylic acids electroca-talytically, with CO and a cobalt imine complex. Vinylic halides were similarly converted with CO and nickel cyanide, under phase-transfer conditions.Allylic (9-phosphates were converted to allylic amides with CO and ClTi=NTMS, in the presence of a palladium catalyst. Terminal alkynes were converted to the alkynyl ester using CO, PdBr2, CuBr2 in methanol and sodium bicarbonate. ... 

Yamazaki s complex (Structure 5) contains two alkyne molecules linked together to form a five-membered metallacycle. Arene-solvated cobalt atoms, obtained by reacting cobalt vapor and arenes, have been used by Italian workers to promote the conversion of a,w-dialkynes and nitriles giving alkynyl-substituted pyridines [20]. -Tolueneiron(0) complexes have also been utilized for the co-cyclotrimerization of acetylene and alkyl cyanides or benzonitrile giving a-substituted pyridine derivatives. However, the catalytic transformation to the industrially important 2-vinylpyridine fails in this case acrylonitrile cannot be co-cyclotrimerized with acetylene at the iron catalyst [17]. 

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