A new synthetic methodology for adding carbon-based nucleophiles to the carbocyclic ring of quinolines has been developed, based on the electron-deficient bonding of the C(8) carbon and the protective coordination of the nitrogen atom to the metal core in the complexes Os 3 (CO) 9 (µ 3 -η 2 -C 9 H 5 (R)N)(µ-H), 1a-1h. These compounds react with a wide range of carbanions (e.g., R′Li) to give the nucleophilic addition products Os 3 (CO) 9 (µ 3 -η 3 -C 9 H 7 (5-R′)N)(µ-H), 2a-2l, and Os 3 (CO) 9 (µ 3 -η 3 -C 9 H 6 (3-, 4-, or 6-R)(5-R′)N)(µ-H), 3b-3g, after quenching with trifluoroacetic acid, in isolated yields of 25-86%. In the 6-substituted derivatives, this addition is stereoselective, forming only the cis-diastereomer. In the case the 6-chloro derivative, a second product is obtained, Os 3 (CO) 9 (µ 3 -η 2 -C 9 H 5 (6-Cl)(5-C(CH 3 ) 2 CN)N)(µ-H) 2 , 4, the result of protonation at the metal core and rearrangement of the carbocyclic ring. The trans-diastereomer of the 6-substituted derivatives can be obtained by quenching the intermediate anion of the unsubstituted complex with (CH 3 O) 2 SO 2 or acetic anhydride. Nucleophilic addition to the 5-chloro complex occurs across the 3,4-bond to give Os 3 (CO) 9 (µ 3η 2 -C 9 H 6 (5-Cl)(4-C(CH 3 ) 2 CN)N)(µ-H), 5. The addition products, types 2 and 3, can be rearomatized by reaction with diazobicyclononane (DBU)/dichlorodicyanoquinone (DDQ) or by reaction of the intermediate anion with trityl cation or DDQ. The resulting rearomatized complexes can be cleanly cleaved from the cluster by heating in acetonitrile under a CO atmosphere, yielding the functionalized quinoline and Os 3 (CO) 12 as the only two products. Solid structures of cis-3e, trans-3e, 4, and 5 are reported.

The reactivity of Os3(CO)9(μ3-η2-C13H8N)(μ-H) (1) is reported. The reaction of 1 with triphenyl phosphine gives Os3(CO)9(μ-η2-C13H8N)(μ-H)(PPh3) (2), in which the phosphine is bonded to the same osmium as the C(10) of the heterocyclic ring, isostructural with the quinoline analogue. Thermal or photochemical decarbonylation of 2 gives moderate yields of Os3(CO)8(μ3-η3-C13H8N)(μ-H)PPh3 (3), which has a σ−π-vinyl bonding mode with the C(9)−C(10) double bond and not the expected μ3-η2 electron-deficient bonding mode. The reactivity of 1 with hydride followed by protonation yields the electron-precise Os3(CO)9(μ3-η2-C13H9N)(μ-H)2 (4), and labeling experiments using D-/H+ indicate direct nucleophilic attack at C(9) by the deuteride followed by protonation at the metal core. The reaction of 1 with lithium isobutyrile nitrile followed by protonation gives Os3(CO)9(C13H9(4-(CH3)2CCN)N)(μ-H) (5), a result of nucleophilic addition across the 3,4 double bond. Reaction with n-butyllithium followed by protonation, on the other hand, gives three products, Os3(CO)9(μ3-η2-C13H8(9-C4H9)N)(μ-H)2 (6), the result of addition at the 9-position, Os3(CO)9(μ3-η2-C13H7(6-C4H9)N)(μ-H) (7), and Os3(CO)9(μ3-η2-C13H7(5-C4H9)N)(μ-H) (8), both the result of nucleophilic substitution for hydrogen. The solid-state structures of 3 and 8 are reported, and the mechanistic implications of these results for the synthetic methodology being developed for these electron-deficient clusters will be discussed.

The past three decades have witnessed an exponential increase in the structural diversity and applications of dendrimers, spanning across drug delivery and diagnostics, protein, and enzyme mimicry, solubility enhancement, coatings, light harvesting, and catalysis. The dendrimer community has recently focused on internally functionalized dendrimers (IFDs) owing to their advanced design and functionality. The synthesis of IFDs relies on advanced orthogonal chemistries and/or (de)protection schemes, as well as careful purification to minimize polydispersity of composition and molecular weight. The studies published on IFDs, however, lay scattered across the chemical literature, and a comprehensive presentation of structural rationale, synthetic procedures, and technologically relevant applications is missing. To address this need, this review presents a comprehensive collection and discussion of all available studies on IFDs, detailing their methods of synthesis and their structure-function correlations. The wide variety of internal functionalities, including hydroxyl, amine, carboxylic acid, allyl, alkyne, and imidazole groups, enables myriad applications in biochemistry, chemical and biomedical engineering, and material science. Particular focus is given to IFDs that are amenable to modular synthetic strategies, which promote higher synthetic yield and scalability, and therefore possess stronger translational and commercial potential. As such, this review guides research groups pursuing the difficult task of IFD rational design and synthesis providing them a concise roadmap to their mission.

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