Organozincs alkenes

The addition usually takes place from the sterically less hindered side of the alkene. The stereochemical course of the addition can be controlled by suitably positioned oxygen center that can coordinate to the organozinc reagent. For example the reaction with 4-hydroxycyclopentene 6 as substrate exclusively yields the c -3-hydroxybicyclo [3.1.0] hexane 7 ... 

The preparation of an unusual organozinc carbenoid 285 from amide 284 has been recently reported.390 The carbenoid 285 was used in amidocyclopropanation reactions of diverse alkenes (Scheme 148).390 Unfortunately, experiments employing other amides yielded the corresponding products in low yields. 

Trisubstituted alkenes. The (Z)-2-bromo-l-alkenylboranes (1), obtained by bromoboration of 1-alkynes with BBr, (13, 43), undergo coupling with organozinc chlorides in the presence of Cl2Pd[P(C6H5)3]2 to provide, after protonolysis, disub-stituted alkenes (3). However, the intermediate alkenylborane (2) can undergo a... 

Coupling of vinyl dichlorides.1 This catalyst effects selective coupling of Grig-nard reagents or organozinc chlorides with only one of the chlorine atoms of 1,1-dichloro-l-alkenes (2) to give (Z)-vinyl chlorides (3). The selective coupling involves the chlorine that is trans to the R substituent, probably because of a steric... 

Activation. Erdik1 has reviewed the methods used since 1970 for activation of zinc and of organozinc reagents. Although chemical activation is still useful, ultrasound activation is being used increasingly. Thus sonic activation allows use of ordinary zinc for cyclopropanation of alkenes with CH2I2 in 67-97% yield and for Reformatsky-type reactions at room temperatures. 

Scheme 7.7. Alkenes as starting materials in 1,4-additions involving (functionalized) organozinc reagents.

Organozinc species RXZnCH2l generated by reacting Zn(CH2l)2 with RXH (e.g. ROH or CF3CO2H) have been explored as effective agents for cyclopropanation of alkenes at room temperature chiral alcohols (RXH) induce asymmetric reaction. 

Enantioselective alkylative ring opening of these oxabicyclic alkenes has also been studied. Lautens and coworkers discovered that palladium complexes efficiently catalyze the addition of organozinc reagents to these activated alkenes with concomitant ring opening. In the presence of (Tol-BINAP)PdCl2, diethylzinc adds to oxabenzonor-... 

Functionalized zinc carbenoids have been prepared from carbonyl compounds by an indirect strategy. The deoxygenation of a carbonyl compound to an organozinc carbenoid can be induced by a reaction with zinc and TMSCl. Therefore, the aldehyde or ketone, when treated with TMSCl or l,2-bis(chlorodimethylsilyl)ethane in the presence of an alkene, generates the cyclopropanation product. This method is quite effective for the production of alkoxy-substituted cyclopropane derivatives. A 55% yield of the... 

Hydrozincation is not as widely observable as that involving , A1 and Zr. This is one of the main reasons why the indirect hydrometallation-transmetallation procedures shown in Schemes 8 and 12 have been developed and used. It is nevertheless highly desirable to directly generate organozincs to be used for the Pd- or Ni-catalyzed cross-coupling from alkenes and alkynes via hydrozincation. Indeed, such reactions have been developed, as shown in Scheme 17. However, further developments are clearly desirable. 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

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