Geminate termination

Double dehydrohalogenation to form terminal alkynes may also be carried out by heating geminal and vicinal dihalides with potassium tert butoxide m dimethyl sulfoxide... 

In decoupling the methyl protons, the NOE difference spectrum shows a nuclear Overhauser enhancement on the cyclopropane proton at = 1.60 and on the terminal vinyl proton with trans coupling at <5// = 5.05 and, because of the geminal coupling, a negative NOE on the other terminal proton at Sh= 4.87. This confirms the trans configuration G. In the cis isomer H no NOE would be expected for the cyclopropane proton, but one would be expected for the alkenyl-// in the a-position indicated by arrows in H. 

The reduction of an azide group with triphenylphosphine in tetrahydrofuran by microwave heating at 130 °C for 5 min has been described by Kihlberg and colleagues (Scheme 6.152) [297]. The use of diethyl 4-(hydrazinosulfonyl)benzyl phosphonate as an in situ diazene precursor for the reduction of trisubstituted gem-diiodoalkenes to terminal geminal diodides under microwave conditions has also been reported [298],... 

Alkylidenemalonates were found to be excellent acceptor molecules (111). Reactions of lithium ylides with dimethyl alkylidenemalonates at —78 °C in THF in the presence of f-BuOH were diastereoselective for all the substituents R except methyl, producing Michael adducts as single diastereomers (Scheme 11.24). The only exception was dimethyl ethylidenemalonate, which produces an 86 14 mixture of diastereomeric adducts, the minor diastereomer being syn-adduct. Since dimethyl alkylidenemalonates bear two geminal methoxycarbonyl moieties, one is cis to the terminal substituent R and the other trans, so of these ester substituents can participate in chelate formation in the transition state. When the terminal substituent R is small, there is a chance for the syn-adduct to be produced, which... 

Scheme 15 Twofold Heok oouplings on acceptor-substituted terminal alkenes to yield geminally diarylated alkenes. ...

Cascade carbopalladation sequences after attack on an alkene are most commonly terminated by dehydropallada-tion, if a / -hydride is available in a yy//-orientation. The intramolecular carbopalladation starting from the monocyclic diene 122 with a geminally disubstituted alkene terminator leads to a neopentylpalladium intermediate 123, which cannot undergo /J-dehydropalladation, but continue the cascade by a carbopalladation to eventually form the... 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

Alkynes are more reactive in hydroalumination than alkenes. Hence, unlike internal alkenes, internal acetylenes readily undergo hydroalumination. Side reactions, however, may occur. Proton-aluminum exchange in terminal alkynes, for instance, leads to substituted products, and geminal dialuminum derivatives are formed as a result of double hydroalumination. In addition, nonsymmetric alkynes usually give mixtures of regioisomers. Appropriate reaction conditions, however, allow selective hydroalumination. Thus, the addition of isoBu2AlH to alkynes is a... 

Substituted cycloalkenes usually react in the ring and not in the side chain. Internal alkenes with CH2 groups in both allylic positions yield a mixture of isomers, whereas terminal alkenes give primary alcohols as a result of allylic rearrangement. Later studies revealed, however, that the reactivity depends on both the structure of the alkene and reaction conditions.674 675 In alcoholic solutions, for example, racemic products are formed. Geminally disubstituted alkenes may exhibit a reactivity sequence CH > CH2 > CH3.675 676... 

The reaction of alkynes and HF more commonly gives geminal difluoroalkanes, usually in high yield. Terminal alkynes afford 2,2-difluoroalkanes (equation 10) and internal alkynes yield geminal difluorides (equation ll).4-23-24 26... 

Yields of geminal difluorides of 70-75% are also obtained from the reaction of terminal or internal alkynes with the (HF)x CjHjN reagent.21... 

There are numerous examples of the addition of HBr to alkynes.10 Addition to acetylene is difficult, tends to produce mixtures of bromide products, and requires a catalyst.94,10 1 22 Simple terminal alkynes react with HBr in the absence of peroxides to produce mixtures of 2-bromo-l -alkenes and geminal dibromides (equation 110).85,159 The latter can be produced in high yield when excess HBr is used. 60... 

Palladium(II) salts, in the form of organic solvent soluble complexes such as PdCl2(RCN)2, Pd(OAc>2 or Li2PdCU, are by far the most extensively utilized transition metal complexes to activate simple (unactivated) alkenes towards nucleophilic attack (Scheme 1). Alkenes rapidly and reversibly complex to pal-ladium(II) species in solution, readily generating alkenepalladium(II) species (1) in situ. Terminal monoalkenes are most strongly complexed, followed by internal cis and trans (respectively) alkenes. Geminally disubstituted, trisubstituted and tetrasubstituted alkenes are only weakly bound, if at all, and intermolecular nucleophilic additions to these alkenes are rare. 

A simple method for introducing a triple bond into an organic compound is to treat an appropriate dihalide with a strong base. Since vicinal dihalides (usually the bromide) are readily formed by reaction of bromine with an alkene, and geminal dihalides from aldehydes or ketones with phosphorus pentachloride, the method is a useful general procedure for the preparation of terminal and non-terminal alkynes from readily available starting materials. 

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