1- alkenylboranes

However, in the reaction of 1-alkenylboranes with aryl- or 1-alkenyi iodides. 2-aryl-l-alkenes 648 are obtained as the main products. When Pd metal produced from Pd(OAc)2 as a catalyst and EtjN as a weak bu.se are u.sed. abnormal products are formed. On the other hand, normal products 649 are obtained by using NaOH[5l7]. 

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

The procedure described here Is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of l-a1kynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1). Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

Coupling of 1-alkenylboranes with 1-bromoalkenes or -alkynes.4,s This reaction fails when catalyzed by Pd(0) alone, but proceeds in high yield in the presence of an added sodium alkoxide or sodium hydroxide in refluxing benzene (or THF). This coupling can be used to prepare (E,E)-, (E,Z)-, (Z,E)-, or (Z,Z)-1,3-dienes stereo- and regiospecifically. 

Another method for the preparation of acetylenic carboxylates involves the reaction of 1-alkenylboranes with CO and alcohols (equation 135). The alkenyl boranes are readily prepared from alkynes by addition of boranes. PdCl2 was again the catalyst, with NaOAc as base and benzoquinone as oxidant.541... 

Potential difference in reactivity between two G-B bonds allowed the transformation of l,2-bis(boryl)-l-alkenes to 1-alkenylboranes via a cross-coupling with the aryl, 1-alkenyl, benzyl, and cinnamyl halides (Equation (23)).211-213 This tandem procedure synthetically equivalent to a yy/z-carboboration of alkynes was used for synthesizing Tamoxifen derivatives via stepwise double coupling with two of the G-B bonds.212,213 Hydrogenation of the resulting bisborylalk-enes with a chiral rhodium catalyst is synthetically equivalent to an asymmetric diboration of alkenes (Equation (24)).214... 

The photocycloaddition of 1-alkenylboranes in cyclohexane provided m-fused head-to-head adducts for 2-cyclopen-tenone and a mixture of cis- and trans-isomer for 2-cyclohexenone (Equation (71)).397... 

Cycloaddition reaction of m-chlorotributylstannane was catalyzed by BC13 or 9-Br-9-BBN at 25 °C (Equation (72)).404 A catalytic cycle involved [2+4]-reaction of 1-alkenylborane, generated in situ by Si-B transmetallation. 

Alkenylboranes were proved to be useful traps for nucleophilic alkyl radicals, thus opening attractive routes to diversely substituted alkyl- and alkenylboronic esters (Equations (191) and (192)).457... 

The Zweifel trans-alkene synthesis from dialkyl-1-alkenylboranes involves the hy-droboration of 1-halo-l-alkynes with dialkylborane (Eq. 69) 126). It is of interest that the base produces a transfer with inversion 126 127). 

Recently, Rossi et al. have applied the cross-coupling of 1-alkenylboranes with 1-alkynyl halides for the synthesis of natural products. Thus, (7 , 9Z)-7,9-dodecadien-1-yl acetate (32), the sex pheromone of Lobesia botrana, has been synthesized by the following sequence involving ... 

As described in previous sections, the palladium catalyzed cross-coupling reaction of stereodefined 1-alkenylboranes with stereodefined 1-alkenyl halides in the presence of relatively strong bases such as sodium alkoxides and hydroxide ( head-to-head... 

Alkenylboron compounds couple with the representative organic halides or triflates (Scheme 28). Hexaalkylbenzene was synthesized by sixfold alkenylation (55) of hexabromobenzene followed by catalytic hydrogenation of the double bonds 11521. The reaction of 1-alkenylborane with 1-bromo-l-alkyne stereose-lectively provided ( )-enyne (56) which was then converted into ( ,Z)-hexa-deca-10,12-dienal, a sex pheromone of the melonworm 11531. Due to the difficulty of purification of a geometrical mixture, the stereoselective synthesis is critical for such dienes or trienes. The PGEi derivatives (57) were synthesized... 

Next, the reactions of 1-alkenyltrialkylborates will be discussed. One of the most interesting and versatile reactions of such 1-alkenylboranes readily obtainable... 

VI Synthesis via 1-Alkenylboranes in the Presence of PaUadium Catalysts and Bases... 

Alkenylboranes also react with carbon monoxide in the primes of j ladium chloride and sodium acetate in methanol to yield oc, -unsaturated otrboxylic acid esters while retaining the original configuration of the double bonds of 1-alkenyl-boranes (Eq. 109)... 

Most recently, Rossi et al. have applied the cross-coupling of 1-alkenylboranes... 

Most recently, it has been disclosed that 1-alkenylboranes react with 3,4-epoxy-1-butene in the presence of a pall ium or nickel complex to form the two corresponding coupling products, 2-ethenyl-3-alken-l-ol (52) and 2,5-alkadien-l-ol (ii) (Eq. 115). ... 

Cross-coupling of aryl halides and 1-alkenylboranes. Aryl iodides and bromides couple efficiently with 1-alkenylboranes (prepared from acetylenes and catecholborane, 4, 70) in the presence of this Pd(0) complex and a base (NaOC2Hs) (equation I). The reaction proceeds with retention of configuration with respect to the alkenylborane to form only (E)-alkenes. Yields are typically 81-100%. One example is reported where the alkenylborane was prepared with disiamylborane (yield 41%)."... 

The versatility of such cross-coupling reactions of 1-alkenylboranes has been... 

Alkynylborates are attacked by many electrophiles at the position f to the boron atom. The rearrangement shown in Scheme 2-9 gives a variety of functionalized 1-alkenylboranes [2, 31]. Tlie stereochemistry can be either E or Z, or a mixture of the two in most cases. 

Cross-coupling reactions between 1-alkynyl halides and 1-alkenylboranes, which are readily available via hydroboration of alkynes, can also be catalyzed by Pd-phosphine complexes. Stereoselective synthesis of conjugated ( )-enynes 143 was achieved by the coupling of the alkynyl bromide with an alkenylborane [Eq. (48)] [69]. 

Miyaura, N., Yamada, K., Suzuki, A. A new stereospecific cross-coupling by the palladium-catalyzed reaction of 1-alkenylboranes with 1-alkenyl or 1-alkynyl halides. Tetrahedron Lett. 1979, 3437-3440. 

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