Methylcopper

The attack of methylcopper reagents from the endo side is sterically hindered, forcing the reagents to attack from the exo side. Similarly, the hindered allylic bromide gives the product derived from a jv -Sn2 reaction311. 

The methyl group is delivered syn to the epoxide via an intermediate chelate between the organometallic reagent and the epoxide oxygen. Consistent with this hypothesis is the observation that the civ selectivity is increased as the solvent polarity is decreased and that addition of trimethylaluminum, which can strongly coordinate to the epoxide, gives nearly exclusively the trans-product. In the latter reaction, it was assumed that the addition of methylcopper occurs anti to the chelated epoxide moiety, possibly via an SN2 mechanism20. 

Vinylic dimerization can be achieved by treatment of divinylchloroboranes (prepared by addition of BH2CI to alkynes see 15-16) with methylcopper. ( , )-1,3-Dienes are prepared in high yields. 

The dry solid is very impact-sensitive and may explode spontaneously on being allowed to dry out at room temperature [1], Methylcopper decomposes explosively at ambient temperature, and violently in presence of a little air [2],... 

Methyl-3-buten-1 -ynyltriethyHead, 3413 Methylcopper, 0440 Methylenedilithium, 0401 Methylenemagnesium, 0402 Methyllithium, 0452 Methylpotassium, 0448 Methylsilver, 0421 Methylsodium, 0463 Octylsodium, 3053 Pentamethyltantalum, 2046 Potassium cyclopentadienide, 1846 Propylcopper(I), 1251 Propyllithium, 1253 Propylsodium, 1269... 

Complex hydrides have been used rather frequently for the conjugate reduction of activated dienes92-95. Just and coworkers92 found that the reduction of a,ft-unsaturated ketene 5,5-acetals with lithium triethylborohydride provided mixtures of 1,4- and 1,6-reduction products which were transformed into enals by treatment with mercuric salts (equation 27). Likewise, tetrahydro-3//-naphthalen-2-ones can be reduced with L-Selectride to the 1,6-reduction products93 -95 this reaction has been utilized in the stereoselective synthesis of several terpenes, e.g. of (/ )-(—)-ligularenolide (equation 28)95. Other methods for the conjugate reduction of acceptor-substituted dienes involve the use of methylcopper/diisobutylaluminum hydride96 and of the Hantzsch ester... 

The transmetalation of dialkenylchloroboranes of type 52 with methylcopper (3 equiv.) provides an alkenylcopper compound 53, which undergoes cross-coupling with allylic halides to produce mixtures of Sn2 and Sn2 products. Interestingly, this method is also useful for the preparation of functionalized alkenylcoppers such as 54 (Scheme 2.14) [31]. 

In the 1952 paper mentioned above [3], Gilman reported on the formation of lithium dimethylcuprate from polymeric methylcopper and methyllithium. These so-called Gilman cuprates were later used for substitution reactions on both saturated [6] and unsaturated [7, 8, 9] substrates. The first example of a cuprate substitution on an allylic acetate (allylic ester) was reported in 1969 [8], while Schlosser reported the corresponding copper-catalyzed reaction between an allylic acetate and a Grignard reagent (Eq. 2) a few years later [10]. 

Table 8. Reactions of diphenylstibanybmethyllithium 25a and-methylcopper 25b with electrophilic compounds )...

When the CuBr-SMe2 complex is used in preparing the organocopper reagent, and Me2S is a cosolvent in the addition, higher yields may be achieved. Under such conditions even the otherwise unreactive methylcopper adds to terminal alkynes to generate 2-methyl-susbtituted alkenylcopper derivatives.520... 

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