Reactions double bonds

Uses ndReactions. Dihydromyrcene is used primarily for manufacture of dihydromyrcenol (25), but there are no known uses for the pseudocitroneUene. Dihydromyrcene can be catalyticaUy hydrated to dihydromyrcenol by a variety of methods (103). Reaction takes place at the more reactive tri-substituted double bond. Reaction of dihydromyrcene with formic acid gives a mixture of the alcohol and the formate ester and hydrolysis of the mixture with base yields dihydromyrcenol (104). The mixture of the alcohol and its formate ester is also a commercially avaUable product known as Dimyrcetol. Sulfuric acid is reported to have advantages over formic acid and hydrogen chloride in that it is less compUcated and gives a higher yield of dihydromyrcenol (105). 

Spirophosphonium ylide 17 undergoes a two-step addition process with diols first the P-N bond is cleaved, followed by addition of the OH bond across the P=N double bond. Reaction with binaphthol stops after the first step to give 137, whereas catechol adds across the double bond to give 138 (Scheme 3) <2004JOC1880>. 

In the next example, the strategy for the formation of fused cyclic ethers utilized the formation of an intermediate a-heterosubstituted carbon radical, generated by the reaction of (TMS)3Si radical with A-(ethoxycarbonyl)-l, 3-thiazolidine derivative [38]. This intermediate gives intramolecular C — C bond formation in the presence of proximate 1,2-disubstituted double bonds (Reaction 7.27). However, when terminal double bonds are used, the hydrosily-lation of the double bond by (TMS)3SiH can compete with the reduction and prevent forming the desired C—C bond formation. 

As has been exemplified in this chapter, fluorinations with fluorine-18 can be classified into two categories (1) the nucleophilic reactions, which usually involve no-carrier-added [ F]fluoride of high-specific radioactivity as its Kf FIF-K complex and include substitutions in the aliphatic and the /lomoaromatic series and (2) the electrophilic reactions, which mainly use moderately low-specific radioactivity molecular [ F]fluorine, or other reagents prepared from it, such as acetyl [ F] hypofluorite, and include addition across double bonds, reactions with carbanions and especially fluorodehydrogenation and fluorodemetallation reactions. 

In this case, the amino acid is already oxidized at the (3-position and elimination of the hydroxy group, as the acetate, generates the double bond. Reaction of the unsaturated azlactone and the free base of an amino acid ester produces the dipeptides containing APhe. This approach has been extensively used in the preparation of a series of APhe containing peptides15 63 ... 

With activated double bonds, reaction with (2) leads to cyclopropane derivatives. 

If the alkene or alkadiene has at least one hydrogen on the carbon adjacent to the double bond, reaction with singlet oxygen may give hydroperoxides. The mechanism of this reaction is related to [4 + 2] cycloadditions and is presumed to occur through a HLickel pericyclic transition state (see Section 21-10D) ... 

Reactions (6) were visualized as an attack by oxygen atoms on a single bond in propylene, while (7) and (8) were regarded as an attack on the olefinic double bond. Reaction (2) was again invoked to explain increase in formaldehyde when O2 was added. 

In the above discussion of stereoselectivity the mechanisms of various reactions have been used to rationalize why some are stereoselective and some are not. Thus the bromination of olefins proceeds via a bridged bromonium ion intermediate and gives only trails addition across the double bond [reactions (6.2) and (6.3)]. In contrast, the addition of HBr across a double bond gives a carboca-tion intermediate that does not maintain the facial integrity of the olefin and is thus much less stereoselective [reaction (6.1)]. In these examples the mechanism of the reaction is used to explain and understand the diastereoselectivity that is observed. There are many other examples (usually in textbooks) where the mechanism of a reaction is used to rationalize the stereoselectivity of the process. To do this requires that the mechanism be known with certainty. 

In the series of hydroxycyclohexadienylperoxyl radicals, one encounters the competition between the H02-/02- elimination leading to phenol [reactions (9) and (14)/(15)] and fragmentation of the ring (Pan et al. 1993). That latter has been attributed to an intramolecular addition of the peroxyl radical function to a diene double bond [reaction (24)]. This reaction is reversible [reaction (-24)], but when 02 adds to the newly created carbon-centered radical the endoperoxidic function is locked in [reaction (25)]. In analogy to reaction (24), the first step of the trichloromethylperoxyl-radical-induced oxidation of indole is its addition to the indole C(2)-C(3) double bond (Shen et al. 1989). 

In allylperoxyl radicals, allylic rearrangement leads to the 1,3-migration of the peroxyl function, with the corresponding shift of the double bond [reaction (28) Schenck et al. 1958],... 

The allyl radical can react with another -CH2OH to form either the hydroxyethyl derivative [reaction (199)] or its isomer with an exocyclic double bond [reactions (200)]. 

The equilibrium is entirely in favour of prenyl bromide because of its more highly substituted double bond. Reactions on the tertiary allylic isomer are very likely to take place by the S l mechanism the cation is stable because it is tertiary and allylic and the equilibration tells us it is already there. Even if the reactions were bimolecular, no mechanism would be necessary for the tertiary... 

In a quantitative theory we shall also have to calculate the energy difference between the normal state and the activated state in which on the reacting atom is localized either a pair of electrons (electrophilic reaction SE), a sextet (pos. charge, nucleophilic reaction SN), or a single electron (radical reaction SR) or in which a pair of electrons is localized in a certain bond (true double bond reactions). The remaining possibilities for resonance will determine mainly which of the possible activated states will have the lowest energy and thus what course the reaction will take (localization hypotheses). 

Epoxidation of vitamin D3 (38) is regio- as well as stereo-selective (equation 17). Though both C(S)—C(6) and C(7)—C(8) are trisubstituted double bonds reaction takes place selectively at C(7)— C(8), since only this route leads to the thermodynamically more stable conjugated diene derivative attack is selectively from the a-face since the 0-face is shielded by the axial methyl at C-13. Epoxidation of (39a) is stereoselective (equation 18). The selectivity is higher in (39a) than in the reaction of (39b R = MOM) due to stereoelectronic repulsive effects involving acetate and peroxy acid in the transition state leading to (41). 

A closer analysis suggests that the Mn=Ge=Mn unit, with Ge=Mn distance of 220.4 pm, is best described by partial triple bonding rather than as double bonding. Reaction of Ge[Mn(CO)2C5H4Me-f/ ]2 with Fe2(CO)9 provides a route to... 

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