1.2- disubstituted olefins

Geminal disubstituted double bonds are inert to [W]2 [61]. However, the reaction of a diene possessing both a geminal disubstituted olefin and a monosubstituted olefin (2-methyl-1,5-hexadiene) with [W]2 yielded exclusively the dimer resulting from the metathesis of the monosubstituted double bond. The reaction with [Ru] 1 produced the same result [53]. 

Schrock s molybdenum catalyst [Mo] 2, however, produced a low molecular weight polymer when exposed to 2-methyl-l,5-hexadiene [61]. Interestingly, it was shown that the monomer initially dimerized to 2,9-dimethyl-l,5,9-decatriene through metathesis of the unsubstituted vinyl group. Over time, however, the substituted vinyl groups underwent CM with the internal olefins to produce 1,4-polyisoprene. This was the first example of the condensation of 

1- disubstituted double bonds via ADMET polymerization. Unlike ROMP using Ph2C=W(CO)5 [62], ADMET produced only pure 1,4-polyisoprene, with no disubstituted or tetra-substituted double bonds detected. 

In ADMET polymerizations that are not amenable to terminal alkenes, 

It may be calculated from thermodynamic data that the metathesis of isobutene in a closed system at 25°C can yield only 1.4% each of ethene and 2,3-dimethylbut-2-ene reaction (22) (Mol 1975). At 200°C the estimated equilibrium conversion is still less than 6% of each product. It is clear that to drive reaction (22) from left to right it is necessary to remove the ethene as fast as it is formed. The reaction is catalyzed by Me4Sn-treated MoO t/Ti02 (x = 2.3-2.9) at 20°C (TOP = 

Rc207/Al203 is very effective for the metathesis of methylenecyclobutane, particularly if the reaction is run at 35°C in the liquid phase under argon with continuous removal of ethene. A 70% yield of dicyclobutylidene can be obtained in 4h reaction (24) (Popov 1973 Strel chik 1976). At 80°C the initial rate of 

For a number of 1,1-disubstituted alkenes metathesis catalysts first bring about conversion to isomers which then undergo cross-metathesis with the remaining substrate (Strel chik 1976 Usov 1983 Kawai 1990). 

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W2 Raney nickel, EtOH, Et3N, rt, 0.5 h, 75-85% yield.A disubstituted olefin was not reduced. 

Initial studies on the Jacobsen-Katsuki epoxidation reaction identified conjugated eyelie and acyelic cw-disubstituted olefins as the class of olefins best suited for the epoxidation reaetion. " Indeed a large variety of c/s-disubstituted olefins have been found to undergo epoxidation with a high degree of enantioselectivity. 2,2"-Dimethylehromene derivatives are especially good substrates for the epoxidation reaetion. Table 1.4.1 lists a variety of examples with their corresponding reference. 

To date, no effieient and general Mn(III)salen-eatalyst exists that effeets epoxidation of 1,1-disubstituted olefins with good enantioseleetivity. 

During the early development of the Jacobsen-Katsuki epoxidation reaetion, it was elear that trans-disubstituted olefins were very poor substrates (slow reaetion rates, low enantioseleetivity) eompared to cis-disubstituted olefins. The side-on approaeh model originally proposed by Groves for porphyrin epoxidation systems was used to rationalize the differenees observed in the epoxidation of the cis and trans-disubstituted elasses (Seheme 1.4.7). ... 

B) is a 1,2-disubstituted olefin and is generally unre-active in free-radical reactions. 

The third and very valuable discovery that the new phthalazine (PHAL) and pyrimidine (PYR) ligand classes (32-35, Figure 2) out-perform the monomeric ligands under identical conditions emerged from a heuristic screening process. The PHAL class in particular has become the first choice for most olefin classes. The PYR class is usually superior for terminal olefins, while the IND class is ideally suited for cA-disubstituted olefins. These ligands are commercially available or can be made easily from relatively inexpensive starting materials. 

