Carbonyl complexes 1,3-dipolar addition

In addition to investigations this important class of compound may also be studied by means of MAS NMR spectroscopy. In fact we have shown that sharp resonances may be obtained from solid state MAS NMR of molecular hydrido carbonyl complexes without recourse either to the multiple pulse methods [22] or to isotopic dilution [23]. This is due to the fact that in these compounds the protons are intramolecularly diluted. We considered again H20s3(CO)io which crystallizes with one molecule as the asymmetric unit, as determined by a neutron diffraction study [24], and displays an interproton distance of 2.38 A. This interproton distance affords a dipolar coupling constant of 8.91 kHz. Now, rotation of the sample at 8.1 kHz allows... 

The stereoselectivity of an addition reaction is considerably lower when the reactions are conducted in polar solvents, complexing additives such as /V./V,A. A, -tetramethylethylenedi-arnine arc used, or when the stereogenic center carries a methoxy group instead of a hydroxy group. This behavior is explained as competition between the cyclic model and a dipolar model, proposed for carbonyl compounds bearing a polar substituent such as chlorine with a highly... 

The chemical behavior of heteroatom-substituted vinylcarbene complexes is similar to that of a,(3-unsaturated carbonyl compounds (Figure 2.17) [206]. It is possible to perform Michael additions [217,230], 1,4-addition of cuprates [151], additions of nucleophilic radicals [231], 1,3-dipolar cycloadditions [232,233], inter-[234-241] or intramolecular [220,242] Diels-Alder reactions, as well as Simmons-Smith- [243], sulfur ylide- [244] or diazomethane-mediated [151] cyclopropanati-ons of the vinylcarbene C-C double bond. The treatment of arylcarbene complexes with organolithium reagents ean lead via conjugate addition to substituted 1,4-cyclohexadien-6-ylidene complexes [245]. 

Simultaneously with the refinement of the EC structure, additional data on the conformation of taxol bound to MT came from ss-NMR REDOR measurements [84]. As introduced above, the REDOR experiment exploits the dipolar interaction between nuclear spins to determine distances between atoms closer than 12 A. A PTX analogue with suitable spin labeling for REDOR was prepared and complexed to MT. Labeling of the amide carbonyl and C3 methine with 13C, and position p- of the 2-O-benzoyl with 19F (Fig. 9, compound 3), allowed the measurement of the two 19F-13C distances I and II (Fig. 9), that could serve, in principle, to discriminate among candidates for PTX conformation. The respective values of 9.8 A 0.5 A and 10.3 A 0.5 A were interpreted by the authors as supportive of the hydrophobi-cally collapsed polar conformation for PTX [84]. 

Cycloaddition reactions, [3 + 2] or [2 + 2] depending on L, of TpRe(CO)(L) (f/ -furan) complexes have been reported. The electrophilic alkene tetracyano-ethylene (TCNE) and the alkyne dimethylacetylenedicarboxylate (DMAD) were used as dienophiles. For L = PMcs and BuNC, 1,3-dipolar cycloaddition of TCNE to / -furan gives [3 + 2] products via a carbonyl ylide (Scheme 47). Addition of... 

Pericyclic Reactions involving Metals.—Further data on the Diels-Alder addition of dienophiles to the cyclopentadiene ring in the complexes [Pt(Tj -C5H5XYXcod)] have appeared reaction occurs from the side remote from the platinum atom. Rate studies reveal that metal carbonyls accelerate the 1,3-dipolar cycloaddition of diazo-esters to norbornadiene. Cycloreversion occurs under extremely mild conditions, affording 3-ethoxycarbonylpyrazole, for example, from the reaction of... 

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