Group 12 unsaturated moiety insertion

The addition of borane to alkenes was first reported by H. C. Brown et al. [3] in 1956. The anti-Markovnikov insertion of an unsaturated moiety into a B-H bond of the borane (R2BH, RBH2 and BH3) proved to be the initial step for introduction of a very wide variety of functional groups. Within the following decade, the same author described the replacement of a boron atom by an amino group, affording a synthetic route from alkenes to amines [4] (Scheme 1). 

In some instances, it is difficult to differentiate between the insertion of an unsaturated moiety into a metal-sulfur bond and addition of the dithiocarbamate to the group, as detailed in Section V.D. The reaction of [CpFe(CO)2(ti -S2CNR2)] (578) with KCN in dry methanol provides a good example of this. Thus, the very air-sensitive metallacyclic complex [CpFe(CO) ri -SC(NR2)SC(=N) ] (579) results (Fig. 306). Overall the process involves... 

Reaction of the stannylborane 9 with an allenyne gives a cyclization product, in which the boryl and stannyl groups are introduced to the acetylenic terminus and the allenic central carbon, respectively (Equation (104)).159 Based on the assumption that an unsaturated functionality initially inserts into the Pd-B bond of (boryl)(stannyl)palladium(n) species, it seems likely that the alkyne moiety is more reactive than the allene moiety in this reaction. 

In view of the remarkable stability of metal homoenolates of esters, the existence of homoenolate species containing a 3-halo substituent, i.e. zinc carbenoid moiety connected to an ester group, appeared to be possible. Indeed, when a silyl ketene acetal is treated with a carbenoid generated from CHBrj and Et2Zn, two types of highly intriguing reactions ensue [58]. With a purely aliphatic substrate, Eq. (61), an alkyl cyclopropylcarboxylate due to intramolecular p-CH-insertion of the intermediate zinc carbenoid formed. When the substrate contained an olefmic double bond in the vicinity of the carbenoid function, Eq. (62), in particular an intermediate derived from an a,P-unsaturated ester, internal cyclo-propanation occurred to give bicyclic or tricyclic product (Table 15). 

A one-pot synthesis of telechelic and semitelechelic poly(alkyl acrylates) with unsaturated end groups has been developed by Bielawski et al. [1]. ATRP of methyl acrylate or -BA was initiated with either ethyl cy-bromomethyl-acrylate or methyl dichloro acetate, as a monofunctional or a difunctional initiator, respectively, and was mediated with various Cu-amine complexes. Addition of excess ethyl 2-bromomethylacrylate was found to immediately quench the polymerization, but also to insert 2-carbethoxyallyl moieties at the ends of the polymer chains (Scheme 24). Thus, the synthesis of telechelic poly(alkyl acrylates) with unsaturated end groups has been accomplished, with very good functionality (f 2) (Scheme 24). 

Interaction of alkynyliodonium compounds with arylsulfinate salts is particularly interesting. When the R group of the alkynyl moiety lacks a y-CH bond, alkynyl sulfones (89, 90) are formed in excellent isolated yields [70, 71] [Eqs. (45), (46)]. When y-CH bonds are available, the intermediate unsaturated carbene (Scheme 3-3) prefers insertion over rearrangement and hence cyclopentenyl sulfones, 91, predominate, although some alkynyl sulfone formation is also observed as illustrated in [Eq. (47)] [72]. 

Finally, the behavior of 90 and its deuterated analogs toward unsaturated substrates was examined as to their stereochemical course. Phenylacetylene was found to insert in a syn-manner with retention of configuration at C-3 (92) 9-fluorenone caused complete loss of configuration of the indanyl moiety in forming the adduct 93 and acetone-dg underwent insignificant addition to its carbonyl group, but rather was transformed into its enol salt with the stereospecific formation of cis-2,3-dideuterio-l,l-dimethylindan (88) (Scheme 12) (25, 26). 

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