Organonickel intermediates

Montgomery and co-workers [36, 42] have shown that organozincs can also couple with alkynes and aldehydes via organonickel intermediates 26 with high degrees of chemo- and stereoselectivities to afford allylic alcohols 27 (Scheme 8.9). Recently, they reported a two-step, four-component synthesis of cyclohexenol de-... 

This result may reflect a hampered P—H elimination in the organonickel intermediates. With other phosphanes or phosphites the yields of 4 and 5 range from 56 to 86 %. The data collected in Table 1 reveal that there is no simple correlation between product distribution and basicity or cone angle 601 of L. 

When phosphane-free nickel complexes, such as bis(cycloocta-l,5-diene)nickel(0) or te-tracarbonylnickel, are employed in the codimerization reaction of acrylic esters, the codimer arising from [2-1-1] addition to the electron-deficient double bond is the main product. The exo-isomer is the only product in these cyclopropanation reactions. This is opposite to the carbene and carbenoid addition reactions to alkenes catalyzed by copper complexes (see previous section) where the thermodynamically less favored e Jo-isomers are formed. This finding indicates that the reaction proceeds via organonickel intermediates rather than carbenoids or carbenes. The introduction of alkyl substituents in the /I-position of the electron-deficient alkenes favors isomerization and/or homo-cyclodimerization of the cyclopropenes. Thus, with methyl crotonate and 3,3-diphenylcyclopropene only 16% of the corresponding ethenylcyc-lopropane was obtained. Methyl 3,3-dimethylacrylate does not react at all with 3,3-dimethyl-cyclopropene, so that the methylester of tra 5-chrysanthemic acid cannot be prepared in this way. This reactivity pattern can be rationalized in terms of a different tendency of the alkenes to coordinate to nickel(O). This tendency decreases in the order un-, mono- < di-< tri- < tet-... 

ISOLATION OF ORGANONICKEL INTERMEDIATES FROM BIPHENYLENE AND (ET3P)4NI. 

Various evidence goes to support the operation of the mechanism outlined below. The strained o- bond of 1 undergoes oxidative addition to form the organonickel intermediate 6. Ligand isomerization followed by extrusion of the Ni(0) catalyst produces the isomeric diene 2 whereas insertion of the coordinated olefin into the Ni-C bond in 6 gives 8, which... 

Evidence for the organonickel intermediate is provided by the hydrolysis products obtained. The [Ni(tmc)] complex reacts with 1,5-dihaloalkane by a similar... 

We found that oxidative addition of aryl, benzyl, and allyl halides to metallic nickel proceeded smoothly to generate the corresponding organonickel intermediates under mild conditions, which afforded homocoupled products [11,41,45] and ketones by the reaction with acid chlorides [42] or alkyl oxalyl... 

Organonickel intermediates formed from nickel(II) acetylacetonate and trimethylaluminum or tetramethylaluminate behave like cuprate reagents and undergo conjugate addition to a,j8-unsaturated ketones (Ashby and Heinsohn, 1974). 

Nickel-catalyzed carbozincation can also proceed smoothly vith alkynes for the synthesis of substituted alkenes regio- and stereoselectively. The reaction may proceed via an intramolecular version leading to tri- or tetrasubstituted alkenes. Thus, treatment of ry-iodoalkyne with RjZn in the presence of 7.5 mol% Ni(acac)2 in THF and NMP yield the cyclopentane derivative in 62% yield [102,112]. Presumably, a reductive elimination step from the corresponding organonickel intermediate may occur. It is noteworthy that no cydization is observed when internal alkyne is used. 

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