Methylmagnesium bromide, reaction with

Recently, Filrstner and coworkers have prepared a super-ate complex of iron(II) as shown in Scheme 5. The structure was fully characterized by X-ray crystallography. They have shown that methylmagnesium bromide reacts with pulegone in the presence of this complex to give the corresponding endocyclic silyl dienol ether. Consequently, they have proposed that a similar ate-complex is probably involved when the reaction is performed under the Kharasch conditions. 

Scheme 9.74. A representation of the reaction of methylmagnesium bromide ( CHsMgBr ) with cyclohexanone to produce 1-methylcyclohexanol and the loss of water from the latter to generate a mixtnre of 1-methylcyclohexene and methylenecyclohexene. The former predominates in the alkene product mixture.

The reaction of l,4-bis(trimethylsilyl)-l,3-butadiyne (174) with disilanes, followed by treatment with methylmagnesium bromide, produces i,l,4,4-tetra(-trimethylsilyl)-l,2,3-butatriene (175) as a major product[96]. The reaction of octaethyltetrasilylane (176) with DMAD proceeds by ring insertion to give the six-membered ring compounds 177 and 178[97], The l-sila-4-stannacyclohexa-2,5-diene 181 was obtained by a two-step reaction of two alkynes with the disilanylstannane 179 via the l-sila-2-stannacyclobutane 180[98],... 

Reaction of 17j -acetoxy-3,3-ethylenedioxy-5a-androstan-l-one (1) with methylmagnesium bromide followed by treatment with acid and reacetylation affords the 1-methyl-A -3-ketone (3). The configuration of carbon-1 of the intermediate (2) has not been established/ ... 

The 11-keto group is relatively difficult to attack, due to steric hindrance. However, reaction of 3j -hydroxy-5a-androstane-l 1,17-dione (37) with methylmagnesium bromide at 25° unexpectedly gives a 30% yield of 1 la,17a-dimethyl-5a-androstane-3j5,lljS,17i -triol (38) in addition to the 17-monomethyl product (39). 

Bromination of the enolate anion from the reaction of 3j -acetoxypregna-5,16-dien-20-one (1) with methylmagnesium bromide in the presence of cuprous chloride affords (after treatment with sodium iodide to dehalo-genate any 5,6-dibromide) a mixture of 17a-bromo- and 17)5-bromo-16a-methyl compounds (11) and (12) in a ratio 9 1. The 17a-iodides can be obtained in an analogous reaction. 

The actual mechanism by which a particular reaction proceeds strongly depends on the nature of the organomagnesium reagent. For instance benzophenone reacts with methylmagnesium bromide by a polar mechanism, while the reaction with t-butylmagnesium chloride proceeds for steric reasons by a SET-mechanism. 

Nitriles (RC=N) react with Grignard reagents (R MgBr). The reaction produc from 2-methylpropanenitrile with methylmagnesium bromide has the fol lowing spectroscopic properties. Propose a structure. 

The teal value of the Wittig reaction is that it yields a pure alkene of defined structure. The C=C bond in the product is always exactly where the OO group was in the reactant, and no alkene isomers (except E,Z isomers) are formed. For example, Wittig reaction of cyclohexanone with methylenetriphenyl-phosphorane yields only the single alkene product methylenecyclohexane. By contrast, addition of methylmagnesium bromide to cyclohexanone, followed by dehydration with POCI3, yields a roughly 9 1 mixture of two alkenes. 

Thiocarbonyl tetrachloride, 46, 21 m Thiocresol (Warning), 47, 107 Thionyl chloride, 46, 16 98 Thiophosgene 46, 21 Thiophosphoryl chloride, reaction with methylmagnesium bromide to i lelci tetramethylbiphosphine disulhdc 46,102... 

When y-lactone 14 is treated with methylmagnesium bromide in THF at -78 °C, a carbonyl addition reaction takes place and gives, after aqueous workup, a hemiketal that is subsequently converted to... 

Ketone 13 possesses the requisite structural features for an a-chelation-controlled carbonyl addition reaction.9-11 Treatment of 13 with 3-methyl-3-butenylmagnesium bromide leads, through the intermediacy of a five-membered chelate, to the formation of intermediate 12 together with a small amount of the C-12 epimer. The degree of stereoselectivity (ca. 50 1 in favor of the desired compound 12) exhibited in this substrate-stereocontrolled addition reaction is exceptional. It is instructive to note that sequential treatment of lactone 14 with 3-methyl-3-butenylmagnesium bromide and tert-butyldimethylsilyl chloride, followed by exposure of the resultant ketone to methylmagnesium bromide, produces the C-12 epimer of intermediate 12 with the same 50 1 stereoselectivity. 

In fact, the highest anti-Cram selectivity reported to date (96% de) was observed with the MAT-mediated addition of methylmagnesium bromide to 2-(l-cyclohexenyl)propanal3 i 36. The stereochemical outcome of this addition reaction can be explained as follows on treatment of the carbonyl compound with the large aluminum reagent, the sterically least hindered complex 9 is formed. Subsequent addition of the nucleophile from the side opposite to the bulky aluminum reagent produces the anti-Cram diastereomer preferentially. 

In contrast to the results obtained with the jS-alkoxy-a-alkyl-y-lactol 16 (vide supra), a chelation-directed, anti-Cram selective nucleophilic addition to the a-methyl-y-lactol 1 was not only observed with methyllithium and methylmagnesium bromide but also with (triisopropoxy)methyl-titanium72. In fact, the highest diastereoselectivity (> 98 % de) was observed with the titanium reagent in dichloromethane as reaction solvent. A seven-membered chelate 3 with the a-methyl substituent in a pscudoequatorial position has been postulated in order to explain the stereochemical outcome. 

A complex reaction takes place when dichlorobis(triphenylphosphine)-nickel (5) is treated with excess methylmagnesium bromide in ether. Detectable amounts of benzene, toluene, and biphenyl are formed, together with mixed phosphines. Nickel appears to be necessary for the substitution reaction since triphenylphosphine alone does not react with the Grignard reagent. 

Scheme 9.4 shows some representative reactions of allylic and alkenyl silanes. Entry 1 involves 3-trimethylsilylcyclopentene, which can be made by hydrosilylation of cyclopentadiene by chlorodimethylsilane, followed by reaction with methylmagnesium bromide. 

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