Allylmetal reagents have been widely used in organic synthesis. [1] In addition, allylic boranes [2] and silanes [3] are highly versatile owing to their wide availability, high stability, and low toxicity as well as excellent chemo-, regio-, and stereoselectivities. In sharp contrast, little attention has been paid to 1-silyl-1-boryl-2-alkenes, though such reagents can be regarded as a hybrid of allylic boranes and silanes and are attractive for the construction of stereochemically complex molecules. [4] Yamamoto, Yatagai, and Maruyama prepared atrimethylsilyl-substituted crotyl-9-BBN (9-BBN 9-borabicyclo[3.3.1]nonane) by deprotonation of crotyl-9-BBN followed by silylation with chlorotrimethylsilane. [5] Similar boronates were synthesized by Tsai and Matteson by the homologation of alkenylboronates with [chloro(trimethylsilyl)methyl]lithium. [6] Both reagents were found to allylate aldehydes [7] in a manner similar to allylic boranes. Although this methodology is effective for acyclic stereocontrol, allylation with allylic silanes and stereospecificity in boronselective allylation remained to be explored. Herein we describe the novel stereocontrolled synthesis of 1-silyl-1boryl-2-alkenes by gem-silylborylation of a-chloroallyllithi-ums and dual stereospecific allylation of aldehydes with silicon or boron functionality of the gem-silylboryl reagents.Recently, the novel gem-silylborylation reaction of 1-halo-1-lithio-1-alkenes with (dimethylphenylsilyl)(pinacolato)borane (1) was disclosed [8] and is considered to proceed via borate formation followed by 1,2-migration of a silyl group from an ate-type boron to a carbenoid carbon. We envisaged that the title gem-silylboryl reagents 2 might be prepared readily by the gem-silylborylation with 1 of vinyl-substituted carbenoids [Eq. (1)]. [9,10] Thus, we treated 1 [11] with a-chloroallyllithiums generated in situ from allylic chlorides and lithium diisopropylamide (LDA) in THF at À 98 8C. The results are summarized in Table 1. Allyl chloride was converted into 2 a in 82 % yield (Entry 1). Substituted allylic chlorides were also gemsilylborylated smoothly in good yields irrespective of the substitution pattern (Entries 2 ± 7). No trace of g-silyl-aboration by 1,4-migration of a silyl group was observed. Noteworthy is that the olefinic configuration was perfectly retained in compounds 2 (Entries 3 ± 5): stereochemically pure allylic chlorides gave single stereoisomers of 2.