Olefins monosubstituted

The mild reaction conditions and the obviously high potential driving force of the ketene Claisen rearrangement recommended the use of the process for more complex systems. The first series of this type of reaction suffered from severe limitations. On the one hand, only electron-deficient ketenes added to the allylamines, and useful yields of the lactams had exclusively been achieved by employing dichloroketene [57, 58 a]. On the other hand, the rearrangement was restricted to either monosubstituted olefins in the amino fragment or the... 

Nitrones, reactive 1,3-dipoles, react with alkenes and alkynes to form isoxazolidines and isoxazolines, respectively. With monosubstituted olefinic dipolarophiles, 5-substituted isoxazolidines are generally formed predominantly however, with olefins bearing strongly electron-withdrawing groups, 4-substituted derivatives may also be formed.631... 

Regio- and stereoselectivity of the process depend on the nature of its participants and are determined by the character of the approach of the dipolarophile to the dipole. (In Scheme 3.127, this is demonstrated for the reaction of mono-substituted nitronates with monosubstituted olefins.)... 

For the overwhelming majority of nitronates, the reactions with monosubstituted olefins are characterized by the head-to-head approach of the olefin, that is, the substituent R is present at the C-5 atom. A general conclusion about stereoselectivity of this reaction (endo or exo approach of olefin to the dipole (Scheme 3.127)) cannot be drawn. However, the exo approach prevails for nitronates. (Possible factors responsible for discrimination of the facial approach will be discussed below in Section 3.4.3.5). 

Points b to d should be explained in more detail for intermolecular cycloaddition reactions of acyclic nitronates A with monosubstituted olefins. Regioselectivity of the process is determined by the character of the approach of olefin to the dipole (head-to-head or head-to-tail, (Chart 3.16, part (1)). In the former case, the substituent R is bound to the C-5 atom in the latter case, to the C-4 atom. 

The reactions shown in Chart 3.16 are characterized by a head-to-head addition of monosubstituted olefins to nitronates, the exo approach of olefins being observed in most cases. 

As in the above described study (337), the addition of monosubstituted olefins (250a,i-m) to nitronates (249) occurs strictly regioselectively in a head-to-head... 

Hirama and co-workers71 developed another chiral bidentate ligand 92 for OsCU-mediated dihydroxylation of /m .v-disubstituted and monosubstituted olefins. As shown in Table 4-14, asymmetric dihydroxylation of olefins using (S,S)-(—)-92b as the chiral ligand provides excellent yield and enantioselectivity. 

The next major contribution in asymmetric cyclopropanation was the introduction of chiral semicorrin ligands 184 by Fritschi et al.95 This ligand has been used for coordinating with copper and has been found to provide improved enantiocontrol in the cyclopropanation of monosubstituted olefins. Copper(I), coordinated by only one semicorrin ligand, is believed96 to be the catalytically active oxidation state. The copper(I) oxidation state can be reached directly... 

The correlation between bulky substituents and stereoselectivity is graphically shown in Figure 3, depicting the possible transition states in the dihydroxylation of a monosubstituted olefin by osmium tetroxide derivatives. This reaction is known to be selective [54], and the selectivity depends on whether the olefin substituent takes a position of type A or B in the transition state. The problem with calculations on a model system where the bulky base is replaced by NH3 is that the positions A and B are completely symmetrical, and thus, they yield the same energy. In other words, the reaction would not be selective with this model system. 

Intramolecular coupling reactions between nucleophilic olefins have also proven to hold potential as synthetically useful reactions. The first example of this type of reaction was reported by Shono and coworkers who examined the intramolecular coupling reaction of an enol acetate and a monosubstituted olefin (Scheme 41) [50]. This reaction was conducted in an effort to probe the nature of the radical cation intermediate generated from the anodic oxidation of... 

The addition of gem-disubstituted olefins, CH2=CXY, on polysilane 2 also worked well [23,24], For example, the addition of 2-methoxypropene and methylenecyclohexane afforded the expected adducts with 73% and 77% degrees of substitution, although a higher loss of molecular weight with respect to the hydrosilylation of monosubstituted olefins is observed. Copolymer 21, containing both mono- and disubstituted olefins, was made from 2 in a single reaction by adding 50 mol% vinyl acetic acid and an excess of 2-methoxypro-pene to the THF-polymer solution [24],... 

The reaction of 4-bromobenzocyclobutene 2 with monosubstituted olefins such as styrenes and alkyl substituted olefins is quite general and provides a... 

The C2-symmetric 2,6-bis(2-oxazolin-2-yl)pyridine (pybox) ligand was originally applied with Rh for enantioselective hydrosilylation of ketones [79], but Nishiyama, Itoh, and co-workers have used the chiral pybox ligands with Ru(II) as an effective cyclopropanation catalyst 31 [80]. The advantages in the use of this catalyst are the high enantiocontrol in product formation (>95 % ee) and the exceptional diastereocontrol for production of the trans-cyclopropane isomer (>92 8) in reactions of diazoacetates with monosubstituted olefins. Electronic influences from 4-substituents of pyridine in 31 affect relative reactivity (p = +1.53) and enantioselectivity, but not diastereoselectivity [81]. The disadvantage in the use of these catalysts, at least for synthetic purposes, is their sluggish reactivity. In fact, the stability of the intermediate metal carbene has allowed their isolation in two cases [82]. 

Three different principles of selectivity are required to achieve this result. First, the difference in rate of epoxidation by the catalyst of a disubstituted versus a monosubstituted olefin must be such that the propenyl group is epoxidized in complete preference to the vinyl group. The effect of this selectivity is to reduce the choice of olefinic faces to four of the two propenyl groups. Second, the inherent enantiofacial selectivity of the catalyst as represented in Figure 6A.1 will narrow the choice of propenyl faces from four to two. Finally, the steric factor responsible for kinetic resolution of 1-substituted allylic alcohols (Fig. 6A.2) will determine the final choice between the propenyl groups in the enantiomers of 80. The net result is the formation of epoxy alcohol 81 and enrichment of the unreacted allylic alcohol in the (35)-enantiomer. 

The enantiomeric excesses obtained to this point for the catalytic AD of monosubstituted olefins (seeTable6D.2 [16,26,29,31,40,44-46,49]) are lower than for frans-disubstituted olefins (Table 6D.3). The entries in Column 9 show enantiomeric purities ranging from 54% ee to 97% ee for dihydroxylations with the (DHQD)2-PHAL and (DHQ)2-PHAL pair of chiral ligands. Several monosubstituted olefins with branching at the a-position (e.g., entries 2-4 and 11) are dihy-droxylated with higher enantioselectivities when DHQD-PHN is used as the chiral ligand instead of (DHQD)2-PHAL. Recently, a new ligand for terminal olefins has been discovered [48b],... 

If steric interactions are the main factors in determining the different free energies for the four possible transition states, the substituent would occupy preferentially that quadrant in which more space is available. If the quadrant preferred by the substituent is known, i.e., if the relative positions of L, S, Z, and H are known, we can predict the enantiomer that is formed prevailingly. The same model also allows us to predict which of the two unsaturated carbon atoms of the substrate will be bound to the metal and carbonylated eventually. For instance, in the case of a monosubstituted olefin (see Scheme VII), Isomer b will prevail over the sum of the antipodes of the other isomer (a + c) and Antipode a will prevail over Antipode c. 

Furthermore, in the case of the asymmetric catalytic system containing rhodium and (—)-DIOP always the same, prochiral face (re) is preferentially formylated for six other monosubstituted olefins (Table 7, column 1). Similar results are obtained with rhodium catalysts when monophosphines are used instead of DIOP. The only... 

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