Alkene strained, complex

Copper(i) triflate, CuSOsF, is an efficient catalyst for the photodimerization of norbornene. The proposed mechanism of this reaction (Scheme 3), which is supported by quantum-yield studies, involves photoexcitation of a 2 1 alkene-CuSOsF complex followed by unimolecular collapse to products. An alternative mechanism involving photoexcitation of a 1 1 copper(i)-alkene complex and a subsequent termolecular interaction with two ground-state alkenes to give the dimer, a mechanism proposed for the photodimerization of norbomene-catalysed by copper(i) halides, is not consistent with the present studies. Photodimerizations catalysed by CuCl have only been observed with strained cycloalkenes. However, CuSOgF is an efficient catalyst for the photodimerization of simple alkenes e.g. cyclopentene. 

The resulting carbene complex 41b bears a hetero substituent and shows activity in the ring-opening/cross metathesis of strained bicyclic alkenes and... 

The phosphine-based platinum(O) catalysts do not catalyze the diboration of alkenes because of the high coordination ability of phosphine over the alkene double bond, but platinum(O) complexes without a phosphine ligand such as Pt(dba)2 [128] and Pt(cod)2 [129] are an excellent catalyst allowing the alkene insertion into the B-Pt bond under mild conditions (Scheme 1-30). The diboration of aliphatic and aromatic terminal alkenes takes place smoothly at 50°C or even at room temperature. The reaction is significantly slow for disubstituted alkenes and cyclic alkenes, but cyclic alkenes having an internal strain afford ds-diboration products in high... 

As invented by Wender,196,197 a variant of the second transformation can take place if the alkene partner is substituted by a participating group such as a strained cyclopropyl or a cyclobutanone (in the case of a 1,6-diene).198 The whole process, which mainly relies on the use of rhodium or ruthenium complexes,1 9 results in the formal... 

Diboration of alkene is catalyzed by Pt(0),42,48-51 Rh(i),52-57 Au(i),52 and Ag(i)58 complexes. Phosphine-free platinum complexes such as Pt(dba)2 and Pt(cod)2 are efficient catalysts for diboration of alkene, whereas those with phosphine ligands show much lower catalytic activities (Equations (3) and (4)).48,49 A PtCl2(cod) complex, which may be readily reduced to Pt(0) species with diboron, also catalyzes the addition of bis(catecholato)diboron to alkenes.42 Platinum-catalyzed diboration has so far been limited to terminal alkenes and strained cyclic alkenes. 

Addition of distannane to alkenes has been achieved only with strained cyclopropenes (Equation (60)).158 3,3-Disubstituted cyclopropenes undergo highly face-selective distannation in the presence of the palladium-isocyanide complex to afford m-adducts. 

In entries 10-13 (Table 21.8) of trisubstituted alkenes, very high diastereo-selectivity is realized by the use of a cationic rhodium catalyst under high hydrogen pressure, and the 1,3-syn- or 1,3-anti-configuration naturally corresponds to the ( )- or (Z)-geometry of the trisubstituted olefin unit [48, 49]. The facial selectivity is rationalized to be controlled by the A(l,3)-allylic strain at the intermediary complex stage (Scheme 21.2) [48]. 

The study of alkene insertions in complexes containing diphosphine ligands turned out to be more complicated than the study of the CO insertion reactions [13], When one attempts to carry out insertion reactions on acetylpalladium complexes decarbonylation takes place. When the reaction is carried out under a pressure of CO the observed rate of alkene insertion depends on the CO pressure due to the competition between CO and ethene coordination. Also, after insertion of the alkene into the acetyl species (3-elimination occurs, except for norbomene or norbomadiene as the alkene. In this instance, as was already reported by Sen [8,27] a syn addition takes place and in this strained skeleton no (3-elimination can take place. Therefore most studies on the alkene insertion and isolation of the intermediates concern the insertion of norbomenes [21,32], The main product observed for norbomene insertion into an acetyl palladium bond is the exo species (see Figure 12.8). 

Strain and steric properties of the alkenes determine the rate of insertion. The carbomethoxy complex (dppp)PdC(0)OCH3+ turned out to be less reactive than the corresponding acetyl-palladium (dppp)PdC(0)CH3+, which was ascribed to the higher nucleophilicity of the acetyl group as compared to the carbomethoxy group. 

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Better results were obtained for the carbamate of 163 (entry 3) [75, 80). Thus, deprotonation of the carbamate 163 with a lithium base, followed by complexation with copper iodide and treatment with one equivalent of an alkyllithium, provided exclusive y-alkylation. Double bond configuration was only partially maintained, however, giving 164 and 165 in a ratio of 89 11. The formation of both alkene isomers is explained in terms of two competing transition states 167 and 168 (Scheme 6.35). Minimization of allylic strain should to some extent favor transition state 167. Employing the enantiomerically enriched carbamate (R)-163 (82% ee) as the starting material, the proposed syn-attack of the organocopper nucleophile could then be as shown. Thus, after substitution and subsequent hydrogenation, R)-2-phenylpentane (169) was obtained in 64% ee [75]. 

The copolymerisation of CO and strained olefins such as norbornene and norbor-nadiene is effectively catalysed by Pd complexes with both phosphines and chelating diphosphines [43-49]. As previously mentioned, many model studies aimed at elucidating the elementary steps of alkene/CO copolymerisation have made use of cyclic alkenes in aprotic media because the products resulting from the insertion of... 

The insertions of CO into Pd-Me and of various strained alkenes into Pd-COR supported by the dinitrogen ligand Ar-BIAN have been studied by Vrieze and Elsevier by means of ex situ NMR spectroscopy [22, 51], For example, the sequential insertions of CO and norbomadiene into Pd-alkyl and Pd-acyl bonds allowed the detection and isolation of several intermediates such as acyl(carbonyl) and P-chelate (Scheme 7.24). The Pd complexes isolated after alkene insertion were found to have a structure arising from cis addition of Pd-C(0)R to the exo face of the olefin. 

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