Metal-catalyzed expansion

Metal-catalyzed expansions of 3- and 4-membered rings including heterocycles (2006—2012) 13ACC272. 

The subjects of structure and bonding in metal isocyanide complexes have been discussed before 90, 156) and will not be treated extensively here. A brief discussion of this subject is presented in Section II of course, special emphasis is given to the more recent information which has appeared. Several areas of current study in the field of transition metal-isocyanide complexes have become particularly important and are discussed in this review in Section III. These include the additions of protonic compounds to coordinated isocyanides, probably the subject most actively being studied at this time insertion reactions into metal-carbon bonded species nucleophilic reactions with metal isocyanide complexes and the metal-catalyzed a-addition reactions. Concurrent with these new developments, there has been a general expansion of descriptive chemistry of isocyanide-metal complexes, and further study of the physical properties of selected species. These developments are summarized in Section IV. 

Abstract Development in the field of transition metal-catalyzed carbonylation of epoxides is reviewed. The reaction is an efficient method to synthesize a wide range of / -hydroxy carbonyl compounds such as small synthetic synthons and polymeric materials. The reaction modes featured in this chapter are ring-expansion carbonylation, alternating copolymerization, formylation, alkoxycarbonylation, and aminocarbonylation. 

Sinfelt et al. (120) observed a twofold increase in the -heptane aromatiza-tion rate when the platinum content of their alumina-supported catalyst increased from 0.10 to 0.60%. At the same time, the rate of methylcyclo-pentane ring expansion remained constant. This result also serves as evidence for metal-catalyzed aromatization over dual-function catalysts without the participation of any Cg cyclic intermediate. The cyclization activity of platinum itself was independent of the nature of the support (109). Pure acidic cyclization prevailed with olefin feed (30, 109). 

The [L]-control maps can be used not only for the first analysis of the mechanism (minimum number of intermediate complexes, their product-determining manifold l nd association processes and their coupling) of homogeneous metal-catalyzed reactions but also for the expansion of catalytic systems to four-, five- or even six-component systems. The role of the new component can in many cases be easily deduced from the chaises of the pattern of the corresponding [L][Pg.87]

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Silirenes (140, equation 32) could also be involved in the transition-metal catalyzed decomposition of bis(diazoketones) 139 which provides the electron-rich [4]radialenes 14266,67. While the formation of 142 directly from silirene 140 cannot be excluded a priori, it is more reasonable to assume that 140 undergoes twofold ring-expansion to form the cyclic cumulene 141, which then provides 142 by a cyclodimerization reaction. The intermediacy of 141 is corroborated by the isolation of the Diels-Alder product 14366. 

Alper reaction. The Alper reaction corresponds to the ring expansion of azir-idines by metal-catalyzed CO insertion (Scheme 3). 

One of the first transition metal-catalyzed ring-expansion reactions of SCBs with the formation of new C-C bonds involved the insertion of acetylenes catalyzed by Pd-complexes to furnish silacyclohexenes (Scheme 46) <1975CL891, 1991BCJ1461>. In addition to the acetylene-insertion products (silacyclohexenes), ring-opened allyl-vinylsilane products that also incorporate the acetylene moieties were observed. The ratio of the two types of the products depends heavily on the nature of acetylenic compounds. 

The situation is much different when an acidic support is used. First, the C8 aromatic products have a distribution that approaches an equilibrium composition. The Pt catalyst on an acidic support is both more active and produces aromatics more selectively than Pt on a nonacidic support (84). It is concluded that the bifunctional mechanism involving cyclization by the acid site followed by a bifunctional ring expansion/dehydrogenation reactions is much more rapid than the monofunctional metal catalyzed dehydrocyclization reaction. For the catalyst based on an acidic support, the tin added initially acts as a catalyst poison (Figure 5), at least during the initial 1-2 weeks of usage. 

The following references contain additional examples of this method and corresponding reaction types Synthesis of 3-fluoroethyl-2-cyclopenten-l-ones [35], preparation of functionalized bicyclo[5.3.0]decane systems and conversion of 1,2-divinylcyclopropanes to functionalized cycloheptanes [36] e.g. karaha-naenone [37], ( )-/Thimachalene [38][39]. Metal-catalyzed ring expansions in which cyclopropane (e.g. [40] [41] [42] [43] [44]) or aziridines (e.g. [45] [46]) and diaziridines (e.g. [45]) function as essential moieties are well known reactions. 

Ring expansion reactions of the type shown in equations (17) and (18), in which a more equitable distribution of carbon atoms between two rings of a polycyclic hydrocarbon is produced, provide the link between the decyclization reactions above and the annulation reactions considered later. It would seem that the greater the strain in the reactant, the larger the number of potential catalysts. The complexes of seven different metals catalyze the reaction shown in equation (17). Of the possible catalysts, only the silver (I) salts bring about any byproduct formation. The isomerization of [l.l.ljpropellane (equation 18) is catalyzed by [RhCl(PPh3)3]. ... 

