Metallacyclic mechanism, cycloaddition

A combined system formed from Co(acac)3, 4 equiv of diethylalu-minum chloride, and chiral diphosphines such as (S,S)-CHIRAPHOS or (/ )-PROPHOS catalyzes homo-Diels-Alder reaction of norbomadiene and terminal acetylenes to give the adducts in reasonable ee (Scheme 109). Use of NORPHOS in the reaction of phenylacetylene affords the cycloadduct in 98.4% ee (268). It has been postulated that the structure of the active metal species involves noibomadiene, acetylene, and the chelating phosphine. The catalyzed cycloaddition may proceed by a metallacycle mechanism (269) rather than via simple [2+2 + 2] pericyclic transition state. 

The mechanism of [3 + 2] reductive cycloadditions clearly is more complex than other aldehyde/alkyne couplings since additional bonds are formed in the process. The catalytic reductive [3 + 2] cycloaddition process likely proceeds via the intermediacy of metallacycle 29, followed by enolate protonation to afford vinyl nickel species 30, alkenyl addition to the aldehyde to afford nickel alkoxide 31, and reduction of the Ni(II) alkoxide 31 back to the catalytically active Ni(0) species by Et3B (Scheme 23). In an intramolecular case, metallacycle 29 was isolated, fully characterized, and illustrated to undergo [3 + 2] reductive cycloaddition upon exposure to methanol [45]. Related pathways have recently been described involving cobalt-catalyzed reductive cyclo additions of enones and allenes [46], suggesting that this novel mechanism may be general for a variety of metals and substrate combinations. 

In order to gain more insight into this proposed mechanism, Montgomery and co-workers tried to isolate the intermediate metallacycle. This effort has also led to the development of a new [2 + 2 + 2]-reaction.226 It has been found that the presence of bipyridine (bpy) or tetramethylethylenediamine (TMEDA) makes the isolation of the desired metallacycles possible, and these metallacycles are characterized by X-ray analysis (Scheme 56).227 Besides important mechanistic implications for enyne isomerizations or intramolecular [4 + 2]-cycloadditions,228 the TMEDA-stabilized seven-membered nickel enolates 224 have been further trapped in aldol reactions, opening an access to complex polycyclic compounds and notably triquinanes. Thus, up to three rings can be generated in the intramolecular version of the reaction, for example, spirocycle 223 was obtained in 49% yield as a single diastereomer from dialdehyde 222 (Scheme 56).229... 

Whereas Fischer-type chromium carbenes react with alkenes, dienes, and alkynes to afford cyclopropanes, vinylcyclopropanes, and aromatic compounds, the iron Fischer-type carbene (47, e.g. R = Ph) reacts with alkenes and dienes to afford primarily coupled products (58) and (59) (Scheme 21). The mechanism proposed involves a [2 -F 2] cycloaddition of the alkene the carbene to form a metallacyclobutane see Metallacycle) (60). This intermediate undergoes jS-hydride elimination followed by reductive elimination to generate the coupled products. Carbenes (47) also react with alkynes under CO pressure (ca. 3.7 atm) to afford 6-ethoxy-o -pyrone complexes (61). The unstable metallacyclobutene (62) is produced by the reaction of (47) with 2-butyne in the absence of CO. Complex (62) decomposes to the pyrone complex (61). It has been suggested that the intermediate (62) is transformed into the vinylketene complex... 

In this general area quite efficient intramolecular examples have been described as well (Scheme 6). A thorough examination of the intermediates and likely mechanisms associated with this system has been published. In particular, it has been found that the type of metallacycle formed depends on the length of the chain separating the two alkyne units in the diyne. Efficient intramolecular cycloaddition to a bicy-clic metallacycle occurs only from 1,6- and 1,7-diynes. The poor results on attempted cyclization of 1,5-diynes to give highly strained bicyclo[3.2.0]hepta-l,4-dien-3-ones result from resistance towards closure of the strained four-membered ring upon metallacycle formation. ... 

Low-valent transition metal catalyzed versions of [2 + 2] cycloadditions. especially with nickel catalysts, were recognized early as useful alternatives to thermal and photochemical methods12-15. The observation of transition metal catalysis, active in [2 + 2]-cycloaddition reactions, originally caused considerable discussion of the mechanism as an inversion of symmetry rules, effected by the transition metal, may be assumed. Thus, it was suggested that, in the presence of the metal catalyst, a forbidden reaction becomes allowed 16,17. This interpretation, however, could not be verified for the overall process, since experimental investigations revealed a stepwise mechanism with metallacycle intermediates18-23. 

Although these novel Rh-catalyzed [2+2+2+1] cycloadditions of endiynes 525 with CO gave similar products to those from CO-SiCaT reactions, the mechanism of the reaction is totally different. The CO-SiCaT reaction is a stepwise process of carbocyclizations, whereas the Rh-catalyzed [2+2+2+1] cycloaddition proceeds via a series of metallacycles (Figure 2-20). ... 