The Arrhenius frequency factors [log(T/M V)] for addition of carbon centered radicals to the unsubstiUited terminus of monosubslituted or 1,1-disubstituted olefins cover a limited range (6.0-9.0), depend primarily on the steric demand of the attacking radical and are generally unaffected by remote alkene substituents. Typical values of log(T/M" V) are ca 6.5 for tertiary polymeric (e.g. PMMA ), ca 7.0 for secondary polymeric (PS, PMA, and ca 7.5, 8.0 and 8.5 for small tertiary (e.g. /-C4H9 ), secondary (i-CiH ) and primary (CHj, CbHs ) radicals respectively (Section 4.5.4).4 For 1,2,2-trisubstituted alkenes the frequency factors arc about an order of magnitude lower.4 The trend in values is consistent with expectation based on Iheoretical calculations. 

The CM reaction between 2-methyl-2-butene (a gera-disubstituted olefin that served in this case also as solvent) and the allylated compound 300, possessing the bicyclo[3.3.1]nonane core of the potential Alzheimer therapeutic garsubellin A (302) [137], underlines the increased activity of the second-generation ruthenium catalysts (Scheme 58). In the presence of 10 mol% of NHC catalyst C, the prenylated compound 301 was formed after only 2 h in 88% yield. 

In effect we postulate that the olefin ion is formed by a 1-3 hydride ion shift accompanied by a beta homolytic bond fission. The fact that olefin ions are formed only at branch points (except methyl branch points) could be explained on an energetic basis if it were not for the contrary fact that the over-all energetics are highly unfavorable. Thus in Reaction 20 we see that a disubstituted olefin ion is formed, and this will be true for any branch other than a methyl branch. Thus ... 

These reactions proceed without solvent as well (Reaction 29)7 On the other hand, reaction in the presence of AICI3 in CH2CI2 gave exclusively gem-disubstituted olefins (Reaction 30)7 The presence of Lewis acid shifts the reaction mechanism from radical to ionic, affording a complementary regios-electivity. 

The exact enthalpy of polymerization for a particular monomer will depend on the steric and electronic effects imposed by the substituents attached to the E=E double bond. For olefins, resonance stabihzation of the double bond and increased strain in the polymer due to substituent interactions are the most important factors governing AHp For example, propylene has a calculated AH of -94.0 kJ moT, whereas the polymerization of the bulkier 2-methylpropene is less exothermic (-78.2 kJ moT ) [63]. Due to resonance effects, the experimentally determined AH of styrene (-72.8 kJ mol ) is less exothermic than that for propylene, while that for bulkier a-methylstyrene is even less favorable (-33.5 kJ moT ) [63]. In general, bulky 1,2-disubstituted olefins (i.e., PhHC= CHPh) are either very difficult or impossible to polymerize. 

Suga and Ibata [44] prepared binaphtyldiimine derivatives 36 (Scheme 19) affording 98% ee as best selectivity for the transformation of 1,1-diphenyl-ethylene with Z-menthyl diazoacetate. The authors performed PM3 calculations and proposed an optimized structure of the copper complex to explain the high enantioselectivity observed with 1,1-disubstituted olefins. 

With this rule in mind the outcome of CM-reactions can often be predicted. In the synthesis of organotrifluoroborate 79 [143] the terminal double bond is a type 1 substrate, while the 1,1-disubstituted olefin can be considered type III. The reaction of 2-methyl-1,4-pentadiene 77 with type II cross-partner 78 furnishes 79 efficiently (only 2 mol% catalyst used) in good yields after two steps. 

The authors beheve this is due to competition of productive and unproductive CM-pathways. While the less encumbered Rul allows larger residues for the generation of disubstituted olefins, it also favours unproductive pathways with 1,2-disubsti-tuted substrates, diminishing the number of effective turnover events. As the size of the ligand decreases, so does the number of unproductive pathways relative to the productive. 

