Alkenyl compounds, preparation

Figure 16.32 represents the alkenylation of an acetylene according to the procedure developed by Cacchi. The coupling partners are an alkenyl triflate (prepared in analogy to Figure 13.23) and an alkyne. The coupling product of these compounds is a 1,3-enyne—in this case conjugated with a cyclohexenone. The C=C triple bond of such an enyne can be hydrogenated in a cis-selective fashion by using Lindlar s Pd catalyst (cf. Figure 17.81). In this way 1,3-dienes are formed that contain at least one cij-configured C=C double bond.

Several of the trialkylaluminum and alkylaluminum halides and hydrides mentioned above are commercially available. Alkynyl, alkenyl, cyclopentadienyl, and aryl derivatives are, in general, not commercially available and must be synthesized for laboratory use. Alkynyl derivatives can be prepared by salt metathesis, as in the reaction of Et2AlCl with NaC=CEt to give Et2AlC=CEt. The acidity of terminal alkynes is sufficient for preparation of alkynyl aluminum compounds by alkane or hydrogen elimination upon reaction with a trialkylaluminum or an aluminum hydride (equation 17), respectively. TriaUcynyl aluminum compounds are typically isolated as Lewis base adducts to stabilize them against otherwise facile polymerization. Alkenyl compounds of aluminnm have similarly been prepared. 

Styryl derivatives of 2-aminofurans, as well as alkenyl compounds, also undergo intramolecular cycloaddition and the alkene function can be introduced by Stille coupling of a suitably functionalised aryl iodide. This approach is illustrated by the tetrahydroquinoline synthesis summarised in Scheme 26 (99JOC3595). The iodo derivative 143 is readily prepared from the carbamate ester 142 (67% yield) and Stille coupling with vinyltributyltin gives the styrene 144 (72% yield). Intramolecular cycloaddition and dehydration is then achieved simply by heating compound 144 in toluene under reflux (24 h) to give the tetrahydroquinoline 145 in 79% yield. 

The use of trifluoroborate salts in couplings, which are very easily prepared from boronic acids by reaction with KHF2, is a useful variant of the Suzuki reaction. These salts have the advantage of enhanced (often considerably) stabihty compared to boronic acids and this is particularly notable for alkenyl compounds, which can be stored for a considerable time. The coupling conditions are very similar to those for boronates and are apphcable to a wide range of heterocyclic substrates, ... 

The alkenyl compounds which can be prepared from 1,5-hexadiene and dialkyl alanes are unstable and undergo intramolecular addition of the A1—C bond to the C=C double bond, which is sterically well sited (293) ... 

Aryl and alkenyl triflates prepared from phenols and carbonyl compounds are reactive substrates which undergo facile oxidative addition. Reactivity of triflates is between that of iodides and bromides. Pd-catalyzed reactions of triflates mean indirect displacement of the phenolic OH group to afford 80 and transformation of carbonyl compounds 81 to the substituted alkenes 82. In classical organic chem-... 

The hydroxyalkyl compounds of the type 120 (where R = GH2(GH2) GH20H and / = 2,3,4, or 6) have been prepared as well as the Gp analogs by the oxidative hydroboration of the 77 -alkenyl compounds Gp""Fe(GO)2 (GH2) GH=GH2 (where Gp = Gp or Gp and n = 2,3,4, or 6). These new cu-hydroxy-alkyl compounds were isolated as low-melting yellow solids or yellow-brown oils and characterized by IR, NMR, and mass spectrometry. 

When valuable alkenyl groups are used, the use of alkyl alkenyl titanocene prepared from the monochlorotitanocene also leads to the unstable titanocene alkenylidene intermediate that is similarly trapped in situ by the carbonyl compound [66]. 

The majority of preparative methods which have been used for obtaining cyclopropane derivatives involve carbene addition to an olefmic bond, if acetylenes are used in the reaction, cyclopropenes are obtained. Heteroatom-substituted or vinyl cydopropanes come from alkenyl bromides or enol acetates (A. de Meijere, 1979 E. J. Corey, 1975 B E. Wenkert, 1970 A). The carbenes needed for cyclopropane syntheses can be obtained in situ by a-elimination of hydrogen halides with strong bases (R. Kdstcr, 1971 E.J. Corey, 1975 B), by copper catalyzed decomposition of diazo compounds (E. Wenkert, 1970 A S.D. Burke, 1979 N.J. Turro, 1966), or by reductive elimination of iodine from gem-diiodides (J. Nishimura, 1969 D. Wen-disch, 1971 J.M. Denis, 1972 H.E. Simmons, 1973 C. Girard, 1974),... 

