Alkenyl lithium reagents

Preparation Using Metallic Lithium. Most simple organolithium reagents can be prepared by reaction of an appropriate halide with lithium metal. The method is applicable to alkyl, aryl, and alkenyl lithium reagents. 

Fluorinated alkenyl zinc reagents can be prepared by two different routes 1) exchange of the corresponding alkenyl lithium reagents at low temperature with zinc chloride 2) direct insertion of zinc into the carbon-halogen bond of fluorinated alkenyl halides. 

The coupling of vinyl bromide with in situ generated terminal alkyne complexes has also been accomplished by the alkylation of ( 75-C5H5)2Zr(Me)Cl 230 with terminal alkenyl lithium reagents.114 The reaction products are either dienes or cyclobutenes, depending upon the substrates employed. 

Recently, Corey has rekindled interest in vinylstannanes as intermediates for alkenyl lithium reagents (79,80,81), and we (52,55) and others (53,56) have utilized this method to prepare various p-chain precursors (Scheme 17). We find hydrostanna-tion of terminal acetylenes to be a facile and high yield reaction, whereas iodovinylation is a capricious and tedious transformation. Thus, treatment of 3-triethylsilyloxy-l-octyne 91 with tributyl-stannane (TBS) in the presence of azobisisobutyronitrile (AIBN) gave stereospecifically an excellent yield of l-tributylstannyl-3-triethylsilyloxy-trans-l-octene (92), which when lithiated with butyllithium and treated with pentynylcopper provided the requisite lithio cuprate 93. The use of TBS to prepare homovinylic ethers such as 96 and 97 gave a 10 1 mixture of trans and cis isomers. 

Stereospecific route to terminal alkenyl-lithium reagents... 

Alkyllithium reagents can also be generated by reduction of sulfides.32 Alkenyl-lithium and substituted alkyllithium reagents can be prepared from sulfides,33 and sulfides can be converted to lithium reagents by the catalytic electron transfer process described for halides.34... 

Corey and Posner discovered that lithium dimethylcuprate could replace iodine or bromine by methyl in a wide variety of compounds, including aryl, alkenyl, and alkyl derivatives. This halogen displacement reaction is more general and gives higher yields than displacements with Grignard or lithium reagents.20... 

Reaction of fluorinated alkenyl Grignard reagents or lithium reagents with organotin halides has been utilized for the preparation of fluorinated alkenyl tin reagents [111, 182-184]. Recently, a novel, general and convenient method has been developed for the preparation of a variety of fluorinated alkenylstannanes... 

Irrespective of the stereochemistry of the ot,p-enones, lithium alkenyltrialkylalanates, e.g. (28) and (29), obtainable by treatment of alkenylalanes with methyllithium (or n-butyllithium), are also excellent 1,4-conjugate alkenyl transfer reagents. The cornerstone of a general prostaglandin synthesis, as exemplified in Scheme 6, employs the alanate conjugate addition process exclusively.13... 

In their work they took advantage of the previously reported syntheses of C-alkenyl- and C-alkynyl-/3-D-glycosides by Kishi [30] and Sinay [31], using vinyl dibromide 2 easily prepared in two steps from glucose derivative 1. Conversion of dibromide 2 to the acetylenic lithium reagent with BuLi, followed by treatment with glucop3ranolactone 3 leads to hemi-acetal 4 in a high yield. Stereospecific reduction of 4 by hydride delivery on the a-face of... 

In the second method, the (l-selanylcyclopropyl)lithium reagent 2 is used to alkylate another a-selanyl aldehyde. Elimination, as above, of the resulting hydroxy diselenide 3 gives the alkenyl(selanyl)cyclopropane 4 with high selectivity for the E -isomer. The yields and isomer distributions of selenides 4 prepared by the two methods are indicated. These products can be converted to allylidene cyclopropanes by two cycles of alkylation and treatment with base (Section 5.2.2.4.). 

This type of approach is inevitably limited and we need to look at more general methods of making alkenyl-lithiums since these can be converted into all the other reagents we might need. 

An analogous vinylketene intermediate (127, see Schemes 57 and 59) as proposed for the Dotz reaction has been assumed in the so-called cyclobutenedione methodology [161]. The key intermediate is a 4-aryl or 4-alkenyl substituted 2-cyclobutenone (128) that can be obtained e.g. by the reaction of the 3-cyclo-butene-1,2-dione (129) with the appropriate lithium reagent or Stille coupling with 4-chloro-3-cyclobutenone. Thermal cyclobutenone ring opening to the vinylketene 130 followed by electrocyclization furnishes the highly substituted aromatic compound 131 (see Scheme 59). 

Equilibria in mutual halogen-lithium exchanges have been measured67 to determine the relative stabilities of various alkyl-, cycloalkyl-, alkenyl-, and aryl-lithium reagents. 

Vinylcyclopropanols have been prepared by the addition of alkenyl Grignard reagents to a variety of cyclopropanone equivalents. Upon treatment with acid, the vinylcyclopropanols rearrange to o-substituted cyclobutanones. Alternatively, a variety of o-heteroatom-substituted cyclopropyl lithium reagents have been developed. These react with aldehydes and ketones to afford cyclopropylcarbinols which also rearrange to cyclobutanones under acid catalysis.Lastly, vinylcyclopropanols and cyclopropylcarbinols have been prepared by the cyclopropanation of enol silyl ethers and allylic alcohols. 

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