Metallacycles hydrolysis

Scheme 7 comprises the following patterns First, a metallacycle gives rise to ketones by CO insertion and reductive elimination. Next, a nickel hydride inserts an unsaturated substrate L, followed by CO. The acyl intermediate can give rise to reductive elimination with formation of acyl halides or acids and esters by hydrolysis, or it can insert a new ligand with subsequent reductive elimination as before. Alternatively, there may be a new insertion of carbon monoxide with final hydrolysis. Third, an intermediate R—Ni—X is formed by oxidative addition. It can react in several ways It can insert a new ligand L, followed by CO to give an... 

In particular, hydrolysis of the y-chelate would give add end groups and Pd-H, while the protonation of the enolate, in equilibrium with the p metallacycle via hydride shift (vide infra) would give keto end groups and Pd-OH (Scheme 7.4) [13c]. 

The seven-membered metallacycles 89 (formed by ketone addition to [(butadiene)ZrCp2] cleanly add a nitrile molecule at elevated temperatures to yield the respective nine-membered metallacyclic products 96. Their hydrolysis then yields the 6-hydroxy-substituted non-conjugated unsaturated imines 97 under kinetic control. Within a few hours at room temperature these rearrange to the thermodynamically favored primary dienamine products 98.107 In this case the thermochemical diene conjugational energy makes the primary dienamines more stable than their conjugated imine tautomers (Scheme 32). 

The fact that very high molecular weight materials form under these aqueous conditions indicates that if a termination reaction involving the hydrolysis of the carbon-metal bonds is occuring in either the metallacycle or metal carbene intermediates, it has a much slower rate (by several orders of magnitude) than the rate of polymer propagation. 

The alkylpyridine ligands of these insertion products can be removed from Zr by normal hydrolysis or other electrophilic Zr—R bond cleavage reactions developed for Cp2Zr(R)Cl compounds (i). The five-membered metallacycles are quite resistant to y -H elimination. The P hydrogens cannot attain the correct orientation for transfer to Zr owing to the chelated structures, and, as for other Zr alkyls, y -H elimination is likely to be endothermic in any case 120). However, P-H elimination can be induced by ligands which open the chelate rings and can trap the cationic Zr... 

The synthesis of bis-aryloxo titanacyclopent-2-ene and titanacyclohept-3-ene (Scheme 93) derivatives has been reported via tricyclization of dienynes. The molecular structure of the titanacyclohept-3-ene has been determined by X-ray diffraction. It is suggested that the formation of the titanacyclohept-3-ene compound proceeds through an insertion of olefin into the Ti-vinyl bond of a titanacyclopent-2-ene intermediate. The metallacycles show interesting reactivity and synthetic usefulness and form novel organic molecules by hydrolysis and thermal catalysis.178... 

Hydrolysis of this metallacycle with aqueous HPF6 led to the formation of the cationic isonitrile complex [TpFe(CNTr)(CO)2]PF6 in 60% yield which is only obtained if the reaction is carried out under an atmosphere of carbon monoxide. 

Although the role of additive BeCl2 in the carboxylation of 45 is not clear, BeClj apparently promotes ring contraction to form an intermediate four-membered ring metallacycle. The same behavior has been mentioned before (see Scheme 6) in the contraction of nickelacyles 23. In fact, carboxylation of 23 in the presence of BeClj leads to the formation of malonic acids after acid hydrolysis of the six-membered ring nickel(II) dicarboxylate 49 (Scheme 15). ° ... 

The reaction of [W(MeCN)(PhCSCPh)3] with diphenylacetylene followed by hydrolysis in alkaline solution afforded [W( n4-C4Ph4)(ri4-C4Ph4H)( n2-MeC(0)NH)], (23), in a reaction which involved alkyne-alkyne coupling to give a metallacycle and hydration of the acetonitrile ligand. The compound was characterised by X-ray diffraction. 

The complex 21 was prepared from 22, X = Cl, Br on reaction with HCl/Brj followed by reaction with MeLi and hydrolysis." The zirconocene RCH2CH(-9BBN)ZrClCp2, prepared from Schwartz s reagent has been used in the synthesis of a-bromoboranes." The cycloaddition of cumulene and heterocumulene with vinylidenetitanocene proceeds with unexpected differences four and five membered metallacycles are formed." Nine complexes have been prepared from the reaction of acyl- and thioacylhydrazones with Cp2ZrCl2." ... 

Hydroboration of carbonyl compounds by pinacolborane is chemoselectively catalysed by titanocene bis(catecholborane) (A). Aryl aldehydes and ketones produce alkoxypinacolboronate esters (B) in moderate to high yields. The facile hydrolysis of B over silica affords alcohols in good yields. The catalytic hydroboration of electron-poor acetophenones is faster than that for electron-rich acetophenones. Computational studies with benzophenone and benzaldehyde indicated that hydroboration is spontaneous and probably proceeds via Ti metallacycle intermediates whose structures... 

The reaction of carbon dioxide with multiple bonded carbon derivatives proceeds in the presence of nickel (o) catalysts to give five-membered ring metallacycles, which on hydrolysis produce carboxylic acid derivatives. This derivatization of olefins and acetylene compounds is of considerable interest in synthetic organic chemistry. 

The reaction of alkynes with carbon dioxide in the presence of nickel (o) catalysts leads to the formation of metallacyclic 1 1 and 2 1 complexes. For example, the reaction of dimethylacetylene with carbon dioxide, in the presence of 1,5,9-cyclododecatrienenickel and Af,Af,A, Af-tetramethylenediamine, affords the five-membered ring metallacycle 20 in 65 % yield Hydrolysis of the metallacycle affords 2-methylcrotonic acid. 

The reaction of ethylene with carbon dioxide in the presence of 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU) and a nickel (o) catalyst affords the metallacycle 25, which on hydrolysis gives propionic acid in 85 % yield. Reaction of the metallacycle with another equivalent of ethylene affords a mixture of unsaturated C-5 carboxylic acids after hydrolysis in 79%yield 2 ... 

Zirconium complexes of dienes react with carbon dioxide to give metallacycles, which also undergo reaction with another equivalent of carbon dioxide to give dicarboxylic acids after hydrolysis" . Oxidative coupling of dienes with carbon dioxide is also observed using nickel (o) catalysts" . 

This is an example of the cycloaddition of carbon dioxide to an M=C bond giving rise to a stable [2+2] cycloadduct. Cyclopentenediisopropoxytitanium reacts with carbon dioxide to give the metallacycle 44 which on hydrolysis affords the cyclopentane carboxylic acid 45. ... 

Aromatic isocyanates react with regular olefins only in the presence of metal catalysts. For example, reaction of ethylene with phenyl isocyanate in the presence of liganded nickel (o) catalysts under argon in THF at -20 °C affords a five-membered ring metalla-cycle, which on hydrolysis gives a Af-substituted carboxylic acid amide. Heating of the metallacycle causes jS-elimination with formation of Af-substituted acrylic acid amides Diolefines and allenes also undergo this reaction with phenyl isocyanate. From 1,1-bis-p-dimethylaminophenylethylene and p-nitrophenyl isocyanate a linear 1 1 adduct is obtained... 

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