Alkenic units

When compound (443), which contains alkene and alkyne moieties, was reacted with benzonitrile oxide, both an isoxazoline (444) and isoxazole (445) were produced, with the former predominating. Oxidation of (444) with permanganate produced 3-phenyl-2-isoxazoline-5-carboxylic acid (446) (67ZOR82i). The reaction of 1-trimethylsilylbut-l-yne-3-ene produced only a compound which reacted at the alkenic unit. Oxidation of the adduct also produced (446) (68ZOB1820). These reactions are shown in Scheme 102. 

Benzene derivatives. Tbe nomenclature is a combination of the lUPAC system and traditional names. Many of the derivatives are named by the substituent group appearing as the prefbt. These may be considered a subclass of the aliphatic-aromatic hydrocarbon family, which contains both aliphatic and aromatic units in its structures. Thus, alkylbenzenes are made up of a benzene ring and alkane units alkenylbenzenes are Composed of a benzene ring and alkene units and alkynylbenzenes comprise a benzene ring and alkyne units. Examples of alkylbenzenes include... 

Alkenes and alkynes can also add to each other to give cyclic products in other ways (see 15-61 and 15-63). The first exclusive exo-dig carbocyclization was reported using HfCU as a catalyst. Alkynes also add to alkenes for form rings in the presence of a palladium catalyst or a zirconium catalyst. " Carbocyclization of an alkene unit to another alkene unit was reported using an yttrium catalyst and alkenes add to alkynes to give cyclic compounds with titanium catalysts. ... 

Similar reactions have been carried out on acetylene. Aldehydes add to alkynes in the presence of a rhodium catalyst to give conjugated ketones. In a cyclic version of the addition of aldehydes, 4-pentenal was converted to cyclopen-tanone with a rhodium-complex catalyst. In the presence of a palladium catalyst, a tosylamide group added to an alkene unit to generate A-tosylpyrrolidine derivatives. ... 

Acyl radicals can be generated and they cyclize in the usual manner. A polyene-cyclization reaction generated four rings, initiating the sequence by treatment of a phenylseleno ester with Bu3SnH/AIBN to form the acyl radical, which added to the first alkene unit. The newly formed carbon radical added to the next alkene, and so on. Acyl radicals generated firom Ts(R)NCOSePh derivatives cyclize to form lactams. ... 

Samarium iodide promotes this addition reaction. In a related reaction, simple alkene units add to esters in the presence of sodium and liquid ammonia to give an alcohol. " °... 

Catalysts have been developed that are compatible with both water and methanol. The reaction is compatible with the presence of other functional groups such as other alkene units,carbonyl units,and amides (see 125). ... 

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

The two reactions shown below are examples of the use of the Wacker reaction in multistep synthesis. In the first case, selectivity is achieved between two terminal alkene units on the basis of a difference in steric accessibility. Both reactions use a reduced amount of Cu(I) salt. In the second reaction this helps to minimize hydrolysis of the acid-sensitive dioxolane ring. 

Interestingly, Hoveyda and coworkers observed a second-order dependence of the reaction rate on the concentration of zirconium in these reactions, suggesting that the zirconacyclopentane is formed from a bimetallic alkene-zirconate complex such as A in Fig. 1 [21]. This finding suggests that olefin alkylations and substitutions occur via reaction of a nucleophilic alkene unit [23]. 

Tsuda and coworkers350 used nickel(O) complexes to effect the [2 + 2 + 2] cycloadditions between two alkyne units and one alkene unit and employed this strategy to synthesize copolymers. Thus, the reaction of diyne 602 with A-octylmaleimide (603) catalyzed by Ni(CO)2(PPh3)2 afforded copolymer 604 with a maximum yield of 60% and a GPC molecular weight of as high as 35,000, which corresponds to n = 64 (equation 172). The exo,exo-bicyclo[2.2.2]oct-7-ene moiety of 604 arises through the reaction of the initially formed [2 + 2 + 2] adduct with another equivalent of A-octylmaleimide. 

Chelating resins have been prepared in which hydroxypyridinones have been incorporated into sepharose gels (66). These materials were developed, along with desferrioxamine analogues (67), with a particular view to immobilizing iron(III). Other specialty hydroxypyridinones have included several containing alkene units, incorporated for their lipophicity, their potential for use in hydroboration (68), or for their intrinsic interest (69). 

The range of the asymmetric epoxidation reaction may be extended still further to include dienes (Entries 7,12,17) and even tetraenes (Entry 26). It is of interest to note that only double bonds adjacent to the carbonyl function are epoxidised and any remaining double bonds are left untouched (Entry 26). This selective reactivity allows for further elaboration of unreacted alkene units at a later stage, (see Sect. 5). Enediones (Entries 21-23) and unsatuxated keto esters (Entries 24 and 25) can also be oxidised in good yields and good to excellent stereoselectivity using polyamino acids. 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Intramolecular reactions of propargylic alcohols bearing an alkene unit at a suitable position proceeded smoothly to give the corresponding substituted chro-... 

The preservation of alkene geometry in these concerted cvcloaddi-tions implies a mode of reaction in which, for each alkene unit, the two new bonds are formed to the same face of the Ji-bond. This mode is called suprafacial-suprafacial cydoaddition, and is one of... 

The addition of organometallic reagents to push-pull alkenes is a synthetically important process in which retention of an alkenic unit in the product occurs this process complements the direct conjugate addition of organometallic reagents to activated alkynes (Section 1.2.5) which is complicated primarily... 

In fact, the (C2)-[(C5)n]-(C2) and (C3)-[(C5) ]-(C3) telomers were produced inparallel with the C4 and Cs alkenes, with the constant statistical telomer ratio of 1 2 1. The result was inconsistent with the pairwise mechanism and suggests that the two ends of the telomer arise from two different alkene units thus, even in the early stages of the reaction, the pent-2-ene can generate all three telomer forms. In other words, a sequential rather than pairwise mechanism seems to have been operative. It was proposed, therefore, that the catalyst acts as a carrier for one half of the alkene and thereby allows sequential reactions of one alkene with... 

The intramolecular nucleophilic addition of the formyl group to the electron deficient alkene unit in the propenoate (10) affords a benzofuranone when catalysed by a thiazolium salt. However, when NaCN is used as the catalyst, an initial Michael addition to the acrylate function is followed by an intramolecular aldol condensation and the chroman (11) is formed. The corresponding butanoates afford chroman-4-ones under the influence of thiazolium cations, but give benzoxepins in the presence of a basic catalyst (95S1311). 

A conjugate diene is made up of two alkene units separated by a single bond [Following fig.(a)]. Dienes are separated by more than one single bond known as non-conjugated dienes [Following fig.(b)]. 

In 1,2-addition, new atoms have been added to each end of one of the alkene units. This is the normal electrophilic addition of an alkene with which we are familiar. In 1,4-addition, new atoms have been added to each end of the entire diene system. Furthermore, the double bond remaining has shifted position (isomerised) to the 2, 3-position. 

The mechanism of 1, 4-addition starts off in the same way as a normal electrophilic addition. We shall consider the reaction of a conjugated diene with hydrogen bromide as an example (Following fig.). One of the alkene units of the diene uses its n electrons to form a bond to the electrophilic hydrogen of HBr. The H-Br bond breaks at the same time to produce a bromide ion. The intermediate carbocation produced has a double bond next to the carbocation centre and is called an allylic carbocation. 

---