Imido compounds cycloadditions

Cycloadditions are among the more common reactions of imido and 0x0 groups in catalytic transformations. Both [2+2] cycloadditions and [3+2] cycloadditions are well established. The [2+2] cycloaddition chemistry occurs during hydroaminations of alkynes, allenes, and strained olefins catalyzed by imido compounds. - The [3+2] cycloaddition reactions appear to occur during dihydroxylation and aminohydroxylation of olefins catalyzed by osmium complexes. ... 

Oxo and imido compounds also react as nucleophiles with compounds containing polar C=X bonds, such as aldehydes, ketones, imines, and heterocumulenes. These reactions generally occur by a formal [2+2] cycloaddition process to generate a metala-cycle that breaks down to the more thermodynamically stable combination of products containing metal-ligand and carbon-heteroatom multiple bonds. Three examples of such reactions are shown in Equations 13.90-13.92. In the first two cases, the four-membered metallacycle was isolated or characterized by low-temp6rature NMR spectroscopy. ... 

Guiducci AE, Boyd CL, Clot E, Mountford P (2009) Reactions of cyclopentadienyl-amidinate titanium imido compounds with CO2 cycloaddition-extnision vs. cycloaddition-insertion. Dalton Trans 5960-5979... 

Tetraazabutadienes174 as such do not exist but complexes can be formed from reaction of organic azides, RN3, with compounds like Ni(COD)3, by interaction of halides with Li2[PhNN=NNPh], or by 1,3-dipolar cycloadditions with imido complexes.175 The compounds have been variously termed as tetraazadiene, tetra-azene, or tetraazenido. They normally have short and long bonds as in (9-XXXII) but there is doubtless electron delocalization over the ring system. 

Reaction of the Cp-amidinato imido titanium complexes Cp Ti[MeC(NPr1)2](NR) (R = But, 2,6-C6H3Me2) with C02 proceed via initial cycloaddition reactions to give N-O-carbamato complexes. The Buc derivative does not react with an excess of C02 and it undergoes a retrocyclization process with the formation of BuNCO and the binuclear fi-oxo compound Cp Ti[MeC(NPr1)2] 2(/i-0). The aryl derivatives exhibit a double CO2 insertion process to give Cp Ti[MeC(NPri)2][0(C0)N(Ar)(C0)0] (Scheme 248).661... 

This sequence was recently used in the synthesis of the alkaloid (+ )-aspidophytine (19). The key sequence of reactions was initiated by treatment of diazo ketoester 15 with Rh2(OAc)4 to generate a transient metallocarbene that reacted with the proximal imido carbonyl group to form dipole 16 (06OL3275). A subsequent 1,3-dipolar cycloaddition across the tethered indole n-bond gave cycloadduct 17 in 97% yield. Oxabicycle 17 was then converted into 18 by the action of BF3 OEt2 in 70% yield and this compound was eventually converted into ( )-aspidophytine (19). 

Half-sandwich terminal early transition metal imido amido complexes have come under considerable scrutiny in recent years, in part due to their participation in C-H bond activation [30] and cycloaddition reactions [31]. According to NMR evidence, many of them are involved in dynamic processes. Compounds of this sort are exemplified by Nb(V) and Ta(V) complexes [(Ti5-C5H5)NbX N(2,6-Me2C6H) ], where X = Cl, NH(2,6-Mc2C6H3), Me [32] and [(Ti5-C5Me5)TaNR2X N(2,6-Me2C6H) ], where R = Me, Pr X = Cl, Me [33]. 

The use of cumulenes has practical S5mthetic use only for the conversion of oxo into imido derivatives by the use of isocyanates. Formation of carbon dioxide provides the necessary driving force to the reaction. An example is the synthesis of compound 88, where the incompleteness of the second step limits the yield. ] a problem of this synthesis is the potential cycloaddition of the imido product with excess isocyanate, leading to met-alated ureas. 

Density functional theory (DPT) calculations were performed to elucidate the relative energies of the intermediates and products of this reaction. There are two reaction pathways that could generate the bis(imido) product 9 and the oxo-imido species 8 from U(N Bu)2(I)2(OPPh3)2 (Paths 1 and 2, Scheme 14.3). See color plate section. The first pathway involves the [2 - - 2] cycloaddition of the C = N bond of the aryl isocyanate to form an AQV-bound ureato intermediate (13), which can isomerize to form species 14 with the -NPh group trans to the terf-butyl imido moiety. Compound 14 can then eliminate BuNCO to generate the unsymmetrical bis(imido) complex 9 (top line in Scheme 14.2). Alternatively, AO-bound carbamate intermediates 15 and 16 could form that result from the [2 + 2] cycloaddition of the C = O bond of the aryl isocyanate across the U = N imido bond. Elimination of a substituted carbodiimide would generate the oxo-imido complex 8 (bottom line in Scheme 14.2). 

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