Regioselective 1,4-hydroboration of pyridine derivatives and quinoline with pinacolborane is catalyzed efficiently by the heterobinuclear catalyst, (IPr)­CuFeCp­(CO)2, at only 2 mol % loading, providing access to valuable 1,4-dihydropyridine (1,4-DHP) products. A variety of reactive functional groups are tolerated in the pyridine 3-position, and sufficient catalytic activity was obtained for reduction of sterically hindered cases such as 3,5-disubstituted pyridines and even 4-substituted pyridines. Mechanistic experiments indicate that the superior catalytic activity and 1,4-regioselectivity of the Cu/Fe heterobinuclear catalyst, compared to the corresponding mononuclear Cu catalyst, is derived from biphilic cooperativity of two key catalytic intermediates: electrophilic CpFe­(CO)2(Bpin) activates the pyridine substrates toward regioselective nucleophilic addition by (IPr)­CuH.

The propeptides of bacterial subtilisin BPN' and Carlsberg were synthesized to investigate their inhibitory function on the enzymes. Kinetically, pro-BPN' inhibits the proteolytic activities of subtilisin BPN' and Carlsberg separately in a slow binding mode. Pro-Carlsberg behaves as a typical rapid equilibrium competitive inhibitor for these two proteases. Functionally, pro-Carlsberg inhibits the subtilisins with moderate selectivity. The inhibition constant Ki of pro-BPN' to subtilisin BPN' is 5.0 nM, and 6.1 nM to subtilisin Carlsberg. The on-rate of pro-BPN' to subtilisin BPN' is 5.8 x 10(5) M(-1)s(-1), and the off-rate 2.9 x 10(-3) s(-1). Similarly, the on-rate of pro-BPN' to subtilisin Carlsberg is 2.2 x 10(5) M(-1)s(-1), and the off-rate 1.3 x 10(-3) s(-1). On the other hand, the Ki of pro-Carlsberg to subtilisin BPN' gives 1.3 x 10(2) nM, and 88 nM to subtilisin Carlsberg. Based on the key features of the interactions between pro-BPN' and subtilisin from X-ray crystallographic results (Gallagher et al., 1995), the correlation between the sequence of subtilisin propeptides and their inhibition abilities on the proteases are compared and discussed.

The mechanism and heterodinuclear cooperative effects for AgRu-catalyzed alkyne semihydrogenation were analyzed with density-functional theory (DFT) and experiment. This combined effort revealed the following: (1) AgRucatalyzed diphenylacetylene hydrogenation initially gives a kinetic mixture of cis-stilbene and trans-stilbene by an ionic Ag−H hydride transfer transition state and post-transition state bifurcation, which was identified by quasiclassical direct dynamics simulations. (2) The hydrogenation reaction exhibits an unexpected inverse kinetic isotope effect (KIE < 1) resulting from an inverse equilibrium isotope effect (EIE) for heterodinuclear H 2 /D 2 activation. (3) The Ag−Ru heterodinuclear cooperative effective is critical for both H 2 activation and vinylsilver protonolysis reaction steps. ( 4) Rate studies and computational analysis show that electron-donating groups accelerate catalysis.

Although quadruple bonding in transition-metal chemistry has been considered a thoroughly studied area, [1a] the concept of multiple bonding [1b] was reinvigorated in 2005 by the seminal discovery of the first CrÀCr quintuple bond in the isolable dimeric chromium compound Ar'CrCrAr' (Ar' = 2,6-(2,6-iPr 2 C 6 H 3 -) 2 C 6 H 3 ) by Power and co-workers. [2] Since then, the structures of several Group 6 homobimetallic compounds with very short CrÀCr (1.73-1.75 ) and MoÀMo (2.02 ) quintuple bonds have been characterized. [3] All these remarkable quintuple-bonded bimetal units are supported by either C-or N-based bridging ligands. Based on their structures, these quintuple-bonded dinuclear compounds can be simply classified into two types as illustrated in Figure 1. The existence of the type I quintuple bond was recently corroborated by experiments, [4a] and the bonding paradigms of both types were realized by theoretical investigations. [4] Preliminary reactivity studies on the type I complexes show that they are reactive towards the activation of small molecules and display interesting complexation with olefins and alkynes. [5] Up to now, both type I and II compounds have been exclusively synthesized by a procedure analogous to the Wurtz reductive coupling reaction of the corresponding chloride coordinated precursors. [2, 3] The previously reported quintuple-bonded dichromium examples were obtained by alkali metal reduction of the mononuclear [LCrCl 2 -(THF) 2 ] [3c,e] or dimeric complexes [LCr(m-Cl)] 2[3] (L = monodentate or bidentate ligand). It should be noted that all these precursors lack Cr À Cr bonding. Besides, we have recently demonstrated that the metal-metal quintuple and quadruple bond can be constructed from the corresponding quadruple and triple bond, respectively. For example, the d bonds in the quintuple-bonded species [Mo 2 {m-h 2 -RC(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] (R = H, Ph) [3h] and quadruple-bonded complex [Mo 2 {m-h 2 -Me 2 Si(N-2,6-iPr 2 C 6 H 3 ) 2 } 2 ] [6] are formed by alkali metal reduction of the corresponding chloride-coordinated quadrupleand triple-bonded species, respectively. However, the formation mechanism of the metalÀmetal quintuple bonds has not been investigated. To this end, continuing our exploration in the field of quintuple-bond chemistry, we herein report the construction of a complex with a Cr À Cr quintuple bond by two subsequent one-electron-reduction steps from a halidefree homo-divalent dichromium complex to a mixed-valent intermediate (Cr I , Cr II ), and then to the final quintuplebonded product. Structural characterization of these dichromium compounds is important to shed light on the formation mechanism of the metal-metal quintuple bonds. Moreover, the metal À metal quadruple bonds can be dramatically elongated by intramolecular axial coordination, but such an interaction in the quintuple-bonding system has not been investigated. We report herein that the CrÀCr quintuple bond can be readily cleaved by disproportionation induced by intramolecular axial coo...

The synthesis of 1-pyrrolines from N-alkenylnitrones and alkynes has been explored as ar etrosynthetic alternative to traditional approaches.T hese cascade reactions are formal [4 + 1] cycloadditions that proceed through ap roposed dipolar cycloaddition and N-alkenylisoxazoline [3,3']sigmatropic rearrangement. Av ariety of cyclic alkynes and terminal alkynes have been shown to undergo the transformation with N-alkenylnitrones under mild conditions to provide the corresponding spirocyclic and densely substituted 1-pyrrolines with high regio-and diastereoselectivity.M echanistic studies provide insight into the balance of steric and electronic effects that promote the cascade process and control the diastereo-and regioisomeric preferences of the 1-pyrroline products.D iastereoselective derivatization of the 1-pyrrolines prepared by the cascade reaction demonstrate the divergent synthetic utility of the new method.Scheme 1. N-Alkenylisoxazoline approach to 1-pyrroline synthesis.LG = leaving group. EWG = electron-withdrawing group.