Crotyl organometallic

The reaction of crotyl organometallic compounds 1 with aldimines 2 results in linear 3 or branched 4 homoallylamines. 

Table 3 Addition of Crotyl Organometallics to Aldehydes (equation 28)...

The reaction of allylic organometallics with electrophilic reagents is a very important tool for the formation of carbon-carbon bonds in acyclic systems and for controlling their stereochemistry. Crotyl organometallic (2-butenylmetal) species undergo a 1,3-shift of the metal at room temperature. For the stereocontrolled use of allylmetals in synthesis, it is important to avoid their equilibration. 

Reactions of Type 111 Crotyl Organometallics with Achiral Aldehydes and Ketones... 

Three classes of crotyl organometallics have now been identified, based on mechanistic and stereochemical preferences. ... 

Type I crotyl organometallics react with aldehydes, presumably via chair-like transition states, such that the stereochemical information present in the reagent is transmitted to an anti (from ( )-alkene precursors) or a syn (from (Z)-alkene precursors) relationship about the new C—C bond of the product (Scheme 3). A detailed analysis of possible transition states appears in a subsequent section. Type I stereoselectivity has been observed for crotyl organometallics incorporating boron, aluminum, silicon and tin (thermal reactions). 

Allylboron compounds have been the most widely studied of the type 1 allyl organometallics. Of these, the dialkylcrotylboranes isomerize most readily and often require handling at temperatures below -78 °C for isomerization to be suppressed. Thus, it is probably more appropriate to view such compounds ie.g. crotyl-9-BBN) as type III crotyl organometallics. The boratropic isomerization of dialkylcrotylboranes, however, is sensitive to steric factors. For example, the ( )- and... 

Transition states for the reactions of type I and type III crotyl organometallics with C=X electrophiles... 

Possible transition states for the reactions of type I and III crotyl organometallics with aldehydes are depicted in Scheme 7. Most of the available stereochemical evidence suggests that these reactions proceed preferentially through transition state (12) in which the metal is coordinated to the carbonyl oxygen syn to the smallest carbonyl substituent, H. This necessitates that R of RCHO adopt an equatorial position if the transition state is chair-like, an arrangement that is structurally similar to the Zimmerman-Traxler model commonly invoked for many aldol reactions. Transition states (13) and (14), however, may potentially intervene and are frequently cited to rationalize the production of minor diastereomers (17). 

Insight into the structure of several allyl/crotyl organometallic reagents has been obtained spectroscopically. Kramer and Brown have examined the NMR spectra of 3-allyl-9-BBN and 3-crotyl-9-BBN... 

Reactions of crotyl organometallics with aldimines can give linear (4) and/or branched (5) homoallyl-amines, a result of addition at the a- and y-crotyl carbon atoms (equation 1). Although branched products usually dominate, the metal (M) and the imine substituents (R and R ) can strongly influence the regiochemical outcome of the reaction similar effects are observed with crotyl in additions to iminium salts and gem-amino ethers. 

Figure 5 Postulated cyclic chair and boat transition states involved in the reactions of crotyl organometallics"...

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