Diene carboxylates can be prepared by the reaction of alkenyl halides with acrylates[34]. For example, pellitorine (30) is prepared by the reaction of I-heptenyl iodide (29) with an acrylate[35]. Enol triflates are reactive pseudo-halides derived from carbonyl compounds, and are utilized extensively for novel transformations. The 3,5-dien-3-ol triflate 31 derived from a 4,5-unsaturated 3-keto steroid is converted into the triene 32 by the reaction of methyl acrylate[36]. 

Formation of carboxylic acids ami their derivatives. Aryl and alkenyl halides undergo Pd-catalyzed carbonylation under mild conditions, offering useful synthetic methods for carbonyl compounds. The facile CO insertion into aryl- or alkenylpalladium complexes, followed by the nucleophilic attack of alcohol or water affords esters or carboxylic acids. Aromatic and a,/ -unsaturated carboxylic acids or esters are prepared by the carbonylation of aryl and alkenyl halides in water or alcohols[30l-305]. 

The Li compound 588 formed by the ort/io-lithiation of A. A -dimethylaniline reacts with vinyl bromide to give the styrene derivative 589(433]. The 2-phe-nylindole 591 is formed by the coupling of l-methyl-2-indolylmagnesium formed in situ from the indolyllithium 590 and MgBr2, with iodobenzene using dppb[434]. 2-Furyl- and 2-thienyllithium in the presence of MgBr2 react with alkenyl halides[435]. The arylallenes 592 and 1,2,4-alkatrienes are prepared by the coupling reaction of the allenyllithium with aryl or alkenyl halides[436]. 

The coupling of alkenylboranes with alkenyl halides is particularly useful for the stereoselective synthesis of conjugated dienes of the four possible double bond isomers[499]. The E and Z forms of vinylboron compounds can be prepared by hydroboration of alkynes and haloalkynes, and their reaction with ( ) or (Z)-vinyl iodides or bromides proceeds without isomerization, and the conjugated dienes of four possible isomeric forms can be prepared in high purity. 

Organophosphorus compounds. Phosphorus-carbon bond fonnation takes place by the reaction of various phosphorus compounds containing a P—H bond with halides or tritlates. Alkylaryl- or alkenylalkylphosphinates are prepared from alkylphosphinate[638]. The optically active isopropyl alkenyl-methylphosphinate 778 is prepared from isopropyl methylphosphinate with retention[639]. The monoaryl and symmetrical and asymmetric diarylphosphi-nates 780, 781, and 782 are prepared by the reaction of the unstable methyl phosphinate 779 with different amounts of aryl iodides. Tnmethyl orthoformate is added to stabilize the methyl phosphinate[640]. 

Dienes and allylarcncs can be prepared by the Pd-catalyzcd coupling of allylic compounds with hard carbon nucleophiles derived from alkenyl and aryl compounds of main group metals. Allylic compounds with various leaving groups can be used. Some of them are unreactive with soft nucleophiles, but... 

The versatility of lithium aluminum hydride permits synthesis of alkyl, alkenyl, and arylsilanes. Silanes containing functional groups, such as chloro, amino, and alkoxyl in the organic substituents, can also be prepared. Mixed compounds containing both SiCl and SiH cannot be prepared from organopolyhalosilanes using lithium aluminum hydride. Reduction is invariably complete. 

C H Sb, however, both possess trigonal-bipyramidal geometry. In addition to compounds of the type R Sb, mixed compounds of the type R R Sb or R3R 25b, where R and R may be alkyl, alkenyl, alkynyl, or aryl groups, are known. Thus triethyl dimethyl antimony [67576-92-5], CgH2 Sb, has been prepared (197) ... 

Polyfiincdonalizeduitro compounds are prepared by the Michael addidon using 2-alkenyl-snbsdtiited 2-siloxycyclopropiinecarboxylates as Michael acceptors fEq. 4.120. ... 

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