This synthetic sequence for an olefin synthesis has been further developed by Kocienski who has shown that eliminative desulphonylations carried out on / -acyloxysulphones are remarkably stereoselective for the synthesis of trans-disubstituted-olefins. The method has wide applicability in that a-lithio phenyl sulphones are readily generated, and are readily coupled to aldehydes or ketones, to give j8-hydroxysulphones. The hydroxyl function of these is then esterified and the synthesis is completed by the reductive elimination with sodium amalgam. Kocienski has prepared two reviews that summarize his syntheses of a range of natural products - one of which is diumycinoP obtained... 

Scheme 10.16 Carbonyl-ene reactions of pyruvates with 1,1-disubstituted olefins with aminosulfoximine ligand.

Prior literature indicated that olefins substituted with chiral sulfoxides could indeed be reduced by hydride or hydrogen with modest stereoselectivity, as summarized in Scheme 5.10. Ogura et al. reported that borane reduction of the unsaturated sulfoxide 42 gave product 43 in 87 13 diastereomer ratio and D20 quench of the borane reduction mixture gave the product 43 deuterated at the a-position to the sulfoxide, consistent with the hydroboration mechanism [10a]. In another paper, Price et al. reported diastereoselective hydrogenation of gem-disubstituted olefin rac-44 to 45 with excellent diastereoselectivity using a rhodium catalyst [10b],... 

For 1,1-disubstituted olefins, the prediction of the stereochemical outcome is based on a modified model (Figure 3.4).73... 

Catalyst 70 is very effective for the reaction of terminal alkenes, however 1,1-disubstituted olefins provide hydrosilylation products presumably, this is due to steric hindrance [45]. When a catalyst with an open geometry (78 or 79) is employed, 1,1-disubstituted alkenes are inserted into C-Y bonds to give quaternary carbon centers with high diastereoselectivities (Scheme 18). As before, initial insertion into the less hindered alkene is followed by cyclic insertion into the more hindered alkene (entry 1) [45]. Catalyst 79 is more active than is 78, operating with shorter reaction times (entries 2 and 3) and reduced temperatures. Transannular cyclization was possible in moderate yield (entry 4), as was formation of spirocyclic or propellane products... 

The electrochemical reduction of activated double bonds may also be used in the characterization of olefins. In all cases in which pairs (124 and 125) of 1,2-disubstituted olefins in which both X and Y are electron-withdrawing... 

In molecules containing a 1,3-diene unit and an isolated double bond, the diene is cyelopropanated preferentially (Scheme 7) 72,82). What has been said about the influence of steric and electronic factors as well as the nature of the catalyst (see above), can also be applied to explain the product distribution in these cases. The inertness of a trisubstituted double bond and the low reactivity of an E-disubstituted olefinic bond are quite obvious in these intramolecular competitions. 

For cyclopropanations with ethyl diazoacetate, a rather weak influence of the olefin structure has been noted 59 60, (Table 7). The preference for the sterically less crowded cyclopropane is more marked for 1,2-disubstituted than for 1,1-disubstituted olefins. The influence of steric factors becomes obvious from the fact that the ratio Z-36/E-36, obtained upon cyclopropanation of silyl enol ethers 35, parallels Knorr s 90> empirical substituent parameter A.d of the group R 60). These ZjE ratios, however, do not represent the thermodynamic equilibrium of both diastereomers. 

In 1986, Pfaltz et al. introduced a new type of pseudo C2-symmetrical copper-semicorrin complex (68) as the catalyst (Scheme 60).227 228 The complexes (68) are reduced in situ by the diazo compound or by pretreatment with phenylhydrazine to give monomeric Cu1 species (69), which catalyze cyclopropanation. Of the semicorrin complexes, complex (68a) (R = CMe2OH) showed the best enantioselectivity in the cyclopropanation of terminal and 1,2-disubstituted olefins.227,228,17 It is noteworthy that complex (68a) catalyzes cyclopropanation, using diazomethane as a carbene source, with good enantioselectivity (70-75% ee).17... 

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