Tetrasubstituted olefin

With regard to the structure of the olefins, tetrasubstituted olefins do not undergo hydroformylation reaction under typical reaction conditions, and olefinic substrates containing functional groups sometimes give poor yields and unexpected products. If there is no plane of symmetry in the substrate across the double bond, at least two isomeric aldehydes are obtained. Although methods for shifting the... 

A major difficulty with the Diels-Alder reaction is its sensitivity to sterical hindrance. Tri- and tetrasubstituted olefins or dienes with bulky substituents at the terminal carbons react only very slowly. Therefore bicyclic compounds with polar reactions are more suitable for such target molecules, e.g. steroids. There exist, however, several exceptions, e. g. a reaction of a tetrasubstituted alkene with a 1,1-disubstituted diene to produce a cyclohexene intermediate containing three contiguous quaternary carbon atoms (S. Danishefsky, 1979). This reaction was assisted by large polarity differences between the electron rich diene and the electron deficient ene component. 

However, when the bulky substituent is no longer present at the electrophilic carbon atom, the addition of the olefin to the morpholine enamine of cyclohexanone leads largely to the tetrasubstituted isomer. For instance the reaction of this enamine with phenyl vinyl sulfone gave a 1 3 mixture of... 

The preferred formation of the tetrasubstituted isomer with the olefin without any bulky substituent at the electrophilic carbon atom is undoubtedly due to the preponderance of that conformation of the dipolar intermediate in which the substituent is syn to the morpholine group, as shown in (41). The situation is, however, reversed in case of the olefin with... 

The alkylation of enamines with nitroolefins, which gives intermediates for reductive cyclization (6S2), also provided an example of a stable cycliza-tion product derived from attack of the intermediate imonium function by the nitro anion (683). A previously claimed tetrasubstituted enamine, which was obtained from addition of a vinylsulfone to morpholinocyclohexene (314), was shown to be the corresponding cyclobutane (684). Perfluoro-olefins also gave alkylation products with enamines (685). Reactions of enamines with diazodicarboxylate (683,686) have been used diagnostically for 6-substituted cyclohexenamines. In a reaction of 2-penten-4-one with a substituted vinylogous amide, stereochemical direction was seen to depend on solvent polarity (687). 

This model prediets that tri-substituted and tetra-substituted olefins would also be poor substrates. Thus it was not until 1994 that a study in the epoxidation of higher substituted olefins appeared. Indeed Jaeobsen revealed that tri-substituted olefins, and even tetra-substituted olefins ean be excellent substratesA new model was put forth that encompasses a skewed side-on approach of tri-substituted olefins to the Mn-oxo eomplex. The observation that certain tetrasubstituted olefins undergo epoxidation with good enantioseleetivity suggests that further studies are needed in order to fully understand the transition state geometry of the catalyst and substrate. 

A common consequence of migration in complex molecules is that tetrasubstituted olefins result, which can be hydrogenated only with difficulty, if at all. It is easier to try to prevent hindered olefin formation than it is to correct it. Attempted hydrogenation of the exocyclic methylene group in 15 proved difficult when using an aged 10% Pd-on-C catalyst there was a... 

Ten years after Sharpless s discovery of the asymmetric epoxidation of allylic alcohols, Jacobsen and Katsuki independently reported asymmetric epoxidations of unfunctionalized olefins by use of chiral Mn-salen catalysts such as 9 (Scheme 9.3) [14, 15]. The reaction works best on (Z)-disubstituted alkenes, although several tri-and tetrasubstituted olefins have been successfully epoxidized [16]. The reaction often requires ligand optimization for each substrate for high enantioselectivity to be achieved. 

The MT0/H202/pyridine system enjoys a broad substrate scope and has become the method of choice for the epoxidation of di-, tri-, and tetrasubstituted olefins. As an added benefit, it gives high diastereoselectivities for a number of cyclic dienes (Table 12.1). 

Normal tetrasubstituted olefins react with Br2 too fast to allow the observation of transient CTC s. We therefore chose an encumbered olefin,... 

In similarly substituted olefins Kf is strongly influenced by steric effects, as shown by the comparison of tetraisobutylethylene with adamantylideneadamantane and (i,/-D3-trishomocubylidene-D3-trishomocubane. In particular, the comparison between cyclohexene and the two tetrasubstituted cage olefins indicates that Kf increases at least by a factor of 103 on passing from a 1,2 disubstituted to a tetrasubstituted olefin. This dependence is likely to be similar in other solvents, because solvent effects on Kf are modest. 

In 2001, Furstner reported the preparation and characterisation of the NHC-Ru complex 22 containing iV,iV -bis[2,6-(diisopropyl)phenyl]imidazolidin-2-ylidene (SIPr) [29] (Fig. 3.6), which is the congener of complex 20. Subsequently, Mol and co-workers revealed that complex 22 was a highly active metathesis initiator [30]. More recent comparative studies showed that catalyst 22 could catalyse the RCM of 1 faster than any other NHC-Ru catalyst, while it was not stable enough to obtain complete conversion in the RCM of 3 and was inefficient for the construction of the tetrasubstituted double bond of cyclic olefin 6 [31]. 

In addition, Blechert and co-workers reported on the highly effective RCM forming tetrasubstituted olefins (such as 6 and 10) in C F by using pre-catalyst 23a prepared via a different procedure [36]. 

The isolation of the -isomer 139 was in fact unexpected, since all tetrasubstituted olefins previously obtained from thiirene dioxide have been assigned the cis-configuration with respect to the two phenyl substituents based on the principle of least motion during the ring opening to olefins ". It might well be, therefore, that the -isomer is obtained through the isomerization of the initially formed Z-isomer. 

The ease with which nitrones undergo cycloadditions to a tetrasubstituted olefin like 3 must be related to the high energy of the HOMO of the dipolarophile (see Sect. 2.1.1). This factor renders comparable the reactivity of BCP (3) and MCP (1), which has a lower lying HOMO, despite the less steric hindrance of 1. 

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

Apparently similar Rh catalysts appear to have Rh(I) resting states of type [Rh(PPh3)2L2]+, which possibly accounts for their very different properties, for example their inability to reduce tri- and tetrasubstituted olefins. 

Catalyst 35b was used by Buchwald and coworkers with [PhMe2NH]+-[(BC6F5)4] to hydrogenate tetrasubstituted unfunctionalized cyclic olefins with... 

Tetrasubstituted alkenes are challenging substrates for enantioselective hydrogenation because of their inherently low reactivity. Crabtree showed that it was possible to hydrogenate unfunctionalized tetrasubstituted alkenes with iridium catalysts [46]. Among the iridium catalysts described in the previous section, several were found to be sufficiently reactive to achieve full conversion with al-kene 77 (Table 30.14). However, the enantioselectivities were significantly lower than with trisubstituted olefins, and higher catalyst loadings were necessary. 

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