In the proposed mechanism, the Rh catalyst selectively reacts with the diyne moiety of 525 to form metallacycle A. Alkene insertion to the Rh-C bond and CO coordination gives metallacycle B. From metallacycle B, CO insertion to the Rh-C bond gives metallacycle C or C , and subsequent reductive elimination affords cycloadduct 526. Reductive elimination of Rh from metallacycle B prior to CO insertion gives the [2+2+2] cycloadduct 527. Kaloko et al. forther expanded the scope of this novel [2+2+2+1] cycloaddition process to cyclohexene-diyne substrates 528, which gave the corresponding tetracyclic products 529 as single diastereomers in high yields (Scheme 2-78). 

In addition to the preceding processes that involve cycloadditions in direct analogy to Diels-Alder-type processes, several formal [4+2] cycloaddition processes have been described that proceed via completely different substrate classes and reaction pathways. In one example, a novel two-carbon ring expansion process was reported by Murakami, wherein the addition of cyclobutanones with alkynes provides cyclohexenones by two-carbon ring expansion of the starting material (Scheme 3-23). The mechanism of this process likely involves oxidative cyclization of die ketone and alkyne with Ni(0) to provide a five-membered metallacycle, followed by a ringexpanding p-carbon elimination as key steps of the process. 

More recent advances in intermolecular [3+2] reductive cycloadditions have involved combinations of enals or enoates with alkynes (Scheme 3-34).l 2 l The initially developed cycloadditions of enals and alkynes likely proceeds by initial formation of a metallacyclic enolate derivative, followed by enolate protonation and addition of the vinyl nickel unit to the resulting carbonyl to produce the boron alkoxide of the observed cyclopentenol product (Scheme 3-35). The analogous transformation with enoates may also proceed by this mechanism, depicted below by the sequence of initial generation of metallacycle 20, followed by enolate protonation to form 21 en route to product generation. Alternatively, the collapse of the metallacycle 20 to a ketene intermediate 22 may occur in the enoate variant. The precise pathway followed likely depends on whether protic or aprotic media are used. 

Unexpected exchange behavior was additionally observed in the P-hydrogen region of the NOESY-2D and ROESY-2D spectra of these metallacycles. For example, when 22c was reacted with 1-butene (30equiv) at —80°C, exchange cross-peaks were clearly visible between the P-protons of the trans and cis ruthenacycles. These results indicate that, not only is olefin rotation possible between degenerate cycloadditions/reversions at —80 C, but stereoisomerization is as well. A hypothetical mechanism for this trans-cis exchange (H tians H ds) is shown in Scheme 8.8, where stereochemical interconversion is depicted to... 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

The rate law does, of course, not decide on the exact pathway by which the olefin dimer is formed from the excited 1 2 complex. One possibility is a radical-radical-dimerization with intermediate formation of a five membered metallacycle that could form the product by reductive elimination. Such a sequence is not subject to the restrictions of a concerted electrocyclic mechanism and the final stereochemistry of the cycloaddition product would be largely determined by the favored stereochemical arrangement in the 1 2 complex. An isolated and structurally characterized intermediate which is cited for support is the Ir complex 5, shown in Scheme 4, formed from [(COD)IrCl]2 and NBD followed by metathesis with 2,5-pentane-dionate [17]. 

In this chapter we described [2 + 2 + 2] and related cycloaddition reactions using palladium, iron, manganese, rhenium, and other transition metals. Palladium complexes are able to catalyze [2 + 2 + 2] and related cycloaddition reactions, which proceed via cascade-type mechanism or metallacycle intermediates. It is worthy of note that arynes are suitable substrates for this palladium catalysis. Iron complexes are promising catalysts for practical [2 + 2 + 2] cycloaddition reactions, owing to their low cost and nontoxicity, although both catalytic activity and substrate scope are not satisfactory. Manganese and rhenium complexes allow the use of 3-keto esters as a cycloaddition partner. To realize the practical process and broaden the product scope, further development of new transition-metal catalysts is expected in this research field. 

Reduction of the bulky terphenyl substituted aluminum iodide Dipp All2 with KCg in the presence of toluene [27] or Me3SiCCSiMe3 [84] afforded novel Al-Al metallacycle species, possibly formed through [2 -i- 4] or [2 + 2] cycloaddition reactions between the dialuminene intermediate Dipp Al=AlDipp and an arene or alkyne source (Scheme 6). However, the exact mechanism remains to be studied and may well involve stepwise ionic or radical processes. For that matter, theoretical calculations suggested the existence of a partial diradical character in dialuminene species [85]. 

Relatively little is known about the mechanisms of the cycloaddition reactions. However, in some cycloaddition reactions metallacyclic intermediates are involved. As shown by reaction 7.5.1.1, the carbon-carbon bond formations in [2-i-2-i-2] cycloaddition probably proceed through the intermediacy of a metallacycle. 

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