Later, /J-lactams have been obtained by regiospecific metal-catalyzed ring expansion of aziridines. Treatment of A -t ri-butyl-2-phenylaziridine (252) with carbon monoxide in benzene, employing chlorodicarbonylrhodium(I) dimer as the catalyst at 90°C and 20 atm, produced the azetidinone 253 in quantitative yield. Several other )S-lactams were also synthesized from the corresponding aziridine derivatives by the application of this method, and it was noted that this reaction was completely regiospecific. The possible reaction mechanism for the formation of fi-btciams is outlined in Scheme 47. 

Combined with the use of precious metal catalyzed washcoats deposited on the walls, microchannel reactors can realize nearly 10 times reduction in reactor size compared with that of a process that utilizes catalyst particles. The washcoat thickness is usually less than lOOjit and provides greater structural stability. This stability arises from smaller thermal expansion ratios and lower temperature gradients. 

A detailed description of the numerous examples of vinylcyclopropropanes used in transition metal mediated organic synthesis is far beyond the scope of this section and can be found in several reviews. Prominent examples are conversions to open-chain products, as well as formation of four-, five-, six- and seven-membered rings via ring expansion or incorporation of other substrates such as carbon monoxide, alkenes or alkynes. Thus divinylcyclopropanes, obtainable via transition metal catalyzed cyclopropanation reactions, undergo a facile thermal Cope rearrangement to form cycloheptadienes. ... 

The metal catalyzed ring opening of epoxides followed by reaction with CO or COj to form P-lactones and carbonates is a useful reaction that continues to attract attention. In an expansion of previous work, Coates and co-workers have developed an improved catalyst, 99, for the carbonylation of epoxides <05JA11426>. These reaction conditions are compatible with a wide variety of side chains, including those bearing Lewis basic functionality. Interestingly, the cyclopentene oxide 97 was readily converted to the P-lactone 98 in excellent yield. 

The transition-metal-catalyzed carbonylation reaction has been extensively investigated, and especially the carbonylative ring expansion reaction of strained heterocycles has been shown to be a useful and efficient procedure to synthesize lactams, lactones, and thiolactones.203 The carbonylation of epoxides and aziridines 450 is a powerful tool to construct the /Mactone and /Mactam skeletons 451 (Scheme 142).204 This type of reactions can be regarded as a hetero-[3 + 1]-cycloaddition. 

Many metal-catalyzed reactions of propargylic esters proceed as if alkylidene-metal carbenoids are involved. In the Pd-catalyzed reaction such species add to norbomenes/ norbomadienes to give ring expansion products. 

Aryl-3,4-dihydro-2//-l,3,4-oxadiazin-2-ones (196) have been prepared for the first time by metal-catalyzed decomposition of A-acyl-A -(a-diazoacyl)hydrazines (192) <88CB887>. A mechanistic rationale involving a carbene intermediate (193) followed by a rearrangement to, and then ring expansion of, oxiranopyrazole intermediates, (194) and (195) is proposed (Scheme 24). 

The resulting expansiveness of this field prevents a comprehensive review. It is our intention, therefore, to highlight some of the most important and recent developments in the chemistry of metal-alkyne complexes. We will largely limit our coverage to that chemistry which clearly involves the intervention of metal- i-bonded alkyne complexes. We thus exclude the chemistry of metal-acetylide derivatives and mention only briefly the burgeoning number of metal-catalyzed reactions for which alkyne complexes are only presumed intermediates. Prior reviews of metal-alkyne chemistry may be consulted for more complete coverage of the older literature [3]. 

Transition metal-catalyzed carbenoid insertion into the k system of aryl compounds followed by ring expansion of the resultant [4.1.0]bicyclohepta-2,4-diene provides the... 

Transition metal catalyzed ring expansions of cyclic ethers to lactones under pressures of CO [51, 52] have been reported for tetrahydrofuran [53], oxetanes, and epoxides [54—56]. Carbonylation of epoxides is particularly important since P-lactone products are challenging synthetic targets (see Section 2.2.5). Using Co(CO)4 in combination with a Lewis acidic Al-salen counterion, the reaction of (R)-propylene oxide and CO occurs with stereochemical retention (Scheme 2.23) [57]. The mechanism is believed to involve Lewis acid activation of the epoxide followed by nucleophilic ring opening with Co(CO)4 [58]. 

One of the key technologies needed to make cyclic conjugated diene polymers useful is an expansion of monomer availability. Presently, neither 1,3-cycloheptadiene nor 1,3-cyclooctadiene has been coordinatively polymerized, even with highly active cationic Ni complexes. The polymerization of functionalized CHDs is, so far, limited to ANiTFA. In order to provide processability and functionality to cyclic conjugated diene polymers, these problems must be overcome. The progress of transition metal-catalyzed polymerization may make this possible in the near future. 

The ring expansion of oxyglycal-derived gm-dihalo 1,2-cyclopropanate oxyglycals provides stable seven-membered halo-oxepines, that are thus suitable precursors for further functionalizations. In a study, vinyl bromide of the bromo-oxepine was used for the metal-catalyzed cross-coupling reactions, namely, Heck, Suzuki, and Sonogashira reactions, so as to generate 2-deoxy-2-C-alkyl/ary/alkynyl septanosides [33]. 

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