Isomerizations of alkynes

Isomerization of alkynes to allenes and then to conjugated dienes is catalyzed by Pd(0) species such as Pd(OAc)2 3.nd PhjP or Pdi(dba)3. The conjugated ketoalkyne 286 was converted into the conjugated dienone 287[141,I42]. 

Morpholine also gives the allyhc amine in high yield. The reaction is thought to involve a known hydridopaUadium-catalyzed isomerization of alkynes to aUenes followed by reaction of the latter with the hydridopalladium complex to give 1-phenyl-substituted q -allylpalladium complexes. These complexes react with amines affording the allylic amines. Primary amines give the diallylic amines. An intramolecular version has been developed for the synthesis of 2-(2-phenyl)-pyrroUdines and -piperidines [319]. 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

Pd-catalyzed isomerization of ynones to furans has been an active area of research over the last decade. Huang et al. described a Pd-catalyzed rearrangement of a,P-acetylenic ketones to furans in moderate yield [102], For example, Pd(dba)2 promoted the isomerization of alkyne 124 to a putative allenyl ketone intermediate 125, which subsequently cyclized to the corresponding furan 126. 

Allenes from Prototropic Isomerizations of Alkynes as Reactive Intermediates... 

Shioiri and co-workers developed a catalytic asymmetric synthesis of allenes by isomerization of the alkyne 240 to allene 242 under the control of a chiral phase-transfer catalyst 241 (Scheme 4.62) [98], Although the enantiomeric excess is not high (35% ee), this is the first example of the asymmetric isomerization of alkynes under phase-transfer catalyzed conditions. 

Scheme 4.62 Asymmetric allene synthesis via isomeration of alkyne 240 with the chiral phase-transfer catalyst 241.

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

An exceptional approach to phosphorus-substituted alkoxyallenes via isomerization of alkynes was introduced by Beletskaya s group. The treatment of 1-alkoxy-l-propynes 23 with l-halo-2,2-bis(trimethylsilyl)phosphaethen 24 furnished alkoxyallenes 25 (Scheme 8.9) [31]. 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

Abstract The computational studies of the isomerization of alkyne into vinylidene and... 

These isomerization reactions are of great interest to theoreticians because the role of many factors (metal, substituents on the organic fragment, ancillary ligands) on the outcome of the reaction can be studied through computations. The purpose of this chapter is to describe the theoretical studies carried out on the isomerization of alkyne to vinylidene and alkene to carbene in the presence of transition metal fragments. 

Additional methods for preparing non-heteroatom-substituted carbene complexes include nucleophilic or electrophilic additions to carbyne complexes (Section 3.1.4), electrophilic additions to alkenyl or alkynyl complexes (Section 3.1.5), and the isomerization of alkyne or cyclopropene complexes (Section 3.1.6). 

Isomerization of alkynes.1 This complex (1) or IrH5[P(/-Pr)3]2 (2) in combination with Bu3P catalyzes isomerization of 2-ynoic esters to (2E,4E)-dienoic esters at 80-100° in yields generally of about 80-90%. 

Isomerization of alkynes to dienes is a useful synthetic approach since alkyne derivatives are readily available. Even though alkyne migrations along the chain under basic conditions are well known, isomerizations of isolated alkyne to 1,3-diene do not take place easily. For some alkynones, this reaction takes place readily with triphenylphosphine . Utilizing isomerization of a yne-one as a key reaction, Guo and Lu reported a three-step synthesis of an anti-cancer agent ostopanic acid starting from pent-4-ynal (equation 166) . ... 

Isomerization of alkynes. This difunetional base induees rapid migration of triple bonds from the interior of the chain to the terminus in seconds at 0 ... 

In contrast to the behavior of free alkynes, 1,3-isomerization of alkyne complexes to produce complexed allenes [140,141) is rather rare. A synthetically useful example [140] is provided by the CpMn(CO)2 complexes of electrophilic alkynes 89 which undergo alumina-promoted isomerization to the corresponding allene derivatives 90 these, in turn, can be oxidatively demetalated to the free allenes 91 (Scheme 4-45). 

Formerly, only very strong bases (Na, NaNHj, KNHj) were found to effect isomerization of alkynes, and then only at elevated temperatures and only of 2-alkynes to 1-alkynes. Since internal alkynes are more stable than terminal alkynes, the isomerization is evidently possible because of formation of a metal acetylide. Migration is blocked by a branch in the chain. The base is also highly active in the exchange of benzene C—-D bonds. It converts limonene to p-cymene with evolution of hydrogen at room temperature in high yield. Brown notes that the base can exist as a chelate (a), a structure that may contribute to the high basicity. 

Isomerization of alkynes Alkynes substituted with a perfluoroalkyl group undergo... 

C24K is a moderately active catalyst for hydrogenation of alkenes. It is a very efficient catalyst for isomerization of cw-stilbene to trcnj-stilbene. It is also effective for isomerization of alkynes 2-octyne-> 1-octyne (86% yield). An allene is a probable intermediate 2-decyne -> 1-decyne (207o) and 1,2-decadiene (9%,). Potassium alone also effects these isomerizations, but only at higher temperatures. Benzene is slowly converted into biphenyl when in contact with C24K in refluxing cyclohexane. This reaction has also been reported for CgK in DMF at 20 (61%, yield).=... 

Wang, Z., Wang, Y., Zhang, L. (2014). Soft Rropargylic Deprotonation Designed Ligand Enables Au-Catalyzed Isomerization of Alkynes to 1,3-Dienes. Journal of the American Chemical Society, 136(25), 8887-8890. 

The isomerization of alkynes using lithium aminopropylamide (LAPA) has seen a number of synthetic applications, but one of the... 

In comparisvjn with the isomerization of alkynes, the base-catalysed r-ear,rangements of alkenes are less well known. Such reactions deserve further attention, however, as recent work demonstrates. Allylic alcohols, for example, rearrange to give aldehydes (25-74% yield) (Scheme 13). Of particular interest is the conversion of undec-10-en-1-ol to undecanal, albeit in low yield (3%). The intermediacy of a stabilized dianion (51) is plausible which is supported by the observations that, e.g. cyclo-hexen-3-ol failed to rearrange. 

Two new series of bifunctional hydrogen-bond-donor asymmetric organocatalysts have been described, based on 2-aminoquinazol-4-(l//)-one (111a) or 3-amino-benzothiadiazine-1,1-dioxide (111b) skeletons. Both types catalyse Michael additions (e.g. nitrostyrene and diethyl malonate), hydrazinations of the a-carbon of ketones and /3-ketoesters, and isomerizations of alkyne esters to aUene esters (mainly... 

KNH2/AI2O3 is effective for the isomerization of alkynes. Thus, 1-hexyne is isomerized exclusively to 2-hexyne in a 92% yield in dioxane at 333 K in 20 h. The isomerization of alkyn-l-yl alcohol to a, /J-unsaturated ketones proceeds over solid-base catalysts (72). Thus, in the following reaction, the a, /3-unsaturated... 

Similarly, a single DA reaction of a l,2-cyclo[3]dendralene was reported by Kanematsu and coworkers in 1986. In this case, a base promoted isomerization of alkyne 100-generated [3]dendralene 101, which subsequently reacted in an intramolecular DA reaction to yield tricycle 102 (Scheme 12.21) [26]. 

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