The road less traveled does indeed make all the difference. The reaction of oxyallylic cations with cyclopentadiene usually yields a [4+3] cycloadduct. Instead, a hydride shift can supersede this cycloadditon process and proceed in synthetically useful yields (see scheme). Computational analyses suggest that substantial electronic effects influence the “road” taken by the oxyallylic cation.

A s y m m e t r i c T r a n s f e r H y d r o g e n a t i o n o f K e t i m i n e s w i t h T r i c h l o r o s i l a n e Abstract: We report structural and mechanistic studies on the organocatalytic asymmetric transfer hydrogenation of ketimines with trichlorosilane. Amines were obtained in good yields and moderate enantioselectivities. Both experiment and computation were utilized to provide an improved understanding of the mechanism.The importance of chiral amines lies in their propensity to be fundamentally important building blocks of natural products, drugs, as well as agrochemicals. 1 The asymmetric reduction of imines with a chiral catalyst represents a direct and most appealing approach to enantiomerically enriched amines and the development of asymmetric catalysts for such processes has been the focus of recent research efforts. 2 However, in contrast to the well-developed, effective, and industrially relevant asymmetric reductions of olefins and ketones, the corresponding imine reductions are less advanced. 3 Among various catalytic methods, hydrogenation or transfer hydrogenation of ketimines with a chiral transition metal catalyst system is currently one of the most accessible protocols. 4 Parallel to the quest for more efficient metal catalysts, the discovery of 'green' catalysts has become another focal point. 5 As a consequence, impressive progress has been made recently in the development of organic catalysts for the enantioselective transfer hydrogenation of ketimines using Hantzsch dihydropyridine or trichlorosilane as the hydride donor in conjunction with a chiral organocatalyst that activates the substrate or the reductant (Scheme 1). 6-16 Scheme 1 Organocatalytic asymetric transfer hydrogenation of iminesOur studies revealed that electron-poor thiourea derivatives can activate imines by hydrogen bonding and thus promote the hydrogen transfer from Hantzsch esters to generate amines, but the substrate scope was limited to aliphatic and aromatic aldimines. 17 Unfortunately, chiral thiourea derivatives fail to promote the hydride transfer from Hantzsch esters to ketimines. As part of our efforts in the development of organocatalytic methods for the reduction of ketimines, we turned our attention to trichlorosilane as an alternative organic reducing agent. 18 Inspired by the reduction of aldehydes, ketones, as well as imines with the combination of trichlorosilane and N,Ndimethylformamide, the Matsumura group were the first to develop chiral N,N-dimethylformamide analogues, namely N-formylproline derivatives, as effective activators for asymmetric reductions of ketimines using trichlorosilane as the reducing agent, giving the corresponding amines in moderate enantiomeric excess. 9 Later, significant improvement of the enantiomeric excess was achieved by Malkov et al. by using a new N-methyl-L-valine derived Lewis basic organocatalyst. 12 Since then, considerable efforts have been devoted to the development of this transformation and a rapid evolution of catalyst structures has been made by Matsum...

We present an analysis of the well-known normal-and inverse-electron-demand hetero-Diels-Alder reaction involving 1,2,4-triazine, 1,3-oxazin-6-one, and 1,4-oxazin-2-one, with alkenes and alkynes, utilizing density functional theory (DFT) at the SCS-MP2/cc-pVDZ//B3LYP/6-31G(d) level to establish a theoretical status quo for synthetic practioners regarding the relative reactivities and stereochemical outcomes for these useful heterocycles. The results suggest that

International audienceWe report a synthetic route to ansa pyrrole amino acids via olefin ring-closing metathesis of diene precursors in the presence of Grubbs I catalyst. The dienes were prepared by Grignard addition to pyrrole sulfinyl imines. The success of the macrocyclic ring closure depends on the dienes structure and only in the case of the 13-membered compound 28 sufficient material could be isolated by preparative HPLC separation to investigate its structure spectroscopically. As also rationalized by our computations at the B3LYP/6-311+G(d.p)//B3LYP/6-31G(d) level of theory 28 is configurationally stable

In this paper we describe the reaction of vinylcarbene complexes 4 and 7 with enamines 5 and 9 to form the η1-acyl complexes 6, 8, and 10. From a mechanistic point of view, the reaction starts with the nucleophilic addition of the β-carbon atom of the enamine to the α-carbon atom of the vinylcarbene complex, generating the zwitterionic structure 12. Instead of the expected ring closure to a metallacyclobutane derivative and concomitant metathesis, intermediate 12 undergoes nucleophilic addition of a carbonyl carbon atom to the iminium carbon atom and rearrangement to the observed η1-acyl complexes. Furthermore the reaction of the η1-vinylidene complex [(CO)(NO)(Cp)WCCH2)] (1) with the enamines 5 and 9 is described. While enamine 5 preferably reacts as a nucleophile and produces finally the alkene complex 11, the reaction of enamine 9 with 1 begins by a proton transfer step and ends with the η1-acyl complex 10. Single-crystal X-ray diffraction data of 6 and 11 are reported.