Cr-alkylpalladium complex

Furthermore, treatment of the aminopalladation product with bromine affords aziridines[176]. The aziridine 160 was obtained stereoselectively from methylamine and 1-decene in 43% yield. The aminopalladation of PdCl2 complexes of ethylene, propylene, and 1-butene with dietliylamine affords the unstable cr-alkylpalladium complex 161, which is converted into the stable chelated acylpalladium complex 162 by treatment with CO[177]. 

Chelating alkenes such as allylic10 and homoallylic11 amines and sulfides underwent alkylation by a range of stabilized carbanions to produce stable cr-alkylpalladium(II) complexes. In these cases the regioselectivity was strictly governed by the inherent stability of a five (versus four or six) membered chelate cr-alkylpalladium complex, with allylic systems (Scheme 5) being alkylated at the more sub-... 

While neopentyl-type and related cr-alkylpalladium complexes are stable toward dehy-dropalladation, all ordinary alkylpalladium complexes formed by carbopalladation of alkenes do undergo /S-hydride eUminalion more or less rapidly. However, under certain structural precondilions and also, to a certain extent, controlled by the external conditions, rearranganents, further carbopalladations (Sect. IV.3), transmetallations, car-bonylations, or nucleophilic substitutions (e.g., by hydride) can occur prior to /3-dehy-dropalladation. 

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

A number of intramolecular Mizoroki-Heck reactions yield the product consistent with a formal a r/-elimination of the HPdX [11], These experimental findings are in opposition to the generally accepted mechanism of a 5y -elimination however, a reasonable explanation is at hand in most cases. There are two main types of alkenyl derivatives which, if added to an CT-aryl- or cr-alkenylpalladium(II) complex, deliver the formal a ft-elimination product. The first case is intramolecular Mizoroki-Heck reactions with o ,jS-unsaturated carbonyl systems which result in the product of a formal 1,4-addition. The initially formed <7-(y3-aryl)- or <7-(/3-alkenyl)alkylpalladium complex should be long-lived enough to epimerize through a palladium(II) enolate intermediate and, thus, deliver the formal anr/-elimination product through conventional 5yn-elimination (Scheme 6.2). 

The reactions catalyzed by cationic palladium complexes are believed to proceed via a different mechanism (Scheme 67).273 Initially, a cationic silylpalladium(n) species is generated by cr-bond metathesis of the Br-Pd+ with a silylstannane. Subsequently, the alkyne and alkene moieties of the 1,6-diyne successively insert into the Pd-Si bond to form a cationic alkylpalladium(n), which then undergoes bond metathesis with silylstannane to liberate the product and regenerate the active catalyst species, S/-Pd+. 

These reactions begin the same way as Hg-mediated nucleophilic addition to alkenes. The alkene coordinates directly to Pd(ll) to form an electrophilic tt complex. The nucleophile attacks one of the C s of the tt complex, and the electrons from the C=C tt bond move to form a cr bond between Pd(ll) and the other C to give an alkylpalladium(II) compound. The alkylpalladium(Il) compound then undergoes /3-hydride elimination to give the observed product and a palladium(ll)... 

---